\input texinfo                @c -*- Texinfo -*-
@c %**start of header
@setfilename make.info

@include version.texi
@set EDITION 0.77

@settitle GNU Make
@setchapternewpage odd
@c Combine the variable and function indices:
@syncodeindex vr fn
@c Combine the program and concept indices:
@syncodeindex pg cp
@c FSF publishers: format makebook.texi instead of using this file directly.
@c ISBN confirmed by Jasimin Huang <jasimin@fsf.org> on 25 Mar 2009
@set ISBN 1-882114-83-3
@c %**end of header

@copying
This file documents the GNU implementation of the @code{make} utility.
@code{make} determines automatically which pieces of a large program need to
be recompiled, and issues the commands to recompile them.

This is Edition @value{EDITION}, last updated @value{UPDATED},
of @cite{The GNU Make Manual}, for GNU @code{make} version @value{VERSION}.

Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995,
1996, 1997, 1998, 1999, 2000, 2002, 2003, 2004, 2005, 2006, 2007,
2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019,
2020, 2021, 2022, 2023, 2024 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
and with the Back-Cover Texts as in (a) below.  A copy of the
license is included in the section entitled ``GNU Free Documentation
License.''

(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
modify this GNU manual.  Buying copies from the FSF supports it in
developing GNU and promoting software freedom.''
@end quotation
@end copying

@c finalout

@c ISPELL CHECK: done, 10 June 1993 --roland
@c ISPELL CHECK: done, 2000-06-25 --Martin Buchholz
@c ISPELL CHECK: done, 2023-08-30 --pds

@dircategory Software development
@direntry
* Make: (make).            Remake files automatically.
@end direntry

@iftex
@shorttitlepage GNU Make
@end iftex
@titlepage
@title GNU Make
@subtitle A Program for Directing Recompilation
@subtitle GNU @code{make} Version @value{VERSION}
@subtitle @value{UPDATED-MONTH}
@author Richard M. Stallman, Roland McGrath, Paul D. Smith
@page
@vskip 0pt plus 1filll
@insertcopying
@sp 2
Published by the Free Software Foundation @*
31 Milk Street, # 960789 @*
Boston, MA 02196 @*
USA  @*
ISBN @value{ISBN} @*
@sp 2
Cover art by Etienne Suvasa.
@end titlepage

@summarycontents
@contents

@ifnottex
@node Top, Overview, (dir), (dir)
@top GNU Make

@insertcopying
@end ifnottex

@menu
* Overview::                    Overview of @code{make}.
* Introduction::                An introduction to @code{make}.
* Makefiles::                   Makefiles tell @code{make} what to do.
* Rules::                       Rules describe when a file must be remade.
* Recipes::                     Recipes say how to remake a file.
* Using Variables::             You can use variables to avoid repetition.
* Conditionals::                Use or ignore parts of the makefile based
                                  on the values of variables.
* Functions::                   Many powerful ways to manipulate text.
* Invoking make: Running.       How to invoke @code{make} on the command line.
* Implicit Rules::              Use implicit rules to treat many files alike,
                                  based on their file names.
* Archives::                    How @code{make} can update library archives.
* Extending make::              Using extensions to @code{make}.
* Integrating make::            Integrating @code{make} with other tools.
* Features::                    Features GNU Make has over other @code{make}s.
* Missing::                     What GNU Make lacks from other @code{make}s.
* Makefile Conventions::        Conventions for writing makefiles for
                                  GNU programs.
* Quick Reference::             A quick reference for experienced users.
* Error Messages::              A list of common errors generated by @code{make}.
* Troubleshooting::             Advice on finding problems.
* Complex Makefile::            A real example of a straightforward,
                                  but nontrivial, makefile.

* GNU Free Documentation License::  License for copying this manual.
* Concept Index::               Index of Concepts.
* Name Index::                  Index of Functions, Variables, & Directives.

@detailmenu
 --- The Detailed Node Listing ---

Overview of @code{make}

* Preparing::                   Preparing and running @code{make}.
* Reading::                     On reading this text.
* Bugs::                        Problems and bugs.

An Introduction to Makefiles

* Rule Introduction::           What a rule looks like.
* Simple Makefile::             A simple makefile.
* How Make Works::              How @code{make} processes this makefile.
* Variables Simplify::          Variables make makefiles simpler.
* make Deduces::                Letting @code{make} deduce the recipes.
* Combine By Prerequisite::     Another style of makefile.
* Cleanup::                     Rules for cleaning the directory.

Writing Makefiles

* Makefile Contents::           What makefiles contain.
* Makefile Names::              How to name your makefile.
* Include::                     How one makefile can use another makefile.
* MAKEFILES Variable::          The environment can specify extra makefiles.
* Remaking Makefiles::          How makefiles get remade.
* Overriding Makefiles::        How to override part of one makefile
                                  with another makefile.
* Reading Makefiles::           How makefiles are read in.
* Parsing Makefiles::           How makefiles are parsed.
* Secondary Expansion::         How and when secondary expansion is performed.

What Makefiles Contain

* Splitting Lines::             Splitting long lines in makefiles

Writing Rules

* Rule Example::                An example explained.
* Rule Syntax::                 General syntax explained.
* Prerequisite Types::          There are two types of prerequisites.
* Wildcards::                   Using wildcard characters such as `*'.
* Directory Search::            Searching other directories for source files.
* Phony Targets::               Using a target that is not a real file's name.
* Force Targets::               You can use a target without a recipe
                                  or prerequisites to mark other targets
                                  as phony.
* Empty Targets::               When only the date matters and the
                                  files are empty.
* Special Targets::             Targets with special built-in meanings.
* Multiple Targets::            When to make use of several targets in a rule.
* Multiple Rules::              How to use several rules with the same target.
* Static Pattern::              Static pattern rules apply to multiple targets
                                  and can vary the prerequisites according to
                                  the target name.
* Double-Colon::                How to use a special kind of rule to allow
                                  several independent rules for one target.
* Automatic Prerequisites::     How to automatically generate rules giving
                                  prerequisites from source files themselves.

Using Wildcard Characters in File Names

* Wildcard Examples::           Several examples.
* Wildcard Pitfall::            Problems to avoid.
* Wildcard Function::           How to cause wildcard expansion where
                                  it does not normally take place.

Searching Directories for Prerequisites

* General Search::              Specifying a search path that applies
                                  to every prerequisite.
* Selective Search::            Specifying a search path
                                  for a specified class of names.
* Search Algorithm::            When and how search paths are applied.
* Recipes/Search::              How to write recipes that work together
                                  with search paths.
* Implicit/Search::             How search paths affect implicit rules.
* Libraries/Search::            Directory search for link libraries.

Static Pattern Rules

* Static Usage::                The syntax of static pattern rules.
* Static versus Implicit::      When are they better than implicit rules?

Writing Recipes in Rules

* Recipe Syntax::               Recipe syntax features and pitfalls.
* Echoing::                     How to control when recipes are echoed.
* Execution::                   How recipes are executed.
* Parallel::                    How recipes can be executed in parallel.
* Errors::                      What happens after a recipe execution error.
* Interrupts::                  What happens when a recipe is interrupted.
* Recursion::                   Invoking @code{make} from makefiles.
* Canned Recipes::              Defining canned recipes.
* Empty Recipes::               Defining useful, do-nothing recipes.

Recipe Syntax

* Splitting Recipe Lines::      Breaking long recipe lines for readability.
* Variables in Recipes::        Using @code{make} variables in recipes.

Recipe Execution

* One Shell::                   One shell for all lines in a recipe.
* Choosing the Shell::          How @code{make} chooses the shell used
                                  to run recipes.

Parallel Execution

* Parallel Disable::            Disabling parallel execution
* Parallel Output::             Handling output during parallel execution
* Parallel Input::              Handling input during parallel execution

Recursive Use of @code{make}

* MAKE Variable::               The special effects of using @samp{$(MAKE)}.
* Variables/Recursion::         How to communicate variables to a sub-@code{make}.
* Options/Recursion::           How to communicate options to a sub-@code{make}.
* -w Option::                   How the @samp{-w} or @samp{--print-directory} option
                                  helps debug use of recursive @code{make} commands.

How to Use Variables

* Variable Naming::             Choosing names for variables.
* Reference::                   How to use the value of a variable.
* Flavors::                     Variables come in two flavors.
* Expanding::                   How text is expanded by @code{make}.
* Values::                      All the ways variables get their values.
* Setting::                     How to set a variable in the makefile.
* Substitution Refs::           Substituting values in variable expansion.
* Override Directive::          How to set a variable in the makefile even if
                                  the user has set it with a command argument.
* Multi-Line::                  An alternate way to set a variable
                                  to a multi-line string.
* Undefine Directive::          How to undefine a variable so that it appears
                                  as if it was never set.
* Environment::                 Variable values can come from the environment.
* Target-specific::             Variable values can be defined on a per-target
                                  basis.
* Pattern-specific::            Target-specific variable values can be applied
                                  to a group of targets that match a pattern.
* Suppressing Inheritance::     Suppress inheritance of variables.
* Special Variables::           Variables with special meaning or behavior.

Basics of Variable References

* Computed Names::              Computing the name of a variable reference.

The Two Flavors of Variables

* Recursive Variables::         Recursive variables delay expansion.
* Simple Variables::            Simple variables expand immediately.

Setting Variables

* Immediate Assignment::        Recursive variables with immediate expansion.
* Shell Assignment::            Assigning variables to shell output.
* Conditional Assignment::      Assigning variables only if not yet defined.
* Appending Assignment::        How to append to the value of a variable.
* Whitespace in Values::        Leading and trailing whitespace in values.

Conditional Parts of Makefiles

* Conditional Example::         Example of a conditional
* Conditional Syntax::          The syntax of conditionals.
* Testing Flags::               Conditionals that test flags.

Functions for Transforming Text

* Syntax of Functions::         How to write a function call.
* Text Functions::              General-purpose text manipulation functions.
* File Name Functions::         Functions for manipulating file names.
* Conditional Functions::       Functions that implement conditions.
* Let Function::                Local variables.
* Foreach Function::            Repeat some text with controlled variation.
* File Function::               Write text to a file.
* Call Function::               Expand a user-defined function.
* Value Function::              Return the un-expanded value of a variable.
* Eval Function::               Evaluate the arguments as makefile syntax.
* Origin Function::             Find where a variable got its value.
* Flavor Function::             Find out the flavor of a variable.
* Make Control Functions::      Functions that control how make runs.
* Shell Function::              Substitute the output of a shell command.
* Guile Function::              Use GNU Guile embedded scripting language.

How to Run @code{make}

* Makefile Arguments::          How to specify which makefile to use.
* Goals::                       How to use goal arguments to specify which
                                  parts of the makefile to use.
* Instead of Execution::        How to use mode flags to specify what
                                  kind of thing to do with the recipes
                                  in the makefile other than simply
                                  execute them.
* Avoiding Compilation::        How to avoid recompiling certain files.
* Overriding::                  How to override a variable to specify
                                  an alternate compiler and other things.
* Testing::                     How to proceed past some errors, to
                                  test compilation.
* Warnings::                    How to control reporting of makefile issues.
* Temporary Files::             Where @code{make} keeps its temporary files.
* Options Summary::             Summary of Options

Using Implicit Rules

* Using Implicit::              How to use an existing implicit rule
                                  to get the recipes for updating a file.
* Catalogue of Rules::          A list of built-in rules.
* Implicit Variables::          How to change what predefined rules do.
* Chained Rules::               How to use a chain of implicit rules.
* Pattern Rules::               How to define new implicit rules.
* Last Resort::                 How to define a recipe for rules which
                                  cannot find any.
* Suffix Rules::                The old-fashioned style of implicit rule.
* Implicit Rule Search::        The precise algorithm for applying
                                  implicit rules.

Defining and Redefining Pattern Rules

* Pattern Intro::               An introduction to pattern rules.
* Pattern Examples::            Examples of pattern rules.
* Automatic Variables::         How to use automatic variables in the
                                  recipe of implicit rules.
* Pattern Match::               How patterns match.
* Match-Anything Rules::        Precautions you should take prior to
                                  defining rules that can match any
                                  target file whatever.
* Canceling Rules::             How to override or cancel built-in rules.

Using @code{make} to Update Archive Files

* Archive Members::             Archive members as targets.
* Archive Update::              The implicit rule for archive member targets.
* Archive Pitfalls::            Dangers to watch out for when using archives.
* Archive Suffix Rules::        You can write a special kind of suffix rule
                                  for updating archives.

Implicit Rule for Archive Member Targets

* Archive Symbols::             How to update archive symbol directories.

Extending GNU @code{make}

* Guile Integration::           Using Guile as an embedded scripting language.
* Loading Objects::             Loading dynamic objects as extensions.

GNU Guile Integration

* Guile Types::                 Converting Guile types to @code{make} strings.
* Guile Interface::             Invoking @code{make} functions from Guile.
* Guile Example::               Example using Guile in @code{make}.

Loading Dynamic Objects

* load Directive::              Loading dynamic objects as extensions.
* Initializing Functions::      How initializing functions are called.
* Remaking Loaded Objects::     How loaded objects get remade.
* Loaded Object API::           Programmatic interface for loaded objects.
* Loaded Object Example::       Example of a loaded object

Integrating GNU @code{make}

* Job Slots::                   Share job slots with GNU Make.
* Terminal Output::             Control output to terminals.

Sharing Job Slots with GNU @code{make}

* POSIX Jobserver::             Using the jobserver on POSIX systems.
* Windows Jobserver::           Using the jobserver on Windows systems.

Quick Reference

* Makefile Directives::         All makefile directives.
* Makefile Functions::          All makefile built-in functions.
* Automatic Variable Reference::  All automatic variables for recipes.
* Special Variable Reference::  All special variables for makefiles.

Troubleshooting Make and Makefiles

* Parse Error::                 Syntax errors when parsing makefiles.
* Command Failure::             Recipe commands exit with error codes.
* Wrong Rule::                  @code{make} chooses the wrong rule.
* No Rule Found::               No rule was found to build a target.
* Extra Rebuilds::              Targets are rebuilt unnecessarily.
* Missing Rebuilds::            Out-of-date targets are not rebuilt.
* Troubleshooting Strategies::  Strategies used for troubleshooting issues.

@end detailmenu
@end menu

@node Overview
@comment  node-name,  next,  previous,  up
@chapter Overview of @code{make}

The @code{make} utility determines which files in a project are out of date,
and runs commands to bring them up to date.  This manual describes the GNU
project's implementation of @code{make}, GNU Make, which was created by
Richard Stallman and Roland McGrath.  Paul D. Smith has handled development
and maintenance since Version 3.76 (1997).

GNU @code{make} conforms to @cite{IEEE Standard 1003.1-2024} (POSIX.1-2024).
@cindex POSIX
@cindex IEEE Standard 1003.1
@cindex standards conformance

Our examples show C programs, since they are common, but you can use
@code{make} with any programming language whose compiler can be run with a
shell command.  Indeed, @code{make} is not limited to compiling programs: it
can be used to automate any task where some files need to be updated
automatically whenever other files have been changed.

@menu
* Preparing::                   Preparing and running @code{make}.
* Reading::                     On reading this text.
* Bugs::                        Problems and bugs.
@end menu

@node Preparing
@section Preparing and Running Make

To use @code{make}, you must write a file, called a @dfn{makefile}, that
describes the relationships among files in your project and provides commands
for updating each file.  For example, in a typical C program the executable
file is updated from object files, which are in turn made by compiling source
files.

Once a suitable makefile exists, each time you change some source files,
this simple shell command:

@example
make
@end example

@noindent
suffices to perform all necessary recompilations.  The @code{make} program
uses the information in the makefile and the last-modification times of the
files to decide which of the files need to be updated.  For each of those
files, it issues the recipes provided in the makefile.

You can provide command line arguments to @code{make} to control which
files should be recompiled, or how.  @xref{Running, ,How to Run
@code{make}}.

@node Reading
@section How to Read This Manual

If you are new to @code{make}, or are looking for a general
introduction, read the first few sections of each chapter, skipping the
later sections.  In each chapter, the first few sections contain
introductory or general information and the later sections contain
specialized or technical information.
@ifnottex
The exception is the second chapter, @ref{Introduction, ,An
Introduction to Makefiles}, all of which is introductory.
@end ifnottex
@iftex
The exception is @ref{Introduction, ,An Introduction to Makefiles},
all of which is introductory.
@end iftex

If you are familiar with other @code{make} programs, see @ref{Features,
,Features of GNU @code{make}}, which lists the enhancements GNU Make has, and
@ref{Missing, ,Incompatibilities and Missing Features}, which explains the few
things GNU Make lacks that others have.

For a quick summary, see @ref{Options Summary}, @ref{Quick Reference},
and @ref{Special Targets}.

If you have a makefile already and it is not working as you expect,
@pxref{Troubleshooting, ,Troubleshooting Make and Makefiles}.

@node Bugs
@section Problems and Bugs
@cindex reporting bugs
@cindex bugs, reporting
@cindex problems and bugs, reporting

If you have problems with GNU @code{make} or think you've found a bug,
please report it to the developers; we cannot promise to do anything but
we might well want to fix it.

Before reporting a bug, make sure you've actually found a real bug.
Carefully reread the documentation and see if it really says you can do
what you're trying to do.  If it's not clear whether you should be able
to do something or not, report that too; it's a bug in the
documentation!

Before reporting a bug or trying to fix it yourself, try to isolate it
to the smallest possible makefile that reproduces the problem.  Then
send us the makefile and the exact results @code{make} gave you,
including any error or warning messages.  Please don't paraphrase
these messages: it's best to cut and paste them into your report.
When generating this small makefile, be sure to not use any non-free
or unusual tools in your recipes: you can almost always emulate what
such a tool would do with simple shell commands.  Finally, be sure to
explain what you expected to occur; this will help us decide whether
the problem was really in the documentation.

Once you have a precise problem you can report it in one of two ways.
Either send electronic mail to:

@example
    bug-make@@gnu.org
@end example

@noindent
or use our Web-based project management tool, at:

@example
    https://savannah.gnu.org/projects/make/
@end example

@noindent
In addition to the information above, please be careful to include the
version number of @code{make} you are using.  You can get this
information with the command @samp{make --version}.  Be sure also to
include the type of machine and operating system you are using.

If you have a code change you'd like to submit, see the @file{README} file
section ``Submitting Patches'' for information.

@node Introduction
@comment  node-name,  next,  previous,  up
@chapter An Introduction to Makefiles

You need a file called a @dfn{makefile} to tell @code{make} what to do.  For
example, the makefile might tell @code{make} how to compile and link a
program.
@cindex makefile

A makefile is a combination of two different ``languages'' in a single file.
Most of the makefile is written to be parsed by @code{make}, but also included
in the makefile are @emph{recipes} which contain the commands used to update
targets.  These commands are passed to a shell to be parsed and run so they
use shell syntax, not @code{make} syntax.  It's important to keep in mind the
difference between these two syntaxes when writing makefiles.

For a detailed description of the content of makefiles, @pxref{Makefile
Contents, ,What Makefiles Contain}.

A makefile is not a procedural list of steps to be taken.  Instead, it
describes a @dfn{directed acyclic graph}, where each node in the graph is a
potential target to be created and each edge in the graph is a prerequisite
relationship.  Every rule in a makefile defines (or updates) a node and
(optionally) some of the edges starting from that node.

In this chapter, we will discuss a simple makefile that describes how to
compile and link a text editor which consists of eight C source files
and three header files.  The makefile can also tell @code{make} how to
run miscellaneous commands when explicitly asked (for example, to remove
certain files as a clean-up operation).  To see a more complex example
of a makefile, see @ref{Complex Makefile}.

When @code{make} recompiles the editor, each changed C source file
must be recompiled.  If a header file has changed, each C source file
that includes the header file must be recompiled to be safe.  Each
compilation produces an object file corresponding to the source file.
Finally, if any source file has been recompiled, all the object files,
whether newly made or saved from previous compilations, must be linked
together to produce the new executable editor.
@cindex recompilation
@cindex editor

@menu
* Rule Introduction::           What a rule looks like.
* Simple Makefile::             A simple makefile.
* How Make Works::              How @code{make} processes this makefile.
* Variables Simplify::          Variables make makefiles simpler.
* make Deduces::                Letting @code{make} deduce the recipes.
* Combine By Prerequisite::     Another style of makefile.
* Cleanup::                     Rules for cleaning the directory.
@end menu

@node Rule Introduction
@comment  node-name,  next,  previous,  up
@section What a Rule Looks Like
@cindex rule, introduction to
@cindex makefile rule parts
@cindex parts of makefile rule

A simple makefile consists of ``rules'' with the following shape:

@cindex targets, introduction to
@cindex prerequisites, introduction to
@cindex recipes, introduction to
@example
@group
@var{target} @dots{} : @var{prerequisites} @dots{}
        @var{recipe}
        @dots{}
        @dots{}
@end group
@end example

A @dfn{target} is usually the name of a file that is generated by a
program; examples of targets are executable or object files.  A target
can also be the name of an action to carry out, such as @samp{clean}
(@pxref{Phony Targets}).

A @dfn{prerequisite} is a file that is used as input to create the
target.  A target often depends on several files.

@cindex tabs in rules
A @dfn{recipe} is an action that @code{make} carries out.  A recipe
may have more than one command, either on the same line or each on its
own line.  @strong{Please note:} you need to put a tab character at
the beginning of every recipe line!  This is an obscurity that catches
the unwary.  If you prefer to prefix your recipes with a character
other than tab, you can set the @code{.RECIPEPREFIX} variable to an
alternate character (@pxref{Special Variables}).

Usually a recipe is in a rule with prerequisites and serves to create a
target file if any of the prerequisites change.  However, the rule that
specifies a recipe for the target need not have prerequisites.  For
example, the rule containing the delete command associated with the
target @samp{clean} does not have prerequisites.

A @dfn{rule}, then, explains how and when to remake certain files
which are the targets of the particular rule.  @code{make} carries out
the recipe on the prerequisites to create or update the target.  A
rule can also explain how and when to carry out an action.
@xref{Rules, , Writing Rules}.

A makefile may contain other text besides rules, but a simple makefile
need only contain rules.  Rules may look somewhat more complicated
than shown in this template, but all fit the pattern more or less.

@node Simple Makefile
@section A Simple Makefile
@cindex simple makefile
@cindex makefile, simple

Here is a straightforward makefile that describes the way an
executable file called @code{edit} depends on eight object files
which, in turn, depend on eight C source and three header files.

In this example, all the C files include @file{defs.h}, but only those
defining editing commands include @file{command.h}, and only low
level files that change the editor buffer include @file{buffer.h}.

@example
@group
edit : main.o kbd.o command.o display.o \
       insert.o search.o files.o utils.o
        cc -o edit main.o kbd.o command.o display.o \
                   insert.o search.o files.o utils.o

main.o : main.c defs.h
        cc -c main.c
kbd.o : kbd.c defs.h command.h
        cc -c kbd.c
command.o : command.c defs.h command.h
        cc -c command.c
display.o : display.c defs.h buffer.h
        cc -c display.c
insert.o : insert.c defs.h buffer.h
        cc -c insert.c
search.o : search.c defs.h buffer.h
        cc -c search.c
files.o : files.c defs.h buffer.h command.h
        cc -c files.c
utils.o : utils.c defs.h
        cc -c utils.c
clean :
        rm edit main.o kbd.o command.o display.o \
           insert.o search.o files.o utils.o
@end group
@end example

@noindent
We split each long line into two lines using backslash/newline; this is
like using one long line, but is easier to read.  @xref{Splitting Lines,
, Splitting Long Lines}.
@cindex continuation lines
@cindex @code{\} (backslash), for continuation lines
@cindex backslash (@code{\}), for continuation lines
@cindex quoting newline, in makefile
@cindex newline, quoting, in makefile

To use this makefile to create the executable file called @file{edit},
type:

@example
make
@end example

To use this makefile to delete the executable file and all the object
files from the directory, type:

@example
make clean
@end example

In the example makefile, the targets include the executable file @samp{edit},
and the object files @samp{main.o}, @samp{kbd.o}, etc.  The prerequisites are
files such as @samp{main.c} and @samp{defs.h}.  You can see that each
@samp{.o} file is both a target and a prerequisite: this is common in
makefiles.  Recipes include @w{@samp{cc -c main.c}} and @w{@samp{cc -c
kbd.c}}.

When a target is a program, it needs to be recompiled or relinked if any of
its prerequisites change.  In addition, any prerequisites that are themselves
automatically generated should be updated first.  In this example, @file{edit}
depends on the eight object files; the object file @file{main.o} depends on
the source file @file{main.c} and on the header file @file{defs.h}.

A recipe may follow each line that contains a target and
prerequisites.  These recipes say how to update the target file.  A
tab character (or whatever character is specified by the
@code{.RECIPEPREFIX} variable; @pxref{Special Variables}) must come at
the beginning of every line in the recipe to distinguish recipes from
other lines in the makefile.  (Bear in mind that @code{make} does not
know anything about how the recipes work.  It is up to you to supply
recipes that will update the target file properly.  All @code{make}
does is execute the recipe you have specified when the target file
needs to be updated.)
@cindex recipe

The target @samp{clean} is not a file, but merely the name of an
action.  Since you normally do not want to carry out the actions in
this rule, @samp{clean} is not a prerequisite of any other rule.
Consequently, @code{make} never does anything with it unless you tell
it specifically.  Note that this rule not only is not a prerequisite,
it also does not have any prerequisites, so the only purpose of the
rule is to run the specified recipe.  Targets that do not refer to
files but are just actions are called @dfn{phony targets}.
@xref{Phony Targets}, for information about this kind of target.
@xref{Errors, , Errors in Recipes}, to see how to cause @code{make}
to ignore errors from @code{rm} or any other command.
@cindex @code{clean} target
@cindex @code{rm} (shell command)

@node How Make Works
@comment  node-name,  next,  previous,  up
@section How @code{make} Processes a Makefile
@cindex processing a makefile
@cindex makefile, how @code{make} processes

By default, @code{make} starts with the first target (not targets whose names
start with @samp{.} unless they also contain one or more @samp{/}).  This is
called the @dfn{default goal}.  (@dfn{Goals} are the targets that @code{make}
strives ultimately to update.  You can override this behavior using the
command line (@pxref{Goals, , Arguments to Specify the Goals}) or with the
@code{.DEFAULT_GOAL} special variable (@pxref{Special Variables, , Other
Special Variables}).
@cindex default goal
@cindex goal, default
@cindex goal

In the simple example of the previous section, the default goal is to
update the executable program @file{edit}; therefore, we put that rule
first.

Thus, when you give the command:

@example
make
@end example

@noindent
@code{make} reads the makefile in the current directory and begins by
processing the first rule.  In the example, this rule is for relinking
@file{edit}; but before @code{make} can fully process this rule, it
must process the rules for the files that @file{edit} depends on,
which in this case are the object files.  Each of these files is
processed according to its own rule.  These rules say to update each
@samp{.o} file by compiling its source file.  The recompilation must
be done if the source file, or any of the header files named as
prerequisites, is more recent than the object file, or if the object
file does not exist.

The other rules are processed because their targets appear as
prerequisites of the goal.  If some other rule is not depended on by the
goal (or anything it depends on, etc.), that rule is not processed,
unless you tell @code{make} to do so (with a command such as
@w{@code{make clean}}).

Before recompiling an object file, @code{make} considers updating its
prerequisites, the source file and header files.  This makefile does not
specify anything to be done for them---the @samp{.c} and @samp{.h} files
are not the targets of any rules---so @code{make} does nothing for these
files.  But @code{make} would update automatically generated C programs,
such as those made by Bison or Yacc, by their own rules at this time.

After recompiling whichever object files need it, @code{make} decides
whether to relink @file{edit}.  This must be done if the file
@file{edit} does not exist, or if any of the object files are newer than
it.  If an object file was just recompiled, it is now newer than
@file{edit}, so @file{edit} is relinked.
@cindex relinking

Thus, if we change the file @file{insert.c} and run @code{make},
@code{make} will compile that file to update @file{insert.o}, and then
link @file{edit}.  If we change the file @file{command.h} and run
@code{make}, @code{make} will recompile the object files @file{kbd.o},
@file{command.o} and @file{files.o} and then link the file @file{edit}.

@node Variables Simplify
@section Variables Make Makefiles Simpler
@cindex variables
@cindex simplifying with variables

In our example, we had to list all the object files twice in the rule for
@file{edit} (repeated here):

@example
@group
edit : main.o kbd.o command.o display.o \
              insert.o search.o files.o utils.o
        cc -o edit main.o kbd.o command.o display.o \
                   insert.o search.o files.o utils.o
@end group
@end example

@cindex @code{objects}
Such duplication is error-prone; if a new object file is added to the
system, we might add it to one list and forget the other.  We can eliminate
the risk and simplify the makefile by using a variable.  @dfn{Variables}
allow a text string to be defined once and substituted in multiple places
later (@pxref{Using Variables, ,How to Use Variables}).

@cindex @code{OBJECTS}
@cindex @code{objs}
@cindex @code{OBJS}
@cindex @code{obj}
@cindex @code{OBJ}
It is standard practice for every makefile to have a variable named
@code{objects}, @code{OBJECTS}, @code{objs}, @code{OBJS}, @code{obj},
or @code{OBJ} which is a list of all object file names.  We would
define such a variable @code{objects} with a line like this in the
makefile:

@example
@group
objects = main.o kbd.o command.o display.o \
          insert.o search.o files.o utils.o
@end group
@end example

@noindent
Then, each place we want to put a list of the object file names, we can
substitute the variable's value by writing @samp{$(objects)}
(@pxref{Using Variables, ,How to Use Variables}).

Here is how the complete simple makefile looks when you use a variable
for the object files:

@example
@group
objects = main.o kbd.o command.o display.o \
          insert.o search.o files.o utils.o

edit : $(objects)
        cc -o edit $(objects)
main.o : main.c defs.h
        cc -c main.c
kbd.o : kbd.c defs.h command.h
        cc -c kbd.c
command.o : command.c defs.h command.h
        cc -c command.c
display.o : display.c defs.h buffer.h
        cc -c display.c
insert.o : insert.c defs.h buffer.h
        cc -c insert.c
search.o : search.c defs.h buffer.h
        cc -c search.c
files.o : files.c defs.h buffer.h command.h
        cc -c files.c
utils.o : utils.c defs.h
        cc -c utils.c
clean :
        rm edit $(objects)
@end group
@end example

@node make Deduces
@section Letting @code{make} Deduce the Recipes
@cindex deducing recipes (implicit rules)
@cindex implicit rule, introduction to
@cindex rule, implicit, introduction to

It is not necessary to spell out the recipes for compiling the individual
C source files, because @code{make} can figure them out: it has an
@dfn{implicit rule} for updating a @samp{.o} file from a correspondingly
named @samp{.c} file using a @samp{cc -c} command.  For example, it will
use the recipe @samp{cc -c main.c -o main.o} to compile @file{main.c} into
@file{main.o}.  We can therefore omit the recipes from the rules for the
object files.  @xref{Implicit Rules, ,Using Implicit Rules}.

When a @samp{.c} file is used automatically in this way, it is also
automatically added to the list of prerequisites.  We can therefore omit
the @samp{.c} files from the prerequisites, provided we omit the recipe.

Here is the entire example, with both of these changes, and a variable
@code{objects} as suggested above:

@example
@group
objects = main.o kbd.o command.o display.o \
          insert.o search.o files.o utils.o

edit : $(objects)
        cc -o edit $(objects)

main.o : defs.h
kbd.o : defs.h command.h
command.o : defs.h command.h
display.o : defs.h buffer.h
insert.o : defs.h buffer.h
search.o : defs.h buffer.h
files.o : defs.h buffer.h command.h
utils.o : defs.h

.PHONY : clean
clean :
        rm edit $(objects)
@end group
@end example

@noindent
This is how we would write the makefile in actual practice.  (The
complications associated with @samp{clean} are described elsewhere.
See @ref{Phony Targets}, and @ref{Errors, ,Errors in Recipes}.)

Because implicit rules are so convenient, they are important.  You
will see them used frequently.

@node Combine By Prerequisite
@section Another Style of Makefile
@cindex combining rules by prerequisite

When the objects of a makefile are created only by implicit rules, an
alternative style of makefile is possible.  In this style of makefile,
you group entries by their prerequisites instead of by their targets.
Here is what one looks like:

@example
@group
objects = main.o kbd.o command.o display.o \
          insert.o search.o files.o utils.o

edit : $(objects)
        cc -o edit $(objects)

$(objects) : defs.h
kbd.o command.o files.o : command.h
display.o insert.o search.o files.o : buffer.h
@end group
@end example

@noindent
Here @file{defs.h} is given as a prerequisite of all the object files;
@file{command.h} and @file{buffer.h} are prerequisites of the specific
object files listed for them.

Whether this is better is a matter of taste: it is more compact, but some
people dislike it because they find it clearer to put all the information
about each target in one place.

@node Cleanup
@section Rules for Cleaning the Directory
@cindex cleaning up
@cindex removing, to clean up

Compiling a program is not the only thing you might want to write rules
for.  Makefiles commonly tell how to do a few other things besides
compiling a program: for example, how to delete all the object files
and executables so that the directory is @samp{clean}.

@cindex @code{clean} target
Here is how we
could write a @code{make} rule for cleaning our example editor:

@example
@group
clean:
        rm edit $(objects)
@end group
@end example

In practice, we might want to write the rule in a somewhat more
complicated manner to handle unanticipated situations.  We would do this:

@example
@group
.PHONY : clean
clean :
        -rm edit $(objects)
@end group
@end example

@noindent
This prevents @code{make} from getting confused by an actual file
called @file{clean} and causes it to continue in spite of errors from
@code{rm}.  (See @ref{Phony Targets}, and @ref{Errors, ,Errors in
Recipes}.)

@noindent
A rule such as this should not be placed at the beginning of the
makefile, because we do not want it to run by default!  Thus, in the
example makefile, we want the rule for @code{edit}, which recompiles
the editor, to remain the default goal.

Since @code{clean} is not a prerequisite of @code{edit}, this rule will not
run at all if we give the command @samp{make} with no arguments.  In
order to make the rule run, we have to type @samp{make clean}.
@xref{Running, ,How to Run @code{make}}.

@node Makefiles
@chapter Writing Makefiles

@cindex makefile, how to write
The information that tells @code{make} how to recompile a system comes from
reading a data base called the @dfn{makefile}.

@menu
* Makefile Contents::           What makefiles contain.
* Makefile Names::              How to name your makefile.
* Include::                     How one makefile can use another makefile.
* MAKEFILES Variable::          The environment can specify extra makefiles.
* Remaking Makefiles::          How makefiles get remade.
* Overriding Makefiles::        How to override part of one makefile
                                  with another makefile.
* Reading Makefiles::           How makefiles are read in.
* Parsing Makefiles::           How makefiles are parsed.
* Secondary Expansion::         How and when secondary expansion is performed.
@end menu

@node Makefile Contents
@section What Makefiles Contain

Makefiles contain five kinds of things: @dfn{explicit rules},
@dfn{implicit rules}, @dfn{variable definitions}, @dfn{directives},
and @dfn{comments}.  Rules, variables, and directives are described at
length in later chapters.

@itemize @bullet
@cindex rule, explicit, definition of
@cindex explicit rule, definition of
@item
An @dfn{explicit rule} says when and how to remake one or more files,
called the rule's @dfn{targets}.  It lists the other files that the
targets depend on, called the @dfn{prerequisites} of the target, and
may also give a recipe to use to create or update the targets.
@xref{Rules, ,Writing Rules}.

@cindex rule, implicit, definition of
@cindex implicit rule, definition of
@item
An @dfn{implicit rule} says when and how to remake a class of files
based on their names.  It describes how a target may depend on a file
with a name similar to the target and gives a recipe to create or
update such a target.  @xref{Implicit Rules, ,Using Implicit Rules}.

@cindex variable definition
@item
A @dfn{variable definition} is a line that specifies a text string
value for a variable that can be substituted into the text later.  The
simple makefile example shows a variable definition for @code{objects}
as a list of all object files (@pxref{Variables Simplify, , Variables
Make Makefiles Simpler}).

@cindex directive
@item
A @dfn{directive} is an instruction for @code{make} to do something
special while reading the makefile.  These include:

@itemize @bullet
@item
Reading another makefile (@pxref{Include, ,Including Other Makefiles}).

@item
Deciding (based on the values of variables) whether to use or
ignore a part of the makefile (@pxref{Conditionals, ,Conditional Parts of Makefiles}).

@item
Defining a variable from a verbatim string containing multiple lines
(@pxref{Multi-Line, ,Defining Multi-Line Variables}).
@end itemize

@cindex comments, in makefile
@cindex @code{#} (comments), in makefile
@item
@samp{#} in a line of a makefile starts a @dfn{comment}.  It and the
rest of the line are ignored, except that a trailing backslash not
escaped by another backslash will continue the comment across multiple
lines.  A line containing just a comment (with perhaps spaces before
it) is effectively blank, and is ignored.  If you want a literal
@code{#}, escape it with a backslash (e.g., @code{\#}).  Comments may
appear on any line in the makefile, although they are treated
specially in certain situations.

You cannot use comments within variable references or function calls:
any instance of @code{#} will be treated literally (rather than as the
start of a comment) inside a variable reference or function call.

Comments within a recipe are passed to the shell, just as with any
other recipe text.  The shell decides how to interpret it: whether or
not this is a comment is up to the shell.

Within a @code{define} directive, comments are not ignored during the
definition of the variable, but rather kept intact in the value of the
variable.  When the variable is expanded they will either be treated
as @code{make} comments or as recipe text, depending on the context in
which the variable is evaluated.
@end itemize

@menu
* Splitting Lines::             Splitting long lines in makefiles
@end menu

@node Splitting Lines
@subsection Splitting Long Lines
@cindex splitting long lines
@cindex long lines, splitting
@cindex backslash (@code{\}), to quote newlines

Makefiles use a ``line-based'' syntax in which the newline character
is special and marks the end of a statement.  GNU @code{make} has no
limit on the length of a statement line, up to the amount of memory in
your computer.

However, it is difficult to read lines which are too long to display
without wrapping or scrolling.  So, you can format your makefiles for
readability by adding newlines into the middle of a statement: you do
this by escaping the internal newlines with a backslash (@code{\})
character.  Where we need to make a distinction we will refer to
``physical lines'' as a single line ending with a newline (regardless
of whether it is escaped) and a ``logical line'' being a complete
statement including all escaped newlines up to the first non-escaped
newline.

The way in which backslash/newline combinations are handled depends on
whether the statement is a recipe line or a non-recipe line.  Handling
of backslash/newline in a recipe line is discussed later
(@pxref{Splitting Recipe Lines}).

Outside of recipe lines, backslash/newlines are converted into a
single space character.  Once that is done, all whitespace around the
backslash/newline is condensed into a single space: this includes all
whitespace preceding the backslash, all whitespace at the beginning of
the line after the backslash/newline, and any consecutive
backslash/newline combinations.

If the @code{.POSIX} special target is defined then backslash/newline handling
is modified slightly to conform to POSIX: first, whitespace preceding a
backslash is not removed and second, consecutive backslash/newlines are not
condensed.

@subsubheading Splitting Without Adding Whitespace
@cindex whitespace, avoiding on line split
@cindex removing whitespace from split lines

If you need to split a line but do @emph{not} want any whitespace
added, you can utilize a subtle trick: replace your backslash/newline
pairs with the three characters dollar sign, backslash, and newline:

@example
var := one$\
       word
@end example

After @code{make} removes the backslash/newline and condenses the
following line into a single space, this is equivalent to:

@example
var := one$ word
@end example

Then @code{make} will perform variable expansion.  The variable
reference @samp{$ } refers to a variable with the one-character name
`` '' (space) which does not exist, and so expands to the empty
string, giving a final assignment which is the equivalent of:

@example
var := oneword
@end example


@node Makefile Names
@section What Name to Give Your Makefile
@cindex makefile name
@cindex name of makefile
@cindex default makefile name
@cindex file name of makefile

@c following paragraph rewritten to avoid overfull hbox
By default, when @code{make} looks for the makefile, it tries the
following names, in order: @file{GNUmakefile}, @file{makefile}
and @file{Makefile}.
@findex Makefile
@findex GNUmakefile
@findex makefile

@cindex @code{README}
Normally you should call your makefile either @file{makefile} or
@file{Makefile}.  (We recommend @file{Makefile} because it appears
prominently near the beginning of a directory listing, right near other
important files such as @file{README}.)  The first name checked,
@file{GNUmakefile}, is not recommended for most makefiles.  You should
use this name if you have a makefile that is specific to GNU
@code{make}, and will not be understood by other versions of
@code{make}.  Other @code{make} programs look for @file{makefile} and
@file{Makefile}, but not @file{GNUmakefile}.

If @code{make} finds none of these names, it does not use any makefile.
Then you must specify a goal with a command argument, and @code{make}
will attempt to figure out how to remake it using only its built-in
implicit rules.  @xref{Implicit Rules, ,Using Implicit Rules}.

@cindex @code{-f}
@cindex @code{--file}
@cindex @code{--makefile}
If you want to use a nonstandard name for your makefile, you can specify
the makefile name with the @samp{-f} or @samp{--file} option.  The
arguments @w{@samp{-f @var{name}}} or @w{@samp{--file=@var{name}}} tell
@code{make} to read the file @var{name} as the makefile.  If you use
more than one @samp{-f} or @samp{--file} option, you can specify several
makefiles.  All the makefiles are effectively concatenated in the order
specified.  The default makefile names @file{GNUmakefile},
@file{makefile} and @file{Makefile} are not checked automatically if you
specify @samp{-f} or @samp{--file}.
@cindex specifying makefile name
@cindex makefile name, how to specify
@cindex name of makefile, how to specify
@cindex file name of makefile, how to specify

@node Include
@section Including Other Makefiles
@cindex including other makefiles
@cindex makefile, including

@findex include
The @code{include} directive tells @code{make} to suspend reading the
current makefile and read one or more other makefiles before continuing.
The directive is a line in the makefile that looks like this:

@example
include @var{filenames}@dots{}
@end example

@noindent
@var{filenames} can contain shell file name patterns.  If
@var{filenames} is empty, nothing is included and no error is printed.
@cindex shell file name pattern (in @code{include})
@cindex shell wildcards (in @code{include})
@cindex wildcard, in @code{include}

Extra spaces are allowed and ignored at the beginning of the line, but
the first character must not be a tab (or the value of
@code{.RECIPEPREFIX})---if the line begins with a tab, it will be
considered a recipe line.  Whitespace is required between
@code{include} and the file names, and between file names; extra
whitespace is ignored there and at the end of the directive.  A
comment starting with @samp{#} is allowed at the end of the line.  If
the file names contain any variable or function references, they are
expanded.  @xref{Using Variables, ,How to Use Variables}.

For example, if you have three @file{.mk} files, @file{a.mk},
@file{b.mk}, and @file{c.mk}, and @code{$(bar)} expands to
@code{bish bash}, then the following expression

@example
include foo *.mk $(bar)
@end example

is equivalent to

@example
include foo a.mk b.mk c.mk bish bash
@end example

When @code{make} processes an @code{include} directive, it suspends
reading of the containing makefile and reads from each listed file in
turn.  When that is finished, @code{make} resumes reading the
makefile in which the directive appears.

One occasion for using @code{include} directives is when several programs,
handled by individual makefiles in various directories, need to use a
common set of variable definitions
(@pxref{Setting, ,Setting Variables}) or pattern rules
(@pxref{Pattern Rules, ,Defining and Redefining Pattern Rules}).

Another such occasion is when you want to generate prerequisites from
source files automatically; the prerequisites can be put in a file that
is included by the main makefile.  This practice is generally cleaner
than that of somehow appending the prerequisites to the end of the main
makefile as has been traditionally done with other versions of
@code{make}.  @xref{Automatic Prerequisites}.
@cindex prerequisites, automatic generation
@cindex automatic generation of prerequisites
@cindex generating prerequisites automatically

@cindex @code{-I}
@cindex @code{--include-dir}
@cindex included makefiles, default directories
@cindex default directories for included makefiles
@findex /usr/gnu/include
@findex /usr/local/include
@findex /usr/include
If the specified name does not start with a slash (or a drive letter and colon
when GNU Make is compiled with MS-DOS / MS-Windows path support), and the file
is not found in the current directory, several other directories are searched.
First, any directories you have specified with the @samp{-I} or
@samp{--include-dir} options are searched (@pxref{Options Summary, ,Summary of
Options}).  Then the following directories (if they exist) are searched, in
this order: @file{@var{prefix}/include} (normally @file{/usr/local/include}
@footnote{GNU Make compiled for MS-DOS and MS-Windows behaves as if
@var{prefix} has been defined to be the root of the DJGPP tree hierarchy.})
@file{/usr/gnu/include}, @file{/usr/local/include}, @file{/usr/include}.

The @code{.INCLUDE_DIRS} variable will contain the current list of
directories that make will search for included files.  @xref{Special
Variables, ,Other Special Variables}.

You can avoid searching in these default directories by adding the
command line option @code{-I} with the special value @code{-} (e.g.,
@code{-I-}) to the command line.  This will cause @code{make} to
forget any already-set include directories, including the default
directories.

If an included makefile cannot be found in any of these directories it is not
an immediately fatal error; processing of the makefile containing the
@code{include} continues.  Once it has finished reading makefiles, @code{make}
will try to remake any that are out of date or don't exist.  @xref{Remaking
Makefiles, ,How Makefiles Are Remade}.  Only after it has failed to find a
rule to remake the makefile, or it found a rule but the recipe failed, will
@code{make} diagnose the missing makefile as a fatal error.

If you want @code{make} to simply ignore a makefile which does not exist
or cannot be remade, with no error message, use the @w{@code{-include}}
directive instead of @code{include}, like this:

@example
-include @var{filenames}@dots{}
@end example

This acts like @code{include} in every way except that there is no
error (not even a warning) if any of the @var{filenames} (or any
prerequisites of any of the @var{filenames}) do not exist or cannot be
remade.

For compatibility with some other @code{make} implementations,
@code{sinclude} is another name for @w{@code{-include}}.

@node MAKEFILES Variable
@section The Variable @code{MAKEFILES}
@cindex makefile, and @code{MAKEFILES} variable
@cindex including (@code{MAKEFILES} variable)

@vindex MAKEFILES
If the environment variable @code{MAKEFILES} is defined, @code{make}
considers its value as a list of names (separated by whitespace) of
additional makefiles to be read before the others.  This works much
like the @code{include} directive: various directories are searched
for those files (@pxref{Include, ,Including Other Makefiles}).  In
addition, the default goal is never taken from one of these makefiles
(or any makefile included by them) and it is not an error if the files
listed in @code{MAKEFILES} are not found.

@cindex recursion, and @code{MAKEFILES} variable
The main use of @code{MAKEFILES} is in communication between recursive
invocations of @code{make} (@pxref{Recursion, ,Recursive Use of
@code{make}}).  It usually is not desirable to set the environment
variable before a top-level invocation of @code{make}, because it is
usually better not to mess with a makefile from outside.  However, if
you are running @code{make} without a specific makefile, a makefile in
@code{MAKEFILES} can do useful things to help the built-in implicit
rules work better, such as defining search paths (@pxref{Directory Search}).

Some users are tempted to set @code{MAKEFILES} in the environment
automatically on login, and program makefiles to expect this to be done.
This is a very bad idea, because such makefiles will fail to work if run by
anyone else.  It is much better to write explicit @code{include} directives
in the makefiles.  @xref{Include, , Including Other Makefiles}.

@node Remaking Makefiles
@section How Makefiles Are Remade
@cindex updating makefiles
@cindex remaking makefiles
@cindex makefile, remaking of
Sometimes makefiles can be remade from other files, such as RCS or SCCS
files.  If a makefile can be remade from other files, you probably want
@code{make} to get an up-to-date version of the makefile to read in.

To this end, after reading in all makefiles @code{make} will consider
each as a goal target, in the order in which they were processed, and
attempt to update it.  If parallel builds (@pxref{Parallel, ,Parallel
Execution}) are enabled then makefiles will be rebuilt in parallel as
well.

If a makefile has a rule which says how to update it (found either in
that very makefile or in another one) or if an implicit rule applies
to it (@pxref{Implicit Rules, ,Using Implicit Rules}), it will be
updated if necessary.  After all makefiles have been checked, if any
have actually been changed, @code{make} starts with a clean slate and
reads all the makefiles over again.  (It will also attempt to update
each of them over again, but normally this will not change them again,
since they are already up to date.)  Each restart will cause the
special variable @code{MAKE_RESTARTS} to be updated (@pxref{Special
Variables}).

If you know that one or more of your makefiles cannot be remade and
you want to keep @code{make} from performing an implicit rule search
on them, perhaps for efficiency reasons, you can use any normal method
of preventing implicit rule look-up to do so.  For example, you can
write an explicit rule with the makefile as the target, and an empty
recipe (@pxref{Empty Recipes, ,Using Empty Recipes}).

If the makefiles specify a double-colon rule to remake a file with a recipe
but no prerequisites, that file will always be remade (@pxref{Double-Colon}).
In the case of makefiles, a makefile that has a double-colon rule with a
recipe but no prerequisites will be remade every time @code{make} is run, and
then again after @code{make} starts over and reads the makefiles in again.
This would cause an infinite loop: @code{make} would constantly remake the
makefile and restart, and never do anything else.  So, to avoid this,
@code{make} will @strong{not} attempt to remake makefiles which are specified
as targets of a double-colon rule with a recipe but no prerequisites.

Phony targets (@pxref{Phony Targets}) have the same effect: they are never
considered up-to-date and so an included file marked as phony would cause
@code{make} to restart continuously.  To avoid this @code{make} will not
attempt to remake makefiles which are marked phony.

You can take advantage of this to optimize startup time: if you know you don't
need your @file{Makefile} to be remade you can prevent make from trying to
remake it by adding either:

@example
.PHONY: Makefile
@end example

or:

@example
Makefile:: ;
@end example

If you do not specify any makefiles to be read with @samp{-f} or
@samp{--file} options, @code{make} will try the default makefile names;
@pxref{Makefile Names, ,What Name to Give Your Makefile}.  Unlike
makefiles explicitly requested with @samp{-f} or @samp{--file} options,
@code{make} is not certain that these makefiles should exist.  However,
if a default makefile does not exist but can be created by running
@code{make} rules, you probably want the rules to be run so that the
makefile can be used.

Therefore, if none of the default makefiles exists, @code{make} will
try to make each of them until it succeeds in making one, or it runs
out of names to try.  Note that it is not an error if @code{make}
cannot find or make any makefile; a makefile is not always
necessary.

When you use the @samp{-t} or @samp{--touch} option
(@pxref{Instead of Execution, ,Instead of Executing Recipes}),
you would not want to use an out-of-date makefile to decide which
targets to touch.  So the @samp{-t} option has no effect on updating
makefiles; they are really updated even if @samp{-t} is specified.
Likewise, @samp{-q} (or @samp{--question}) and @samp{-n} (or
@samp{--just-print}) do not prevent updating of makefiles, because an
out-of-date makefile would result in the wrong output for other targets.
Thus, @samp{make -f mfile -n foo} will update @file{mfile}, read it in,
and then print the recipe to update @file{foo} and its prerequisites
without running it.  The recipe printed for @file{foo} will be the one
specified in the updated contents of @file{mfile}.

However, on occasion you might actually wish to prevent updating of even
the makefiles.  You can do this by specifying the makefiles as goals in
the command line as well as specifying them as makefiles.  When the
makefile name is specified explicitly as a goal, the options @samp{-t}
and so on do apply to them.

Thus, @samp{make -f mfile -n mfile foo} would read the makefile
@file{mfile}, print the recipe needed to update it without actually
running it, and then print the recipe needed to update @file{foo}
without running that.  The recipe for @file{foo} will be the one
specified by the existing contents of @file{mfile}.

@node Overriding Makefiles
@section Overriding Part of Another Makefile

@cindex overriding makefiles
@cindex makefile, overriding
Sometimes it is useful to have a makefile that is mostly just like
another makefile.  You can often use the @samp{include} directive to
include one in the other, and add more targets or variable definitions.
However, it is invalid for two makefiles to give different recipes for
the same target.  But there is another way.

@cindex match-anything rule, used to override
In the containing makefile (the one that wants to include the other),
you can use a match-anything pattern rule to say that to remake any
target that cannot be made from the information in the containing
makefile, @code{make} should look in another makefile.
@xref{Pattern Rules}, for more information on pattern rules.

For example, if you have a makefile called @file{Makefile} that says how
to make the target @samp{foo} (and other targets), you can write a
makefile called @file{GNUmakefile} that contains:

@example
foo:
        frobnicate > foo

%: force
        @@$(MAKE) -f Makefile $@@
force: ;
@end example

If you say @samp{make foo}, @code{make} will find @file{GNUmakefile},
read it, and see that to make @file{foo}, it needs to run the recipe
@samp{frobnicate > foo}.  If you say @samp{make bar}, @code{make} will
find no way to make @file{bar} in @file{GNUmakefile}, so it will use the
recipe from the pattern rule: @samp{make -f Makefile bar}.  If
@file{Makefile} provides a rule for updating @file{bar}, @code{make}
will apply the rule.  And likewise for any other target that
@file{GNUmakefile} does not say how to make.

The way this works is that the pattern rule has a pattern of just
@samp{%}, so it matches any target whatever.  The rule specifies a
prerequisite @file{force}, to guarantee that the recipe will be run even
if the target file already exists.  We give the @file{force} target an
empty recipe to prevent @code{make} from searching for an implicit rule to
build it---otherwise it would apply the same match-anything rule to
@file{force} itself and create a prerequisite loop!

@node Reading Makefiles
@section How @code{make} Reads a Makefile
@cindex reading makefiles
@cindex makefile, reading

GNU @code{make} does its work in two distinct phases.  During the
first phase it reads all the makefiles, included makefiles, etc. and
internalizes all the variables and their values and implicit and
explicit rules, and builds a dependency graph of all the targets and
their prerequisites.  During the second phase, @code{make} uses this
internalized data to determine which targets need to be updated and
run the recipes necessary to update them.

It's important to understand this two-phase approach because it has a
direct impact on how variable and function expansion happens; this is
often a source of some confusion when writing makefiles.  Below is a
summary of the different constructs that can be found in a makefile,
and the phase in which expansion happens for each part of the
construct.

We say that expansion is @dfn{immediate} if it happens during the
first phase: @code{make} will expand that part of the construct as the
makefile is parsed.  We say that expansion is @dfn{deferred} if it is
not immediate.  Expansion of a deferred construct part is delayed
until the construct is used: either when it is referenced in an
immediate context, or when it is needed during the second phase.

You may not be familiar with some of these constructs yet.  You can
reference this section as you become familiar with them, in later
chapters.

@subheading Variable Assignment
@cindex =, expansion
@cindex :=, expansion
@cindex ::=, expansion
@cindex :::=, expansion
@cindex !=, expansion
@cindex +=, expansion
@cindex define, expansion

Variable definitions are parsed as follows:

@example
@var{immediate} = @var{deferred}
@var{immediate} := @var{immediate}
@var{immediate} ::= @var{immediate}
@var{immediate} :::= @var{immediate-with-escape}
@var{immediate} += @var{deferred} or @var{immediate}
@var{immediate} != @var{immediate}

define @var{immediate}
  @var{deferred}
endef

define @var{immediate} =
  @var{deferred}
endef

define @var{immediate} :=
  @var{immediate}
endef

define @var{immediate} ::=
  @var{immediate}
endef

define @var{immediate} :::=
  @var{immediate-with-escape}
endef

define @var{immediate} +=
  @var{deferred} or @var{immediate}
endef

define @var{immediate} !=
  @var{immediate}
endef
@end example

For the append operator @samp{+=}, the right-hand side is considered
immediate if the variable was previously set as a simple variable
(@samp{:=} or @samp{::=}), and deferred otherwise.

For the @var{immediate-with-escape} operator @samp{:::=}, the value on
the right-hand side is immediately expanded but then escaped (that is,
all instances of @code{$} in the result of the expansion are replaced
with @code{$$}).

For the shell assignment operator @samp{!=}, the right-hand side is
evaluated immediately and handed to the shell.  The result is stored
in the variable named on the left, and that variable is considered a
recursively expanded variable (and will thus be re-evaluated on each
reference).

@subsubheading Conditional Assignment Modifier
@cindex ?=, expansion
@cindex ?:=, expansion
@cindex ?::=, expansion
@cindex ?:::=, expansion
@cindex ?!=, expansion

Adding a conditional modifier (@pxref{Conditional Assignment, ,Conditional
Variable Assignment}) to an assignment operator does not change the expansion
style for that operator: if the variable is not defined then the assignment
will be made using expansion as defined above.

If the variable is already defined, then the right-hand side of the assignment
is ignored and not expanded.

@subheading Conditional Directives
@cindex ifdef, expansion
@cindex ifeq, expansion
@cindex ifndef, expansion
@cindex ifneq, expansion

Conditional directives are parsed immediately.  This means, for
example, that automatic variables cannot be used in conditional
directives, as automatic variables are not set until the recipe for
that rule is invoked.  If you need to use automatic variables in a
conditional directive you @emph{must} move the condition into the
recipe and use shell conditional syntax instead.

@subheading Rule Definition
@cindex target, expansion
@cindex prerequisite, expansion
@cindex implicit rule, expansion
@cindex pattern rule, expansion
@cindex explicit rule, expansion

A rule is always expanded the same way, regardless of the form:

@example
@var{immediate} : @var{immediate} ; @var{deferred}
        @var{deferred}
@end example

That is, the target and prerequisite sections are expanded
immediately, and the recipe used to build the target is always
deferred.  This is true for explicit rules, pattern rules, suffix
rules, static pattern rules, and simple prerequisite definitions.

@node Parsing Makefiles
@section How Makefiles Are Parsed
@cindex parsing makefiles
@cindex makefiles, parsing

GNU @code{make} parses makefiles line-by-line.  Parsing proceeds using
the following steps:

@enumerate
@item
Read in a full logical line, including backslash-escaped lines
(@pxref{Splitting Lines, , Splitting Long Lines}).

@item
Remove comments (@pxref{Makefile Contents, , What Makefiles Contain}).

@item
If the line begins with the recipe prefix character and we are in a
rule context, add the line to the current recipe and read the next
line (@pxref{Recipe Syntax}).

@item
Expand elements of the line which appear in an @emph{immediate}
expansion context (@pxref{Reading Makefiles, , How @code{make} Reads a
Makefile}).

@item
Scan the line for a separator character, such as @samp{:} or @samp{=},
to determine whether the line is a macro assignment or a rule
(@pxref{Recipe Syntax}).

@item
Internalize the resulting operation and read the next line.
@end enumerate

An important consequence of this is that a macro can expand to an
entire rule, @emph{if it is one line long}.  This will work:

@example
myrule = target : ; echo built

$(myrule)
@end example

However, this will not work because @code{make} does not re-split lines
after it has expanded them:

@example
define myrule
target:
        echo built
endef

$(myrule)
@end example

The above makefile results in the definition of a target @samp{target}
with prerequisites @samp{echo} and @samp{built}, as if the makefile
contained @code{target: echo built}, rather than a rule with a recipe.
Newlines still present in a line after expansion is complete are
ignored as normal whitespace.

In order to properly expand a multi-line macro you must use the
@code{eval} function: this causes the @code{make} parser to be run on
the results of the expanded macro (@pxref{Eval Function}).

@node Secondary Expansion
@section Secondary Expansion
@cindex secondary expansion
@cindex expansion, secondary

@findex .SECONDEXPANSION
Previously we learned that GNU @code{make} works in two distinct
phases: a read-in phase and a target-update phase (@pxref{Reading
Makefiles, , How @code{make} Reads a Makefile}).  GNU Make also has
the ability to enable a @emph{second expansion} of the prerequisites
(only) for some or all targets defined in the makefile.  In order for
this second expansion to occur, the special target
@code{.SECONDEXPANSION} must be defined before the first prerequisite
list that makes use of this feature.

If @code{.SECONDEXPANSION} is defined then when GNU @code{make} needs to check
the prerequisites of a target, the prerequisites are expanded a @emph{second
time}.  In most circumstances this secondary expansion will have no effect,
since all variable and function references will have been expanded during the
initial parsing of the makefiles.  In order to take advantage of the secondary
expansion phase of the parser, then, it's necessary to @emph{escape} the
variable or function reference in the makefile.  In this case the first
expansion merely un-escapes the reference but doesn't expand it, and expansion
is left to the secondary expansion phase.  For example, consider this
makefile:

@example
.SECONDEXPANSION:
ONEVAR = onefile
TWOVAR = twofile
myfile: $(ONEVAR) $$(TWOVAR)
@end example

After the first expansion phase the prerequisites list of the
@file{myfile} target will be @code{onefile} and @code{$(TWOVAR)}; the
first (un-escaped) variable reference to @var{ONEVAR} is expanded,
while the second (escaped) variable reference is simply un-escaped,
without being recognized as a variable reference.  Now during the
secondary expansion the first word is expanded again but since it
contains no variable or function references it remains the value
@file{onefile}, while the second word is now a normal reference to the
variable @var{TWOVAR}, which is expanded to the value @file{twofile}.
The final result is that there are two prerequisites, @file{onefile}
and @file{twofile}.

Obviously, this is not a very interesting case since the same result
could more easily have been achieved simply by having both variables
appear, un-escaped, in the prerequisites list.  One difference becomes
apparent if the variables are reset; consider this example:

@example
.SECONDEXPANSION:
AVAR = top
onefile: $(AVAR)
twofile: $$(AVAR)
AVAR = bottom
@end example

Here the prerequisite of @file{onefile} will be expanded immediately,
and resolve to the value @file{top}, while the prerequisite of
@file{twofile} will not be full expanded until the secondary expansion
and yield a value of @file{bottom}.

This is marginally more exciting, but the true power of this feature
only becomes apparent when you discover that secondary expansions
always take place within the scope of the automatic variables for that
target.  This means that you can use variables such as @code{$@@},
@code{$*}, etc. during the second expansion and they will have their
expected values, just as in the recipe.  All you have to do is defer
the expansion by escaping the @code{$}.  Also, secondary expansion
occurs for both explicit and implicit (pattern) rules.  Knowing this,
the possible uses for this feature increase dramatically.  For
example:

@example
.SECONDEXPANSION:
main_OBJS := main.o try.o test.o
lib_OBJS := lib.o api.o

main lib: $$($$@@_OBJS)
@end example

Here, after the initial expansion the prerequisites of both the
@file{main} and @file{lib} targets will be @code{$($@@_OBJS)}.  During
the secondary expansion, the @code{$@@} variable is set to the name of
the target and so the expansion for the @file{main} target will yield
@code{$(main_OBJS)}, or @code{main.o try.o test.o}, while the
secondary expansion for the @file{lib} target will yield
@code{$(lib_OBJS)}, or @code{lib.o api.o}.

You can also mix in functions here, as long as they are properly escaped:

@example
main_SRCS := main.c try.c test.c
lib_SRCS := lib.c api.c

.SECONDEXPANSION:
main lib: $$(patsubst %.c,%.o,$$($$@@_SRCS))
@end example

This version allows users to specify source files rather than object
files, but gives the same resulting prerequisites list as the previous
example.

Evaluation of automatic variables during the secondary expansion
phase, especially of the target name variable @code{$$@@}, behaves
similarly to evaluation within recipes.  However, there are some
subtle differences and ``corner cases'' which come into play for the
different types of rule definitions that @code{make} understands.  The
subtleties of using the different automatic variables are described
below.

@subheading Secondary Expansion of Explicit Rules
@cindex secondary expansion and explicit rules
@cindex explicit rules, secondary expansion of

During the secondary expansion of explicit rules, @code{$$@@} and
@code{$$%} evaluate, respectively, to the file name of the target and,
when the target is an archive member, the target member name.  The
@code{$$<} variable evaluates to the first prerequisite in the first
rule for this target.  @code{$$^} and @code{$$+} evaluate to the list
of all prerequisites of rules @emph{that have already appeared} for
the same target (@code{$$+} with repetitions and @code{$$^}
without).  The following example will help illustrate these behaviors:

@example
.SECONDEXPANSION:

foo: foo.1 bar.1 $$< $$^ $$+    # line #1

foo: foo.2 bar.2 $$< $$^ $$+    # line #2

foo: foo.3 bar.3 $$< $$^ $$+    # line #3
@end example

In the first prerequisite list, all three variables (@code{$$<},
@code{$$^}, and @code{$$+}) expand to the empty string.  In the
second, they will have values @code{foo.1}, @code{foo.1 bar.1}, and
@code{foo.1 bar.1} respectively.  In the third they will have values
@code{foo.1}, @code{foo.1 bar.1 foo.2 bar.2}, and @code{foo.1 bar.1
foo.2 bar.2 foo.1 foo.1 bar.1 foo.1 bar.1} respectively.

Rules undergo secondary expansion in makefile order, except that
the rule with the recipe is always evaluated last.

The variables @code{$$?} and @code{$$*} are not available and expand
to the empty string.

@subheading Secondary Expansion of Static Pattern Rules
@cindex secondary expansion and static pattern rules
@cindex static pattern rules, secondary expansion of

Rules for secondary expansion of static pattern rules are identical to
those for explicit rules, above, with one exception: for static
pattern rules the @code{$$*} variable is set to the pattern stem.  As
with explicit rules, @code{$$?} is not available and expands to the
empty string.

@subheading Secondary Expansion of Implicit Rules
@cindex secondary expansion and implicit rules
@cindex implicit rules, secondary expansion of

As @code{make} searches for an implicit rule, it substitutes the stem
and then performs secondary expansion for every rule with a matching
target pattern.  The value of the automatic variables is derived in
the same fashion as for static pattern rules.  As an example:

@example
.SECONDEXPANSION:

foo: bar

foo foz: fo%: bo%

%oo: $$< $$^ $$+ $$*
@end example

When the implicit rule is tried for target @file{foo}, @code{$$<}
expands to @file{bar}, @code{$$^} expands to @file{bar boo},
@code{$$+} also expands to @file{bar boo}, and @code{$$*} expands to
@file{f}.

Note that the directory prefix (D), as described in @ref{Implicit Rule
Search, ,Implicit Rule Search Algorithm}, is appended (after
expansion) to all the patterns in the prerequisites list.  As an
example:

@example
.SECONDEXPANSION:

/tmp/foo.o:

%.o: $$(addsuffix /%.c,foo bar) foo.h
        @@echo $^
@end example

The prerequisite list printed, after the secondary expansion and
directory prefix reconstruction, will be @file{/tmp/foo/foo.c
/tmp/bar/foo.c foo.h}.  If you are not interested in this
reconstruction, you can use @code{$$*} instead of @code{%} in the
prerequisites list.

@node Rules
@chapter Writing Rules
@cindex writing rules
@cindex rule, how to write
@cindex target
@cindex prerequisite

A @dfn{rule} appears in the makefile and says when and how to remake
certain files, called the rule's @dfn{targets} (most often only one per rule).
It lists the other files that are the @dfn{prerequisites} of the target, and
the @dfn{recipe} to use to create or update the target.

@cindex default goal
@cindex goal, default
The order of rules is not significant, except for determining the @dfn{default
goal}: the target for @code{make} to consider, if you do not otherwise specify
one.  The default goal is the first target of the first rule in the first
makefile.  There are two exceptions: a target starting with a period is not a
default unless it also contains one or more slashes, @samp{/}; and, a target
that defines a pattern rule has no effect on the default goal.  (@xref{Pattern
Rules, ,Defining and Redefining Pattern Rules}.)

Therefore, we usually write the makefile so that the first rule is the
one for compiling the entire program or all the programs described by
the makefile (often with a target called @samp{all}).
@xref{Goals, ,Arguments to Specify the Goals}.

@menu
* Rule Example::                An example explained.
* Rule Syntax::                 General syntax explained.
* Prerequisite Types::          There are two types of prerequisites.
* Wildcards::                   Using wildcard characters such as `*'.
* Directory Search::            Searching other directories for source files.
* Phony Targets::               Using a target that is not a real file's name.
* Force Targets::               You can use a target without a recipe
                                  or prerequisites to mark other targets
                                  as phony.
* Empty Targets::               When only the date matters and the
                                  files are empty.
* Special Targets::             Targets with special built-in meanings.
* Multiple Targets::            When to make use of several targets in a rule.
* Multiple Rules::              How to use several rules with the same target.
* Static Pattern::              Static pattern rules apply to multiple targets
                                  and can vary the prerequisites according to
                                  the target name.
* Double-Colon::                How to use a special kind of rule to allow
                                  several independent rules for one target.
* Automatic Prerequisites::     How to automatically generate rules giving
                                  prerequisites from source files themselves.
@end menu

@ifnottex
@node Rule Example
@section Rule Example

Here is an example of a rule:

@example
foo.o : foo.c defs.h       # module for twiddling the frobs
        cc -c -g foo.c
@end example

Its target is @file{foo.o} and its prerequisites are @file{foo.c} and
@file{defs.h}.  It has one command in the recipe: @samp{cc -c -g foo.c}.
The recipe starts with a tab to identify it as a recipe.

This rule says two things:

@itemize @bullet
@item
How to decide whether @file{foo.o} is out of date: it is out of date
if it does not exist, or if either @file{foo.c} or @file{defs.h} is
more recent than it.

@item
How to update the file @file{foo.o}: by running @code{cc} as stated.
The recipe does not explicitly mention @file{defs.h}, but we presume
that @file{foo.c} includes it, and that is why @file{defs.h} was added
to the prerequisites.
@end itemize
@end ifnottex

@node Rule Syntax
@section Rule Syntax

@cindex rule syntax
@cindex syntax of rules
In general, a rule looks like this:

@example
@var{targets} : @var{prerequisites}
        @var{recipe}
        @dots{}
@end example

@noindent
or like this:

@example
@var{targets} : @var{prerequisites} ; @var{recipe}
        @var{recipe}
        @dots{}
@end example

@cindex targets
@cindex rule targets
The @var{targets} are file names, separated by spaces.  Wildcard
characters may be used (@pxref{Wildcards, ,Using Wildcard Characters
in File Names}) and a name of the form @file{@var{a}(@var{m})}
represents member @var{m} in archive file @var{a}
(@pxref{Archive Members, ,Archive Members as Targets}).
Usually there is only one
target per rule, but occasionally there is a reason to have more
(@pxref{Multiple Targets, , Multiple Targets in a Rule}).

@cindex recipes
@cindex tab character (in commands)
The @var{recipe} lines start with a tab character (or the first
character in the value of the @code{.RECIPEPREFIX} variable;
@pxref{Special Variables}).  The first recipe line may appear on the line
after the prerequisites, with a tab character, or may appear on the
same line, with a semicolon.  Either way, the effect is the same.
There are other differences in the syntax of recipes.
@xref{Recipes, ,Writing Recipes in Rules}.

@cindex dollar sign (@code{$}), in rules
@cindex @code{$}, in rules
@cindex rules, and @code{$}
Because dollar signs are used to start @code{make} variable
references, if you really want a dollar sign in a target or
prerequisite you must write two of them, @samp{$$} (@pxref{Using
Variables, ,How to Use Variables}).  If you have enabled secondary
expansion (@pxref{Secondary Expansion}) and you want a literal dollar
sign in the prerequisites list, you must actually write @emph{four}
dollar signs (@samp{$$$$}).

You may split a long line by inserting a backslash followed by a
newline, but this is not required, as @code{make} places no limit on
the length of a line in a makefile.

A rule tells @code{make} two things: when the targets are out of date,
and how to update them when necessary.

@cindex prerequisites
@cindex rule prerequisites
The criterion for being out of date is specified in terms of the
@var{prerequisites}, which consist of file names separated by spaces.
(Wildcards and archive members (@pxref{Archives}) are allowed here too.)
A target is out of date if it does not exist or if it is older than any
of the prerequisites (by comparison of last-modification times).  The
idea is that the contents of the target file are computed based on
information in the prerequisites, so if any of the prerequisites changes,
the contents of the existing target file are no longer necessarily
valid.

How to update is specified by a @var{recipe}.  This is one or more
lines to be executed by the shell (normally @samp{sh}), but with some
extra features (@pxref{Recipes, ,Writing Recipes in Rules}).

@node Prerequisite Types
@comment  node-name,  next,  previous,  up
@section Types of Prerequisites
@cindex prerequisite types
@cindex types of prerequisites

@cindex prerequisites, normal
@cindex normal prerequisites
@cindex prerequisites, order-only
@cindex order-only prerequisites
There are two different types of prerequisites understood by GNU @code{make}:
normal prerequisites, described in the previous section, and @dfn{order-only}
prerequisites.  A normal prerequisite makes two statements: first, it imposes
an order in which recipes will be invoked: the recipes for all prerequisites
of a target will be completed before the recipe for the target is started.
Second, it imposes a dependency relationship: if any prerequisite is newer
than the target, then the target is considered out-of-date and must be
rebuilt.

Normally, this is exactly what you want: if a target's prerequisite is
updated, then the target should also be updated.

Occasionally you may want to ensure that a prerequisite is built before a
target, but @emph{without} forcing the target to be updated if the
prerequisite is updated.  @dfn{Order-only} prerequisites are used to create
this type of relationship.  Order-only prerequisites can be specified by
placing a pipe symbol (@code{|}) in the prerequisites list: any prerequisites
to the left of the pipe symbol are normal; any prerequisites to the right are
order-only:

@example
@var{targets} : @var{normal-prerequisites} | @var{order-only-prerequisites}
@end example

The normal prerequisites section may of course be empty.  Also, you
may still declare multiple lines of prerequisites for the same target:
they are appended appropriately (normal prerequisites are appended to
the list of normal prerequisites; order-only prerequisites are
appended to the list of order-only prerequisites).  Note that if you
declare the same file to be both a normal and an order-only
prerequisite, the normal prerequisite takes precedence (since they
have a strict superset of the behavior of an order-only prerequisite).

Order-only prerequisites are never checked when determining if the
target is out of date; even order-only prerequisites marked as phony
(@pxref{Phony Targets}) will not cause the target to be rebuilt.

Consider an example where your targets are to be placed in a separate
directory, and that directory might not exist before @code{make} is
run.  In this situation, you want the directory to be created before
any targets are placed into it but, because the timestamps on
directories change whenever a file is added, removed, or renamed, we
certainly don't want to rebuild all the targets whenever the
directory's timestamp changes.  One way to manage this is with
order-only prerequisites: make the directory an order-only
prerequisite on all the targets:

@example
OBJDIR := objdir
OBJS := $(addprefix $(OBJDIR)/,foo.o bar.o baz.o)

$(OBJDIR)/%.o : %.c
        $(COMPILE.c) $(OUTPUT_OPTION) $<

all: $(OBJS)

$(OBJS): | $(OBJDIR)

$(OBJDIR):
        mkdir $(OBJDIR)
@end example

Now the rule to create the @file{objdir} directory will be run, if
needed, before any @samp{.o} is built, but no @samp{.o} will be built
because the @file{objdir} directory timestamp changed.

@node Wildcards
@section Using Wildcard Characters in File Names
@cindex wildcard
@cindex file name with wildcards
@cindex globbing (wildcards)

@cindex @code{*} (wildcard character)
@cindex @code{?} (wildcard character)
@cindex @code{[@dots{}]} (wildcard characters)
A single file name can specify many files using @dfn{wildcard characters}.
The wildcard characters in @code{make} are @samp{*}, @samp{?} and
@samp{[@dots{}]}, the same as in the Bourne shell.  For example, @file{*.c}
specifies a list of all the files (in the working directory) whose names
end in @samp{.c}.

If an expression matches multiple files then the results will be
sorted.@footnote{Some older versions of GNU @code{make} did not sort the
results of wildcard expansion.}  However multiple expressions will not be
globally sorted.  For example, @file{*.c *.h} will list all the files whose
names end in @samp{.c}, sorted, followed by all the files whose names end in
@samp{.h}, sorted.

@cindex @code{~} (tilde)
@cindex tilde (@code{~})
@cindex home directory
The character @samp{~} at the beginning of a file name also has special
significance.  If alone, or followed by a slash, it represents your home
directory.  For example @file{~/bin} expands to @file{/home/you/bin}.
If the @samp{~} is followed by a word, the string represents the home
directory of the user named by that word.  For example @file{~john/bin}
expands to @file{/home/john/bin}.  On systems which don't have a home
directory for each user (such as MS-DOS or MS-Windows), this
functionality can be simulated by setting the environment variable
@var{HOME}.

Wildcard expansion is performed by @code{make} automatically in
targets and in prerequisites.  In recipes, the shell is responsible
for wildcard expansion.  In other contexts, wildcard expansion happens
only if you request it explicitly with the @code{wildcard} function.

The special significance of a wildcard character can be turned off by
preceding it with a backslash.  Thus, @file{foo\*bar} would refer to a
specific file whose name consists of @samp{foo}, an asterisk, and
@samp{bar}.

@menu
* Wildcard Examples::           Several examples.
* Wildcard Pitfall::            Problems to avoid.
* Wildcard Function::           How to cause wildcard expansion where
                                  it does not normally take place.
@end menu

@node Wildcard Examples
@subsection Wildcard Examples

Wildcards can be used in the recipe of a rule, where they are expanded
by the shell.  For example, here is a rule to delete all the object files:

@example
@group
clean:
        rm -f *.o
@end group
@end example
@cindex @code{rm} (shell command)

Wildcards are also useful in the prerequisites of a rule.  With the
following rule in the makefile, @samp{make print} will print all the
@samp{.c} files that have changed since the last time you printed them:

@example
print: *.c
        lpr -p $?
        touch print
@end example

@cindex @code{print} target
@cindex @code{lpr} (shell command)
@cindex @code{touch} (shell command)
@noindent
This rule uses @file{print} as an empty target file; see @ref{Empty
Targets, ,Empty Target Files to Record Events}.  (The automatic variable
@samp{$?} is used to print only those files that have changed; see
@ref{Automatic Variables}.)

Wildcard expansion does not happen when you define a variable.  Thus, if
you write this:

@example
objects = *.o
@end example

@noindent
then the value of the variable @code{objects} is the actual string
@samp{*.o}.  However, if you use the value of @code{objects} in a
target or prerequisite, wildcard expansion will take place there.  If
you use the value of @code{objects} in a recipe, the shell may perform
wildcard expansion when the recipe runs.  To set @code{objects} to the
expansion, instead use:

@example
objects := $(wildcard *.o)
@end example

@noindent
@xref{Wildcard Function}.

@node Wildcard Pitfall
@subsection Pitfalls of Using Wildcards
@cindex wildcard pitfalls
@cindex pitfalls of wildcards
@cindex mistakes with wildcards
@cindex errors with wildcards
@cindex problems with wildcards

Now here is an example of a naive way of using wildcard expansion, that
does not do what you would intend.  Suppose you would like to say that the
executable file @file{foo} is made from all the object files in the
directory, and you write this:

@example
objects = *.o

foo : $(objects)
        cc -o foo $(CFLAGS) $(objects)
@end example

@noindent
The value of @code{objects} is the actual string @samp{*.o}.  Wildcard
expansion happens in the rule for @file{foo}, so that each @emph{existing}
@samp{.o} file becomes a prerequisite of @file{foo} and will be recompiled if
necessary.

But what if you delete all the @samp{.o} files?  When a wildcard matches
no files, it is left as it is, so then @file{foo} will depend on the
oddly-named file @file{*.o}.  Since no such file is likely to exist,
@code{make} will give you an error saying it cannot figure out how to
make @file{*.o}.  This is not what you want!

Actually it is possible to obtain the desired result with wildcard
expansion, but you need more sophisticated techniques, including the
@code{wildcard} function and string substitution.
@ifnottex
@xref{Wildcard Function, ,The Function @code{wildcard}}.
@end ifnottex
@iftex
These are described in the following section.
@end iftex

@cindex wildcards and MS-DOS/MS-Windows backslashes
@cindex backslashes in pathnames and wildcard expansion

Microsoft operating systems (MS-DOS and MS-Windows) use backslashes to
separate directories in pathnames, like so:

@example
  c:\foo\bar\baz.c
@end example

This is equivalent to the Unix-style @file{c:/foo/bar/baz.c} (the
@file{c:} part is the so-called drive letter).  When @code{make} runs on
these systems, it supports backslashes as well as the Unix-style forward
slashes in pathnames.  However, this support does @emph{not} include the
wildcard expansion, where backslash is a quote character.  Therefore,
you @emph{must} use Unix-style slashes in these cases.


@node Wildcard Function
@subsection The Function @code{wildcard}
@findex wildcard

Wildcard expansion happens automatically in rules.  But wildcard expansion
does not normally take place when a variable is set, or inside the
arguments of a function.  If you want to do wildcard expansion in such
places, you need to use the @code{wildcard} function, like this:

@example
$(wildcard @var{pattern}@dots{})
@end example

@noindent
This string, used anywhere in a makefile, is replaced by a
space-separated list of names of existing files that match one of the
given file name patterns.  If no existing file name matches a pattern,
then that pattern is omitted from the output of the @code{wildcard}
function.  Note that this is different from how unmatched wildcards
behave in rules, where they are used verbatim rather than ignored
(@pxref{Wildcard Pitfall}).

As with wildcard expansion in rules, the results of the @code{wildcard}
function are sorted.  But again, each individual expression is sorted
separately, so @samp{$(wildcard *.c *.h)} will expand to all files matching
@samp{.c}, sorted, followed by all files matching @samp{.h}, sorted.

One use of the @code{wildcard} function is to get a list of all the C source
files in a directory, like this:

@example
$(wildcard *.c)
@end example

We can change the list of C source files into a list of object files by
replacing the @samp{.c} suffix with @samp{.o} in the result, like this:

@example
$(patsubst %.c,%.o,$(wildcard *.c))
@end example

@noindent
(Here we have used another function, @code{patsubst}.
@xref{Text Functions, ,Functions for String Substitution and Analysis}.)

Thus, a makefile to compile all C source files in the directory and then
link them together could be written as follows:

@example
objects := $(patsubst %.c,%.o,$(wildcard *.c))

foo : $(objects)
        cc -o foo $(objects)
@end example

@noindent
(This takes advantage of the implicit rule for compiling C programs, so
there is no need to write explicit rules for compiling the files.
@xref{Flavors, ,The Two Flavors of Variables}, for an explanation of
@samp{:=}, which is a variant of @samp{=}.)

@node Directory Search
@section Searching Directories for Prerequisites
@cindex vpath
@cindex search path for prerequisites (@code{VPATH})
@cindex directory search (@code{VPATH})

For large systems, it is often desirable to put sources in a separate
directory from the binaries.  The @dfn{directory search} features of
@code{make} facilitate this by searching several directories
automatically to find a prerequisite.  When you redistribute the files
among directories, you do not need to change the individual rules,
just the search paths.

@menu
* General Search::              Specifying a search path that applies
                                  to every prerequisite.
* Selective Search::            Specifying a search path
                                  for a specified class of names.
* Search Algorithm::            When and how search paths are applied.
* Recipes/Search::              How to write recipes that work together
                                  with search paths.
* Implicit/Search::             How search paths affect implicit rules.
* Libraries/Search::            Directory search for link libraries.
@end menu

@node General Search
@subsection @code{VPATH}: Search Path for All Prerequisites
@vindex VPATH

The value of the @code{make} variable @code{VPATH} specifies a list of
directories that @code{make} should search.  Most often, the
directories are expected to contain prerequisite files that are not in the
current directory; however, @code{make} uses @code{VPATH} as a search
list for both prerequisites and targets of rules.

Thus, if a file that is listed as a target or prerequisite does not exist
in the current directory, @code{make} searches the directories listed in
@code{VPATH} for a file with that name.  If a file is found in one of
them, that file may become the prerequisite (see below).  Rules may then
specify the names of files in the prerequisite list as if they all
existed in the current directory.  @xref{Recipes/Search, ,Writing Recipes with Directory Search}.

In the @code{VPATH} variable, directory names are separated by colons or
blanks.  The order in which directories are listed is the order followed
by @code{make} in its search.  (On MS-DOS and MS-Windows, semi-colons
are used as separators of directory names in @code{VPATH}, since the
colon can be used in the pathname itself, after the drive letter.)

For example,

@example
VPATH = src:../headers
@end example

@noindent
specifies a path containing two directories, @file{src} and
@file{../headers}, which @code{make} searches in that order.

With this value of @code{VPATH}, the following rule,

@example
foo.o : foo.c
@end example

@noindent
is interpreted as if it were written like this:

@example
foo.o : src/foo.c
@end example

@noindent
assuming the file @file{foo.c} does not exist in the current directory but
is found in the directory @file{src}.

@node Selective Search
@subsection The @code{vpath} Directive
@findex vpath

Similar to the @code{VPATH} variable, but more selective, is the
@code{vpath} directive (note lower case), which allows you to specify a
search path for a particular class of file names: those that match a
particular pattern.  Thus you can supply certain search directories for
one class of file names and other directories (or none) for other file
names.

There are three forms of the @code{vpath} directive:

@table @code
@item vpath @var{pattern} @var{directories}
Specify the search path @var{directories} for file names that match
@var{pattern}.

The search path, @var{directories}, is a list of directories to be
searched, separated by colons (semi-colons on MS-DOS and MS-Windows) or
blanks, just like the search path used in the @code{VPATH} variable.

@item vpath @var{pattern}
Clear out the search path associated with @var{pattern}.

@c Extra blank line makes sure this gets two lines.
@item vpath

Clear all search paths previously specified with @code{vpath} directives.
@end table

A @code{vpath} pattern is a string containing a @samp{%} character.  The
string must match the file name of a prerequisite that is being searched
for, the @samp{%} character matching any sequence of zero or more
characters (as in pattern rules; @pxref{Pattern Rules, ,Defining and
Redefining Pattern Rules}).  For example, @code{%.h} matches files that
end in @code{.h}.  (If there is no @samp{%}, the pattern must match the
prerequisite exactly, which is not useful very often.)

@cindex @code{%}, quoting in @code{vpath}
@cindex @code{\} (backslash), to quote @code{%}
@cindex backslash (@code{\}), to quote @code{%}
@cindex quoting @code{%}, in @code{vpath}
@samp{%} characters in a @code{vpath} directive's pattern can be quoted
with preceding backslashes (@samp{\}).  Backslashes that would otherwise
quote @samp{%} characters can be quoted with more backslashes.
Backslashes that quote @samp{%} characters or other backslashes are
removed from the pattern before it is compared to file names.  Backslashes
that are not in danger of quoting @samp{%} characters go unmolested.

When a prerequisite fails to exist in the current directory, if the
@var{pattern} in a @code{vpath} directive matches the name of the
prerequisite file, then the @var{directories} in that directive are searched
just like (and before) the directories in the @code{VPATH} variable.

For example,

@example
vpath %.h ../headers
@end example

@noindent
tells @code{make} to look for any prerequisite whose name ends in @file{.h}
in the directory @file{../headers} if the file is not found in the current
directory.

If several @code{vpath} patterns match the prerequisite file's name, then
@code{make} processes each matching @code{vpath} directive one by one,
searching all the directories mentioned in each directive.  @code{make}
handles multiple @code{vpath} directives in the order in which they
appear in the makefile; multiple directives with the same pattern are
independent of each other.

@need 750
Thus,

@example
@group
vpath %.c foo
vpath %   blish
vpath %.c bar
@end group
@end example

@noindent
will look for a file ending in @samp{.c} in @file{foo}, then
@file{blish}, then @file{bar}, while

@example
@group
vpath %.c foo:bar
vpath %   blish
@end group
@end example

@noindent
will look for a file ending in @samp{.c} in @file{foo}, then
@file{bar}, then @file{blish}.

@node Search Algorithm
@subsection How Directory Searches are Performed
@cindex algorithm for directory search
@cindex directory search algorithm

When a prerequisite is found through directory search, regardless of type
(general or selective), the pathname located may not be the one that
@code{make} actually provides you in the prerequisite list.  Sometimes
the path discovered through directory search is thrown away.

The algorithm @code{make} uses to decide whether to keep or abandon a
path found via directory search is as follows:

@enumerate
@item
If a target file does not exist at the path specified in the makefile,
directory search is performed.

@item
If the directory search is successful, that path is kept and this file
is tentatively stored as the target.

@item
All prerequisites of this target are examined using this same method.

@item
After processing the prerequisites, the target may or may not need to be
rebuilt:

@enumerate a
@item
If the target does @emph{not} need to be rebuilt, the path to the file
found during directory search is used for any prerequisite lists which
contain this target.  In short, if @code{make} doesn't need to rebuild
the target then you use the path found via directory search.

@item
If the target @emph{does} need to be rebuilt (is out-of-date), the
pathname found during directory search is @emph{thrown away}, and the
target is rebuilt using the file name specified in the makefile.  In
short, if @code{make} must rebuild, then the target is rebuilt locally,
not in the directory found via directory search.
@end enumerate
@end enumerate

This algorithm may seem complex, but in practice it is quite often
exactly what you want.

@cindex traditional directory search (GPATH)
@cindex directory search, traditional (GPATH)
Other versions of @code{make} use a simpler algorithm: if the file does
not exist, and it is found via directory search, then that pathname is
always used whether or not the target needs to be built.  Thus, if the
target is rebuilt it is created at the pathname discovered during
directory search.

@vindex GPATH
If, in fact, this is the behavior you want for some or all of your
directories, you can use the @code{GPATH} variable to indicate this to
@code{make}.

@code{GPATH} has the same syntax and format as @code{VPATH} (that is, a
space- or colon-delimited list of pathnames).  If an out-of-date target
is found by directory search in a directory that also appears in
@code{GPATH}, then that pathname is not thrown away.  The target is
rebuilt using the expanded path.

@node Recipes/Search
@subsection Writing Recipes with Directory Search
@cindex recipes, and directory search
@cindex directory search (@code{VPATH}), and recipes

When a prerequisite is found in another directory through directory search,
this cannot change the recipe of the rule; they will execute as written.
Therefore, you must write the recipe with care so that it will look for
the prerequisite in the directory where @code{make} finds it.

This is done with the @dfn{automatic variables} such as @samp{$^}
(@pxref{Automatic Variables}).
For instance, the value of @samp{$^} is a
list of all the prerequisites of the rule, including the names of
the directories in which they were found, and the value of
@samp{$@@} is the target.  Thus:

@example
foo.o : foo.c
        cc -c $(CFLAGS) $^ -o $@@
@end example

@noindent
(The variable @code{CFLAGS} exists so you can specify flags for C
compilation by implicit rules; we use it here for consistency so it will
affect all C compilations uniformly;
@pxref{Implicit Variables, ,Variables Used by Implicit Rules}.)

Often the prerequisites include header files as well, which you do not
want to mention in the recipe.  The automatic variable @samp{$<} is
just the first prerequisite:

@example
VPATH = src:../headers
foo.o : foo.c defs.h hack.h
        cc -c $(CFLAGS) $< -o $@@
@end example

@node Implicit/Search
@subsection Directory Search and Implicit Rules
@cindex @code{VPATH}, and implicit rules
@cindex directory search (@code{VPATH}), and implicit rules
@cindex search path for prerequisites (@code{VPATH}), and implicit rules
@cindex implicit rule, and directory search
@cindex implicit rule, and @code{VPATH}
@cindex rule, implicit, and directory search
@cindex rule, implicit, and @code{VPATH}

The search through the directories specified in @code{VPATH} or with
@code{vpath} also happens during consideration of implicit rules
(@pxref{Implicit Rules, ,Using Implicit Rules}).

For example, when a file @file{foo.o} has no explicit rule, @code{make}
considers implicit rules, such as the built-in rule to compile
@file{foo.c} if that file exists.  If such a file is lacking in the
current directory, the appropriate directories are searched for it.  If
@file{foo.c} exists (or is mentioned in the makefile) in any of the
directories, the implicit rule for C compilation is applied.

The recipes of implicit rules normally use automatic variables as a
matter of necessity; consequently they will use the file names found by
directory search with no extra effort.

@node Libraries/Search
@subsection Directory Search for Link Libraries
@cindex link libraries, and directory search
@cindex libraries for linking, directory search
@cindex directory search (@code{VPATH}), and link libraries
@cindex @code{VPATH}, and link libraries
@cindex search path for prerequisites (@code{VPATH}), and link libraries
@cindex @code{-l} (library search)
@cindex link libraries, patterns matching
@cindex @code{.LIBPATTERNS}, and link libraries
@vindex .LIBPATTERNS

Directory search applies in a special way to libraries used with the
linker.  This special feature comes into play when you write a prerequisite
whose name is of the form @samp{-l@var{name}}.  (You can tell something
strange is going on here because the prerequisite is normally the name of a
file, and the @emph{file name} of a library generally looks like
@file{lib@var{name}.a}, not like @samp{-l@var{name}}.)

When a prerequisite's name has the form @samp{-l@var{name}}, @code{make}
handles it specially by searching for the file @file{lib@var{name}.so},
and, if it is not found, for the file @file{lib@var{name}.a} in the current
directory, in directories specified by matching @code{vpath}
search paths and the @code{VPATH} search path, and then in the
directories @file{/lib}, @file{/usr/lib}, and @file{@var{prefix}/lib}
(normally @file{/usr/local/lib}, but MS-DOS/MS-Windows versions of
@code{make} behave as if @var{prefix} is defined to be the root of the
DJGPP installation tree).

For example, if there is a @file{/usr/lib/libcurses.a} library on your
system (and no @file{/usr/lib/libcurses.so} file), then

@example
@group
foo : foo.c -lcurses
        cc $^ -o $@@
@end group
@end example

@noindent
would cause the command @samp{cc foo.c /usr/lib/libcurses.a -o foo} to
be executed when @file{foo} is older than @file{foo.c} or than
@file{/usr/lib/libcurses.a}.

Although the default set of files to be searched for is
@file{lib@var{name}.so} and @file{lib@var{name}.a}, this is customizable
via the @code{.LIBPATTERNS} variable.  Each word in the value of this
variable is a pattern string.  When a prerequisite like
@samp{-l@var{name}} is seen, @code{make} will replace the percent in
each pattern in the list with @var{name} and perform the above directory
searches using each library file name.

The default value for @code{.LIBPATTERNS} is @samp{lib%.so lib%.a},
which provides the default behavior described above.

You can turn off link library expansion completely by setting this
variable to an empty value.

@node Phony Targets
@section Phony Targets
@cindex phony targets
@cindex targets, phony
@cindex targets without a file

A phony target is one that is not really the name of a file; rather it
is just a name for a recipe to be executed when you make an explicit
request.  There are two reasons to use a phony target: to avoid a
conflict with a file of the same name, and to improve performance.

If you write a rule whose recipe will not create the target file, the
recipe will be executed every time the target comes up for remaking.
Here is an example:

@example
@group
clean:
        rm *.o temp
@end group
@end example

@noindent
Because the @code{rm} command does not create a file named @file{clean},
probably no such file will ever exist.  Therefore, the @code{rm} command
will be executed every time you say @samp{make clean}.
@cindex @code{rm} (shell command)

@cindex using .PHONY
In this example, the @file{clean} target will not work properly if a
file named @file{clean} is ever created in this directory.  Since it
has no prerequisites, @file{clean} would always be considered up to
date and its recipe would not be executed.  To avoid this problem you
can explicitly declare the target to be phony by making it a
prerequisite of the special target @code{.PHONY}
(@pxref{Special Targets, ,Special Built-in Target Names}) as follows:

@example
@group
.PHONY: clean
clean:
        rm *.o temp
@end group
@end example

@noindent
Once this is done, @samp{make clean} will run the recipe regardless of
whether there is a file named @file{clean}.

Prerequisites of @code{.PHONY} are always interpreted as literal
target names, never as patterns (even if they contain @samp{%}
characters).  To always rebuild a pattern rule consider using a
``force target'' (@pxref{Force Targets, ,Rules without Recipes or
Prerequisites}).

Phony targets are also useful in conjunction with recursive
invocations of @code{make} (@pxref{Recursion, ,Recursive Use of @code{make}}).
In this situation the makefile will often contain a variable which
lists a number of sub-directories to be built.  A simplistic way to
handle this is to define one rule with a recipe that loops over the
sub-directories, like this:

@example
@group
SUBDIRS = foo bar baz

subdirs:
        for dir in $(SUBDIRS); do \
          $(MAKE) -C $$dir; \
        done
@end group
@end example

There are problems with this method, however.  First, any error detected in a
sub-make is ignored by this rule, so it will continue to build the rest of the
directories even when one fails.  This can be overcome by adding shell
commands to note the error and exit, but then it will do so even if
@code{make} is invoked with the @code{-k} option, which is unfortunate.
Second, and perhaps more importantly, you cannot take full advantage of
@code{make}'s ability to build targets in parallel (@pxref{Parallel, ,Parallel
Execution}), since there is only one rule.  Each individual makefile's targets
will be built in parallel, but only one sub-directory will be built at a time.

By declaring the sub-directories as @code{.PHONY} targets (you must do
this as the sub-directory obviously always exists; otherwise it won't
be built) you can remove these problems:

@example
@group
SUBDIRS = foo bar baz

.PHONY: subdirs $(SUBDIRS)

subdirs: $(SUBDIRS)

$(SUBDIRS):
        $(MAKE) -C $@@

foo: baz
@end group
@end example

Here we've also declared that the @file{foo} sub-directory cannot be
built until after the @file{baz} sub-directory is complete; this kind of
relationship declaration is particularly important when attempting
parallel builds.

The implicit rule search (@pxref{Implicit Rules}) is skipped for
@code{.PHONY} targets.  This is why declaring a target as
@code{.PHONY} is good for performance, even if you are not worried
about the actual file existing.

A phony target should not be a prerequisite of a real target file; if it is,
its recipe will be run every time @code{make} considers that file.  As long as
a phony target is never a prerequisite of a real target, the phony target
recipe will be executed only when the phony target is a specified goal
(@pxref{Goals, ,Arguments to Specify the Goals}).

You should not declare an included makefile as phony.  Phony targets are not
intended to represent real files, and because the target is always considered
out of date make will always rebuild it then re-execute itself
(@pxref{Remaking Makefiles, ,How Makefiles Are Remade}).  To avoid this,
@code{make} will not re-execute itself if an included file marked as phony is
re-built.

Phony targets can have prerequisites.  When one directory contains multiple
programs, it is most convenient to describe all of the programs in one
makefile @file{./Makefile}.  Since the target remade by default will be the
first one in the makefile, it is common to make this a phony target named
@samp{all} and give it, as prerequisites, all the individual programs.  For
example:

@example
all : prog1 prog2 prog3
.PHONY : all

prog1 : prog1.o utils.o
        cc -o prog1 prog1.o utils.o

prog2 : prog2.o
        cc -o prog2 prog2.o

prog3 : prog3.o sort.o utils.o
        cc -o prog3 prog3.o sort.o utils.o
@end example

@noindent
Now you can say just @samp{make} to remake all three programs, or
specify as arguments the ones to remake (as in @samp{make prog1
prog3}).  Phoniness is not inherited: the prerequisites of a phony
target are not themselves phony, unless explicitly declared to be so.

When one phony target is a prerequisite of another, it serves as a subroutine
of the other.  For example, here @samp{make cleanall} will delete the
object files, the difference files, and the file @file{program}:

@example
.PHONY: cleanall cleanobj cleandiff

cleanall : cleanobj cleandiff
        rm program

cleanobj :
        rm *.o

cleandiff :
        rm *.diff
@end example

@node Force Targets
@section Rules without Recipes or Prerequisites
@cindex force targets
@cindex targets, force
@cindex @code{FORCE}
@cindex rule, no recipe or prerequisites

If a rule has no prerequisites or recipe, and the target of the rule
is a nonexistent file, then @code{make} imagines this target to have
been updated whenever its rule is run.  This implies that all targets
depending on this one will always have their recipe run.

An example will illustrate this:

@example
@group
clean: FORCE
        rm $(objects)
FORCE:
@end group
@end example

Here the target @samp{FORCE} satisfies the special conditions, so the
target @file{clean} that depends on it is forced to run its recipe.
There is nothing special about the name @samp{FORCE}, but that is one
name commonly used this way.

As you can see, using @samp{FORCE} this way has the same results as using
@samp{.PHONY: clean}.

Using @samp{.PHONY} is more explicit and more efficient.  However,
other versions of @code{make} do not support @samp{.PHONY}; thus
@samp{FORCE} appears in many makefiles.  @xref{Phony Targets}.

@node Empty Targets
@section Empty Target Files to Record Events
@cindex empty targets
@cindex targets, empty
@cindex recording events with empty targets

The @dfn{empty target} is a variant of the phony target; it is used to hold
recipes for an action that you request explicitly from time to time.
Unlike a phony target, this target file can really exist; but the file's
contents do not matter, and usually are empty.

The purpose of the empty target file is to record, with its
last-modification time, when the rule's recipe was last executed.  It
does so because one of the commands in the recipe is a @code{touch}
command to update the target file.

The empty target file should have some prerequisites (otherwise it
doesn't make sense).  When you ask to remake the empty target, the
recipe is executed if any prerequisite is more recent than the target;
in other words, if a prerequisite has changed since the last time you
remade the target.  Here is an example:

@example
print: foo.c bar.c
        lpr -p $?
        touch print
@end example
@cindex @code{print} target
@cindex @code{lpr} (shell command)
@cindex @code{touch} (shell command)

@noindent
With this rule, @samp{make print} will execute the @code{lpr} command if
either source file has changed since the last @samp{make print}.  The
automatic variable @samp{$?} is used to print only those files that have
changed (@pxref{Automatic Variables}).

@node Special Targets
@section Special Built-in Target Names
@cindex special targets
@cindex built-in special targets
@cindex targets, built-in special

Certain names have special meanings if they appear as targets.

@table @code
@findex .PHONY
@item .PHONY

The prerequisites of the special target @code{.PHONY} are considered to
be phony targets.  When it is time to consider such a target,
@code{make} will run its recipe unconditionally, regardless of
whether a file with that name exists or what its last-modification
time is.  @xref{Phony Targets, ,Phony Targets}.

@findex .SUFFIXES
@item .SUFFIXES

The prerequisites of the special target @code{.SUFFIXES} are the list
of suffixes to be used in checking for suffix rules.
@xref{Suffix Rules, , Old-Fashioned Suffix Rules}.

@findex .DEFAULT@r{, special target}
@item .DEFAULT

The recipe specified for @code{.DEFAULT} is used for any target for
which no rules are found (either explicit rules or implicit rules).
@xref{Last Resort}.  If a @code{.DEFAULT} recipe is specified, every
file mentioned as a prerequisite, but not as a target in a rule, will have
that recipe executed on its behalf.  @xref{Implicit Rule Search,
,Implicit Rule Search Algorithm}.

@findex .PRECIOUS
@item .PRECIOUS
@cindex precious targets
@cindex preserving with @code{.PRECIOUS}

The targets which @code{.PRECIOUS} depends on are given the following
special treatment: if @code{make} is killed or interrupted during the
execution of their recipes, the target is not deleted.
@xref{Interrupts, ,Interrupting or Killing @code{make}}.  Also, if the
target is an intermediate file, it will not be deleted after it is no
longer needed, as is normally done.  @xref{Chained Rules, ,Chains of
Implicit Rules}.  In this latter respect it overlaps with the
@code{.SECONDARY} special target.

You can also list the target pattern of an implicit rule (such as
@samp{%.o}) as a prerequisite file of the special target @code{.PRECIOUS}
to preserve intermediate files created by rules whose target patterns
match that file's name.

@findex .INTERMEDIATE
@item .INTERMEDIATE
@cindex intermediate targets, explicit

The targets which @code{.INTERMEDIATE} depends on are treated as
intermediate files.  @xref{Chained Rules, ,Chains of Implicit Rules}.
@code{.INTERMEDIATE} with no prerequisites has no effect.

@findex .NOTINTERMEDIATE
@item .NOTINTERMEDIATE
@cindex not intermediate targets, explicit

Prerequisites of the special target @code{.NOTINTERMEDIATE} are never
considered intermediate files.  @xref{Chained Rules, ,Chains of Implicit Rules}.
@code{.NOTINTERMEDIATE} with no prerequisites causes all targets to be treated
as not intermediate.

If the prerequisite is a target pattern then targets that are built using that
pattern rule are not considered intermediate.

@findex .SECONDARY
@item .SECONDARY
@cindex secondary targets
@cindex preserving with @code{.SECONDARY}

The targets which @code{.SECONDARY} depends on are treated as
intermediate files, except that they are never automatically deleted.
@xref{Chained Rules, ,Chains of Implicit Rules}.

@code{.SECONDARY} can be used to avoid redundant rebuilds in some unusual
situations.  For example:

@example
@group
hello.bin: hello.o bye.o
        $(CC) -o $@@ $^

%.o: %.c
        $(CC) -c -o $@@ $<

.SECONDARY: hello.o bye.o
@end group
@end example

Suppose @file{hello.bin} is up to date in regards to the source files,
@emph{but} the object file @file{hello.o} is missing.  Without
@code{.SECONDARY} make would rebuild @file{hello.o} then rebuild
@file{hello.bin} even though the source files had not changed.  By declaring
@file{hello.o} as @code{.SECONDARY} @code{make} will not need to rebuild it
and won't need to rebuild @file{hello.bin} either.  Of course, if one of the
source files @emph{were} updated then all object files would be rebuilt so
that the creation of @file{hello.bin} could succeed.

@code{.SECONDARY} with no prerequisites causes all targets to be treated
as secondary (i.e., no target is removed because it is considered
intermediate).

@item .SECONDEXPANSION

If @code{.SECONDEXPANSION} is mentioned as a target anywhere in the
makefile, then all prerequisite lists defined @emph{after} it appears
will be expanded a second time after all makefiles have been read in.
@xref{Secondary Expansion, ,Secondary Expansion}.

@findex .DELETE_ON_ERROR
@item .DELETE_ON_ERROR
@cindex removing targets on failure

If @code{.DELETE_ON_ERROR} is mentioned as a target anywhere in the
makefile, then @code{make} will delete the target of a rule if it has
changed and its recipe exits with a nonzero exit status, just as it
does when it receives a signal.  @xref{Errors, ,Errors in Recipes}.

@findex .IGNORE
@item .IGNORE

If you specify prerequisites for @code{.IGNORE}, then @code{make} will
ignore errors in execution of the recipe for those particular files.
The recipe for @code{.IGNORE} (if any) is ignored.

If mentioned as a target with no prerequisites, @code{.IGNORE} says to
ignore errors in execution of recipes for all files.  This usage of
@samp{.IGNORE} is supported only for historical compatibility.  Since
this affects every recipe in the makefile, it is not very useful; we
recommend you use the more selective ways to ignore errors in specific
recipes.  @xref{Errors, ,Errors in Recipes}.

@findex .LOW_RESOLUTION_TIME
@item .LOW_RESOLUTION_TIME

If you specify prerequisites for @code{.LOW_RESOLUTION_TIME},
@command{make} assumes that these files are created by commands that
generate low resolution time stamps.  The recipe for the
@code{.LOW_RESOLUTION_TIME} target are ignored.

The high resolution file time stamps of many modern file systems
lessen the chance of @command{make} incorrectly concluding that a file
is up to date.  Unfortunately, some hosts do not provide a way to set a
high resolution file time stamp, so commands like @samp{cp -p} that
explicitly set a file's time stamp must discard its sub-second part.
If a file is created by such a command, you should list it as a
prerequisite of @code{.LOW_RESOLUTION_TIME} so that @command{make}
does not mistakenly conclude that the file is out of date.  For
example:

@example
@group
.LOW_RESOLUTION_TIME: dst
dst: src
        cp -p src dst
@end group
@end example

Since @samp{cp -p} discards the sub-second part of @file{src}'s time
stamp, @file{dst} is typically slightly older than @file{src} even when
it is up to date.  The @code{.LOW_RESOLUTION_TIME} line causes
@command{make} to consider @file{dst} to be up to date if its time stamp
is at the start of the same second that @file{src}'s time stamp is in.

Due to a limitation of the archive format, archive member time stamps
are always low resolution.  You need not list archive members as
prerequisites of @code{.LOW_RESOLUTION_TIME}, as @command{make} does this
automatically.

@findex .SILENT
@item .SILENT

If you specify prerequisites for @code{.SILENT}, then @code{make} will
not print the recipe used to remake those particular files before
executing them.  The recipe for @code{.SILENT} is ignored.

If mentioned as a target with no prerequisites, @code{.SILENT} says
not to print any recipes before executing them.  You may also use more
selective ways to silence specific recipe command lines.
@xref{Echoing, ,Recipe Echoing}.  If you want to silence all recipes
for a particular run of @code{make}, use the @samp{-s} or
@w{@samp{--silent}} option (@pxref{Options Summary}).

@findex .EXPORT_ALL_VARIABLES
@item .EXPORT_ALL_VARIABLES

Simply by being mentioned as a target, this tells @code{make} to export all
variables to child processes by default.  This is an alternative to using
@code{export} with no arguments.  @xref{Variables/Recursion, ,Communicating
Variables to a Sub-@code{make}}.

@findex .NOTPARALLEL
@item .NOTPARALLEL
@cindex parallel execution, overriding

If @code{.NOTPARALLEL} is mentioned as a target with no prerequisites, all
targets in this invocation of @code{make} will be run serially, even if the
@samp{-j} option is given.  Any recursively invoked @code{make} command will
still run recipes in parallel (unless its makefile also contains this target).

If @code{.NOTPARALLEL} has targets as prerequisites, then all the
prerequisites of those targets will be run serially.  This implicitly adds a
@code{.WAIT} between each prerequisite of the listed targets.  @xref{Parallel
Disable, , Disabling Parallel Execution}.

@item .ONESHELL
@cindex recipe execution, single invocation

If @code{.ONESHELL} is mentioned as a target, then when a target is
built all lines of the recipe will be given to a single invocation of
the shell rather than each line being invoked separately.
@xref{Execution, ,Recipe Execution}.

@findex .POSIX
@item .POSIX
@cindex POSIX-conforming mode, setting

If @code{.POSIX} is mentioned as a target, then the makefile will be
parsed and run in POSIX-conforming mode.  This does @emph{not} mean
that only POSIX-conforming makefiles will be accepted: all advanced
GNU @code{make} features are still available.  Rather, this target
causes @code{make} to behave as required by POSIX in those areas
where @code{make}'s default behavior differs.

In particular, if this target is mentioned then recipes will be
invoked as if the shell had been passed the @code{-e} flag: the first
failing command in a recipe will cause the recipe to fail immediately.
@end table

Any defined implicit rule suffix also counts as a special target if it
appears as a target, and so does the concatenation of two suffixes, such
as @samp{.c.o}.  These targets are suffix rules, an obsolete way of
defining implicit rules (but a way still widely used).  In principle, any
target name could be special in this way if you break it in two and add
both pieces to the suffix list.  In practice, suffixes normally begin with
@samp{.}, so these special target names also begin with @samp{.}.
@xref{Suffix Rules, ,Old-Fashioned Suffix Rules}.

@node Multiple Targets
@section Multiple Targets in a Rule
@cindex multiple targets
@cindex several targets in a rule
@cindex targets, multiple
@cindex rule, with multiple targets

When an explicit rule has multiple targets they can be treated in one
of two possible ways: as independent targets or as grouped targets.
The manner in which they are treated is determined by the separator that
appears after the list of targets.

@subsubheading Rules with Independent Targets
@cindex independent targets
@cindex targets, independent

Rules that use the standard target separator, @code{:}, define
independent targets.  This is equivalent to writing the same rule once
for each target, with duplicated prerequisites and recipes.  Typically,
the recipe would use automatic variables such as @samp{$@@} to specify
which target is being built.

Rules with independent targets are useful in two cases:

@itemize @bullet
@item
You want just prerequisites, no recipe.  For example:

@example
kbd.o command.o files.o: command.h
@end example

@noindent
gives an additional prerequisite to each of the three object files
mentioned.  It is equivalent to writing:

@example
kbd.o: command.h
command.o: command.h
files.o: command.h
@end example

@item
Similar recipes work for all the targets.  The automatic variable
@samp{$@@} can be used to substitute the particular target to be
remade into the commands (@pxref{Automatic Variables}).  For example:

@example
@group
bigoutput littleoutput : text.g
        generate text.g -$(subst output,,$@@) > $@@
@end group
@end example

@noindent
is equivalent to

@example
bigoutput : text.g
        generate text.g -big > bigoutput
littleoutput : text.g
        generate text.g -little > littleoutput
@end example

@noindent
Here we assume the hypothetical program @code{generate} makes two
types of output, one if given @samp{-big} and one if given
@samp{-little}.
@xref{Text Functions, ,Functions for String Substitution and Analysis},
for an explanation of the @code{subst} function.
@end itemize

Suppose you would like to vary the prerequisites according to the
target, much as the variable @samp{$@@} allows you to vary the recipe.
You cannot do this with multiple targets in an ordinary rule, but you
can do it with a @dfn{static pattern rule}.  @xref{Static Pattern,
,Static Pattern Rules}.

@subsubheading Rules with Grouped Targets
@cindex grouped targets
@cindex targets, grouped

If instead of independent targets you have a recipe that generates multiple
files from a single invocation, you can express that relationship by declaring
your rule to use @emph{grouped targets}.  A grouped target rule uses the
separator @code{&:} (the @samp{&} here is used to imply ``all'').

When @code{make} builds any one of the grouped targets, it understands that
all the other targets in the group are also updated as a result of the
invocation of the recipe.  Furthermore, if only some of the grouped targets
are out of date or missing @code{make} will realize that running the recipe
will update all of the targets.  Finally, if any of the grouped targets are
out of date, all the grouped targets are considered out of date.

As an example, this rule defines a grouped target:

@example
@group
foo bar biz &: baz boz
        echo $^ > foo
        echo $^ > bar
        echo $^ > biz
@end group
@end example

During the execution of a grouped target's recipe, the automatic
variable @samp{$@@} is set to the name of the particular target in the
group which triggered the rule.  Caution must be used if relying on
this variable in the recipe of a grouped target rule.

Unlike independent targets, a grouped target rule @emph{must} include
a recipe.  However, targets that are members of a grouped target may
also appear in independent target rule definitions that do not have
recipes.

Each target may have only one recipe associated with it.  If a grouped
target appears in either an independent target rule or in another
grouped target rule with a recipe, you will get a warning and the
latter recipe will replace the former recipe.  Additionally the target
will be removed from the previous group and appear only in the new
group.

If you would like a target to appear in multiple groups, then you must
use the double-colon grouped target separator, @code{&::} when
declaring all of the groups containing that target.  Grouped
double-colon targets are each considered independently, and each
grouped double-colon rule's recipe is executed at most once, if at
least one of its multiple targets requires updating.

@node Multiple Rules
@section Multiple Rules for One Target
@cindex multiple rules for one target
@cindex several rules for one target
@cindex rule, multiple for one target
@cindex target, multiple rules for one

One file can be the target of several rules.  All the prerequisites
mentioned in all the rules are merged into one list of prerequisites for
the target.  If the target is older than any prerequisite from any rule,
the recipe is executed.

There can only be one recipe to be executed for a file.  If more than
one rule gives a recipe for the same file, @code{make} uses the last
one given and prints an error message.  (As a special case, if the
file's name begins with a dot, no error message is printed.  This odd
behavior is only for compatibility with other implementations of
@code{make}@dots{} you should avoid using it).  Occasionally it is
useful to have the same target invoke multiple recipes which are
defined in different parts of your makefile; you can use
@dfn{double-colon rules} (@pxref{Double-Colon}) for this.

An extra rule with just prerequisites can be used to give a few extra
prerequisites to many files at once.  For example, makefiles often
have a variable, such as @code{objects}, containing a list of all the
compiler output files in the system being made.  An easy way to say
that all of them must be recompiled if @file{config.h} changes is to
write the following:

@example
objects = foo.o bar.o
foo.o : defs.h
bar.o : defs.h test.h
$(objects) : config.h
@end example

This could be inserted or taken out without changing the rules that really
specify how to make the object files, making it a convenient form to use if
you wish to add the additional prerequisite intermittently.

Another wrinkle is that the additional prerequisites could be
specified with a variable that you set with a command line argument to
@code{make} (@pxref{Overriding, ,Overriding Variables}).  For example,

@example
@group
extradeps=
$(objects) : $(extradeps)
@end group
@end example

@noindent
means that the command @samp{make extradeps=foo.h} will consider
@file{foo.h} as a prerequisite of each object file, but plain @samp{make}
will not.

If none of the explicit rules for a target has a recipe, then @code{make}
searches for an applicable implicit rule to find one
@pxref{Implicit Rules, ,Using Implicit Rules}).

@node Static Pattern
@section Static Pattern Rules
@cindex static pattern rule
@cindex rule, static pattern
@cindex pattern rules, static (not implicit)
@cindex varying prerequisites
@cindex prerequisites, varying (static pattern)

@dfn{Static pattern rules} are rules which specify multiple targets and
construct the prerequisite names for each target based on the target name.
They are more general than ordinary rules with multiple targets because the
targets do not have to have identical prerequisites.  Their prerequisites must
be @emph{analogous}, but not necessarily @emph{identical}.

@menu
* Static Usage::                The syntax of static pattern rules.
* Static versus Implicit::      When are they better than implicit rules?
@end menu

@node Static Usage
@subsection Syntax of Static Pattern Rules
@cindex static pattern rule, syntax of
@cindex pattern rules, static, syntax of

Here is the syntax of a static pattern rule:

@example
@var{targets} @dots{}: @var{target-pattern}: @var{prereq-patterns} @dots{}
        @var{recipe}
        @dots{}
@end example

@noindent
The @var{targets} list specifies the targets that the rule applies to.
The targets can contain wildcard characters, just like the targets of
ordinary rules (@pxref{Wildcards, ,Using Wildcard Characters in File
Names}).

@cindex target pattern, static (not implicit)
@cindex stem
The @var{target-pattern} and @var{prereq-patterns} say how to compute the
prerequisites of each target.  Each target is matched against the
@var{target-pattern} to extract a part of the target name, called the
@dfn{stem}.  This stem is substituted into each of the @var{prereq-patterns}
to make the prerequisite names (one from each @var{prereq-pattern}).

Each pattern normally contains the character @samp{%} just once.  When the
@var{target-pattern} matches a target, the @samp{%} can match any part of
the target name; this part is called the @dfn{stem}.  The rest of the
pattern must match exactly.  For example, the target @file{foo.o} matches
the pattern @samp{%.o}, with @samp{foo} as the stem.  The targets
@file{foo.c} and @file{foo.out} do not match that pattern.

@cindex prerequisite pattern, static (not implicit)
The prerequisite names for each target are made by substituting the stem
for the @samp{%} in each prerequisite pattern.  For example, if one
prerequisite pattern is @file{%.c}, then substitution of the stem
@samp{foo} gives the prerequisite name @file{foo.c}.  It is legitimate
to write a prerequisite pattern that does not contain @samp{%}; then this
prerequisite is the same for all targets.

@cindex @code{%}, quoting in static pattern
@cindex @code{\} (backslash), to quote @code{%}
@cindex backslash (@code{\}), to quote @code{%}
@cindex quoting @code{%}, in static pattern
@samp{%} characters in pattern rules can be quoted with preceding
backslashes (@samp{\}).  Backslashes that would otherwise quote @samp{%}
characters can be quoted with more backslashes.  Backslashes that quote
@samp{%} characters or other backslashes are removed from the pattern
before it is compared to file names or has a stem substituted into it.
Backslashes that are not in danger of quoting @samp{%} characters go
unmolested.  For example, the pattern @file{the\%weird\\%pattern\\} has
@samp{the%weird\} preceding the operative @samp{%} character, and
@samp{pattern\\} following it.  The final two backslashes are left alone
because they cannot affect any @samp{%} character.

Here is an example, which compiles each of @file{foo.o} and @file{bar.o}
from the corresponding @file{.c} file:

@example
@group
objects = foo.o bar.o

all: $(objects)

$(objects): %.o: %.c
        $(CC) -c $(CFLAGS) $< -o $@@
@end group
@end example

@noindent
Here @samp{$<} is the automatic variable that holds the name of the
prerequisite and @samp{$@@} is the automatic variable that holds the name
of the target; see @ref{Automatic Variables}.

Each target specified must match the target pattern; a warning is issued
for each target that does not.  If you have a list of files, only some of
which will match the pattern, you can use the @code{filter} function to
remove non-matching file names (@pxref{Text Functions, ,Functions for String Substitution and Analysis}):

@example
files = foo.elc bar.o lose.o

$(filter %.o,$(files)): %.o: %.c
        $(CC) -c $(CFLAGS) $< -o $@@
$(filter %.elc,$(files)): %.elc: %.el
        emacs -f batch-byte-compile $<
@end example

@noindent
In this example the result of @samp{$(filter %.o,$(files))} is
@file{bar.o lose.o}, and the first static pattern rule causes each of
these object files to be updated by compiling the corresponding C source
file.  The result of @w{@samp{$(filter %.elc,$(files))}} is
@file{foo.elc}, so that file is made from @file{foo.el}.

Another example shows how to use @code{$*} in static pattern rules:
@vindex $*@r{, and static pattern}

@example
@group
bigoutput littleoutput : %output : text.g
        generate text.g -$* > $@@
@end group
@end example

@noindent
When the @code{generate} command is run, @code{$*} will expand to the
stem, either @samp{big} or @samp{little}.

@node Static versus Implicit
@subsection Static Pattern Rules versus Implicit Rules
@cindex rule, static pattern versus implicit
@cindex static pattern rule, versus implicit

A static pattern rule has much in common with an implicit rule defined as a
pattern rule (@pxref{Pattern Rules, ,Defining and Redefining Pattern Rules}).
Both have a pattern for the target and patterns for constructing the
names of prerequisites.  The difference is in how @code{make} decides
@emph{when} the rule applies.

An implicit rule @emph{can} apply to any target that matches its pattern,
but it @emph{does} apply only when the target has no recipe otherwise
specified, and only when the prerequisites can be found.  If more than one
implicit rule appears applicable, only one applies; the choice depends on
the order of rules.

By contrast, a static pattern rule applies to the precise list of targets
that you specify in the rule.  It cannot apply to any other target and it
invariably does apply to each of the targets specified.  If two conflicting
rules apply, and both have recipes, that's an error.

The static pattern rule can be better than an implicit rule for these
reasons:

@itemize @bullet
@item
You may wish to override the usual implicit rule for a few
files whose names cannot be categorized syntactically but
can be given in an explicit list.

@item
If you cannot be sure of the precise contents of the directories
you are using, you may not be sure which other irrelevant files
might lead @code{make} to use the wrong implicit rule.  The choice
might depend on the order in which the implicit rule search is done.
With static pattern rules, there is no uncertainty: each rule applies
to precisely the targets specified.
@end itemize

@node Double-Colon
@section Double-Colon Rules
@cindex double-colon rules
@cindex rule, double-colon (@code{::})
@cindex multiple rules for one target (@code{::})
@cindex @code{::} rules (double-colon)

@dfn{Double-colon} rules are explicit rules written with @samp{::}
instead of @samp{:} after the target names.  They are handled
differently from ordinary rules when the same target appears in more
than one rule.  Pattern rules with double-colons have an entirely
different meaning (@pxref{Match-Anything Rules}).

When a target appears in multiple rules, all the rules must be the same
type: all ordinary, or all double-colon.  If they are double-colon, each
of them is independent of the others.  Each double-colon rule's recipe
is executed if the target is older than any prerequisites of that rule.
If there are no prerequisites for that rule, its recipe is always
executed (even if the target already exists).  This can result in
executing none, any, or all of the double-colon rules.

Double-colon rules with the same target are in fact completely separate
from one another.  Each double-colon rule is processed individually, just
as rules with different targets are processed.

The double-colon rules for a target are executed in the order they appear
in the makefile.  However, the cases where double-colon rules really make
sense are those where the order of executing the recipes would not matter.

Double-colon rules are somewhat obscure and not often very useful; they
provide a mechanism for cases in which the method used to update a target
differs depending on which prerequisite files caused the update, and such
cases are rare.

Each double-colon rule should specify a recipe; if it does not, an
implicit rule will be used if one applies.
@xref{Implicit Rules, ,Using Implicit Rules}.

@node Automatic Prerequisites
@section Generating Prerequisites Automatically
@cindex prerequisites, automatic generation
@cindex automatic generation of prerequisites
@cindex generating prerequisites automatically

In the makefile for a program, many of the rules you need to write often
say only that some object file depends on some header
file.  For example, if @file{main.c} uses @file{defs.h} via an
@code{#include}, you would write:

@example
main.o: defs.h
@end example

@noindent
You need this rule so that @code{make} knows that it must remake
@file{main.o} whenever @file{defs.h} changes.  You can see that for a
large program you would have to write dozens of such rules in your
makefile.  And, you must always be very careful to update the makefile
every time you add or remove an @code{#include}.
@cindex @code{#include}

@cindex @code{-M} (to compiler)
To avoid this hassle, most modern C compilers can write these rules for
you, by looking at the @code{#include} lines in the source files.
Usually this is done with the @samp{-M} option to the compiler.
For example, the command:

@example
cc -M main.c
@end example

@noindent
generates the output:

@example
main.o : main.c defs.h
@end example

@noindent
Thus you no longer have to write all those rules yourself.
The compiler will do it for you.

Note that such a rule constitutes mentioning @file{main.o} in a
makefile, so it can never be considered an intermediate file by
implicit rule search.  This means that @code{make} won't ever remove
the file after using it; @pxref{Chained Rules, ,Chains of Implicit
Rules}.

@cindex @code{make depend}
With old @code{make} programs, it was traditional practice to use this
compiler feature to generate prerequisites on demand with a command like
@samp{make depend}.  That command would create a file @file{depend}
containing all the automatically-generated prerequisites; then the
makefile could use @code{include} to read them in (@pxref{Include}).

In GNU @code{make}, the feature of remaking makefiles makes this
practice obsolete---you need never tell @code{make} explicitly to
regenerate the prerequisites, because it always regenerates any makefile
that is out of date.  @xref{Remaking Makefiles}.

The practice we recommend for automatic prerequisite generation is to have
one makefile corresponding to each source file.  For each source file
@file{@var{name}.c} there is a makefile @file{@var{name}.d} which lists
what files the object file @file{@var{name}.o} depends on.  That way
only the source files that have changed need to be rescanned to produce
the new prerequisites.

Here is the pattern rule to generate a file of prerequisites (i.e., a makefile)
called @file{@var{name}.d} from a C source file called @file{@var{name}.c}:

@smallexample
@group
%.d: %.c
        @@set -e; rm -f $@@; \
         $(CC) -M $(CPPFLAGS) $< > $@@.$$$$; \
         sed 's,\($*\)\.o[ :]*,\1.o $@@ : ,g' < $@@.$$$$ > $@@; \
         rm -f $@@.$$$$
@end group
@end smallexample

@noindent
@xref{Pattern Rules}, for information on defining pattern rules.  The
@samp{-e} flag to the shell causes it to exit immediately if the
@code{$(CC)} command (or any other command) fails (exits with a
nonzero status).
@cindex @code{-e} (shell flag)

@cindex @code{-MM} (to GNU compiler)
With the GNU C compiler, you may wish to use the @samp{-MM} flag instead
of @samp{-M}.  This omits prerequisites on system header files.
@xref{Preprocessor Options, , Options Controlling the Preprocessor,
gcc, Using GNU CC}, for details.

@cindex @code{sed} (shell command)
The purpose of the @code{sed} command is to translate (for example):

@example
main.o : main.c defs.h
@end example

@noindent
into:

@example
main.o main.d : main.c defs.h
@end example

@noindent
@cindex @code{.d}
This makes each @samp{.d} file depend on all the source and header files
that the corresponding @samp{.o} file depends on.  @code{make} then
knows it must regenerate the prerequisites whenever any of the source or
header files changes.

Once you've defined the rule to remake the @samp{.d} files,
you then use the @code{include} directive to read them all in.
@xref{Include}.  For example:

@example
@group
sources = foo.c bar.c

include $(sources:.c=.d)
@end group
@end example

@noindent
(This example uses a substitution variable reference to translate the
list of source files @samp{foo.c bar.c} into a list of prerequisite
makefiles, @samp{foo.d bar.d}.  @xref{Substitution Refs}, for full
information on substitution references.)  Since the @samp{.d} files are
makefiles like any others, @code{make} will remake them as necessary
with no further work from you.  @xref{Remaking Makefiles}.

Note that the @samp{.d} files contain target definitions; you should
be sure to place the @code{include} directive @emph{after} the first,
default goal in your makefiles or run the risk of having a random
object file become the default goal.
@xref{How Make Works}.

@node Recipes
@chapter Writing Recipes in Rules
@cindex recipes
@cindex recipes, how to write
@cindex writing recipes

The recipe of a rule consists of one or more shell command lines to
be executed, one at a time, in the order they appear.  Typically, the
result of executing these commands is that the target of the rule is
brought up to date.

Users use many different shell programs, but recipes in makefiles are
always interpreted by @file{/bin/sh} unless the makefile specifies
otherwise.  @xref{Execution, ,Recipe Execution}.

@menu
* Recipe Syntax::               Recipe syntax features and pitfalls.
* Echoing::                     How to control when recipes are echoed.
* Execution::                   How recipes are executed.
* Parallel::                    How recipes can be executed in parallel.
* Errors::                      What happens after a recipe execution error.
* Interrupts::                  What happens when a recipe is interrupted.
* Recursion::                   Invoking @code{make} from makefiles.
* Canned Recipes::              Defining canned recipes.
* Empty Recipes::               Defining useful, do-nothing recipes.
@end menu

@node Recipe Syntax
@section Recipe Syntax
@cindex recipe syntax
@cindex syntax of recipe

Makefiles have the unusual property that there are really two distinct
syntaxes in one file.  Most of the makefile uses @code{make} syntax
(@pxref{Makefiles, ,Writing Makefiles}).  However, recipes are meant
to be interpreted by the shell and so they are written using shell
syntax.  The @code{make} program does not try to understand shell
syntax: it performs only a very few specific translations on the
content of the recipe before handing it to the shell.

Each line in the recipe must start with a tab (or the first character
in the value of the @code{.RECIPEPREFIX} variable; @pxref{Special
Variables}), except that the first recipe line may be attached to the
target-and-prerequisites line with a semicolon in between.  @emph{Any}
line in the makefile that begins with a tab and appears in a ``rule
context'' (that is, after a rule has been started until another rule
or variable definition) will be considered part of a recipe for that
rule.  Blank lines and lines of just comments may appear among the
recipe lines; they are ignored.

Some consequences of these rules include:

@itemize @bullet
@item
A blank line that begins with a tab is not blank: it's an empty
recipe (@pxref{Empty Recipes}).

@cindex comments, in recipes
@cindex recipes, comments in
@cindex @code{#} (comments), in recipes
@item
A comment in a recipe is not a @code{make} comment; it will be
passed to the shell as-is.  Whether the shell treats it as a comment
or not depends on your shell.

@item
A variable definition in a ``rule context'' which is indented by a tab
as the first character on the line, will be considered part of a
recipe, not a @code{make} variable definition, and passed to the
shell.

@item
A conditional expression (@code{ifdef}, @code{ifeq},
etc. @pxref{Conditional Syntax, ,Syntax of Conditionals}) in a ``rule
context'' which is indented by a tab as the first character on the
line, will be considered part of a recipe and be passed to the shell.

@end itemize

@menu
* Splitting Recipe Lines::      Breaking long recipe lines for readability.
* Variables in Recipes::        Using @code{make} variables in recipes.
@end menu

@node Splitting Recipe Lines
@subsection Splitting Recipe Lines
@cindex recipes, splitting
@cindex splitting recipes
@cindex recipes, backslash (@code{\}) in
@cindex recipes, quoting newlines in
@cindex backslash (@code{\}), in recipes
@cindex @code{\} (backslash), in recipes
@cindex quoting newline, in recipes
@cindex newline, quoting, in recipes

One of the few ways in which @code{make} does interpret recipes is
checking for a backslash just before the newline.  As in normal
makefile syntax, a single logical recipe line can be split into
multiple physical lines in the makefile by placing a backslash before
each newline.  A sequence of lines like this is considered a single
recipe line, and one instance of the shell will be invoked to run it.

However, in contrast to how they are treated in other places in a
makefile (@pxref{Splitting Lines, , Splitting Long Lines}),
backslash/newline pairs are @emph{not} removed from the recipe.  Both
the backslash and the newline characters are preserved and passed to
the shell.  How the backslash/newline is interpreted depends on your
shell.  If the first character of the next line after the
backslash/newline is the recipe prefix character (a tab by default;
@pxref{Special Variables}), then that character (and only that
character) is removed.  Whitespace is never added to the recipe.

For example, the recipe for the all target in this makefile:

@example
@group
all :
        @@echo no\
space
        @@echo no\
        space
        @@echo one \
        space
        @@echo one\
         space
@end group
@end example

@noindent
consists of four separate shell commands where the output is:

@example
@group
nospace
nospace
one space
one space
@end group
@end example

As a more complex example, this makefile:

@example
@group
all : ; @@echo 'hello \
        world' ; echo "hello \
    world"
@end group
@end example

@noindent
will invoke one shell with a command of:

@example
@group
echo 'hello \
world' ; echo "hello \
    world"
@end group
@end example

@noindent
which, according to shell quoting rules, will yield the following output:

@example
@group
hello \
world
hello     world
@end group
@end example

@noindent
Notice how the backslash/newline pair was removed inside the string
quoted with double quotes (@code{"@dots{}"}), but not from the string
quoted with single quotes (@code{'@dots{}'}).  This is the way the
default shell (@file{/bin/sh}) handles backslash/newline pairs.  If
you specify a different shell in your makefiles it may treat them
differently.

Sometimes you want to split a long line inside of single quotes, but
you don't want the backslash/newline to appear in the quoted content.
This is often the case when passing scripts to languages such as Perl,
where extraneous backslashes inside the script can change its meaning
or even be a syntax error.  One simple way of handling this is to
place the quoted string, or even the entire command, into a
@code{make} variable then use the variable in the recipe.  In this
situation the newline quoting rules for makefiles will be used, and
the backslash/newline will be removed.  If we rewrite our example
above using this method:

@example
@group
HELLO = 'hello \
world'

all : ; @@echo $(HELLO)
@end group
@end example

@noindent
we will get output like this:

@example
@group
hello world
@end group
@end example

If you like, you can also use target-specific variables
(@pxref{Target-specific, ,Target-specific Variable Values}) to obtain
a tighter correspondence between the variable and the recipe that
uses it.

@node Variables in Recipes
@subsection Using Variables in Recipes
@cindex variable references in recipes
@cindex recipes, using variables in

The other way in which @code{make} processes recipes is by expanding
any variable references in them (@pxref{Reference,Basics of Variable
References}).  This occurs after make has finished reading all the
makefiles and the target is determined to be out of date; so, the
recipes for targets which are not rebuilt are never expanded.

Variable and function references in recipes have identical syntax and
semantics to references elsewhere in the makefile.  They also have the
same quoting rules: if you want a dollar sign to appear in your
recipe, you must double it (@samp{$$}).  For shells like the default
shell, that use dollar signs to introduce variables, it's important to
keep clear in your mind whether the variable you want to reference is
a @code{make} variable (use a single dollar sign) or a shell variable
(use two dollar signs).  For example:

@example
@group
LIST = one two three
all:
        for i in $(LIST); do \
            echo $$i; \
        done
@end group
@end example

@noindent
results in the following command being passed to the shell:

@example
@group
for i in one two three; do \
    echo $i; \
done
@end group
@end example

@noindent
which generates the expected result:

@example
@group
one
two
three
@end group
@end example

@node Echoing
@section Recipe Echoing
@cindex echoing of recipes
@cindex silent operation
@cindex @code{@@} (in recipes)
@cindex recipes, echoing
@cindex printing of recipes

Normally @code{make} prints each line of the recipe before it is
executed.  We call this @dfn{echoing} because it gives the appearance
that you are typing the lines yourself.

When a line starts with @samp{@@}, the echoing of that line is suppressed.
The @samp{@@} is discarded before the line is passed to the shell.
Typically you would use this for a command whose only effect is to print
something, such as an @code{echo} command to indicate progress through
the makefile:

@example
@@echo About to make distribution files
@end example

Note that the start of a recipe line is interpreted differently when
using @code{.ONESHELL} (@pxref{One Shell, ,Using One Shell}).

@cindex @code{-n}
@cindex @code{--just-print}
@cindex @code{--dry-run}
@cindex @code{--recon}
When @code{make} is given the flag @samp{-n} or @samp{--just-print} it
only echoes most recipes, without executing them.  @xref{Options
Summary, ,Summary of Options}.  In this case even the recipe lines
starting with @samp{@@} are printed.  This flag is useful for finding
out which recipes @code{make} thinks are necessary without actually
doing them.

@cindex @code{-s}
@cindex @code{--silent}
@cindex @code{--quiet}
The @samp{-s} or @samp{--silent}
flag to @code{make} prevents all echoing, as if all recipes
started with @samp{@@}.  A rule in the makefile for the special target
@code{.SILENT} without prerequisites has the same effect
(@pxref{Special Targets, ,Special Built-in Target Names}).

@node Execution
@section Recipe Execution
@cindex recipe, execution
@cindex execution, of recipes
@vindex @code{SHELL} @r{(recipe execution)}

When it is time to execute recipes to update a target, they are
executed by invoking a new sub-shell for each line of the recipe,
unless the @code{.ONESHELL} special target is in effect
(@pxref{One Shell, ,Using One Shell})  (In practice, @code{make} may
take shortcuts that do not affect the results.)

@cindex @code{cd} (shell command)
@cindex shell variables, setting in recipes
@cindex recipes setting shell variables
@strong{Please note:} this implies that setting shell variables and
invoking shell commands such as @code{cd} that set a context local to
each process will not affect the following lines in the recipe.@footnote{On
MS-DOS, the value of current working directory is @strong{global}, so
changing it @emph{will} affect the following recipe lines on those
systems.}  If you want to use @code{cd} to affect the next statement,
put both statements in a single recipe line.  Then @code{make} will
invoke one shell to run the entire line, and the shell will execute
the statements in sequence.  For example:

@example
foo : bar/lose
        cd $(<D) && gobble $(<F) > ../$@@
@end example

@noindent
Here we use the shell AND operator (@code{&&}) so that if the
@code{cd} command fails, the script will fail without trying to invoke
the @code{gobble} command in the wrong directory, which could cause
problems (in this case it would certainly cause @file{../foo} to be
truncated, at least).

@menu
* One Shell::                   One shell for all lines in a recipe.
* Choosing the Shell::          How @code{make} chooses the shell used
                                  to run recipes.
@end menu

@node One Shell
@subsection Using One Shell
@cindex recipe lines, single shell
@cindex @code{.ONESHELL}, use of
@findex .ONESHELL

Sometimes you would prefer that all the lines in the recipe be passed
to a single invocation of the shell.  There are generally two
situations where this is useful: first, it can improve performance in
makefiles where recipes consist of many command lines, by avoiding
extra processes.  Second, you might want newlines to be included in
your recipe command (for example perhaps you are using a very
different interpreter as your @code{SHELL}).  If the @code{.ONESHELL}
special target appears anywhere in the makefile then @emph{all}
recipe lines for each target will be provided to a single invocation
of the shell.  Newlines between recipe lines will be preserved.  For
example:

@example
.ONESHELL:
foo : bar/lose
        cd $(<D)
        gobble $(<F) > ../$@@
@end example

@noindent
would now work as expected even though the commands are on different
recipe lines.

If @code{.ONESHELL} is provided, then only the first line of the
recipe will be checked for the special prefix characters (@samp{@@},
@samp{-}, and @samp{+}).  Subsequent lines will include the special
characters in the recipe line when the @code{SHELL} is invoked.  If
you want your recipe to start with one of these special characters
you'll need to arrange for them to not be the first characters on the
first line, perhaps by adding a comment or similar.  For example, this
would be a syntax error in Perl because the first @samp{@@} is removed
by make:

@example
.ONESHELL:
SHELL = /usr/bin/perl
.SHELLFLAGS = -e
show :
        @@f = qw(a b c);
        print "@@f\n";
@end example

@noindent
However, either of these alternatives would work properly:

@example
.ONESHELL:
SHELL = /usr/bin/perl
.SHELLFLAGS = -e
show :
        # Make sure "@@" is not the first character on the first line
        @@f = qw(a b c);
        print "@@f\n";
@end example

@noindent
or

@example
.ONESHELL:
SHELL = /usr/bin/perl
.SHELLFLAGS = -e
show :
        my @@f = qw(a b c);
        print "@@f\n";
@end example

As a special feature, if @code{SHELL} is determined to be a
POSIX-style shell, the special prefix characters in ``internal''
recipe lines will be @emph{removed} before the recipe is processed.
This feature is intended to allow existing makefiles to add the
@code{.ONESHELL} special target and still run properly without
extensive modifications.  Since the special prefix characters are not
legal at the beginning of a line in a POSIX shell script this is not a
loss in functionality.  For example, this works as expected:

@example
.ONESHELL:
foo : bar/lose
        @@cd $(@@D)
        @@gobble $(@@F) > ../$@@
@end example

Even with this special feature, however, makefiles with
@code{.ONESHELL} will behave differently in ways that could be
noticeable.  For example, normally if any line in the recipe fails,
that causes the rule to fail and no more recipe lines are processed.
Under @code{.ONESHELL} a failure of any but the final recipe line will
not be noticed by @code{make}.  You can modify @code{.SHELLFLAGS} to
add the @code{-e} option to the shell which will cause any failure
anywhere in the command line to cause the shell to fail, but this
could itself cause your recipe to behave differently.  Ultimately you
may need to harden your recipe lines to allow them to work with
@code{.ONESHELL}.

@node Choosing the Shell
@subsection Choosing the Shell
@cindex shell, choosing the
@cindex @code{SHELL}, value of
@cindex @code{.SHELLFLAGS}, value of

@vindex SHELL
@vindex .SHELLFLAGS
The program used as the shell is taken from the variable @code{SHELL}.
If this variable is not set in your makefile, the program
@file{/bin/sh} is used as the shell.  The argument(s) passed to the
shell are taken from the variable @code{.SHELLFLAGS}.  The default
value of @code{.SHELLFLAGS} is @code{-c} normally, or @code{-ec} in
POSIX-conforming mode.

@cindex environment, @code{SHELL} in
Unlike most variables, the variable @code{SHELL} is never set from the
environment.  This is because the @code{SHELL} environment variable is
used to specify your personal choice of shell program for interactive
use.  It would be very bad for personal choices like this to affect the
functioning of makefiles.  @xref{Environment, ,Variables from the
Environment}.

Furthermore, when you do set @code{SHELL} in your makefile that value
is @emph{not} exported in the environment to recipe lines that
@code{make} invokes.  Instead, the value inherited from the user's
environment, if any, is exported.  You can override this behavior by
explicitly exporting @code{SHELL} (@pxref{Variables/Recursion,
,Communicating Variables to a Sub-@code{make}}), forcing it to be
passed in the environment to recipe lines.

However, on MS-DOS and MS-Windows the value of @code{SHELL} in the
environment @strong{is} used, since on those systems most users do not
set this variable, and therefore it is most likely set specifically to
be used by @code{make}.  On MS-DOS, if the setting of @code{SHELL} is
not suitable for @code{make}, you can set the variable
@code{MAKESHELL} to the shell that @code{make} should use; if set it
will be used as the shell instead of the value of @code{SHELL}.

@subsubheading Choosing a Shell in DOS and Windows
@cindex shell, in DOS and Windows
@cindex DOS, choosing a shell in
@cindex Windows, choosing a shell in

Choosing a shell in MS-DOS and MS-Windows is much more complex than on
other systems.

@vindex COMSPEC
On MS-DOS, if @code{SHELL} is not set, the value of the variable
@code{COMSPEC} (which is always set) is used instead.

@cindex @code{SHELL}, MS-DOS specifics
The processing of lines that set the variable @code{SHELL} in Makefiles
is different on MS-DOS.  The stock shell, @file{command.com}, is
ridiculously limited in its functionality and many users of @code{make}
tend to install a replacement shell.  Therefore, on MS-DOS, @code{make}
examines the value of @code{SHELL}, and changes its behavior based on
whether it points to a Unix-style or DOS-style shell.  This allows
reasonable functionality even if @code{SHELL} points to
@file{command.com}.

If @code{SHELL} points to a Unix-style shell, @code{make} on MS-DOS
additionally checks whether that shell can indeed be found; if not, it
ignores the line that sets @code{SHELL}.  In MS-DOS, GNU @code{make}
searches for the shell in the following places:

@enumerate
@item
In the precise place pointed to by the value of @code{SHELL}.  For
example, if the makefile specifies @samp{SHELL = /bin/sh}, @code{make}
will look in the directory @file{/bin} on the current drive.

@item
In the current directory.

@item
In each of the directories in the @code{PATH} variable, in order.

@end enumerate

In every directory it examines, @code{make} will first look for the
specific file (@file{sh} in the example above).  If this is not found,
it will also look in that directory for that file with one of the known
extensions which identify executable files.  For example @file{.exe},
@file{.com}, @file{.bat}, @file{.btm}, @file{.sh}, and some others.

If any of these attempts is successful, the value of @code{SHELL} will
be set to the full pathname of the shell as found.  However, if none of
these is found, the value of @code{SHELL} will not be changed, and thus
the line that sets it will be effectively ignored.  This is so
@code{make} will only support features specific to a Unix-style shell if
such a shell is actually installed on the system where @code{make} runs.

Note that this extended search for the shell is limited to the cases
where @code{SHELL} is set from the Makefile; if it is set in the
environment or command line, you are expected to set it to the full
pathname of the shell, exactly as things are on Unix.

The effect of the above DOS-specific processing is that a Makefile that
contains @samp{SHELL = /bin/sh} (as many Unix makefiles do), will work
on MS-DOS unaltered if you have e.g.@: @file{sh.exe} installed in some
directory along your @code{PATH}.

@node Parallel
@section Parallel Execution
@cindex recipes, execution in parallel
@cindex parallel execution
@cindex execution, in parallel
@cindex job slots
@cindex @code{-j}
@cindex @code{--jobs}

GNU @code{make} knows how to execute several recipes at once.  Normally,
@code{make} will execute only one recipe at a time, waiting for it to finish
before executing the next.  However, the @samp{-j} or @samp{--jobs} option
tells @code{make} to execute many recipes simultaneously.  You can inhibit
parallelism for some or all targets from within the makefile (@pxref{Parallel
Disable, ,Disabling Parallel Execution}).

On MS-DOS, the @samp{-j} option has no effect, since that system doesn't
support multi-processing.

Providing @samp{-j} with no argument tells @code{make} that it should run as
many jobs in parallel as are allowed by the dependency graph.  This is usually
not what you want as it can overload your system very quickly.  For example
suppose you have a large program that is built from 900 object files.  Since
object files normally do not depend on each other, using @samp{-j} by itself
will attempt to create all 900 object files in parallel, starting 900
instances of the compiler in simultaneously.  However, @samp{-j} with no
argument is useful in conjunction with @samp{-l} (see below).

If the @samp{-j} option is followed by an integer, this is the maximum number
of recipes to execute at once; this is called the number of @dfn{job slots}.

Handling recursive @code{make} invocations raises issues for parallel
execution.  For more information on this, see @ref{Options/Recursion,
,Communicating Options to a Sub-@code{make}}.

If a recipe fails (is killed by a signal or exits with a nonzero
status), and errors are not ignored for that recipe (@pxref{Errors,
,Errors in Recipes}), the remaining recipe lines to remake the same
target will not be run.  If a recipe fails and the @samp{-k} or
@samp{--keep-going} option was not given (@pxref{Options Summary,
,Summary of Options}), @code{make} aborts execution.  If make
terminates for any reason (including a signal) with child processes
running, it waits for them to finish before actually exiting.

@cindex load average
@cindex limiting jobs based on load
@cindex jobs, limiting based on load
@cindex @code{-l} (load average)
@cindex @code{--max-load}
@cindex @code{--load-average}
When the system is heavily loaded, you will probably want to run fewer jobs
than when it is lightly loaded.  You can use the @samp{-l} option to tell
@code{make} to limit the number of jobs to run at once, based on the load
average.  The @samp{-l} or @samp{--max-load}
option is followed by a floating-point number.  For
example,

@example
-l 2.5
@end example

@noindent
will not let @code{make} start more than one job if the load average is
above 2.5.  The @samp{-l} option with no following number removes the
load limit, if one was given with a previous @samp{-l} option.

More precisely, when @code{make} goes to start up a job, and it already has
at least one job running, it checks the current load average; if it is not
lower than the limit given with @samp{-l}, @code{make} waits until the load
average goes below that limit, or until all the other jobs finish.

By default, there is no load limit.

@menu
* Parallel Disable::            Disabling parallel execution
* Parallel Output::             Handling output during parallel execution
* Parallel Input::              Handling input during parallel execution
@end menu

@node Parallel Disable
@subsection Disabling Parallel Execution
@cindex disabling parallel execution
@cindex parallel execution, disabling

If a makefile completely and accurately defines the dependency relationships
between all of its targets, then @code{make} will correctly build the goals
regardless of whether parallel execution is enabled or not.  This is the ideal
way to write makefiles.

However, sometimes some or all of the targets in a makefile cannot be executed
in parallel and it's not feasible to add the prerequisites needed to inform
@code{make}.  In that case the makefile can use various methods to disable
parallel execution.

@cindex .NOTPARALLEL special target
If the @code{.NOTPARALLEL} special target with no prerequisites is specified
anywhere then the entire instance of @code{make} will be run serially,
regardless of the parallel setting.  For example:

@example
@group
all: one two three
one two three: ; @@sleep 1; echo $@@

.NOTPARALLEL:
@end group
@end example

Regardless of how @code{make} is invoked, the targets @file{one}, @file{two},
and @file{three} will be run serially.

If the @code{.NOTPARALLEL} special target has prerequisites, then each of
those prerequisites will be considered a target and all prerequisites of these
targets will be run serially.  Note that only when building this target will
the prerequisites be run serially: if some other target lists the same
prerequisites and is not in @code{.NOTPARALLEL} then these prerequisites may
be run in parallel.  For example:

@example
@group
all: base notparallel

base: one two three
notparallel: one two three

one two three: ; @@sleep 1; echo $@@

.NOTPARALLEL: notparallel
@end group
@end example

Here @samp{make -j base} will run the targets @file{one}, @file{two}, and
@file{three} in parallel, while @samp{make -j notparallel} will run them
serially.  If you run @samp{make -j all} then they @emph{will} be run in
parallel since @file{base} lists them as prerequisites and is not serialized.

The @code{.NOTPARALLEL} target should not have commands.

@cindex .WAIT special target
@findex .WAIT
Finally you can control the serialization of specific prerequisites in a
fine-grained way using the @code{.WAIT} special target.  When this target
appears in a prerequisite list and parallel execution is enabled, @code{make}
will not build any of the prerequisites to the @emph{right} of @code{.WAIT}
until all prerequisites to the @emph{left} of @code{.WAIT} have completed.
For example:

@example
@group
all: one two .WAIT three
one two three: ; @@sleep 1; echo $@@
@end group
@end example

If parallel execution is enabled, @code{make} will try to build @file{one} and
@file{two} in parallel but will not try to build @file{three} until both are
complete.

As with targets provided to @code{.NOTPARALLEL}, @code{.WAIT} takes effect
only when building the target in whose prerequisite list it appears.  If the
same prerequisites are present in other targets, without @code{.WAIT}, then
they may still be run in parallel.  Because of this, neither
@code{.NOTPARALLEL} with targets nor @code{.WAIT} are as reliable for
controlling parallel execution as defining a prerequisite relationship.
However they are easy to use and may be sufficient in less complex situations.

The @code{.WAIT} prerequisite will not be present in any of the automatic
variables for the rule.

You can create an actual target @code{.WAIT} in your makefile for portability
but this is not required to use this feature.  If a @code{.WAIT} target is
created it should not have prerequisites or commands.

The @code{.WAIT} feature is also implemented in other versions of @code{make}
and it's specified in the POSIX standard for @code{make}.

@node Parallel Output
@subsection Output During Parallel Execution
@cindex output during parallel execution
@cindex parallel execution, output during

When running several recipes in parallel the output from each
recipe appears as soon as it is generated, with the result that
messages from different recipes may be interspersed, sometimes even
appearing on the same line.  This can make reading the output very
difficult.

@cindex @code{--output-sync}
@cindex @code{-O}
To avoid this you can use the @samp{--output-sync} (@samp{-O}) option.
This option instructs @code{make} to save the output from the commands
it invokes and print it all once the commands are completed.
Additionally, if there are multiple recursive @code{make} invocations
running in parallel, they will communicate so that only one of them is
generating output at a time.

If working directory printing is enabled (@pxref{-w Option, ,The
@samp{--print-directory} Option}), the enter/leave messages are
printed around each output grouping.  If you prefer not to see these
messages add the @samp{--no-print-directory} option to @code{MAKEFLAGS}.

There are four levels of granularity when synchronizing output,
specified by giving an argument to the option (e.g.,  @samp{-Oline} or
@samp{--output-sync=recurse}).

@table @code
@item none
This is the default: all output is sent directly as it is generated and
no synchronization is performed.

@item line
Output from each individual line of the recipe is grouped and printed
as soon as that line is complete.  If a recipe consists of multiple
lines, they may be interspersed with lines from other recipes.

@item target
Output from the entire recipe for each target is grouped and printed
once the target is complete.  This is the default if the
@code{--output-sync} or @code{-O} option is given with no argument.

@item recurse
Output from each recursive invocation of @code{make} is grouped and
printed once the recursive invocation is complete.

@end table

Regardless of the mode chosen, the total build time will be the same.
The only difference is in how the output appears.

The @samp{target} and @samp{recurse} modes both collect the output of
the entire recipe of a target and display it uninterrupted when the
recipe completes.  The difference between them is in how recipes that
contain recursive invocations of @code{make} are treated
(@pxref{Recursion, ,Recursive Use of @code{make}}).  For all recipes
which have no recursive lines, the @samp{target} and @samp{recurse}
modes behave identically.

If the @samp{recurse} mode is chosen, recipes that contain recursive
@code{make} invocations are treated the same as other targets: the
output from the recipe, including the output from the recursive
@code{make}, is saved and printed after the entire recipe is complete.
This ensures output from all the targets built by a given recursive
@code{make} instance are grouped together, which may make the output
easier to understand.  However it also leads to long periods of time
during the build where no output is seen, followed by large bursts of
output.  If you are not watching the build as it proceeds, but instead
viewing a log of the build after the fact, this may be the best option
for you.

If you are watching the output, the long gaps of quiet during the
build can be frustrating.  The @samp{target} output synchronization
mode detects when @code{make} is going to be invoked recursively,
using the standard methods, and it will not synchronize the output of
those lines.  The recursive @code{make} will perform the
synchronization for its targets and the output from each will be
displayed immediately when it completes.  Be aware that output from
recursive lines of the recipe are not synchronized (for example if
the recursive line prints a message before running @code{make}, that
message will not be synchronized).

The @samp{line} mode can be useful for front-ends that are watching
the output of @code{make} to track when recipes are started and
completed.

Some programs invoked by @code{make} may behave differently if they
determine they're writing output to a terminal versus a file (often
described as ``interactive'' vs. ``non-interactive'' modes).  For
example, many programs that can display colorized output will not do
so if they determine they are not writing to a terminal.  If your
makefile invokes a program like this then using the output
synchronization options will cause the program to believe it's running
in ``non-interactive'' mode even though the output will ultimately go
to the terminal.

@node Parallel Input
@subsection Input During Parallel Execution
@cindex input during parallel execution
@cindex parallel execution, input during
@cindex standard input

Two processes cannot both take input from the same device at the same
time.  To make sure that only one recipe tries to take input from the
terminal at once, @code{make} will invalidate the standard input
streams of all but one running recipe.  If another recipe attempts to
read from standard input it will usually incur a fatal error (a
@samp{Broken pipe} signal).
@cindex broken pipe

It is unpredictable which recipe will have a valid standard input stream
(which will come from the terminal, or wherever you redirect the standard
input of @code{make}).  The first recipe run will always get it first, and
the first recipe started after that one finishes will get it next, and so
on.

We will change how this aspect of @code{make} works if we find a better
alternative.  In the mean time, you should not rely on any recipe using
standard input at all if you are using the parallel execution feature; but
if you are not using this feature, then standard input works normally in
all recipes.

@node Errors
@section Errors in Recipes
@cindex errors (in recipes)
@cindex recipes, errors in
@cindex exit status (errors)

After each shell invocation returns, @code{make} looks at its exit
status.  If the shell completed successfully (the exit status is
zero), the next line in the recipe is executed in a new shell; after
the last line is finished, the rule is finished.

If there is an error (the exit status is nonzero), @code{make} gives up on
the current rule, and perhaps on all rules.

Sometimes the failure of a certain recipe line does not indicate a problem.
For example, you may use the @code{mkdir} command to ensure that a
directory exists.  If the directory already exists, @code{mkdir} will
report an error, but you probably want @code{make} to continue regardless.

@cindex @code{-} (in recipes)
To ignore errors in a recipe line, write a @samp{-} at the beginning
of the line's text (after the initial tab).  The @samp{-} is discarded
before the line is passed to the shell for execution.

Note that the beginning of a recipe line is interpreted differently when
using @code{.ONESHELL} (@pxref{One Shell, ,Using One Shell}).

For example,

@example
@group
clean:
        -rm -f *.o
@end group
@end example
@cindex @code{rm} (shell command)

@noindent
This causes @code{make} to continue even if @code{rm} is unable to
remove a file.

@cindex @code{-i}
@cindex @code{--ignore-errors}
When you run @code{make} with the @samp{-i} or @samp{--ignore-errors}
flag, errors are ignored in all recipes of all rules.  A rule in the
makefile for the special target @code{.IGNORE} has the same effect, if
there are no prerequisites.  This is less flexible but sometimes useful.

When errors are to be ignored, because of either a @samp{-} or the
@samp{-i} flag, @code{make} treats an error return just like success,
except that it prints out a message that tells you the status code
the shell exited with, and says that the error has been ignored.

When an error happens that @code{make} has not been told to ignore,
it implies that the current target cannot be correctly remade, and neither
can any other that depends on it either directly or indirectly.  No further
recipes will be executed for these targets, since their preconditions
have not been achieved.


@cindex @code{-k}
@cindex @code{--keep-going}
Normally @code{make} gives up immediately in this circumstance, returning a
nonzero status.  However, if the @samp{-k} or @samp{--keep-going}
flag is specified, @code{make}
continues to consider the other prerequisites of the pending targets,
remaking them if necessary, before it gives up and returns nonzero status.
For example, after an error in compiling one object file, @samp{make -k}
will continue compiling other object files even though it already knows
that linking them will be impossible.  @xref{Options Summary, ,Summary of Options}.

The usual behavior assumes that your purpose is to get the specified
targets up to date; once @code{make} learns that this is impossible, it
might as well report the failure immediately.  The @samp{-k} option says
that the real purpose is to test as many of the changes made in the
program as possible, perhaps to find several independent problems so
that you can correct them all before the next attempt to compile.  This
is why Emacs' @code{compile} command passes the @samp{-k} flag by
default.
@cindex Emacs (@code{M-x compile})

@findex .DELETE_ON_ERROR@r{, errors in recipes}
@cindex deletion of target files
@cindex removal of target files
@cindex target, deleting on error
Usually when a recipe line fails, if it has changed the target file at all,
the file is corrupted and cannot be used---or at least it is not
completely updated.  Yet the file's time stamp says that it is now up to
date, so the next time @code{make} runs, it will not try to update that
file.  The situation is just the same as when the shell is killed by a
signal; @pxref{Interrupts}.  So generally the right thing to do is to
delete the target file if the recipe fails after beginning to change
the file.  @code{make} will do this if @code{.DELETE_ON_ERROR} appears
as a target.  This is almost always what you want @code{make} to do, but
it is not historical practice; so for compatibility, you must explicitly
request it.

@node Interrupts
@section Interrupting or Killing @code{make}
@cindex interrupt
@cindex signal
@cindex deletion of target files
@cindex removal of target files
@cindex target, deleting on interrupt
@cindex killing (interruption)

If @code{make} gets a fatal signal while a shell is executing, it may
delete the target file that the recipe was supposed to update.  This is
done if the target file's last-modification time has changed since
@code{make} first checked it.

The purpose of deleting the target is to make sure that it is remade from
scratch when @code{make} is next run.  Why is this?  Suppose you type
@kbd{Ctrl-c} while a compiler is running, and it has begun to write an
object file @file{foo.o}.  The @kbd{Ctrl-c} kills the compiler, resulting
in an incomplete file whose last-modification time is newer than the source
file @file{foo.c}.  But @code{make} also receives the @kbd{Ctrl-c} signal
and deletes this incomplete file.  If @code{make} did not do this, the next
invocation of @code{make} would think that @file{foo.o} did not require
updating---resulting in a strange error message from the linker when it
tries to link an object file half of which is missing.

@cindex .PRECIOUS, preserving targets
You can prevent the deletion of a target file in this way by making the
special target @code{.PRECIOUS} depend on it.  Before remaking a target,
@code{make} checks to see whether it appears on the prerequisites of
@code{.PRECIOUS}, and thereby decides whether the target should be deleted
if a signal happens.  Some reasons why you might do this are that the
target is updated in some atomic fashion, or exists only to record a
modification-time (its contents do not matter), or must exist at all
times to prevent other sorts of trouble.

Although @code{make} does its best to clean up there are certain situations
in which cleanup is impossible.  For example, @code{make} may be killed by
an uncatchable signal.  Or, one of the programs make invokes may be killed
or crash, leaving behind an up-to-date but corrupt target file: @code{make}
will not realize that this failure requires the target to be cleaned.  Or
@code{make} itself may encounter a bug and crash.

For these reasons it's best to write @emph{defensive recipes}, which won't
leave behind corrupted targets even if they fail.  Most commonly these
recipes create temporary files rather than updating the target directly,
then rename the temporary file to the final target name.  Some compilers
already behave this way, so that you don't need to write a defensive recipe.

@node Recursion
@section Recursive Use of @code{make}
@cindex recursion
@cindex subdirectories, recursion for

Recursive use of @code{make} means using @code{make} as a command in a
makefile.  This technique is useful when you want separate makefiles for
various subsystems that compose a larger system.  For example, suppose you
have a sub-directory @file{subdir} which has its own makefile, and you would
like the containing directory's makefile to run @code{make} on the
sub-directory.  You can do it by writing this:

@example
subsystem:
        cd subdir && $(MAKE)
@end example

@noindent
or, equivalently, this (@pxref{Options Summary, ,Summary of Options}):

@example
subsystem:
        $(MAKE) -C subdir
@end example
@cindex @code{-C}
@cindex @code{--directory}

You can write recursive @code{make} commands just by copying this example,
but there are many things to know about how they work and why, and about
how the sub-@code{make} relates to the top-level @code{make}.  You may
also find it useful to declare targets that invoke recursive
@code{make} commands as @samp{.PHONY} (for more discussion on when
this is useful, see @ref{Phony Targets}).

@vindex @code{CURDIR}
For your convenience, when GNU @code{make} starts (after it has
processed any @code{-C} options) it sets the variable @code{CURDIR} to
the pathname of the current working directory.  This value is never
touched by @code{make} again: in particular note that if you include
files from other directories the value of @code{CURDIR} does not
change.  The value has the same precedence it would have if it were
set in the makefile (by default, an environment variable @code{CURDIR}
will not override this value).  Note that setting this variable has no
impact on the operation of @code{make} (it does not cause @code{make}
to change its working directory, for example).

@menu
* MAKE Variable::               The special effects of using @samp{$(MAKE)}.
* Variables/Recursion::         How to communicate variables to a sub-@code{make}.
* Options/Recursion::           How to communicate options to a sub-@code{make}.
* -w Option::                   How the @samp{-w} or @samp{--print-directory} option
                                  helps debug use of recursive @code{make} commands.
@end menu

@node MAKE Variable
@subsection How the @code{MAKE} Variable Works
@vindex MAKE
@cindex recursion, and @code{MAKE} variable

Recursive @code{make} commands should always use the variable @code{MAKE},
not the explicit command name @samp{make}, as shown here:

@example
@group
subsystem:
        cd subdir && $(MAKE)
@end group
@end example

The value of this variable is the file name with which @code{make} was
invoked.  If this file name was @file{/bin/make}, then the recipe executed
is @samp{cd subdir && /bin/make}.  If you use a special version of
@code{make} to run the top-level makefile, the same special version will be
executed for recursive invocations.
@cindex @code{cd} (shell command)

@cindex +, and recipes
As a special feature, using the variable @code{MAKE} in the recipe of
a rule alters the effects of the @samp{-t} (@samp{--touch}), @samp{-n}
(@samp{--just-print}), or @samp{-q} (@w{@samp{--question}}) option.
Using the @code{MAKE} variable has the same effect as using a @samp{+}
character at the beginning of the recipe line.  @xref{Instead of
Execution, ,Instead of Executing the Recipes}.  This special feature
is only enabled if the @code{MAKE} variable appears directly in the
recipe: it does not apply if the @code{MAKE} variable is referenced
through expansion of another variable.  In the latter case you must
use the @samp{+} token to get these special effects.

Note that the beginning of a recipe line is interpreted differently when
using @code{.ONESHELL} (@pxref{One Shell, ,Using One Shell}).

Consider the command @samp{make -t} in the above example.  (The
@samp{-t} option marks targets as up to date without actually running
any recipes; see @ref{Instead of Execution}.)  Following the usual
definition of @samp{-t}, a @samp{make -t} command in the example would
create a file named @file{subsystem} and do nothing else.  What you
really want it to do is run @samp{@w{cd subdir &&} @w{make -t}}; but
that would require executing the recipe, and @samp{-t} says not to
execute recipes.
@cindex @code{-t}, and recursion
@cindex recursion, and @code{-t}
@cindex @code{--touch}, and recursion

The special feature makes this do what you want: whenever a recipe
line of a rule contains the variable @code{MAKE}, the flags @samp{-t},
@samp{-n} and @samp{-q} do not apply to that line.  Recipe lines
containing @code{MAKE} are executed normally despite the presence of a
flag that causes most recipes not to be run.  The usual
@code{MAKEFLAGS} mechanism passes the flags to the sub-@code{make}
(@pxref{Options/Recursion, ,Communicating Options to a
Sub-@code{make}}), so your request to touch the files, or print the
recipes, is propagated to the subsystem.

@node Variables/Recursion
@subsection Communicating Variables to a Sub-@code{make}
@cindex sub-@code{make}
@cindex environment, and recursion
@cindex exporting variables
@cindex variables, environment
@cindex variables, exporting
@cindex recursion, and environment
@cindex recursion, and variables

Variable values of the top-level @code{make} can be passed to the
sub-@code{make} through the environment by explicit request.  These
variables are defined in the sub-@code{make} as defaults, but they do
not override variables defined in the makefile used by
the sub-@code{make} unless you use the @samp{-e} switch (@pxref{Options
Summary, ,Summary of Options}).

To pass down, or @dfn{export}, a variable, @code{make} adds the
variable and its value to the environment for running each line of the
recipe.  The sub-@code{make}, in turn, uses the environment to
initialize its table of variable values.  @xref{Environment,
,Variables from the Environment}.

Except by explicit request, @code{make} exports a variable only if it
is either defined in the environment initially, or if set on the command
line and its name consists only of letters, numbers, and underscores.

@cindex SHELL, exported value
The value of the @code{make} variable @code{SHELL} is not exported.
Instead, the value of the @code{SHELL} variable from the invoking
environment is passed to the sub-@code{make}.  You can force
@code{make} to export its value for @code{SHELL} by using the
@code{export} directive, described below.  @xref{Choosing the Shell}.

The special variable @code{MAKEFLAGS} is always exported (unless you
unexport it).  @code{MAKEFILES} is exported if you set it to anything.

@code{make} automatically passes down variable values that were defined
on the command line, by putting them in the @code{MAKEFLAGS} variable.
@iftex
See the next section.
@end iftex
@ifnottex
@xref{Options/Recursion}.
@end ifnottex

Variables are @emph{not} normally passed down if they were created by
default by @code{make} (@pxref{Implicit Variables, ,Variables Used by
Implicit Rules}).  The sub-@code{make} will define these for
itself.

@findex export
If you want to export specific variables to a sub-@code{make}, use the
@code{export} directive, like this:

@example
export @var{variable} @dots{}
@end example

@noindent
@findex unexport
If you want to @emph{prevent} a variable from being exported, use the
@code{unexport} directive, like this:

@example
unexport @var{variable} @dots{}
@end example

@noindent
In both of these forms, the arguments to @code{export} and
@code{unexport} are expanded, and so could be variables or functions
which expand to a (list of) variable names to be (un)exported.

GNU Make will add any variable you export to the environment of all child
processes.  However, be aware that some child processes themselves will not
accept all types of variables.  For example, many modern shells will clean the
incoming environment by removing or hiding any environment variable whose name
is invalid for the shell (e.g., alphanumeric plus underscore).  There is
nothing GNU Make can do about the behavior of these child processes: if you
want your makefile to be maximally portable you should choose simple variable
names for all exported variables.

As a convenience, you can define a variable and export it at the same
time by doing:

@example
export @var{variable} = value
@end example

@noindent
has the same result as:

@example
@var{variable} = value
export @var{variable}
@end example

@noindent
and

@example
export @var{variable} := value
@end example

@noindent
has the same result as:

@example
@var{variable} := value
export @var{variable}
@end example

Likewise,

@example
export @var{variable} += value
@end example

@noindent
is just like:

@example
@var{variable} += value
export @var{variable}
@end example

@noindent
@xref{Appending Assignment, ,Appending More Text to Variables}.

You may notice that the @code{export} and @code{unexport} directives
work in @code{make} in the same way they work in the shell, @code{sh}.

If you want all variables to be exported by default, you can use
@code{export} by itself:

@example
export
@end example

@noindent
This tells @code{make} that variables which are not explicitly mentioned in an
@code{export} or @code{unexport} directive should be exported.  Any variable
given in an @code{unexport} directive will still @emph{not} be exported.

@findex .EXPORT_ALL_VARIABLES
Another way to export variables by default is to add the special target
@code{.EXPORT_ALL_VARIABLES} to your makefile.  This will be ignored by other
variants of @code{make}.

When using @code{export} by itself or @code{.EXPORT_ALL_VARIABLES} to export
variables by default, only variables whose names consist solely of
alphanumerics and underscores will be exported.  To export other variables you
must specifically mention them in an @code{export} directive.

Adding a variable's value to the environment requires it to be expanded.  If
expanding a variable has side-effects (such as the @code{info} or @code{eval}
or similar functions) then these side-effects will be seen every time a
command is invoked.  You can avoid this by ensuring that such variables have
names which are not exportable by default.  However, a better solution is to
@emph{not} use this ``export by default'' facility at all, and instead
explicitly @code{export} the relevant variables by name.

Be aware that exporting all variables can have serious performance impacts,
especially for recursive variables used in conjunction with the @code{shell}
function.  We recommend you avoid this and instead export only the specific
variables needed to run your commands.  If you do need to export all
variables, use simple variable assignment wherever possible, especially when
invoking the @code{shell} function.

You can use @code{unexport} by itself to tell @code{make} @emph{not} to export
variables by default.  Since this is the default behavior, you would only need
to do this if @code{export} had been used by itself earlier (in an included
makefile, perhaps).  You @strong{cannot} use @code{export} and @code{unexport}
by themselves to have variables exported for some recipes and not for others.
The last @code{export} or @code{unexport} directive that appears by itself
determines the behavior for the entire run of @code{make}.

@vindex MAKELEVEL
@cindex recursion, level of
As a special feature, the variable @code{MAKELEVEL} is changed when it
is passed down from level to level.  This variable's value is a string
which is the depth of the level as a decimal number.  The value is
@samp{0} for the top-level @code{make}; @samp{1} for a sub-@code{make},
@samp{2} for a sub-sub-@code{make}, and so on.  The incrementation
happens when @code{make} sets up the environment for a recipe.

The main use of @code{MAKELEVEL} is to test it in a conditional
directive (@pxref{Conditionals, ,Conditional Parts of Makefiles}); this
way you can write a makefile that behaves one way if run recursively and
another way if run directly by you.

@vindex MAKEFILES
You can use the variable @code{MAKEFILES} to cause all sub-@code{make}
commands to use additional makefiles.  The value of @code{MAKEFILES} is
a whitespace-separated list of file names.  This variable, if defined in
the outer-level makefile, is passed down through the environment; then
it serves as a list of extra makefiles for the sub-@code{make} to read
before the usual or specified ones.  @xref{MAKEFILES Variable, ,The
Variable @code{MAKEFILES}}.

@node Options/Recursion
@subsection Communicating Options to a Sub-@code{make}
@cindex options, and recursion
@cindex recursion, and options

@vindex MAKEFLAGS
Flags such as @samp{-s} and @samp{-k} are passed automatically to the
sub-@code{make} through the variable @code{MAKEFLAGS}.  This variable is
set up automatically by @code{make} to contain the flag letters that
@code{make} received.  Thus, if you do @w{@samp{make -ks}} then
@code{MAKEFLAGS} gets the value @samp{ks}.

As a consequence, every sub-@code{make} gets a value for @code{MAKEFLAGS} in
its environment.  In response, it takes the flags from that value and
processes them as if they had been given as arguments.  @xref{Options Summary,
,Summary of Options}.  This means that, unlike other environment variables,
@code{MAKEFLAGS} specified in the environment take precedence over
@code{MAKEFLAGS} specified in the makefile.

The value of @code{MAKEFLAGS} is a possibly empty group of characters
representing single-letter options that take no argument, followed by a space
and any options that take arguments or which have long option names.  If an
option has both single-letter and long options, the single-letter option is
always preferred.  If there are no single-letter options on the command line,
then the value of @code{MAKEFLAGS} starts with a space.

@cindex command line variable definitions, and recursion
@cindex variables, command line, and recursion
@cindex recursion, and command line variable definitions
Likewise variables defined on the command line are passed to the
sub-@code{make} through @code{MAKEFLAGS}.  Words in the value of
@code{MAKEFLAGS} that contain @samp{=}, @code{make} treats as variable
definitions just as if they appeared on the command line.
@xref{Overriding, ,Overriding Variables}.

@cindex @code{-C}, and recursion
@cindex @code{-f}, and recursion
@cindex @code{-o}, and recursion
@cindex @code{-W}, and recursion
@cindex @code{--directory}, and recursion
@cindex @code{--file}, and recursion
@cindex @code{--old-file}, and recursion
@cindex @code{--assume-old}, and recursion
@cindex @code{--assume-new}, and recursion
@cindex @code{--new-file}, and recursion
@cindex recursion, and @code{-C}
@cindex recursion, and @code{-f}
@cindex recursion, and @code{-o}
@cindex recursion, and @code{-W}
The options @samp{-C}, @samp{-f}, @samp{-o}, and @samp{-W} are not put
into @code{MAKEFLAGS}; these options are not passed down.

@cindex @code{-j}, and recursion
@cindex @code{--jobs}, and recursion
@cindex recursion, and @code{-j}
@cindex job slots, and recursion
The @samp{-j} option is a special case (@pxref{Parallel, ,Parallel Execution}).
If you set it to some numeric value @samp{N} and your operating system
supports it (most any UNIX system will; others typically won't), the
parent @code{make} and all the sub-@code{make}s will communicate to
ensure that there are only @samp{N} jobs running at the same time
between them all.  Note that any job that is marked recursive
(@pxref{Instead of Execution, ,Instead of Executing Recipes})
doesn't count against the total jobs (otherwise we could get @samp{N}
sub-@code{make}s running and have no slots left over for any real work!)

If your operating system doesn't support the above communication, then
no @samp{-j} is added to @code{MAKEFLAGS}, so that sub-@code{make}s
run in non-parallel mode.  If the @w{@samp{-j}} option were passed down
to sub-@code{make}s you would get many more jobs running in parallel
than you asked for.  If you give @samp{-j} with no numeric argument,
meaning to run as many jobs as possible in parallel, this is passed
down, since multiple infinities are no more than one.

If you do not want to pass the other flags down, you must change the
value of @code{MAKEFLAGS}, for example like this:

@example
subsystem:
        cd subdir && $(MAKE) MAKEFLAGS=
@end example

@vindex MAKEOVERRIDES
The command line variable definitions really appear in the variable
@code{MAKEOVERRIDES}, and @code{MAKEFLAGS} contains a reference to this
variable.  If you do want to pass flags down normally, but don't want to
pass down the command line variable definitions, you can reset
@code{MAKEOVERRIDES} to empty, like this:

@example
MAKEOVERRIDES =
@end example

@noindent
@cindex Arg list too long
@cindex E2BIG
This is not usually useful to do.  However, some systems have a small
fixed limit on the size of the environment, and putting so much
information into the value of @code{MAKEFLAGS} can exceed it.  If you
see the error message @samp{Arg list too long}, this may be the problem.
(For strict compliance with POSIX, changing @code{MAKEOVERRIDES} does
not affect @code{MAKEFLAGS} if the special target @samp{.POSIX} appears
in the makefile.  You probably do not care about this.)

@vindex MFLAGS
A similar variable @code{MFLAGS} exists also, for historical compatibility.
It has the same value as @code{MAKEFLAGS} except that it does not contain the
command line variable definitions, and it always begins with a hyphen unless
it is empty.  @code{MFLAGS} was traditionally used explicitly in the recursive
@code{make} command, like this:

@example
subsystem:
        cd subdir && $(MAKE) $(MFLAGS)
@end example

@noindent
but now @code{MAKEFLAGS} makes this usage redundant.  If you want your
makefiles to be compatible with old @code{make} programs, use this
technique; it will work fine with more modern @code{make} versions too.

@cindex setting options from environment
@cindex options, setting from environment
@cindex setting options in makefiles
@cindex options, setting in makefiles
The @code{MAKEFLAGS} variable can also be useful if you want to have
certain options, such as @samp{-k} (@pxref{Options Summary, ,Summary of
Options}), set each time you run @code{make}.  You simply put a value for
@code{MAKEFLAGS} in your environment.  You can also set @code{MAKEFLAGS} in
a makefile, to specify additional flags that should also be in effect for
that makefile.  (Note that you cannot use @code{MFLAGS} this way.  That
variable is set only for compatibility; @code{make} does not interpret a
value you set for it in any way.)

When @code{make} interprets the value of @code{MAKEFLAGS} (either from the
environment or from a makefile), it first prepends a hyphen if the value
does not already begin with one.  Then it chops the value into words
separated by blanks, and parses these words as if they were options given
on the command line (except that @samp{-C}, @samp{-f}, @samp{-h},
@samp{-o}, @samp{-W}, and their long-named versions are ignored; and there
is no error for an invalid option).

If you do put @code{MAKEFLAGS} in your environment, you should be sure not
to include any options that will drastically affect the actions of
@code{make} and undermine the purpose of makefiles and of @code{make}
itself.  For instance, the @samp{-t}, @samp{-n}, and @samp{-q} options, if
put in one of these variables, could have disastrous consequences and would
certainly have at least surprising and probably annoying effects.

If you'd like to run other implementations of @code{make} in addition to GNU
Make, and hence do not want to add GNU Make-specific flags to your
@code{MAKEFLAGS} environment variable, you can add them to the
@code{GNUMAKEFLAGS} environment variable instead.  This is parsed just before
@code{MAKEFLAGS}, in the same way as @code{MAKEFLAGS}.  When @code{make}
constructs @code{MAKEFLAGS} to pass to a recursive @code{make} it will include
all flags, even those taken from @code{GNUMAKEFLAGS}.  As a result, after
parsing @code{GNUMAKEFLAGS} GNU Make sets this variable to the empty string to
avoid duplicating flags during recursion.  Only @code{GNUMAKEFLAGS}
environment variables are treated specially: setting the @code{make} variable
@code{GNUMAKEFLAGS} has no effect.

It's best to use @code{GNUMAKEFLAGS} only with flags which won't
materially change the behavior of your makefiles.  If your makefiles
require GNU Make anyway then simply use @code{MAKEFLAGS}.  Flags such
as @samp{--no-print-directory} or @samp{--output-sync} may be
appropriate for @code{GNUMAKEFLAGS}.

@node -w Option
@subsection The @samp{--print-directory} Option
@cindex directories, printing them
@cindex printing directories
@cindex recursion, and printing directories

If you use several levels of recursive @code{make} invocations, the
@samp{-w} or @w{@samp{--print-directory}} option can make the output a
lot easier to understand by showing each directory as @code{make}
starts processing it and as @code{make} finishes processing it.  For
example, if @samp{make -w} is run in the directory @file{/u/gnu/make},
@code{make} will print a line of the form:

@example
make: Entering directory `/u/gnu/make'.
@end example

@noindent
before doing anything else, and a line of the form:

@example
make: Leaving directory `/u/gnu/make'.
@end example

@noindent
when processing is completed.

@cindex @code{-C}, and @code{-w}
@cindex @code{--directory}, and @code{--print-directory}
@cindex recursion, and @code{-w}
@cindex @code{-w}, and @code{-C}
@cindex @code{-w}, and recursion
@cindex @code{--print-directory}, and @code{--directory}
@cindex @code{--print-directory}, and recursion
@cindex @code{--no-print-directory}
@cindex @code{--print-directory}, disabling
@cindex @code{-w}, disabling
Normally, you do not need to specify this option because @samp{make}
does it for you: @samp{-w} is turned on automatically when you use the
@samp{-C} option, and in sub-@code{make}s.  @code{make} will not
automatically turn on @samp{-w} if you also use @samp{-s}, which says to
be silent, or if you use @samp{--no-print-directory} to explicitly
disable it.

@node Canned Recipes
@section Defining Canned Recipes
@cindex canned recipes
@cindex recipes, canned
@cindex sequences of commands
@cindex commands, sequences of

When the same sequence of commands is useful in making various
targets, you can define it as a canned sequence with the @code{define}
directive, and refer to the canned sequence from the recipes for those
targets.  The canned sequence is actually a variable, so the name must
not conflict with other variable names.

Here is an example of defining a canned recipe:

@example
define run-yacc =
yacc $(firstword $^)
mv y.tab.c $@@
endef
@end example
@cindex @code{yacc}

@noindent
Here @code{run-yacc} is the name of the variable being defined;
@code{endef} marks the end of the definition; the lines in between are the
commands.  The @code{define} directive does not expand variable references
and function calls in the canned sequence; the @samp{$} characters,
parentheses, variable names, and so on, all become part of the value of the
variable you are defining.
@xref{Multi-Line, ,Defining Multi-Line Variables},
for a complete explanation of @code{define}.

The first command in this example runs Yacc on the first prerequisite of
whichever rule uses the canned sequence.  The output file from Yacc is
always named @file{y.tab.c}.  The second command moves the output to the
rule's target file name.

To use the canned sequence, substitute the variable into the recipe of a
rule.  You can substitute it like any other variable
(@pxref{Reference, ,Basics of Variable References}).
Because variables defined by @code{define} are recursively expanded
variables, all the variable references you wrote inside the @code{define}
are expanded now.  For example:

@example
foo.c : foo.y
        $(run-yacc)
@end example

@noindent
@samp{foo.y} will be substituted for the variable @samp{$^} when it occurs in
@code{run-yacc}'s value, and @samp{foo.c} for @samp{$@@}.

This is a realistic example, but this particular one is not needed in
practice because @code{make} has an implicit rule to figure out these
commands based on the file names involved
(@pxref{Implicit Rules, ,Using Implicit Rules}).

@cindex @@, and @code{define}
@cindex -, and @code{define}
@cindex +, and @code{define}
In recipe execution, each line of a canned sequence is treated just as
if the line appeared on its own in the rule, preceded by a tab.  In
particular, @code{make} invokes a separate sub-shell for each line.  You
can use the special prefix characters that affect command lines
(@samp{@@}, @samp{-}, and @samp{+}) on each line of a canned sequence.
@xref{Recipes, ,Writing Recipes in Rules}.
For example, using this canned sequence:

@example
define frobnicate =
@@echo "frobnicating target $@@"
frob-step-1 $< -o $@@-step-1
frob-step-2 $@@-step-1 -o $@@
endef
@end example

@noindent
@code{make} will not echo the first line, the @code{echo} command.
But it @emph{will} echo the following two recipe lines.

On the other hand, prefix characters on the recipe line that refers to
a canned sequence apply to every line in the sequence.  So the rule:

@example
frob.out: frob.in
        @@$(frobnicate)
@end example

@noindent
does not echo @emph{any} recipe lines.
(@xref{Echoing, ,Recipe Echoing}, for a full explanation of @samp{@@}.)

@node Empty Recipes
@section Using Empty Recipes
@cindex empty recipes
@cindex recipes, empty

It is sometimes useful to define recipes which do nothing.  This is done
simply by giving a recipe that consists of nothing but whitespace.  For
example:

@example
target: ;
@end example

@noindent
defines an empty recipe for @file{target}.  You could also use a line
beginning with a recipe prefix character to define an empty recipe,
but this would be confusing because such a line looks empty.

@findex .DEFAULT@r{, and empty recipes}
You may be wondering why you would want to define a recipe that does
nothing.  One reason this is useful is to prevent a target from
getting implicit recipes (from implicit rules or the @code{.DEFAULT}
special target; @pxref{Implicit Rules} and @pxref{Last Resort,
,Defining Last-Resort Default Rules}).

Empty recipes can also be used to avoid errors for targets that will
be created as a side-effect of another recipe: if the target does not
exist the empty recipe ensures that @code{make} won't complain that it
doesn't know how to build the target, and @code{make} will assume the
target is out of date.

You may be inclined to define empty recipes for targets that are not
actual files, but only exist so that their prerequisites can be
remade.  However, this is not the best way to do that, because the
prerequisites may not be remade properly if the target file actually
does exist.  @xref{Phony Targets, ,Phony Targets}, for a better way to
do this.

@node Using Variables
@chapter How to Use Variables
@cindex variable
@cindex value

A makefile @dfn{variable} is a name defined to represent a string of text,
called the variable's @dfn{value}.  (In some other versions of @code{make},
variables are called @dfn{macros}.)
@cindex macro

Variable values can contain lists of file names, options to pass to compilers,
programs to run, directories to look in for source files, directories to write
output in, or anything else you can imagine.

There are two different ways, called @dfn{flavors}, that a variable can hold
its value internally (@pxref{Flavors, The Two Flavors of Variables}).  The
flavor of the variable is specified by the assignment operator used when it is
defined (@pxref{Setting, ,Setting Variables}).

When a variable is @dfn{referenced} in targets, prerequisites, recipes, and
other parts of the makefile (@pxref{Reference, ,Basics of Variable
References}), the reference is replaced with the value of the variable.  This
is called @dfn{expanding} the variable (@pxref{Expanding, ,How Expansion
Works}).  A variable reference is not always expanded immediately as the
makefile is parsed; expansion can be deferred until later depending on the
context.  @xref{Reading Makefiles, ,How @code{make} Reads a Makefile}.

When a variable reference is expanded, the value it expands to is that of its
most recent definition at the time of expansion.  In other words, variables
in @code{make} are @dfn{dynamically scoped}.

@menu
* Variable Naming::             Choosing names for variables.
* Reference::                   How to use the value of a variable.
* Flavors::                     Variables come in two flavors.
* Expanding::                   How text is expanded by @code{make}.
* Values::                      All the ways variables get their values.
* Setting::                     How to set a variable in the makefile.
* Substitution Refs::           Substituting values in variable expansion.
* Override Directive::          How to set a variable in the makefile even if
                                  the user has set it with a command argument.
* Multi-Line::                  An alternate way to set a variable
                                  to a multi-line string.
* Undefine Directive::          How to undefine a variable so that it appears
                                  as if it was never set.
* Environment::                 Variable values can come from the environment.
* Target-specific::             Variable values can be defined on a per-target
                                  basis.
* Pattern-specific::            Target-specific variable values can be applied
                                  to a group of targets that match a pattern.
* Suppressing Inheritance::     Suppress inheritance of variables.
* Special Variables::           Variables with special meaning or behavior.
@end menu

@node Variable Naming
@section Names of Variables
@cindex names of variables
@cindex variables, names of

A variable name may be any sequence of characters not containing @samp{:},
@samp{#}, @samp{=}, or whitespace.

Variable names are case-sensitive.  For instance the names @samp{foo},
@samp{FOO}, and @samp{Foo} all refer to different variables.

It is traditional to use upper case letters in variable names, but we
recommend using lower case letters for variable names that serve internal
purposes in the makefile, and reserving upper case for parameters that
control implicit rules or for parameters that the user should override with
command options. @xref{Overriding, ,Overriding Variables}.

Although GNU Make allows most characters to be part of a variable name, shells
generally only support variable names containing letters, numbers, and
underscores.  Consider this when defining variables that need to be exported
to the shell.  @xref{Variables/Recursion, ,Communicating Variables to a
Sub-@code{make}}.

Variable names beginning with @samp{.} followed by uppercase letters are
reserved by the POSIX standard.  GNU Make reserves variable names beginning
with @samp{.} and containing uppercase letters and non-alphabetic characters.
These variables may be given special meaning in future versions of GNU Make.

A few variables have names that are a single punctuation character or
just a few characters.  These are the @dfn{automatic variables}, and
they have particular specialized uses.  @xref{Automatic Variables}.

@node Reference
@section Basics of Variable References
@cindex variables, how to reference
@cindex reference to variables
@cindex @code{$}, in variable reference
@cindex dollar sign (@code{$}), in variable reference

To reference a variable's value, write a dollar sign (@samp{$}) followed by
the name of the variable in parentheses or braces: both @code{$(foo)} and
@code{$@{foo@}} are references to the variable @samp{foo}.

If the variable name consists of a single character then the parenthesis or
braces can be omitted.  Thus @code{$A} is a reference to the variable
@samp{A}.  We recommend using parenthesis or braces even for single-letter
variable names to avoid confusion (e.g., @code{$foo} refers to the variable
@samp{f} followed by the string @samp{oo} which may not be clear) unless
omitting them gives significant readability improvements.  One place where
readability is often improved by omitting parentheses and braces is with
automatic variables (@pxref{Automatic Variables}).

This special behavior of @samp{$} is why you must write @samp{$$} to have the
effect of a single dollar sign in a makefile.

Variable references can be used in any context: targets, prerequisites,
recipes, most directives, and new variable values.  Here is an
example of a common case, where a variable holds the names of all the
object files in a program:

@example
@group
objects = program.o foo.o utils.o
program : $(objects)
        cc -o program $(objects)

$(objects) : defs.h
@end group
@end example

@menu
* Computed Names::              Computing the name of a variable reference.
@end menu

@node Computed Names
@subsection Computed Variable Names
@cindex nested variable reference
@cindex computed variable name
@cindex variables, computed names
@cindex variables, nested references
@cindex variables, @samp{$} in name
@cindex @code{$}, in variable name
@cindex dollar sign (@code{$}), in variable name

Most of the time static variable names are sufficient, but in more complex
situations it can be extremely useful to dynamically construct variable names.
This can be achieved by using a variable reference inside a reference to a
variable.  This is referred to as a @dfn{computed variable name} or a
@dfn{nested variable reference}.  For example,

@example
x = y
y = z
a := $($(x))
@end example

@noindent
defines the variable @samp{a} to have the value @code{z}, in this way:
@code{make} first expands @code{$(x)} to the value @code{y}, which means
@code{$($(x))} expands to @code{$(y)}, and @code{$(y)} expands to the string
@code{z}.  Thus the name of the variable to reference is not stated
explicitly; it is computed by expansion of @samp{$(x)}.

The previous example shows two levels of nesting, but any number of levels
are possible.  For example, here are three levels:

@example
x = y
y = z
z = u
a := $($($(x)))
@end example

@noindent
Here the innermost @code{$(x)} expands to @samp{y}, so @code{$($(x))}
expands to @code{$(y)} which in turn expands to @samp{z}; now we have
@code{$(z)}, which expands to @samp{u}.

References to recursively-expanded variables within a variable name are
re-expanded in the usual fashion.  For example:

@example
x = $(y)
y = z
z = Hello
a := $($(x))
@end example

@noindent
defines @samp{a} as @code{Hello}: @code{$($(x))} becomes @code{$($(y))}
which becomes @code{$(z)} which becomes @samp{Hello}.

Nested variable references can also contain modified references and function
invocations (@pxref{Functions, ,Functions for Transforming Text}), just like
any other reference.  For example, using the @code{subst} function
(@pxref{Text Functions, ,Functions for String Substitution and Analysis}):

@example
@group
x = variable1
variable2 := Hello
y = $(subst 1,2,$(x))
z = y
a := $($($(z)))
@end group
@end example

@noindent
eventually defines @code{a} as @samp{Hello}.  It is doubtful that anyone
would ever want to write a nested reference as convoluted as this one, but
it works: @code{$($($(z)))} expands to @code{$($(y))} which becomes
@code{$($(subst 1,2,$(x)))}.  This gets the value @samp{variable1} from
@samp{x} and changes it by substitution to @samp{variable2}, so that the
entire string becomes @code{$(variable2)}, a simple variable reference
whose value is @samp{Hello}.

A computed variable name need not consist entirely of a single variable
reference.  It can contain several variable references, as well as some
invariant text.  For example,

@example
@group
a_dirs := dira dirb
1_dirs := dir1 dir2
@end group

@group
a_files := filea fileb
1_files := file1 file2
@end group

@group
ifeq "$(use_a)" "yes"
a1 := a
else
a1 := 1
endif
@end group

@group
ifeq "$(use_dirs)" "yes"
df := dirs
else
df := files
endif

dirs := $($(a1)_$(df))
@end group
@end example

@noindent
will give @samp{dirs} the same value as @samp{a_dirs}, @samp{1_dirs},
@samp{a_files} or @samp{1_files} depending on the settings of @samp{use_a}
and @samp{use_dirs}.

Computed variable names can also be used in substitution references:

@example
@group
a_objects := a.o b.o c.o
1_objects := 1.o 2.o 3.o

sources := $($(a1)_objects:.o=.c)
@end group
@end example

@noindent
defines @samp{sources} as either @samp{a.c b.c c.c} or @samp{1.c 2.c 3.c},
depending on the value of @samp{a1}.

The only restriction on this sort of use of nested variable references is that
they cannot specify part of the name of a function to be called.  This is
because the test for a recognized function name is done before the expansion
of nested references.  For example,

@example
@group
ifdef do_sort
func := sort
else
func := strip
endif
@end group

@group
bar := a d b g q c
@end group

@group
foo := $($(func) $(bar))
@end group
@end example

@noindent
attempts to give @samp{foo} the value of the variable @samp{sort a d b g q c}
or @samp{strip a d b g q c}, rather than giving @samp{a d b g q c} as the
argument to either the @samp{sort} or the @samp{strip} function.  This
restriction could be removed in the future if that change is shown to be a
good idea.

You can also use computed variable names in the left-hand side of a variable
assignment, or in a @samp{define} directive, as in:

@example
dir = foo
$(dir)_sources := $(wildcard $(dir)/*.c)
define $(dir)_print =
lpr $($(dir)_sources)
endef
@end example

@noindent
This example defines the variables @samp{dir}, @samp{foo_sources}, and
@samp{foo_print}.

Note that @dfn{nested variable references} are quite different from
@dfn{recursively expanded variables}
(@pxref{Flavors, ,The Two Flavors of Variables}), though both are
used together in complex ways when doing makefile programming.

@node Flavors
@section The Two Flavors of Variables
@cindex flavors of variables
@cindex variables, flavors

There are two different ways that a variable in GNU Make can behave when a
reference is expanded; we call them the @dfn{flavors} of variables.

The assignment operator used to set the variable's value (@pxref{Setting, ,
Setting Variables}) determines both the way in which the value is handled
during assignment, and also the flavor of the variable's value after
assignment.  Although there are a number of different assignment operators,
all variables will end up as one of the two flavors.

@menu
* Recursive Variables::         Recursive variables delay expansion.
* Simple Variables::            Simple variables expand immediately.
@end menu

@node Recursive Variables
@subsection Recursively Expanded Variable Assignment
@cindex recursive variable expansion
@cindex recursively expanded variables
@cindex variables, recursively expanded

The first flavor of variable is a @dfn{recursively expanded} variable.  This
is the standard type of variable supported by all versions of @code{make} and
defined by POSIX.

When a recursively expanded variable is expanded due to a variable reference,
the value of the variable is also expanded.  This is called @dfn{recursive
expansion}.

Consider this makefile:

@example
foo = $(bar)
bar = $(ugh)
ugh = Huh?

all: ; @@echo $(foo)
@end example

After @code{make} parses this makefile the value of the variable @code{foo}
will be the string @samp{$(bar)}, the value of the variable @code{bar} will be
the string @samp{$(ugh)}, and the value of the variable @code{ugh} will be the
string @samp{Huh?}.

If you run @code{make} with this makefile, it will print @samp{Huh?}: the
variable reference @samp{$(foo)} expands to @samp{$(bar)} which expands to
@samp{$(ugh)} which finally expands to @samp{Huh?}.

The big advantage of this flavor of variable is that it can refer to variables
which have not been defined yet.  This means:

@example
CFLAGS = $(include_dirs) -O
include_dirs = -Ifoo -Ibar
@end example

@noindent
will do what was intended, even though when @code{CFLAGS} was set the variable
@samp{include_dirs} was not set yet: when @code{CFLAGS} is expanded in a
recipe it will expand to the value of @code{include_dirs} at that time,
yielding @samp{-Ifoo -Ibar -O}.

This flavor also allows variables to contain references to automatic variables
(@pxref{Automatic Variables}); since automatic variables are not set until a
recipe is going to be run it would not work if the variable's value was
expanded when the variable was assigned.

There are also a number of disadvantages.  The first one is that programmers
are not used to this type of assignment and may be confused.  In a makefile
like this:

@example
@group
CFLAGS = -g
DEBUG_FLAGS = $(CFLAGS)

CFLAGS = -O2
OPT_FLAGS = $(CFLAGS)
@end group
@end example

@noindent
when @code{DEBUG_FLAGS} and @code{OPT_FLAGS} are expanded later they will
@emph{both} contain the value @samp{-O2}; each assignment to @code{CFLAGS}
overwrites the previous assignment and whatever the last value was, will be
seen when references to @samp{$(CFLAGS)} are expanded.

Another disadvantage is that you cannot reference a variable in its own value;
for instance this:
@cindex loops in variable expansion
@cindex variables, loops in expansion

@example
CFLAGS = $(CFLAGS) -O
@end example

@noindent
will generate an error due to an infinite loop in the variable expansion.  If
you want to append to the end of the variable you can use the @samp{+=}
assignment operator (@pxref{Appending Assignment, , Appending More Text to
Variables}) to deal with this.  The fact that a ``recursively expanded''
variable cannot contain a recursive reference to itself is one of those
idiosyncratic proofs that naming is hard in computer science.

A third disadvantage is that the value will be re-expanded every time the
variable is referenced.  If the expansion of that variable is expensive--for
example if it contains functions (@pxref{Functions, ,Functions for
Transforming Text}) which are expensive to execute--this can impact the
performance of @code{make}.  It may also cause functions like @code{wildcard}
and @code{shell} to give unpredictable results since you cannot easily control
when they are called, or how many times.

@node Simple Variables
@subsection Simply Expanded Variable Assignment
To avoid the problems and inconveniences of recursively expanded variables,
GNU Make provides another flavor of variable value: @dfn{simply expanded
variables}.

The value of a simply expanded variable is expanded only once, immediately
when the variable is assigned in the makefile.  All references to variables
and functions in its value are resolved at the time the variable is assigned.
When the variable is later expanded it is not re-expanded (thus it is not
``recursive''): the stored value is used verbatim as the result of the
expansion.

If we write the example from the previous section using simply expanded
variables (note the use of @samp{:=} for assignment), we will get more
traditional behavior:

@example
@group
CFLAGS = -g
DEBUG_FLAGS := $(CFLAGS)

CFLAGS = -O2
OPT_FLAGS := $(CFLAGS)
@end group
@end example

Now the value of @code{DEBUG_FLAGS} will be @samp{-g} while the value of
@code{OPT_FLAGS} is @samp{-O2} as expected.

Simply expanded variables simplify makefile programming because they work like
variables in most other programming languages.  They allow you to redefine a
variable using its own value.  And they make the use of functions more
efficient since they are expanded only one time, not each time the variable is
referenced.

@node Expanding
@section How Expansion Works
@cindex expanding variables
@cindex variables, expanding

The process of @dfn{expanding} a string in @code{make} means, to replace each
variable reference (@pxref{Reference, ,Basics of Variable References}) with
the value of that variable.  All text which is not part of a variable
reference is preserved in the result.

Expansion proceeds from @emph{right to left} and from @emph{inside out}.  For
example, given the following text:

@example
save the $(shell $(date)) for $(event)
@end example

@noindent
the expansion would keep the static string @samp{save the } (starting from
right to left), then expand the variable named @code{date} (going from inside
out) and use the result as the argument to the GNU Make function @code{shell}
(@pxref{Shell Function, ,The @code{shell} Function}).  The output of that
shell command is appended into the result of the expansion.  Then the static
string @samp{ for } is added, and finally the expansion of the variable
@code{event}.

The expansion of a variable that has not been defined is not an error: instead
it silently expands to an empty string.  This behavior is required by POSIX;
however it can be controlled in GNU Make (@pxref{Warnings, ,Makefile
Warnings}).

Although expansion always proceeds in the same way, deciding @emph{when} text
is expanded depends on the context in which it's used: often text is expanded
as the makefile is parsed.  In other situations the expansion is delayed (for
example the text of a recipe is not expanded until the recipe is invoked).
@xref{Reading Makefiles, ,How @code{make} Reads a Makefile}.

Also, deciding how to interpret the value of a variable during expansion
depends on the type of variable.  @xref{Flavors, ,The Two Flavors of Variables}.

@node Values
@section How Variables Get Their Values
@cindex variables, how they get their values
@cindex value, how a variable gets it

Variables can get values in several different ways:

@itemize @bullet
@item
You can specify an overriding value on the command line when you run
@code{make}.  @xref{Overriding, ,Overriding Variables}.

@item
You can specify a value in the makefile, either with an assignment
(@pxref{Setting, ,Setting Variables}) or with a verbatim definition
(@pxref{Multi-Line, ,Defining Multi-Line Variables}).

@item
You can specify a short-lived value with the @code{let} function
(@pxref{Let Function}) or with the @code{foreach} function
(@pxref{Foreach Function}).

@item
Variables and their values in the environment when @code{make} is invoked are
imported as @code{make} variables.  @xref{Environment, ,Variables from the
Environment}.

@item
Several variables are given new values for each rule that @code{make} invokes,
before the recipe for that rule is expanded.  @xref{Automatic Variables}.

@item
Several variables have default initial values.
@xref{Implicit Variables, ,Variables Used by Implicit Rules}.
@end itemize

@node Setting
@section Setting Variables
@cindex setting variables
@cindex variables, setting
@cindex =
@cindex :=
@cindex ::=

A makefile variable is set by writing a line starting with the variable name
followed by one of the assignment operators @code{=}, @code{:=}, @code{::=},
@code{:::=}, or @code{!=}.  Whitespace can optionally come before the variable
name, between the variable name and the operator, and after the operator but
before the value, and is discarded.  Whatever text follows becomes the value
of the variable.  For example:

@example
objects = main.o foo.o bar.o utils.o
@end example

@noindent
defines a variable named @code{objects} to contain the value @samp{main.o
foo.o bar.o utils.o}.  The whitespace before and after the assignment operator
is ignored.

If there is ambiguity between the end of the variable name and the start of
the assignment operator (@pxref{Setting, ,Setting Variables}), GNU Make always
prefers the operator.  For example:

@example
FOO?=bar
@end example

@noindent
will always be parsed into the tokens @samp{FOO}, @samp{?=}, and @samp{bar}.
On the other hand, this:

@example
FOO? = bar
@end example

@noindent
will be parsed as @samp{FOO?}, @samp{=}, and @samp{bar}.  For this reason,
among others, it's a best practice to always add whitespace around operators
when setting @code{make} variables.

There is no limit on the length of the value of a variable, except the amount
of memory on the computer.  You can split the value of a variable into
multiple physical lines, for readability, by adding backslashes
(@pxref{Splitting Lines, ,Splitting Long Lines}).

The assignment operator controls two things: first, how the value (the
string after the operator) is handled when the variable is set, and second how
the value is handled when the variable is expanded (i.e., its flavor:
@pxref{Flavors, The Two Flavors of Variables}).

The @samp{=} operator creates a @emph{recursively expanded} variable.  The
value of the variable @emph{is not} expanded when the variable is defined: the
right hand side of the assignment operator is used verbatim as the value of
the variable.

The @samp{:=} and @samp{::=} operators create a @emph{simply expanded}
variable.  The value of the variable @emph{is} expanded when the variable is
defined.  These operators are interchangeable: @samp{:=} is specific to GNU
Make while @samp{::=} has the same behavior and is specified by POSIX.

The @samp{:::=} operator creates a @emph{recursively expanded} variable, like
@samp{=}.  The value of the variable @emph{is} expanded when the variable is
defined, like @samp{:=} / @samp{::=}.  After the expansion, the resulting
value is @emph{escaped} by substituting each dollar sign in the value with two
dollar signs (@samp{$$}).  @xref{Immediate Assignment, ,Immediately Expanded
Variable Assignment}.

The shell assignment operator @samp{!=} creates a @emph{recursively expanded}
variable.  The value of the variable is obtained by passing the expanded
right-hand side to the shell as a command, and collecting its output.
@xref{Shell Assignment, Shell Variable Assignment}.

Each of these assignment operators can also be prefixed with a question mark
(@samp{?}) to make them @dfn{conditional}.  The right-hand side is only
considered if the variable is not yet set.  If it is set, the value of the
assignment is ignored.  @xref{Conditional Assignment, ,Conditional Variable
Assignment}.

Finally, more text can be added to an already-set variable value using the
appending assignment operator @samp{+=}.  @xref{Appending Assignment,
,Appending More Text to Variables}.

@menu
* Immediate Assignment::        Recursive variables with immediate expansion.
* Shell Assignment::            Assigning variables to shell output.
* Conditional Assignment::      Assigning variables only if not yet defined.
* Appending Assignment::        How to append to the value of a variable.
* Whitespace in Values::        Leading and trailing whitespace in values.
@end menu

@node Immediate Assignment
@subsection Immediately Expanded Variable Assignment
@cindex immediate variable assignment
@cindex variables, immediate assignment
@cindex :::=

The @samp{:::=} assignment operator allows for immediate expansion, but unlike
simple assignment the resulting variable is recursive and will be re-expanded
again on every use.  In order to avoid unexpected results, after the value is
immediately expanded it will automatically be quoted: all instances of
@samp{$} in the value after expansion will be converted into @samp{$$}.  This
type of assignment uses the @samp{:::=} operator.  For example,

@example
@group
var = first
OUT :::= $(var)
var = second
@end group
@end example

@noindent
results in the @code{OUT} variable containing the text @samp{first}, while here:

@example
@group
var = one$$two
OUT :::= $(var)
var = three
@end group
@end example

@noindent
results in the @code{OUT} variable containing the text @samp{one$$two}.  The
value is expanded when the variable is assigned, so the result is the
expansion of the first value of @code{var}, @samp{one$two}; then the value is
re-escaped before the assignment is complete giving the final result of
@samp{one$$two}.

This is generally equivalent to the GNU Make @samp{:=} / @samp{::=} assignment
operators, but there are a few differences:

First, since the variable is a recursive variable when appending to it with
the @samp{+=} operator the value on the right-hand side is not expanded
immediately as it would be using @samp{:=} / @samp{::=}.

Second, expansion of these variables is slightly less efficient since they
will be re-expanded when they are referenced.  However since all dollar signs
are escaped the expansion simply un-escapes the value, it won't expand any
variables or run any functions.

Here is another example:

@example
@group
var = one$$two
OUT :::= $(var)
OUT += $(var)
var = three$$four
@end group
@end example

After this, the value of @code{OUT} is the text @samp{one$$two $(var)}.  When
this variable is used it will be expanded and the result will be
@samp{one$two three$four}.

This style of assignment is equivalent to the traditional BSD @code{make}
@samp{:=} assignment operator, and different from GNU Make's @samp{:=}
assignment operator.  The @samp{:::=} assignment operator was added to POSIX
to provide portability.

@node Shell Assignment
@subsection Shell Variable Assignment
@cindex shell variable assignment
@cindex variables, shell assignment
@cindex !=

The shell assignment operator @samp{!=} creates a @emph{recursively expanded}
variable.  The value of the variable is computed by first expanding the
right-hand side of the assignment then invoking a shell and passing the
expanded value to the shell as a script to run.  The output generated by the
shell (to standard out) is used as the value of the variable.

If the last character of output is a newline, that character is removed; all
other newlines are replaced by spaces.  For example:

@example
hash != printf '\043'
file_list != find . -name '*.c'
@end example

Because the variable is recursively expanded you must take care to ensure
that, if the shell script outputs dollar signs which you don't want to be
expanded by @code{make} when the variable is expanded, you escape them with
two dollar signs.

The @samp{!=} operator is portable to some other versions of @code{make} and
is defined in the latest POSIX specifications.

Alternatively, you can set a simply expanded variable to the result of running
a program using the @code{shell} function call.  @xref{Shell Function, , The
@code{shell} Function}.  For example:

@example
hash := $(shell printf '\043')
var := $(shell find . -name "*.c")
@end example

As with the @code{shell} function, the exit status of the just-invoked
shell script is stored in the @code{.SHELLSTATUS} variable when assigning with
@samp{!=}.

@node Conditional Assignment
@subsection Conditional Variable Assignment
@cindex conditional variable assignment
@cindex variables, conditional assignment
@cindex ?=
@cindex ?:=
@cindex ?::=
@cindex ?:::=
@cindex ?!=

Any non-appending assignment operator (@samp{=}, @samp{:=}, @samp{::=},
@samp{:::=}, @samp{!=}) can be prefixed with a conditional modifier, @samp{?}.
When this modifier is used, the assignment will proceed normally according to
the base assignment operator, @emph{only if} the variable is not already
defined.

If the variable is already defined, the assignment is ignored and the
right-hand side (value) of the assignment is not processed.

For example this statement:

@example
FOO ?= bar
@end example

@noindent
is equivalent to this (@pxref{Origin Function, ,The @code{origin} Function}):

@example
ifeq ($(origin FOO), undefined)
  FOO = bar
endif
@end example

More generally a statement of the form:

@example
NAME ?<op> VALUE
@end example

will do nothing if the variable @code{NAME} is defined, and will perform the
operation @samp{NAME <op> VALUE} for any assignment operation @samp{<op>} if
@code{NAME} is not defined.

Note that a variable set to an empty value is still considered to be defined,
so assignments modified with @samp{?} will not set that variable.

@node Appending Assignment
@subsection Appending More Text to Variables
@cindex +=
@cindex appending to variables
@cindex variables, appending to

Often it is useful to add more text to the value of a variable already defined.
You do this with a line containing @samp{+=}, like this:

@example
objects += another.o
@end example

@noindent
This takes the value of the variable @code{objects}, and adds the text
@samp{another.o} to it (preceded by a single space, if it has a value
already).  Thus:

@example
objects = main.o foo.o bar.o utils.o
objects += another.o
@end example

@noindent
sets @code{objects} to @samp{main.o foo.o bar.o utils.o another.o}.

Using @samp{+=} is similar to:

@example
objects = main.o foo.o bar.o utils.o
objects := $(objects) another.o
@end example

@noindent
but differs in ways that become important when you use more complex values.

When the variable in question has not been defined before, @samp{+=}
acts just like normal @samp{=}: it defines a recursively-expanded
variable.  However, when there @emph{is} a previous definition, exactly
what @samp{+=} does depends on what flavor of variable you defined
originally.  @xref{Flavors, ,The Two Flavors of Variables}, for an
explanation of the two flavors of variables.

When you add to a variable's value with @samp{+=}, @code{make} acts
essentially as if you had included the extra text in the initial definition of
the variable.  If you defined it first with @samp{:=} or @samp{::=}, making it
a simply-expanded variable, @samp{+=} adds to that simply-expanded definition,
and expands the new text before appending it to the old value just as
@samp{:=} does (see @ref{Setting, ,Setting Variables}, for a full explanation
of @samp{:=} or @samp{::=}).  In fact,

@example
variable := value
variable += more
@end example

@noindent
is exactly equivalent to:

@noindent
@example
variable := value
variable := $(variable) more
@end example

On the other hand, when you use @samp{+=} with a variable that you defined
first to be recursively-expanded using plain @samp{=} or @samp{:::=},
@code{make} appends the un-expanded text to the existing value, whatever it
is.  This means that

@example
@group
variable = value
variable += more
@end group
@end example

@noindent
is roughly equivalent to:

@example
@group
temp = value
variable = $(temp) more
@end group
@end example

@noindent
except that of course it never defines a variable called @code{temp}.
The importance of this comes when the variable's old value contains
variable references.  Take this common example:

@example
CFLAGS = $(includes) -O
@dots{}
CFLAGS += -pg # enable profiling
@end example

@noindent
The first line defines the @code{CFLAGS} variable with a reference to another
variable, @code{includes}.  (@code{CFLAGS} is used by the rules for C
compilation; @pxref{Catalogue of Rules, ,Catalogue of Built-In Rules}.)
Using @samp{=} for the definition makes @code{CFLAGS} a recursively-expanded
variable, meaning @w{@samp{$(includes) -O}} is @emph{not} expanded when
@code{make} processes the definition of @code{CFLAGS}.  Thus, @code{includes}
need not be defined yet for its value to take effect.  It only has to be
defined before any reference to @code{CFLAGS}.  If we tried to append to the
value of @code{CFLAGS} without using @samp{+=}, we might do it like this:

@example
CFLAGS := $(CFLAGS) -pg # enable profiling
@end example

@noindent
This is pretty close, but not quite what we want.  Using @samp{:=}
redefines @code{CFLAGS} as a simply-expanded variable; this means
@code{make} expands the text @w{@samp{$(CFLAGS) -pg}} before setting the
variable.  If @code{includes} is not yet defined, we get @w{@samp{ -O
-pg}}, and a later definition of @code{includes} will have no effect.
Conversely, by using @samp{+=} we set @code{CFLAGS} to the
@emph{unexpanded} value @w{@samp{$(includes) -O -pg}}.  Thus we preserve
the reference to @code{includes}, so if that variable gets defined at
any later point, a reference like @samp{$(CFLAGS)} still uses its
value.

@node Whitespace in Values
@subsection Whitespace in Variable Values
@cindex spaces, in variable values
@cindex whitespace, in variable values
@cindex variables, spaces in values

As described above, leading whitespace (whitespace between the assignment
operator and the first non-whitespace character of the value) is discarded.
All whitespace after this is preserved, @emph{including} trailing spaces.

Unwanted trailing whitespace can cause your makefile to behave in unexpected
ways: consider a variable reference such as @samp{$(dir)/file.c} if the
variable @samp{dir} had an unexpected trailing space in its value.  If your
text editor has settings to automatically strip all trailing spaces from files
when they are saved, or to make trailing whitespace visible somehow, it's a
good idea to enable these facilities for makefiles.

You may occasionally need to define a variable value that begins with
whitespace.  To force whitespace at the beginning of a value you can use a
variable reference that expands to the empty string.  For example:

@example
@group
emptystring :=
space := $(emptystring) $(emptystring)
@end group
@end example

The space between the assignment operator and the variable reference
@samp{$(emptystring)} will be discarded, then the rest of the string is
expanded.  The @samp{emptystring} variables expand to no text, leaving just
the space as the value.

The second reference to @samp{$(emptystring)} is used to avoid having the
trailing space stripped by editors and to make the intent simpler to
understand.  You don't have to use a second variable reference; you could also
use a comment such as:

@example
@group
emptystring :=
space := $(emptystring) #<-one space
@end group
@end example

@noindent
to achieve the same effect.

Conversely, if you do @emph{not} want any whitespace at the end of the
variable value you must not put whitespace plus a comment on the end of the
variable value, such as this:

@example
dir := /foo/bar    # directory to put the frobs in
@end example

@noindent
Here the value of the variable @code{dir} is @w{@samp{/foo/bar    }} (with
four trailing spaces), which was probably not the intention.  For this reason
adding comments to the end of variable assignment lines is discouraged in
makefiles, and using preceding comment lines is preferred; for example:

@example
@group
# directory to put the frobs in
dir := /foo/bar
@end group
@end example

Another option is to make extensive use of the @code{strip} function
(@pxref{Text Functions}) to ensure variable expansions have no leading or
trailing whitespace.

@node Substitution Refs
@section Substitution References
@cindex modified variable reference
@cindex substitution variable reference
@cindex variables, modified reference
@cindex variables, substitution reference

@cindex variables, substituting suffix in
@cindex suffix, substituting in variables
A @dfn{substitution reference} substitutes the value of a variable with
alterations that you specify.  It has the form
@samp{$(@var{var}:@var{a}=@var{b})} (or
@samp{$@{@var{var}:@var{a}=@var{b}@}}) and its meaning is to take the value
of the variable @var{var}, replace every @var{a} at the end of a word with
@var{b} in that value, and substitute the resulting string.

When we say ``at the end of a word'', we mean that @var{a} must appear
either followed by whitespace or at the end of the value in order to be
replaced; other occurrences of @var{a} in the value are unaltered.  For
example:

@example
foo := a.o b.o l.a c.o
bar := $(foo:.o=.c)
@end example

@noindent
sets @samp{bar} to @samp{a.c b.c l.a c.c}.  @xref{Setting, ,Setting Variables}.

A substitution reference is shorthand for the @code{patsubst}
expansion function (@pxref{Text Functions, ,Functions for String Substitution and Analysis}):
@samp{$(@var{var}:@var{a}=@var{b})} is equivalent to
@samp{$(patsubst %@var{a},%@var{b},@var{var})}.  We provide
substitution references as well as @code{patsubst} for compatibility
with other implementations of @code{make}.

Another type of substitution reference lets you use the full power of
the @code{patsubst} function.  It has the same form
@samp{$(@var{var}:@var{a}=@var{b})} described above, except that now
@var{a} must contain a single @samp{%} character.  This case is
equivalent to @samp{$(patsubst @var{a},@var{b},$(@var{var}))}.
@xref{Text Functions, ,Functions for String Substitution and Analysis},
for a description of the @code{patsubst} function.  For example:

@example
@group
foo := a.o b.o l.a c.o
bar := $(foo:%.o=%.c)
@end group
@end example

@noindent
sets @samp{bar} to @samp{a.c b.c l.a c.c}.

@node Override Directive
@section The @code{override} Directive
@findex override
@cindex overriding with @code{override}
@cindex variables, overriding

If a variable has been set with a command argument
(@pxref{Overriding, ,Overriding Variables}),
then ordinary assignments in the makefile are ignored.  If you want to set
the variable in the makefile even though it was set with a command
argument, you can use an @code{override} directive, which is a line that
looks like this:

@example
override @var{variable} = @var{value}
@end example

@noindent
or

@example
override @var{variable} := @var{value}
@end example

To append more text to a variable defined on the command line, use:

@example
override @var{variable} += @var{more text}
@end example

@noindent
@xref{Appending Assignment, ,Appending More Text to Variables}.

Variable assignments marked with the @code{override} flag have a
higher priority than all other assignments, except another
@code{override}.  Subsequent assignments or appends to this variable
which are not marked @code{override} will be ignored.

The @code{override} directive was not invented for escalation in the war
between makefiles and command arguments.  It was invented so you can alter
and add to values that the user specifies with command arguments.

For example, suppose you always want the @samp{-g} switch when you run the
C compiler, but you would like to allow the user to specify the other
switches with a command argument just as usual.  You could use this
@code{override} directive:

@example
override CFLAGS += -g
@end example

You can also use @code{override} directives with @code{define} directives
(@pxref{Multi-Line, , Defining Multi-Line Variables}).  This is done as you
might expect:

@example
override define foo =
bar
endef
@end example

@node Multi-Line
@section Defining Multi-Line Variables
@findex define
@findex endef
@cindex multi-line variable definition
@cindex variables, multi-line
@cindex verbatim variable definition
@cindex defining variables verbatim
@cindex variables, defining verbatim

Another way to set the value of a variable is to use the @code{define}
directive.  This directive has an unusual syntax which allows newline
characters to be included in the value, which is convenient for
defining both canned sequences of commands (@pxref{Canned Recipes,
,Defining Canned Recipes}), and also sections of makefile syntax to
use with @code{eval} (@pxref{Eval Function}).

The @code{define} directive is followed on the same line by the name
of the variable being defined and an (optional) assignment operator,
and nothing more.  The value to give the variable appears on the
following lines.  The end of the value is marked by a line containing
just the word @code{endef}.

Aside from this difference in syntax, @code{define} works just like
any other variable definition.  The variable name may contain function
and variable references, which are expanded when the directive is read
to find the actual variable name to use.

The final newline before the @code{endef} is not included in the
value; if you want your value to contain a trailing newline you must
include a blank line.  For example in order to define a variable that
contains a newline character you must use @emph{two} empty lines, not one:

@example
define newline


endef
@end example

You may omit the variable assignment operator if you prefer.  If
omitted, @code{make} assumes it to be @samp{=} and creates a
recursively-expanded variable (@pxref{Flavors, ,The Two Flavors of Variables}).
When using a @samp{+=} operator, the value is appended to the previous
value as with any other append operation: with a single space
separating the old and new values.

You may nest @code{define} directives: @code{make} will keep track of
nested directives and report an error if they are not all properly
closed with @code{endef}.  Note that lines beginning with the recipe
prefix character are considered part of a recipe, so any @code{define}
or @code{endef} strings appearing on such a line will not be
considered @code{make} directives.

@example
define two-lines
echo foo
echo $(bar)
endef
@end example

@need 800
When used in a recipe, the previous example is functionally equivalent
to this:

@example
two-lines = echo foo; echo $(bar)
@end example

@noindent
since two commands separated by semicolon behave much like two separate
shell commands.  However, note that using two separate lines means
@code{make} will invoke the shell twice, running an independent sub-shell
for each line.  @xref{Execution, ,Recipe Execution}.

If you want variable definitions made with @code{define} to take
precedence over command-line variable definitions, you can use the
@code{override} directive together with @code{define}:

@example
override define two-lines =
foo
$(bar)
endef
@end example

@noindent
@xref{Override Directive, ,The @code{override} Directive}.

@node Undefine Directive
@section Undefining Variables
@findex undefine
@cindex undefining variable

If you want to clear a variable, setting its value to empty is usually
sufficient. Expanding such a variable will yield the same result (empty
string) regardless of whether it was set or not. However, if you are
using the @code{flavor} (@pxref{Flavor Function}) and
@code{origin} (@pxref{Origin Function}) functions, there is a difference
between a variable that was never set and a variable with an empty value.
In such situations you may want to use the @code{undefine} directive to
make a variable appear as if it was never set. For example:

@example
@group
foo := foo
bar = bar

undefine foo
undefine bar

$(info $(origin foo))
$(info $(flavor bar))
@end group
@end example

This example will print ``undefined'' for both variables.

If you want to undefine a command-line variable definition, you can use
the @code{override} directive together with @code{undefine}, similar to
how this is done for variable definitions:

@example
override undefine CFLAGS
@end example

@node Environment
@section Variables from the Environment

@cindex variables, environment
@cindex environment
Variables in @code{make} can come from the environment in which
@code{make} is run.  Every environment variable that @code{make} sees
when it starts up is transformed into a @code{make} variable with the
same name and value.  However, an explicit assignment in the makefile,
or with a command argument, overrides the environment.  (If the
@samp{-e} flag is specified, then values from the environment override
assignments in the makefile.  @xref{Options Summary, ,Summary of
Options}.  But this is not recommended practice.)

Thus, by setting the variable @code{CFLAGS} in your environment, you can
cause all C compilations in most makefiles to use the compiler switches you
prefer.  This is safe for variables with standard or conventional meanings
because you know that no makefile will use them for other things.  (Note
this is not totally reliable; some makefiles set @code{CFLAGS} explicitly
and therefore are not affected by the value in the environment.)

When @code{make} runs a recipe, some variables defined in the makefile
are placed into the environment of each command @code{make} invokes.
By default, only variables that came from the @code{make}'s
environment or set on its command line are placed into the environment
of the commands.  You can use the @code{export} directive to pass
other variables.  @xref{Variables/Recursion, , Communicating Variables
to a Sub-@code{make}}, for full details.

Other use of variables from the environment is not recommended.  It is not
wise for makefiles to depend for their functioning on environment variables
set up outside their control, since this would cause different users to get
different results from the same makefile.  This is against the whole
purpose of most makefiles.

@cindex SHELL, import from environment
Such problems would be especially likely with the variable
@code{SHELL}, which is normally present in the environment to specify
the user's choice of interactive shell.  It would be very undesirable
for this choice to affect @code{make}; so, @code{make} handles the
@code{SHELL} environment variable in a special way; see @ref{Choosing
the Shell}.

@node Target-specific
@section Target-specific Variable Values
@cindex target-specific variables
@cindex variables, target-specific

Variable values in @code{make} are usually global; that is, they are the
same regardless of where they are evaluated (unless they're reset, of
course).  Exceptions to that are variables defined with the @code{let}
function (@pxref{Let Function}) or the @code{foreach} function
(@pxref{Foreach Function}, and automatic variables
(@pxref{Automatic Variables}).

Another exception are @dfn{target-specific variable values}.  This
feature allows you to define different values for the same variable,
based on the target that @code{make} is currently building.  As with
automatic variables, these values are only available within the context
of a target's recipe (and in other target-specific assignments).

Set a target-specific variable value like this:

@example
@var{target} @dots{} : @var{variable-assignment}
@end example

Target-specific variable assignments can be prefixed with any or all of the
special keywords @code{export}, @code{unexport}, @code{override}, or
@code{private}; these apply their normal behavior to this instance of the
variable only.

Multiple @var{target} values create a target-specific variable value for
each member of the target list individually.

The @var{variable-assignment} can be any valid form of assignment; recursive
(@samp{=}), simple (@samp{:=} or @samp{::=}), immediate (@samp{::=}), shell
(@samp{!=}), or appending (@samp{+=}), and may be preceded by a conditional
modifier (@samp{?}).  The variable that appears within the
@var{variable-assignment} is evaluated within the context of the target: thus,
any previously-defined target-specific variable values will be in effect.
Note that this variable is actually distinct from any ``global'' value: the
two variables do not have to have the same flavor (recursive vs.@: simple).

Target-specific variables have the same priority as any other makefile
variable.  Variables provided on the command line (and in the
environment if the @samp{-e} option is in force) will take precedence.
Specifying the @code{override} directive will allow the target-specific
variable value to be preferred.

There is one more special feature of target-specific variables: when
you define a target-specific variable that variable value is also in
effect for all prerequisites of this target, and all their
prerequisites, etc.@: (unless those prerequisites override that variable
with their own target-specific variable value).  So, for example, a
statement like this:

@example
prog : CFLAGS = -g
prog : prog.o foo.o bar.o
@end example

@noindent
will set @code{CFLAGS} to @samp{-g} in the recipe for @file{prog}, but
it will also set @code{CFLAGS} to @samp{-g} in the recipes that create
@file{prog.o}, @file{foo.o}, and @file{bar.o}, and any recipes which
create their prerequisites.

Be aware that a given prerequisite will only be built once per
invocation of make, at most.  If the same file is a prerequisite of
multiple targets, and each of those targets has a different value for
the same target-specific variable, then the first target to be built
will cause that prerequisite to be built and the prerequisite will
inherit the target-specific value from the first target.  It will
ignore the target-specific values from any other targets.

@node Pattern-specific
@section Pattern-specific Variable Values
@cindex pattern-specific variables
@cindex variables, pattern-specific

In addition to target-specific variable values
(@pxref{Target-specific, ,Target-specific Variable Values}), GNU
@code{make} supports pattern-specific variable values.  In this form,
the variable is defined for any target that matches the pattern
specified.

Set a pattern-specific variable value like this:

@example
@var{pattern} @dots{} : @var{variable-assignment}
@end example
where @var{pattern} is a %-pattern.  As with target-specific variable
values, multiple @var{pattern} values create a pattern-specific variable
value for each pattern individually.  The @var{variable-assignment} can
be any valid form of assignment.  Any command line variable setting will
take precedence, unless @code{override} is specified.

For example:

@example
%.o : CFLAGS = -O
@end example

@noindent
will assign @code{CFLAGS} the value of @samp{-O} for all targets
matching the pattern @samp{%.o}.

If a target matches more than one pattern, the matching pattern-specific
variables with longer stems are interpreted first. This results in more
specific variables taking precedence over the more generic ones, for
example:

@example
%.o: %.c
        $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@@

lib/%.o: CFLAGS := -fPIC -g
%.o: CFLAGS := -g

all: foo.o lib/bar.o
@end example

In this example the first definition of the @code{CFLAGS} variable
will be used to update @file{lib/bar.o} even though the second one
also applies to this target. Pattern-specific variables which result
in the same stem length are considered in the order in which they
were defined in the makefile.

Pattern-specific variables are searched after any target-specific
variables defined explicitly for that target, and before target-specific
variables defined for the parent target.

@node Suppressing Inheritance
@section Suppressing Inheritance
@findex private
@cindex suppressing inheritance
@cindex inheritance, suppressing

As described in previous sections, @code{make} variables are inherited
by prerequisites.  This capability allows you to modify the behavior
of a prerequisite based on which targets caused it to be rebuilt.  For
example, you might set a target-specific variable on a @code{debug}
target, then running @samp{make debug} will cause that variable to be
inherited by all prerequisites of @code{debug}, while just running
@samp{make all} (for example) would not have that assignment.

Sometimes, however, you may not want a variable to be inherited.  For
these situations, @code{make} provides the @code{private} modifier.
Although this modifier can be used with any variable assignment, it
makes the most sense with target- and pattern-specific variables.  Any
variable marked @code{private} will be visible to its local target but
will not be inherited by prerequisites of that target.  A global
variable marked @code{private} will be visible in the global scope but
will not be inherited by any target, and hence will not be visible
in any recipe.

As an example, consider this makefile:
@example
EXTRA_CFLAGS =

prog: private EXTRA_CFLAGS = -L/usr/local/lib
prog: a.o b.o
@end example

Due to the @code{private} modifier, @samp{a.o} and @samp{b.o} will not
inherit the @code{EXTRA_CFLAGS} variable assignment from the
@code{prog} target.

@node Special Variables
@comment  node-name,  next,  previous,  up
@section Other Special Variables
@cindex makefiles, and special variables
@cindex special variables

GNU @code{make} supports some variables that have special properties.

@table @code

@vindex MAKEFILE_LIST
@cindex makefiles, and @code{MAKEFILE_LIST} variable
@cindex including (@code{MAKEFILE_LIST} variable)
@item MAKEFILE_LIST
Contains the name of each makefile that is parsed by @code{make}, in
the order in which it was parsed.  The name is appended just
before @code{make} begins to parse the makefile.  Thus, if the first
thing a makefile does is examine the last word in this variable, it
will be the name of the current makefile.  Once the current makefile
has used @code{include}, however, the last word will be the
just-included makefile.

If a makefile named @code{Makefile} has this content:

@example
@group
name1 := $(lastword $(MAKEFILE_LIST))

include inc.mk

name2 := $(lastword $(MAKEFILE_LIST))

all:
        @@echo name1 = $(name1)
        @@echo name2 = $(name2)
@end group
@end example

@noindent
then you would expect to see this output:

@example
@group
name1 = Makefile
name2 = inc.mk
@end group
@end example

@vindex .DEFAULT_GOAL
@item .DEFAULT_GOAL
Sets the default goal to be used if no targets were specified on the
command line (@pxref{Goals, , Arguments to Specify the Goals}).  The
@code{.DEFAULT_GOAL} variable allows you to discover the current
default goal, restart the default goal selection algorithm by clearing
its value, or to explicitly set the default goal.  The following
example illustrates these cases:

@example
@group
# Query the default goal.
ifeq ($(.DEFAULT_GOAL),)
  $(warning no default goal is set)
endif

.PHONY: foo
foo: ; @@echo $@@

$(warning default goal is $(.DEFAULT_GOAL))

# Reset the default goal.
.DEFAULT_GOAL :=

.PHONY: bar
bar: ; @@echo $@@

$(warning default goal is $(.DEFAULT_GOAL))

# Set our own.
.DEFAULT_GOAL := foo
@end group
@end example

This makefile prints:

@example
@group
no default goal is set
default goal is foo
default goal is bar
foo
@end group
@end example

Note that assigning more than one target name to @code{.DEFAULT_GOAL} is
invalid and will result in an error.

@vindex MAKE_RESTARTS
@item MAKE_RESTARTS
This variable is set only if this instance of @code{make} has
restarted (@pxref{Remaking Makefiles, , How Makefiles Are Remade}): it
will contain the number of times this instance has restarted.  Note
this is not the same as recursion (counted by the @code{MAKELEVEL}
variable).  You should not set, modify, or export this variable.

@vindex MAKE_TERMOUT
@vindex MAKE_TERMERR
@item MAKE_TERMOUT
@itemx MAKE_TERMERR
When @code{make} starts it will check whether stdout and stderr will
show their output on a terminal.  If so, it will set
@code{MAKE_TERMOUT} and @code{MAKE_TERMERR}, respectively, to the name
of the terminal device (or @code{true} if this cannot be determined).
If set these variables will be marked for export.  These variables
will not be changed by @code{make} and they will not be modified if
already set.

These values can be used (particularly in combination with output
synchronization (@pxref{Parallel Output, ,Output During Parallel
Execution}) to determine whether @code{make} itself is writing to a
terminal; they can be tested to decide whether to force recipe
commands to generate colorized output for example.

If you invoke a sub-@code{make} and redirect its stdout or stderr it
is your responsibility to reset or unexport these variables as well,
if your makefiles rely on them.

@vindex .RECIPEPREFIX
@item .RECIPEPREFIX
The first character of the value of this variable is used as the
character make assumes is introducing a recipe line.  If the variable
is empty (as it is by default) that character is the standard tab
character.  For example, this is a valid makefile:

@example
@group
.RECIPEPREFIX = >
all:
> @@echo Hello, world
@end group
@end example

The value of @code{.RECIPEPREFIX} can be changed multiple times; once set
it stays in effect for all rules parsed until it is modified.

@vindex .VARIABLES
@item .VARIABLES
Expands to a list of the @emph{names} of all global variables defined
so far.  This includes variables which have empty values, as well as
built-in variables (@pxref{Implicit Variables, , Variables Used by
Implicit Rules}), but does not include any variables which are only
defined in a target-specific context.  Note that any value you assign
to this variable will be ignored; it will always return its special
value.

@c @vindex .TARGETS
@c @item .TARGETS
@c The second special variable is @code{.TARGETS}.  When expanded, the
@c value consists of a list of all targets defined in all makefiles read
@c up until that point.  Note it's not enough for a file to be simply
@c mentioned in the makefile to be listed in this variable, even if it
@c would match an implicit rule and become an ``implicit target''.  The
@c file must appear as a target, on the left-hand side of a ``:'', to be
@c considered a target for the purposes of this variable.

@vindex .FEATURES
@item .FEATURES
Expands to a list of special features supported by this version of
@code{make}.  Possible values include, but are not limited to:

@table @samp

@item archives
Supports @code{ar} (archive) files using special file name syntax.
@xref{Archives, ,Using @code{make} to Update Archive Files}.

@item check-symlink
Supports the @code{-L} (@code{--check-symlink-times}) flag.
@xref{Options Summary, ,Summary of Options}.

@item else-if
Supports ``else if'' non-nested conditionals.  @xref{Conditional
Syntax, ,Syntax of Conditionals}.

@item extra-prereqs
Supports the @code{.EXTRA_PREREQS} special target.

@item grouped-target
Supports grouped target syntax for explicit rules.  @xref{Multiple Targets,
,Multiple Targets in a Rule}.

@item guile
Has GNU Guile available as an embedded extension language.
@xref{Guile Integration, ,GNU Guile Integration}.

@item jobserver
Supports ``job server'' enhanced parallel builds.  @xref{Parallel,
,Parallel Execution}.

@item jobserver-fifo
Supports ``job server'' enhanced parallel builds using named pipes.
@xref{Integrating make, ,Integrating GNU @code{make}}.

@item load
Supports dynamically loadable objects for creating custom extensions.
@xref{Loading Objects, ,Loading Dynamic Objects}.

@item notintermediate
Supports the @code{.NOTINTERMEDIATE} special target.
@xref{Integrating make, ,Integrating GNU @code{make}}.

@item oneshell
Supports the @code{.ONESHELL} special target.  @xref{One Shell, ,Using
One Shell}.

@item order-only
Supports order-only prerequisites.  @xref{Prerequisite Types, ,Types
of Prerequisites}.

@item output-sync
Supports the @code{--output-sync} command line option.  @xref{Options Summary,
,Summary of Options}.

@item second-expansion
Supports secondary expansion of prerequisite lists.

@item shell-export
Supports exporting @code{make} variables to @code{shell} functions.

@item shortest-stem
Uses the ``shortest stem'' method of choosing which pattern, of
multiple applicable options, will be used.  @xref{Pattern Match, ,How
Patterns Match}.

@item target-specific
Supports target-specific and pattern-specific variable assignments.
@xref{Target-specific, ,Target-specific Variable Values}.

@item undefine
Supports the @code{undefine} directive.  @xref{Undefine Directive}.
@end table

@vindex .INCLUDE_DIRS
@item .INCLUDE_DIRS
Expands to a list of directories that @code{make} searches for
included makefiles (@pxref{Include, , Including Other Makefiles}).
Note that modifying this variable's value does not change the list of
directories which are searched.

@vindex .EXTRA_PREREQS
@item .EXTRA_PREREQS
Each word in this variable is a new prerequisite which is added to
targets for which it is set.  These prerequisites differ from normal
prerequisites in that they do not appear in any of the automatic
variables (@pxref{Automatic Variables}).  This allows prerequisites to
be defined which do not impact the recipe.

Consider a rule to link a program:

@example
myprog: myprog.o file1.o file2.o
       $(CC) $(CFLAGS) $(LDFLAGS) -o $@@ $^ $(LDLIBS)
@end example

Now suppose you want to enhance this makefile to ensure that updates
to the compiler cause the program to be re-linked.  You can add the
compiler as a prerequisite, but you must ensure that it's not passed
as an argument to link command.  You'll need something like this:

@example
myprog: myprog.o file1.o file2.o $(CC)
       $(CC) $(CFLAGS) $(LDFLAGS) -o $@@ \
           $(filter-out $(CC),$^) $(LDLIBS)
@end example

Then consider having multiple extra prerequisites: they would all have
to be filtered out.  Using @code{.EXTRA_PREREQS} and target-specific
variables provides a simpler solution:

@example
myprog: myprog.o file1.o file2.o
       $(CC) $(CFLAGS) $(LDFLAGS) -o $@@ $^ $(LDLIBS)
myprog: .EXTRA_PREREQS = $(CC)
@end example

This feature can also be useful if you want to add prerequisites to a
makefile you cannot easily modify: you can create a new file such as
@file{extra.mk}:

@example
myprog: .EXTRA_PREREQS = $(CC)
@end example

then invoke @code{make -f extra.mk -f Makefile}.

Setting @code{.EXTRA_PREREQS} globally will cause those prerequisites
to be added to all targets (which did not themselves override it with
a target-specific value).  Note @code{make} is smart enough not to add
a prerequisite listed in @code{.EXTRA_PREREQS} as a prerequisite to
itself.

@item .WARNINGS
Changes the actions taken when @code{make} detects warning conditions in the
makefile.  @xref{Warnings, ,Makefile Warnings}.

@end table

@node Conditionals
@chapter Conditional Parts of Makefiles

@cindex conditionals
A @dfn{conditional} directive causes part of a makefile to be obeyed
or ignored depending on the values of variables.  Conditionals can
compare the value of one variable to another, or the value of a
variable to a constant string.  Conditionals control what @code{make}
actually ``sees'' in the makefile, so they @emph{cannot} be used to
control recipes at the time of execution.

@menu
* Conditional Example::         Example of a conditional
* Conditional Syntax::          The syntax of conditionals.
* Testing Flags::               Conditionals that test flags.
@end menu

@node Conditional Example
@section Example of a Conditional

The following example of a conditional tells @code{make} to use one
set of libraries if the @code{CC} variable is @samp{gcc}, and a
different set of libraries otherwise.  It works by controlling which
of two recipe lines will be used for the rule.  The result is that
@samp{CC=gcc} as an argument to @code{make} changes not only which
compiler is used but also which libraries are linked.

@example
libs_for_gcc = -lgnu
normal_libs =

foo: $(objects)
ifeq ($(CC),gcc)
        $(CC) -o foo $(objects) $(libs_for_gcc)
else
        $(CC) -o foo $(objects) $(normal_libs)
endif
@end example

This conditional uses three directives: one @code{ifeq}, one @code{else}
and one @code{endif}.

The @code{ifeq} directive begins the conditional, and specifies the
condition.  It contains two arguments, separated by a comma and surrounded
by parentheses.  Variable substitution is performed on both arguments and
then they are compared.  The lines of the makefile following the
@code{ifeq} are obeyed if the two arguments match; otherwise they are
ignored.

The @code{else} directive causes the following lines to be obeyed if the
previous conditional failed.  In the example above, this means that the
second alternative linking command is used whenever the first alternative
is not used.  It is optional to have an @code{else} in a conditional.

The @code{endif} directive ends the conditional.  Every conditional must
end with an @code{endif}.  Unconditional makefile text follows.

As this example illustrates, conditionals work at the textual level:
the lines of the conditional are treated as part of the makefile, or
ignored, according to the condition.  This is why the larger syntactic
units of the makefile, such as rules, may cross the beginning or the
end of the conditional.

When the variable @code{CC} has the value @samp{gcc}, the above example has
this effect:

@example
foo: $(objects)
        $(CC) -o foo $(objects) $(libs_for_gcc)
@end example

@noindent
When the variable @code{CC} has any other value, the effect is this:

@example
foo: $(objects)
        $(CC) -o foo $(objects) $(normal_libs)
@end example

Equivalent results can be obtained in another way by conditionally assigning a
variable and then using the variable unconditionally:

@example
libs_for_gcc = -lgnu
normal_libs =

ifeq ($(CC),gcc)
  libs = $(libs_for_gcc)
else
  libs = $(normal_libs)
endif

foo: $(objects)
        $(CC) -o foo $(objects) $(libs)
@end example

@node Conditional Syntax
@section Syntax of Conditionals
@findex ifdef
@findex ifeq
@findex ifndef
@findex ifneq
@findex else
@findex endif

The syntax of a simple conditional with no @code{else} is as follows:

@example
@var{conditional-directive}
@var{text-if-true}
endif
@end example

@noindent
The @var{text-if-true} may be any lines of text, to be considered as part
of the makefile if the condition is true.  If the condition is false, no
text is used instead.

The syntax of a complex conditional is as follows:

@example
@var{conditional-directive}
@var{text-if-true}
else
@var{text-if-false}
endif
@end example

or:

@example
@var{conditional-directive-one}
@var{text-if-one-is-true}
else @var{conditional-directive-two}
@var{text-if-two-is-true}
else
@var{text-if-one-and-two-are-false}
endif
@end example

@noindent
There can be as many ``@code{else} @var{conditional-directive}''
clauses as necessary.  Once a given condition is true,
@var{text-if-true} is used and no other clause is used; if no
condition is true then @var{text-if-false} is used.  The
@var{text-if-true} and @var{text-if-false} can be any number of lines
of text.

The syntax of the @var{conditional-directive} is the same whether the
conditional is simple or complex; after an @code{else} or not.  There
are four different directives that test different conditions.  Here is
a table of them:

@table @code
@item ifeq (@var{arg1}, @var{arg2})
@itemx ifeq '@var{arg1}' '@var{arg2}'
@itemx ifeq "@var{arg1}" "@var{arg2}"
@itemx ifeq "@var{arg1}" '@var{arg2}'
@itemx ifeq '@var{arg1}' "@var{arg2}"
Expand all variable references in @var{arg1} and @var{arg2} and
compare them.  If they are identical, the @var{text-if-true} is
effective; otherwise, the @var{text-if-false}, if any, is effective.

Often you want to test if a variable has a non-empty value.  When the
value results from complex expansions of variables and functions,
expansions you would consider empty may actually contain whitespace
characters and thus are not seen as empty.  However, you can use the
@code{strip} function (@pxref{Text Functions}) to avoid interpreting
whitespace as a non-empty value.  For example:

@example
@group
ifeq ($(strip $(foo)),)
@var{text-if-empty}
endif
@end group
@end example

@noindent
will evaluate @var{text-if-empty} even if the expansion of
@code{$(foo)} contains whitespace characters.

@item ifneq (@var{arg1}, @var{arg2})
@itemx ifneq '@var{arg1}' '@var{arg2}'
@itemx ifneq "@var{arg1}" "@var{arg2}"
@itemx ifneq "@var{arg1}" '@var{arg2}'
@itemx ifneq '@var{arg1}' "@var{arg2}"
Expand all variable references in @var{arg1} and @var{arg2} and
compare them.  If they are different, the @var{text-if-true} is
effective; otherwise, the @var{text-if-false}, if any, is effective.

@item ifdef @var{variable-name}
The @code{ifdef} form takes the @emph{name} of a variable as its
argument, not a reference to a variable.  If the value of that
variable has a non-empty value, the @var{text-if-true} is effective;
otherwise, the @var{text-if-false}, if any, is effective.  Variables
that have never been defined have an empty value.  The text
@var{variable-name} is expanded, so it could be a variable or function
that expands to the name of a variable.  For example:

@example
bar = true
foo = bar
ifdef $(foo)
frobozz = yes
endif
@end example

The variable reference @code{$(foo)} is expanded, yielding @code{bar},
which is considered to be the name of a variable.  The variable
@code{bar} is not expanded, but its value is examined to determine if
it is non-empty.

Note that @code{ifdef} only tests whether a variable has a value.  It
does not expand the variable to see if that value is nonempty.
Consequently, tests using @code{ifdef} return true for all definitions
except those like @code{foo =}.  To test for an empty value, use
@w{@code{ifeq ($(foo),)}}.  For example,

@example
bar =
foo = $(bar)
ifdef foo
frobozz = yes
else
frobozz = no
endif
@end example

@noindent
sets @samp{frobozz} to @samp{yes}, while:

@example
foo =
ifdef foo
frobozz = yes
else
frobozz = no
endif
@end example

@noindent
sets @samp{frobozz} to @samp{no}.

@item ifndef @var{variable-name}
If the variable @var{variable-name} has an empty value, the
@var{text-if-true} is effective; otherwise, the @var{text-if-false},
if any, is effective.  The rules for expansion and testing of
@var{variable-name} are identical to the @code{ifdef} directive.
@end table

Extra spaces are allowed and ignored at the beginning of the
conditional directive line, but a tab is not allowed.  (If the line
begins with a tab, it will be considered part of a recipe for a rule.)
Aside from this, extra spaces or tabs may be inserted with no effect
anywhere except within the directive name or within an argument.  A
comment starting with @samp{#} may appear at the end of the line.

The other two directives that play a part in a conditional are @code{else}
and @code{endif}.  Each of these directives is written as one word, with no
arguments.  Extra spaces are allowed and ignored at the beginning of the
line, and spaces or tabs at the end.  A comment starting with @samp{#} may
appear at the end of the line.

Conditionals affect which lines of the makefile @code{make} uses.  If
the condition is true, @code{make} reads the lines of the
@var{text-if-true} as part of the makefile; if the condition is false,
@code{make} ignores those lines completely.  It follows that syntactic
units of the makefile, such as rules, may safely be split across the
beginning or the end of the conditional.

@code{make} evaluates conditionals when it reads a makefile.
Consequently, you cannot use automatic variables in the tests of
conditionals because they are not defined until recipes are run
(@pxref{Automatic Variables}).

To prevent intolerable confusion, it is not permitted to start a
conditional in one makefile and end it in another.  However, you may
write an @code{include} directive within a conditional, provided you do
not attempt to terminate the conditional inside the included file.

@node Testing Flags
@section Conditionals that Test Flags

You can write a conditional that tests @code{make} command flags such as
@samp{-t} by using the variable @code{MAKEFLAGS} together with the
@code{findstring} function
(@pxref{Text Functions, , Functions for String Substitution and Analysis}).
This is useful when @code{touch} is not enough to make a file appear up
to date.

Recall that @code{MAKEFLAGS} will put all single-letter options (such as
@samp{-t}) into the first word, and that word will be empty if no
single-letter options were given.  To work with this, it's helpful to add a
value at the start to ensure it's considered a word: for example
@samp{-$(MAKEFLAGS)}.

The @code{findstring} function determines whether one string appears as a
substring of another.  If you want to test for the @samp{-t} flag, use
@samp{t} as the first string and the first word of @code{MAKEFLAGS} as the
other.

For example, here is how to arrange to use @samp{ranlib -t} to finish
marking an archive file up to date:

@example
archive.a: @dots{}
ifneq (,$(findstring t,$(firstword -$(MAKEFLAGS))))
        +touch archive.a
        +ranlib -t archive.a
else
        ranlib archive.a
endif
@end example

@noindent
The @samp{+} prefix marks those recipe lines as ``recursive'' so that
they will be executed despite use of the @samp{-t} flag.
@xref{Recursion, ,Recursive Use of @code{make}}.

@node Functions
@chapter Functions for Transforming Text
@cindex functions

@dfn{Functions} allow you to do text processing in the makefile to
compute the files to operate on or the commands to use in recipes.
You use a function in a @dfn{function call}, where you give the name
of the function and some text (the @dfn{arguments}) for the function
to operate on.  The result of the function's processing is substituted
into the makefile at the point of the call, just as a variable might
be substituted.

@menu
* Syntax of Functions::         How to write a function call.
* Text Functions::              General-purpose text manipulation functions.
* File Name Functions::         Functions for manipulating file names.
* Conditional Functions::       Functions that implement conditions.
* Let Function::                Local variables.
* Foreach Function::            Repeat some text with controlled variation.
* File Function::               Write text to a file.
* Call Function::               Expand a user-defined function.
* Value Function::              Return the un-expanded value of a variable.
* Eval Function::               Evaluate the arguments as makefile syntax.
* Origin Function::             Find where a variable got its value.
* Flavor Function::             Find out the flavor of a variable.
* Make Control Functions::      Functions that control how make runs.
* Shell Function::              Substitute the output of a shell command.
* Guile Function::              Use GNU Guile embedded scripting language.
@end menu

@node Syntax of Functions
@section Function Call Syntax
@cindex @code{$}, in function call
@cindex dollar sign (@code{$}), in function call
@cindex arguments of functions
@cindex functions, syntax of

A function call resembles a variable reference.  It can appear
anywhere a variable reference can appear, and it is expanded using the
same rules as variable references.  A function call looks like this:

@example
$(@var{function} @var{arguments})
@end example

@noindent
or like this:

@example
$@{@var{function} @var{arguments}@}
@end example

Here @var{function} is a function name; one of a short list of names
that are part of @code{make}.  You can also essentially create your own
functions by using the @code{call} built-in function.

The @var{arguments} are the arguments of the function.  They are separated
from the function name by one or more spaces or tabs, and if there is more
than one argument, then they are separated by commas.  Such whitespace and
commas are not part of an argument's value.  The delimiters which you use to
surround the function call, whether parentheses or braces, can appear in an
argument only in matching pairs; the other kind of delimiters may appear
singly.  If the arguments themselves contain other function calls or variable
references, it is wisest to use the same kind of delimiters for all the
references; write @w{@samp{$(subst a,b,$(x))}}, not @w{@samp{$(subst
a,b,$@{x@})}}.  This is because it is clearer, and because only one type of
delimiter is matched to find the end of the reference.

Each argument is expanded before the function is invoked, unless otherwise
noted below.  The substitution is done in the order in which the arguments
appear.

@subsubheading Special Characters
@cindex special characters in function arguments
@cindex function arguments, special characters in

When using characters that are special to @code{make} as function arguments,
you may need to hide them.  GNU @code{make} doesn't support escaping
characters with backslashes or other escape sequences; however, because
arguments are split before they are expanded you can hide them by putting them
into variables.

Characters you may need to hide include:

@itemize @bullet
@item
Commas
@item
Initial whitespace in the first argument
@item
Unmatched open parenthesis or brace
@item
An open parenthesis or brace if you don't want it to start a matched pair
@end itemize

For example, you can define variables @code{comma} and @code{space} whose
values are isolated comma and space characters, then substitute these
variables where such characters are wanted, like this:

@example
@group
comma:= ,
empty:=
space:= $(empty) $(empty)
foo:= a b c
bar:= $(subst $(space),$(comma),$(foo))
# @r{bar is now `a,b,c'.}
@end group
@end example

@noindent
Here the @code{subst} function replaces each space with a comma, through
the value of @code{foo}, and substitutes the result.

@node Text Functions
@section Functions for String Substitution and Analysis
@cindex functions, for text

Here are some functions that operate on strings:

@table @code
@item $(subst @var{from},@var{to},@var{text})
@findex subst
Performs a textual replacement on the text @var{text}: each occurrence
of @var{from} is replaced by @var{to}.  The result is substituted for
the function call.  For example,

@example
$(subst ee,EE,feet on the street)
@end example

produces the value @samp{fEEt on the strEEt}.

@item $(patsubst @var{pattern},@var{replacement},@var{text})
@findex patsubst
Finds whitespace-separated words in @var{text} that match @var{pattern} and
replaces them with @var{replacement}.  Here @var{pattern} may contain a
@samp{%} which acts as a wildcard, matching any number of any characters
within a word.  If @var{replacement} also contains a @samp{%}, the @samp{%} is
replaced by the text that matched the @samp{%} in @var{pattern}.  Words that
do not match the pattern are kept without change in the output.  Only the
first @samp{%} in the @var{pattern} and @var{replacement} is treated this way;
any subsequent @samp{%} is unchanged.

If @var{pattern} does not contain a @samp{%} then the entire word must compare
equal to be a match.

@cindex @code{%}, quoting in @code{patsubst}
@cindex @code{\} (backslash), to quote @code{%}
@cindex backslash (@code{\}), to quote @code{%}
@cindex quoting @code{%}, in @code{patsubst}
@samp{%} characters in @code{patsubst} function invocations can be
quoted with preceding backslashes (@samp{\}).  Backslashes that would
otherwise quote @samp{%} characters can be quoted with more backslashes.
Backslashes that quote @samp{%} characters or other backslashes are
removed from the pattern before it is compared to file names or has a stem
substituted into it.  Backslashes that are not in danger of quoting
@samp{%} characters go unmolested.  For example, the pattern
@file{the\%weird\\%pattern\\} has @samp{the%weird\} preceding the
operative @samp{%} character, and @samp{pattern\\} following it.  The
final two backslashes are left alone because they cannot affect any
@samp{%} character.

Whitespace between words is folded into single space characters;
leading and trailing whitespace is discarded.

For example,

@example
$(patsubst %.c,%.o,x.c.c bar.c)
@end example

@noindent
produces the value @samp{x.c.o bar.o}, while

@example
$(patsubst foo.c,foo.o,foo.c foobar.c)
@end example

@noindent
produces the value @samp{foo.o foobar.c}.

Substitution references (@pxref{Substitution Refs, ,Substitution
References}) are a simpler way to get the effect of the @code{patsubst}
function.

@item $(strip @var{string})
@cindex stripping whitespace
@cindex whitespace, stripping
@cindex spaces, stripping
@findex strip
Removes leading and trailing whitespace from @var{string} and replaces
each internal sequence of one or more whitespace characters with a
single space.  Thus, @samp{$(strip a b  c )} results in @w{@samp{a b c}}.

The function @code{strip} can be very useful when used in conjunction
with conditionals.  When comparing something with the empty string
@samp{} using @code{ifeq} or @code{ifneq}, you usually want a string of
just whitespace to match the empty string (@pxref{Conditionals}).

Thus, the following may fail to have the desired results:

@example
.PHONY: all
ifneq   "$(needs_made)" ""
all: $(needs_made)
else
all:;@@echo 'Nothing to make!'
endif
@end example

@noindent
Replacing the variable reference @w{@samp{$(needs_made)}} with the
function call @w{@samp{$(strip $(needs_made))}} in the @code{ifneq}
directive would make it more robust.

@item $(findstring @var{find},@var{in})
@findex findstring
@cindex searching for strings
@cindex finding strings
@cindex strings, searching for
Searches @var{in} for an occurrence of @var{find}.  If it occurs, the
value is @var{find}; otherwise, the value is empty.  You can use this
function in a conditional to test for the presence of a specific
substring in a given string.  Thus, the two examples,

@example
$(findstring a,a b c)
$(findstring a,b c)
@end example

@noindent
produce the values @samp{a} and @samp{} (the empty string),
respectively.  @xref{Testing Flags}, for a practical application of
@code{findstring}.

@need 750
@findex filter
@cindex filtering words
@cindex words, filtering
@item $(filter @var{pattern}@dots{},@var{text})
Returns all whitespace-separated words in @var{text} that @emph{do} match
any of the @var{pattern} words, removing any words that @emph{do not}
match.  Each word in @var{pattern} is compared to every word in @var{text}
using the same algorithm as the @code{patsubst} function above.

@example
sources := foo.c bar.c baz.s ugh.h
foo: $(sources)
        cc $(filter %.c %.s,$(sources)) -o foo
@end example

@noindent
says that @file{foo} depends on @file{foo.c}, @file{bar.c},
@file{baz.s} and @file{ugh.h} but only @file{foo.c}, @file{bar.c} and
@file{baz.s} should be specified in the command to the
compiler.

@item $(filter-out @var{pattern}@dots{},@var{text})
@findex filter-out
@cindex filtering out words
@cindex words, filtering out
Returns all whitespace-separated words in @var{text} that @emph{do not} match
any of the @var{pattern} words, removing the words that @emph{do} match one or
more.  Each word in @var{pattern} is compared to every word in @var{text}
using the same algorithm as the @code{patsubst} function above.  This is the
exact opposite of the @code{filter} function.

For example, given:

@example
@group
objects = main1.o foo.o main2.o bar.o remain1.o
mains = main1.o main2.o
@end group
@end example

@noindent
the following generates a list which contains all the object files not
in @samp{mains}:

@example
$(filter-out $(mains),$(objects))
@end example

@noindent
This would expand to @samp{foo.o bar.o remain1.o}.

@need 1500
@findex sort
@cindex sorting words
@item $(sort @var{list})
Sorts the words of @var{list} in lexical order, removing duplicate
words.  The output is a list of words separated by single spaces.
Thus,

@example
$(sort foo bar lose)
@end example

@noindent
returns the value @samp{bar foo lose}.

@cindex removing duplicate words
@cindex duplicate words, removing
@cindex words, removing duplicates
Incidentally, since @code{sort} removes duplicate words, you can use
it for this purpose even if you don't care about the sort order.

@item $(word @var{n},@var{text})
@findex word
@cindex word, selecting a
@cindex selecting a word
Returns the @var{n}th word of @var{text}.  The legitimate values of
@var{n} start from 1.  If @var{n} is bigger than the number of words
in @var{text}, the value is empty.  For example,

@example
$(word 2, foo bar baz)
@end example

@noindent
returns @samp{bar}.

@item $(wordlist @var{s},@var{e},@var{text})
@findex wordlist
@cindex words, selecting lists of
@cindex selecting word lists
Returns the list of words in @var{text} starting with word @var{s} and
ending with word @var{e} (inclusive).  The legitimate values of @var{s}
start from 1; @var{e} may start from 0.  If @var{s} is bigger than the
number of words in @var{text}, the value is empty.  If @var{e} is
bigger than the number of words in @var{text}, words up to the end of
@var{text} are returned.  If @var{s} is greater than @var{e}, nothing
is returned.  For example,

@example
$(wordlist 2, 3, foo bar baz)
@end example

@noindent
returns @samp{bar baz}.

@item $(words @var{text})
@findex words
@cindex words, finding number
Returns the number of words in @var{text}.  Thus, the last word of @var{text}
is @w{@code{$(word $(words @var{text}),@var{text})}}.

@item $(firstword @var{names}@dots{})
@findex firstword
@cindex words, extracting first
The argument @var{names} is regarded as a series of names, separated
by whitespace.  The value is the first name in the series.  The rest
of the names are ignored.

For example,

@example
$(firstword foo bar)
@end example

@noindent
produces the result @samp{foo}.  Although @code{$(firstword
@var{text})} is the same as @code{$(word 1,@var{text})}, the
@code{firstword} function is retained for its simplicity.


@item $(lastword @var{names}@dots{})
@findex lastword
@cindex words, extracting last
The argument @var{names} is regarded as a series of names, separated
by whitespace.  The value is the last name in the series.

For example,

@example
$(lastword foo bar)
@end example

@noindent
produces the result @samp{bar}.  Although @code{$(lastword
@var{text})} is the same as @code{$(word $(words @var{text}),@var{text})},
the @code{lastword} function was added for its simplicity and better
performance.
@end table


Here is a realistic example of the use of @code{subst} and
@code{patsubst}.  Suppose that a makefile uses the @code{VPATH} variable
to specify a list of directories that @code{make} should search for
prerequisite files
(@pxref{General Search, , @code{VPATH} Search Path for All Prerequisites}).
This example shows how to
tell the C compiler to search for header files in the same list of
directories.

The value of @code{VPATH} is a list of directories separated by colons,
such as @samp{src:../headers}.  First, the @code{subst} function is used to
change the colons to spaces:

@example
$(subst :, ,$(VPATH))
@end example

@noindent
This produces @samp{src ../headers}.  Then @code{patsubst} is used to turn
each directory name into a @samp{-I} flag.  These can be added to the
value of the variable @code{CFLAGS}, which is passed automatically to the C
compiler, like this:

@example
override CFLAGS += $(patsubst %,-I%,$(subst :, ,$(VPATH)))
@end example

@noindent
The effect is to append the text @samp{-Isrc -I../headers} to the
previously given value of @code{CFLAGS}.  The @code{override} directive is
used so that the new value is assigned even if the previous value of
@code{CFLAGS} was specified with a command argument (@pxref{Override
Directive, , The @code{override} Directive}).

@node File Name Functions
@section Functions for File Names
@cindex functions, for file names
@cindex file name functions

Several of the built-in expansion functions relate specifically to
taking apart file names or lists of file names.

Each of the following functions performs a specific transformation on a
file name.  The argument of the function is regarded as a series of file
names, separated by whitespace.  (Leading and trailing whitespace is
ignored.)  Each file name in the series is transformed in the same way and
the results are concatenated with single spaces between them.

@table @code
@item $(dir @var{names}@dots{})
@findex dir
@cindex directory part
@cindex file name, directory part
Extracts the directory-part of each file name in @var{names}.  The
directory-part of the file name is everything up through (and
including) the last slash in it.  If the file name contains no slash,
the directory part is the string @samp{./}.  For example,

@example
$(dir src/foo.c hacks)
@end example

@noindent
produces the result @samp{src/ ./}.

@item $(notdir @var{names}@dots{})
@findex notdir
@cindex file name, nondirectory part
@cindex nondirectory part
Extracts all but the directory-part of each file name in @var{names}.
If the file name contains no slash, it is left unchanged.  Otherwise,
everything through the last slash is removed from it.

A file name that ends with a slash becomes an empty string.  This is
unfortunate, because it means that the result does not always have the
same number of whitespace-separated file names as the argument had;
but we do not see any other valid alternative.

For example,

@example
$(notdir src/foo.c hacks)
@end example

@noindent
produces the result @samp{foo.c hacks}.

@item $(suffix @var{names}@dots{})
@findex suffix
@cindex suffix, function to find
@cindex file name suffix
Extracts the suffix of each file name in @var{names}.  If the file name
contains a period, the suffix is everything starting with the last
period.  Otherwise, the suffix is the empty string.  This frequently
means that the result will be empty when @var{names} is not, and if
@var{names} contains multiple file names, the result may contain fewer
file names.

For example,

@example
$(suffix src/foo.c src-1.0/bar.c hacks)
@end example

@noindent
produces the result @samp{.c .c}.

@item $(basename @var{names}@dots{})
@findex basename
@cindex basename
@cindex file name, basename of
Extracts all but the suffix of each file name in @var{names}.  If the file
name contains a period, the basename is everything up to (and not including)
the last period.  Periods in the directory part are ignored.  If there is no
period, the basename is the entire file name.  For example,

@example
$(basename src/foo.c src-1.0/bar hacks)
@end example

@noindent
produces the result @samp{src/foo src-1.0/bar hacks}.

@c plural convention with dots (be consistent)
@item $(addsuffix @var{suffix},@var{names}@dots{})
@findex addsuffix
@cindex suffix, adding
@cindex file name suffix, adding
The argument @var{names} is regarded as a series of names, separated
by whitespace; @var{suffix} is used as a unit.  The value of
@var{suffix} is appended to the end of each individual name and the
resulting larger names are concatenated with single spaces between
them.  For example,

@example
$(addsuffix .c,foo bar)
@end example

@noindent
produces the result @samp{foo.c bar.c}.

@item $(addprefix @var{prefix},@var{names}@dots{})
@findex addprefix
@cindex prefix, adding
@cindex file name prefix, adding
The argument @var{names} is regarded as a series of names, separated
by whitespace; @var{prefix} is used as a unit.  The value of
@var{prefix} is prepended to the front of each individual name and the
resulting larger names are concatenated with single spaces between
them.  For example,

@example
$(addprefix src/,foo bar)
@end example

@noindent
produces the result @samp{src/foo src/bar}.

@item $(join @var{list1},@var{list2})
@findex join
@cindex joining lists of words
@cindex words, joining lists
Concatenates the two arguments word by word: the two first words (one
from each argument) concatenated form the first word of the result, the
two second words form the second word of the result, and so on.  So the
@var{n}th word of the result comes from the @var{n}th word of each
argument.  If one argument has more words than the other, the extra
words are copied unchanged into the result.

For example, @samp{$(join a b,.c .o)} produces @samp{a.c b.o}.

Whitespace between the words in the lists is not preserved; it is
replaced with a single space.

This function can merge the results of the @code{dir} and
@code{notdir} functions, to produce the original list of files which
was given to those two functions.

@item $(wildcard @var{pattern})
@cindex wildcard, function
The argument @var{pattern} is a file name pattern, typically containing
wildcard characters (as in shell file name patterns).  The result of
@code{wildcard} is a space-separated list of the names of existing files
that match the pattern.
@xref{Wildcards, ,Using Wildcard Characters in File Names}.

@item $(realpath @var{names}@dots{})
@findex realpath
@cindex realpath
@cindex file name, realpath of
For each file name in @var{names} return the canonical absolute name.
A canonical name does not contain any @code{.} or @code{..} components,
nor any repeated path separators (@code{/}) or symlinks.  In case of a
failure the empty string is returned.  Consult the @code{realpath(3)}
documentation for a list of possible failure causes.

@item $(abspath @var{names}@dots{})
@findex abspath
@cindex abspath
@cindex file name, abspath of
For each file name in @var{names} return an absolute name that does
not contain any @code{.} or @code{..} components, nor any repeated path
separators (@code{/}).  Note that, in contrast to the @code{realpath}
function, @code{abspath} does not resolve symlinks and does not require
the file names to refer to existing files or directories.  Use the
@code{wildcard} function to test for existence.
@end table

@node Conditional Functions
@section Functions for Conditionals
@cindex conditional expansion
There are four functions that provide conditional expansion.  A key
aspect of these functions is that not all of the arguments are
expanded initially.  Only those arguments which need to be expanded,
will be expanded.

@table @code
@item $(if @var{condition},@var{then-part}[,@var{else-part}])
@findex if
The @code{if} function provides support for conditional expansion in a
functional context (as opposed to the GNU @code{make} makefile
conditionals such as @code{ifeq} (@pxref{Conditional Syntax, ,Syntax of
Conditionals})).

The first argument, @var{condition}, first has all preceding and
trailing whitespace stripped, then is expanded.  If it expands to any
non-empty string, then the condition is considered to be true.  If it
expands to an empty string, the condition is considered to be false.

If the condition is true then the second argument, @var{then-part}, is
evaluated and this is used as the result of the evaluation of the entire
@code{if} function.

If the condition is false then the third argument, @var{else-part}, is
evaluated and this is the result of the @code{if} function.  If there is
no third argument, the @code{if} function evaluates to nothing (the
empty string).

Note that only one of the @var{then-part} or the @var{else-part} will be
evaluated, never both.  Thus, either can contain side-effects (such as
@code{shell} function calls, etc.)

@item $(or @var{condition1}[,@var{condition2}[,@var{condition3}@dots{}]])
@findex or
The @code{or} function provides a ``short-circuiting'' OR operation.
Each argument is expanded, in order.  If an argument expands to a
non-empty string the processing stops and the result of the expansion
is that string.  If, after all arguments are expanded, all of them are
false (empty), then the result of the expansion is the empty string.

@item $(and @var{condition1}[,@var{condition2}[,@var{condition3}@dots{}]])
@findex and
The @code{and} function provides a ``short-circuiting'' AND operation.
Each argument is expanded, in order.  If an argument expands to an
empty string the processing stops and the result of the expansion is
the empty string.  If all arguments expand to a non-empty string then
the result of the expansion is the expansion of the last argument.

@item $(intcmp @var{lhs},@var{rhs}[,@var{lt-part}[,@var{eq-part}[,@var{gt-part}]]])
@findex intcmp
The @code{intcmp} function provides support for numerical comparison of
integers.  This function has no counterpart among the GNU @code{make} makefile
conditionals.

The left-hand side, @var{lhs}, and right-hand side, @var{rhs}, are expanded
and parsed as integral numbers in base 10.  Expansion of the remaining
arguments is controlled by how the numerical left-hand side compares to the
numerical right-hand side.

If there are no further arguments, then the function expands to empty if the
left-hand side and right-hand side do not compare equal, or to their numerical
value if they do compare equal.

Else if the left-hand side is strictly less than the right-hand side, the
@code{intcmp} function evaluates to the expansion of the third argument,
@var{lt-part}.  If both sides compare equal, then the @code{intcmp} function
evaluates to the expansion of the fourth argument, @var{eq-part}.  If the
left-hand side is strictly greater than the right-hand side, then the
@code{intcmp} function evaluates to the expansion of the fifth argument,
@var{gt-part}.

If @var{gt-part} is missing, it defaults to @var{eq-part}.  If @var{eq-part}
is missing, it defaults to the empty string.  Thus both @samp{$(intcmp
9,7,hello)} and @samp{$(intcmp 9,7,hello,world,)} evaluate to the empty
string, while @samp{$(intcmp 9,7,hello,world)} (notice the absence of a comma
after @code{world}) evaluates to @samp{world}.

@end table

@node Let Function
@section The @code{let} Function
@findex let
@cindex variables, local

The @code{let} function provides a means to limit the scope of a
variable.  The assignment of the named variables in a @code{let}
expression is in effect only within the text provided by the
@code{let} expression, and this assignment doesn't impact that named
variable in any outer scope.

Additionally, the @code{let} function enables list unpacking by
assigning all unassigned values to the last named variable.

The syntax of the @code{let} function is:

@example
$(let @var{var} [@var{var} ...],[@var{list}],@var{text})
@end example

@noindent
The first two arguments, @var{var} and @var{list}, are expanded before
anything else is done; note that the last argument, @var{text}, is
@strong{not} expanded at the same time.  Next, each word of the
expanded value of @var{list} is bound to each of the variable names,
@var{var}, in turn, with the final variable name being bound to the
remainder of the expanded @var{list}.  In other words, the first word
of @var{list} is bound to the first variable @var{var}, the second
word to the second variable @var{var}, and so on.

If there are more variable names in @var{var} than there are words in
@var{list}, the remaining @var{var} variable names are set to the
empty string.  If there are fewer @var{var}s than words in @var{list}
then the last @var{var} is set to all remaining words in @var{list}.

The variables in @var{var} are assigned as simply-expanded variables
during the execution of @code{let}.  @xref{Flavors, ,The Two Flavors
of Variables}.

After all variables are thus bound, @var{text} is expanded to provide
the result of the @code{let} function.

For example, this macro reverses the order of the words in the list
that it is given as its first argument:

@example
reverse = $(let first rest,$1,\
            $(if $(rest),$(call reverse,$(rest)) )$(first))

all: ; @@echo $(call reverse,d c b a)
@end example

@noindent
will print @code{a b c d}.  When first called, @code{let} will expand
@var{$1} to @code{d c b a}.  It will then assign @var{first} to
@code{d} and assign @var{rest} to @code{c b a}.  It will then expand
the if-statement, where @code{$(rest)} is not empty so we recursively
invoke the @var{reverse} function with the value of @var{rest} which
is now @code{c b a}.  The recursive invocation of @code{let} assigns
@var{first} to @code{c} and @var{rest} to @code{b a}.  The recursion
continues until @code{let} is called with just a single value,
@code{a}.  Here @var{first} is @code{a} and @var{rest} is empty, so we
do not recurse but simply expand @code{$(first)} to @code{a} and
return, which adds @code{ b}, etc.

After the @var{reverse} call is complete, the @var{first} and
@var{rest} variables are no longer set.  If variables by those names
existed beforehand, they are not affected by the expansion of the
@code{reverse} macro.

@node Foreach Function
@section The @code{foreach} Function
@findex foreach
@cindex words, iterating over

The @code{foreach} function is similar to the @code{let} function, but very
different from other functions.  It causes one piece of text to be used
repeatedly, each time with a different substitution performed on it.  The
@code{foreach} function resembles the @code{for} command in the
shell @code{sh} and the @code{foreach} command in the C-shell @code{csh}.

The syntax of the @code{foreach} function is:

@example
$(foreach @var{var},@var{list},@var{text})
@end example

@noindent
The first two arguments, @var{var} and @var{list}, are expanded before
anything else is done; note that the last argument, @var{text}, is
@strong{not} expanded at the same time.  Then for each word of the expanded
value of @var{list}, the variable named by the expanded value of @var{var}
is set to that word, and @var{text} is expanded.  Presumably @var{text}
contains references to that variable, so its expansion will be different
each time.

The result is that @var{text} is expanded as many times as there are
whitespace-separated words in @var{list}.  The multiple expansions of
@var{text} are concatenated, with spaces between them, to make the result
of @code{foreach}.

This simple example sets the variable @samp{files} to the list of all files
in the directories in the list @samp{dirs}:

@example
dirs := a b c d
files := $(foreach dir,$(dirs),$(wildcard $(dir)/*))
@end example

Here @var{text} is @samp{$(wildcard $(dir)/*)}.  The first repetition
finds the value @samp{a} for @code{dir}, so it produces the same result
as @samp{$(wildcard a/*)}; the second repetition produces the result
of @samp{$(wildcard b/*)}; and the third, that of @samp{$(wildcard c/*)}.

This example has the same result (except for setting @samp{dirs}) as
the following example:

@example
files := $(wildcard a/* b/* c/* d/*)
@end example

When @var{text} is complicated, you can improve readability by giving it
a name, with an additional variable:

@example
find_files = $(wildcard $(dir)/*)
dirs := a b c d
files := $(foreach dir,$(dirs),$(find_files))
@end example

@noindent
Here we use the variable @code{find_files} this way.  We use plain @samp{=}
to define a recursively-expanding variable, so that its value contains an
actual function call to be re-expanded under the control of @code{foreach};
a simply-expanded variable would not do, since @code{wildcard} would be
called only once at the time of defining @code{find_files}.

Like the @code{let} function, the @code{foreach} function has no permanent
effect on the variable @var{var}; its value and flavor after the
@code{foreach} function call are the same as they were beforehand.  The
other values which are taken from @var{list} are in effect only
temporarily, during the execution of @code{foreach}.  The variable
@var{var} is a simply-expanded variable during the execution of
@code{foreach}.  If @var{var} was undefined before the @code{foreach}
function call, it is undefined after the call.
@xref{Flavors, ,The Two Flavors of Variables}.

You must take care when using complex variable expressions that result in
variable names because many strange things are valid variable names, but
are probably not what you intended.  For example,

@smallexample
files := $(foreach Esta-escrito-en-espanol!,b c ch,$(find_files))
@end smallexample

@noindent
might be useful if the value of @code{find_files} references the variable
whose name is @samp{Esta-escrito-en-espanol!} (es un nombre bastante largo,
no?), but it is more likely to be a mistake.

@node File Function
@section The @code{file} Function
@findex file
@cindex writing to a file
@cindex file, writing to
@cindex reading from a file
@cindex file, reading from

The @code{file} function allows the makefile to write to or read from
a file.  Two modes of writing are supported: overwrite, where the text
is written to the beginning of the file and any existing content is
lost, and append, where the text is written to the end of the file,
preserving the existing content.  In both cases the file is created if
it does not exist.  It is a fatal error if the file cannot be opened
for writing, or if the write operation fails.  The @code{file}
function expands to the empty string when writing to a file.

When reading from a file, the @code{file} function expands to the
verbatim contents of the file, except that the final newline (if there
is one) will be stripped.  Attempting to read from a non-existent file
expands to the empty string.

The syntax of the @code{file} function is:

@example
$(file @var{op} @var{filename}[,@var{text}])
@end example

When the @code{file} function is evaluated all its arguments are
expanded first, then the file indicated by @var{filename} will be
opened in the mode described by @var{op}.

The operator @var{op} can be @code{>} to indicate the file will be
overwritten with new content, @code{>>} to indicate the current
contents of the file will be appended to, or @code{<} to indicate the
contents of the file will be read in.  The @var{filename} specifies
the file to be written to or read from.  There may optionally be
whitespace between the operator and the file name.

When reading files, it is an error to provide a @var{text} value.

When writing files, @var{text} will be written to the file.  If
@var{text} does not already end in a newline a final newline will be
written (even if @var{text} is the empty string).  If the @var{text}
argument is not given at all, nothing will be written.

For example, the @code{file} function can be useful if your build
system has a limited command line size and your recipe runs a command
that can accept arguments from a file as well.  Many commands use the
convention that an argument prefixed with an @code{@@} specifies a
file containing more arguments.  Then you might write your recipe in
this way:

@example
@group
program: $(OBJECTS)
        $(file >$@@.in,$^)
        $(CMD) $(CMDFLAGS) @@$@@.in
        @@rm $@@.in
@end group
@end example

If the command required each argument to be on a separate line of the
input file, you might write your recipe like this:

@example
@group
program: $(OBJECTS)
        $(file >$@@.in) $(foreach O,$^,$(file >>$@@.in,$O))
        $(CMD) $(CMDFLAGS) @@$@@.in
        @@rm $@@.in
@end group
@end example

@node Call Function
@section The @code{call} Function
@findex call
@cindex functions, user defined
@cindex user defined functions

The @code{call} function is unique in that it can be used to create new
parameterized functions.  You can write a complex expression as the
value of a variable, then use @code{call} to expand it with different
values.

The syntax of the @code{call} function is:

@example
$(call @var{variable},@var{param},@var{param},@dots{})
@end example

When @code{make} expands this function, it assigns each @var{param} to
temporary variables @code{$(1)}, @code{$(2)}, etc.  The variable
@code{$(0)} will contain @var{variable}.  There is no maximum number of
parameter arguments.  There is no minimum, either, but it doesn't make
sense to use @code{call} with no parameters.

Then @var{variable} is expanded as a @code{make} variable in the context
of these temporary assignments.  Thus, any reference to @code{$(1)} in
the value of @var{variable} will resolve to the first @var{param} in the
invocation of @code{call}.

Note that @var{variable} is the @emph{name} of a variable, not a
@emph{reference} to that variable.  Therefore you would not normally use
a @samp{$} or parentheses when writing it.  (You can, however, use a
variable reference in the name if you want the name not to be a
constant.)

If @var{variable} is the name of a built-in function, the built-in function
is always invoked (even if a @code{make} variable by that name also
exists).

The @code{call} function expands the @var{param} arguments before
assigning them to temporary variables.  This means that @var{variable}
values containing references to built-in functions that have special
expansion rules, like @code{foreach} or @code{if}, may not work as you
expect.

Some examples may make this clearer.

This macro simply reverses its arguments:

@smallexample
reverse = $(2) $(1)

foo = $(call reverse,a,b)
@end smallexample

@noindent
Here @code{foo} will contain @samp{b a}.

This one is slightly more interesting: it defines a macro to search for
the first instance of a program in @code{PATH}:

@smallexample
pathsearch = $(firstword $(wildcard $(addsuffix /$(1),$(subst :, ,$(PATH)))))

LS := $(call pathsearch,ls)
@end smallexample

@noindent
Now the variable @code{LS} contains @code{/bin/ls} or similar.

The @code{call} function can be nested.  Each recursive invocation gets its
own local values for @code{$(1)}, etc.@: that mask the values of higher-level
@code{call} functions.  For example, here is an implementation of a @dfn{map}
function:

@smallexample
map = $(foreach a,$(2),$(call $(1),$(a)))
@end smallexample

Now you can @code{map} a function that normally takes only one argument,
such as @code{origin}, to multiple values in one step:

@smallexample
o = $(call map,origin,o map MAKE)
@end smallexample

and end up with @code{o} containing something like @samp{file file default}.

A final caution: be careful when adding whitespace to the arguments to
@code{call}.  As with other functions, any whitespace contained in the
second and subsequent arguments is kept; this can cause strange
effects.  It's generally safest to remove all extraneous whitespace when
providing parameters to @code{call}.

@node Value Function
@comment  node-name,  next,  previous,  up
@section The @code{value} Function
@findex value
@cindex variables, unexpanded value

The @code{value} function provides a way for you to use the value of a
variable @emph{without} having it expanded.  Please note that this
does not undo expansions which have already occurred; for example if
you create a simply expanded variable its value is expanded during the
definition; in that case the @code{value} function will return the
same result as using the variable directly.

The syntax of the @code{value} function is:

@example
$(value @var{variable})
@end example

Note that @var{variable} is the @emph{name} of a variable, not a
@emph{reference} to that variable.  Therefore you would not normally
use a @samp{$} or parentheses when writing it.  (You can, however, use
a variable reference in the name if you want the name not to be a
constant.)

The result of this function is a string containing the value of
@var{variable}, without any expansion occurring.  For example, in this
makefile:

@example
@group
FOO = $PATH

all:
        @@echo $(FOO)
        @@echo $(value FOO)
@end group
@end example

@noindent
The first output line would be @code{ATH}, since the ``$P'' would be
expanded as a @code{make} variable, while the second output line would
be the current value of your @code{$PATH} environment variable, since
the @code{value} function avoided the expansion.

The @code{value} function is most often used in conjunction with the
@code{eval} function (@pxref{Eval Function}).

@node Eval Function
@comment  node-name,  next,  previous,  up
@section The @code{eval} Function
@findex eval
@cindex evaluating makefile syntax
@cindex makefile syntax, evaluating

The @code{eval} function is very special: it allows you to define new
makefile constructs that are not constant; which are the result of
evaluating other variables and functions.  The argument to the
@code{eval} function is expanded, then the results of that expansion
are parsed as makefile syntax.  The expanded results can define new
@code{make} variables, targets, implicit or explicit rules, etc.

The result of the @code{eval} function is always the empty string;
thus, it can be placed virtually anywhere in a makefile without
causing syntax errors.

It's important to realize that the @code{eval} argument is expanded
@emph{twice}; first by the @code{eval} function, then the results of
that expansion are expanded again when they are parsed as makefile
syntax.  This means you may need to provide extra levels of escaping
for ``$'' characters when using @code{eval}.  The @code{value}
function (@pxref{Value Function}) can sometimes be useful in these
situations, to circumvent unwanted expansions.

Here is an example of how @code{eval} can be used; this example
combines a number of concepts and other functions.  Although it might
seem overly complex to use @code{eval} in this example, rather than
just writing out the rules, consider two things: first, the template
definition (in @code{PROGRAM_template}) could need to be much more
complex than it is here; and second, you might put the complex,
``generic'' part of this example into another makefile, then include
it in all the individual makefiles.  Now your individual makefiles are
quite straightforward.

@example
@group
PROGRAMS    = server client

server_OBJS = server.o server_priv.o server_access.o
server_LIBS = priv protocol

client_OBJS = client.o client_api.o client_mem.o
client_LIBS = protocol

# Everything after this is generic

.PHONY: all
all: $(PROGRAMS)

define PROGRAM_template =
 $(1): $$($(1)_OBJS) $$($(1)_LIBS:%=-l%)
 ALL_OBJS   += $$($(1)_OBJS)
endef

$(foreach prog,$(PROGRAMS),$(eval $(call PROGRAM_template,$(prog))))

$(PROGRAMS):
        $(LINK.o) $^ $(LDLIBS) -o $@@

clean:
        rm -f $(ALL_OBJS) $(PROGRAMS)
@end group
@end example

@node Origin Function
@section The @code{origin} Function
@findex origin
@cindex variables, origin of
@cindex origin of variable

The @code{origin} function is unlike most other functions in that it does
not operate on the values of variables; it tells you something @emph{about}
a variable.  Specifically, it tells you where it came from.

The syntax of the @code{origin} function is:

@example
$(origin @var{variable})
@end example

Note that @var{variable} is the @emph{name} of a variable to inquire about,
not a @emph{reference} to that variable.  Therefore you would not normally
use a @samp{$} or parentheses when writing it.  (You can, however, use a
variable reference in the name if you want the name not to be a constant.)

The result of this function is a string telling you how the variable
@var{variable} was defined:

@table @samp
@item undefined

if @var{variable} was never defined.

@item default

if @var{variable} has a default definition, as is usual with @code{CC}
and so on.  @xref{Implicit Variables, ,Variables Used by Implicit Rules}.
Note that if you have redefined a default variable, the @code{origin}
function will return the origin of the later definition.

@item environment

if @var{variable} was inherited from the environment provided to
@code{make}.

@item environment override

if @var{variable} was inherited from the environment provided to
@code{make}, and is overriding a setting for @var{variable} in the
makefile as a result of the @w{@samp{-e}} option (@pxref{Options
Summary, ,Summary of Options}).

@item file

if @var{variable} was defined in a makefile.

@item command line

if @var{variable} was defined on the command line.

@item override

if @var{variable} was defined with an @code{override} directive in a
makefile (@pxref{Override Directive, ,The @code{override} Directive}).

@item automatic

if @var{variable} is an automatic variable defined for the execution
of the recipe for each rule (@pxref{Automatic Variables}).
@end table

This information is primarily useful (other than for your curiosity) to
determine if you want to believe the value of a variable.  For example,
suppose you have a makefile @file{foo} that includes another makefile
@file{bar}.  You want a variable @code{bletch} to be defined in @file{bar}
if you run the command @w{@samp{make -f bar}}, even if the environment contains
a definition of @code{bletch}.  However, if @file{foo} defined
@code{bletch} before including @file{bar}, you do not want to override that
definition.  This could be done by using an @code{override} directive in
@file{foo}, giving that definition precedence over the later definition in
@file{bar}; unfortunately, the @code{override} directive would also
override any command line definitions.  So, @file{bar} could
include:

@example
@group
ifdef bletch
ifeq "$(origin bletch)" "environment"
bletch = barf, gag, etc.
endif
endif
@end group
@end example

@noindent
If @code{bletch} has been defined from the environment, this will redefine
it.

If you want to override a previous definition of @code{bletch} if it came
from the environment, even under @samp{-e}, you could instead write:

@example
@group
ifneq "$(findstring environment,$(origin bletch))" ""
bletch = barf, gag, etc.
endif
@end group
@end example

Here the redefinition takes place if @samp{$(origin bletch)} returns either
@samp{environment} or @samp{environment override}.
@xref{Text Functions, , Functions for String Substitution and Analysis}.

@node Flavor Function
@section The @code{flavor} Function
@findex flavor
@cindex variables, flavor of
@cindex flavor of variable

The @code{flavor} function, like the @code{origin} function, does not
operate on the values of variables but rather it tells you something
@emph{about} a variable.  Specifically, it tells you the flavor of a
variable (@pxref{Flavors, ,The Two Flavors of Variables}).

The syntax of the @code{flavor} function is:

@example
$(flavor @var{variable})
@end example

Note that @var{variable} is the @emph{name} of a variable to inquire about,
not a @emph{reference} to that variable.  Therefore you would not normally
use a @samp{$} or parentheses when writing it.  (You can, however, use a
variable reference in the name if you want the name not to be a constant.)

The result of this function is a string that identifies the flavor of the
variable @var{variable}:

@table @samp
@item undefined

if @var{variable} was never defined.

@item recursive

if @var{variable} is a recursively expanded variable.

@item simple

if @var{variable} is a simply expanded variable.

@end table

@node Make Control Functions
@section Functions That Control Make
@cindex functions, for controlling make
@cindex controlling make

These functions control the way make runs.  Generally, they are used to
provide information to the user of the makefile or to cause make to stop
if some sort of environmental error is detected.

@table @code
@item $(error @var{text}@dots{})
@findex error
@cindex error, stopping on
@cindex stopping make
Generates a fatal error where the message is @var{text}.  Note that
the error is generated whenever this function is evaluated.  So, if
you put it inside a recipe or on the right side of a recursive
variable assignment, it won't be evaluated until later.  The
@var{text} will be expanded before the error is generated.

For example,

@example
ifdef ERROR1
$(error error is $(ERROR1))
endif
@end example

@noindent
will generate a fatal error during the read of the makefile if the
@code{make} variable @code{ERROR1} is defined.  Or,

@example
ERR = $(error found an error!)

.PHONY: err
err: ; $(ERR)
@end example

@noindent
will generate a fatal error while @code{make} is running, if the
@code{err} target is invoked.

@item $(warning @var{text}@dots{})
@findex warning
@cindex warnings, printing
@cindex printing user warnings
This function works similarly to the @code{error} function, above,
except that @code{make} doesn't exit.  Instead, @var{text} is expanded
and the resulting message is displayed, but processing of the makefile
continues.

The result of the expansion of this function is the empty string.

@item $(info @var{text}@dots{})
@findex info
@cindex printing messages
This function does nothing more than print its (expanded) argument(s)
to standard output.  No makefile name or line number is added.  The
result of the expansion of this function is the empty string.
@end table

@node Shell Function
@section The @code{shell} Function
@findex shell
@cindex command expansion
@cindex backquotes
@cindex shell command, function for

The @code{shell} function is unlike any other function other than the
@code{wildcard} function
(@pxref{Wildcard Function, ,The Function @code{wildcard}}) in that it
communicates with the world outside of @code{make}.

The @code{shell} function provides for @code{make} the same facility that
backquotes (@samp{`}) provide in most shells: it does @dfn{command expansion}.
This means that it takes as an argument a shell command and expands to the
output of the command.  The only processing @code{make} does on the result is
to convert each newline (or carriage-return / newline pair) to a single space.
If there is a trailing (carriage-return and) newline it will simply be
removed.

The commands run by calls to the @code{shell} function are run when the
function calls are expanded (@pxref{Reading Makefiles, , How @code{make} Reads
a Makefile}).  Because this function involves spawning a new shell, you should
carefully consider the performance implications of using the @code{shell}
function within recursively expanded variables vs.@: simply expanded variables
(@pxref{Flavors, ,The Two Flavors of Variables}).

An alternative to the @code{shell} function is the @samp{!=} assignment
operator; it provides a similar behavior but has subtle differences
(@pxref{Setting, , Setting Variables}).  The @samp{!=} assignment operator is
included in newer POSIX standards.

@vindex .SHELLSTATUS
After the @code{shell} function or @samp{!=} assignment operator is
used, its exit status is placed in the @code{.SHELLSTATUS} variable.

Here are some examples of the use of the @code{shell} function:

@example
contents := $(shell cat foo)
@end example

@noindent
sets @code{contents} to the contents of the file @file{foo}, with a space
(rather than a newline) separating each line.

@example
files := $(shell echo *.c)
@end example

@noindent
sets @code{files} to the expansion of @samp{*.c}.  Unless @code{make} is
using a very strange shell, this has the same result as
@w{@samp{$(wildcard *.c)}} (as long as at least one @samp{.c} file
exists).

All variables that are marked as @code{export} will also be passed to the
shell started by the @code{shell} function.  It is possible to create a
variable expansion loop: consider this @file{makefile}:

@example
export HI = $(shell echo hi)
all: ; @@echo $$HI
@end example

When @code{make} wants to run the recipe it must add the variable @var{HI} to
the environment; to do so it must be expanded.  The value of this variable
requires an invocation of the @code{shell} function, and to invoke it we must
create its environment.  Since @var{HI} is exported, we need to expand it to
create its environment.  And so on.  In this obscure case @code{make} will use
the value of the variable from the environment provided to @code{make}, or
else the empty string if there was none, rather than looping or issuing an
error.  This is often what you want; for example:

@example
export PATH = $(shell echo /usr/local/bin:$$PATH)
@end example

However, it would be simpler and more efficient to use a simply-expanded
variable here (@samp{:=}) in the first place.

@node Guile Function
@section The @code{guile} Function
@findex guile
@cindex Guile

If GNU @code{make} is built with support for GNU Guile as an embedded
extension language then the @code{guile} function will be available.
The @code{guile} function takes one argument which is first expanded
by @code{make} in the normal fashion, then passed to the GNU Guile
evaluator.  The result of the evaluator is converted into a string and
used as the expansion of the @code{guile} function in the makefile.
See @ref{Guile Integration, ,GNU Guile Integration} for details on
writing extensions to @code{make} in Guile.

You can determine whether GNU Guile support is available by checking
the @code{.FEATURES} variable for the word @var{guile}.

@node Running
@chapter How to Run @code{make}

A makefile that says how to recompile a program can be used in more
than one way.  The simplest use is to recompile every file that is out
of date.  Usually, makefiles are written so that if you run
@code{make} with no arguments, it does just that.

But you might want to update only some of the files; you might want to use
a different compiler or different compiler options; you might want just to
find out which files are out of date without changing them.

By giving arguments when you run @code{make}, you can do any of these
things and many others.

@cindex exit status of make
The exit status of @code{make} is always one of three values:
@table @code
@item 0
The exit status is zero if @code{make} is successful.
@item 2
The exit status is two if @code{make} encounters any errors.
It will print messages describing the particular errors.
@item 1
The exit status is one if you use the @samp{-q} flag and @code{make}
determines that some target is not already up to date.
@xref{Instead of Execution, ,Instead of Executing Recipes}.
@end table

@menu
* Makefile Arguments::          How to specify which makefile to use.
* Goals::                       How to use goal arguments to specify which
                                  parts of the makefile to use.
* Instead of Execution::        How to use mode flags to specify what
                                  kind of thing to do with the recipes
                                  in the makefile other than simply
                                  execute them.
* Avoiding Compilation::        How to avoid recompiling certain files.
* Overriding::                  How to override a variable to specify
                                  an alternate compiler and other things.
* Testing::                     How to proceed past some errors, to
                                  test compilation.
* Warnings::                    How to control reporting of makefile issues.
* Temporary Files::             Where @code{make} keeps its temporary files.
* Options Summary::             Summary of Options
@end menu

@node Makefile Arguments
@section Arguments to Specify the Makefile
@cindex @code{--file}
@cindex @code{--makefile}
@cindex @code{-f}

The way to specify the name of the makefile is with the @samp{-f} or
@samp{--file} option (@samp{--makefile} also works).  For example,
@samp{-f altmake} says to use the file @file{altmake} as the makefile.

If you use the @samp{-f} flag several times and follow each @samp{-f}
with an argument, all the specified files are used jointly as
makefiles.

If you do not use the @samp{-f} or @samp{--file} flag, the default is
to try @file{GNUmakefile}, @file{makefile}, and @file{Makefile}, in
that order, and use the first of these three which exists or can be made
(@pxref{Makefiles, ,Writing Makefiles}).

@node Goals
@section Arguments to Specify the Goals
@cindex goal, how to specify

The @dfn{goals} are the targets that @code{make} should strive ultimately
to update.  Other targets are updated as well if they appear as
prerequisites of goals, or prerequisites of prerequisites of goals, etc.

By default, the goal is the first target in the makefile (not counting
targets that start with a period).  Therefore, makefiles are usually
written so that the first target is for compiling the entire program or
programs they describe.  If the first rule in the makefile has several
targets, only the first target in the rule becomes the default goal, not
the whole list.  You can manage the selection of the default goal from
within your makefile using the @code{.DEFAULT_GOAL} variable
(@pxref{Special Variables, , Other Special Variables}).

You can also specify a different goal or goals with command line
arguments to @code{make}.  Use the name of the goal as an argument.
If you specify several goals, @code{make} processes each of them in
turn, in the order you name them.

Any target in the makefile may be specified as a goal (unless it
starts with @samp{-} or contains an @samp{=}, in which case it will be
parsed as a switch or variable definition, respectively).  Even
targets not in the makefile may be specified, if @code{make} can find
implicit rules that say how to make them.

@vindex MAKECMDGOALS
@code{Make} will set the special variable @code{MAKECMDGOALS} to the
list of goals you specified on the command line.  If no goals were given
on the command line, this variable is empty.  Note that this variable
should be used only in special circumstances.

An example of appropriate use is to avoid including @file{.d} files
during @code{clean} rules (@pxref{Automatic Prerequisites}), so
@code{make} won't create them only to immediately remove them
again:

@example
@group
sources = foo.c bar.c

ifeq (,$(filter clean,$(MAKECMDGOALS)))
include $(sources:.c=.d)
endif
@end group
@end example

One use of specifying a goal is if you want to compile only a part of
the program, or only one of several programs.  Specify as a goal each
file that you wish to remake.  For example, consider a directory containing
several programs, with a makefile that starts like this:

@example
.PHONY: all
all: size nm ld ar as
@end example

If you are working on the program @code{size}, you might want to say
@w{@samp{make size}} so that only the files of that program are recompiled.

Another use of specifying a goal is to make files that are not normally
made.  For example, there may be a file of debugging output, or a
version of the program that is compiled specially for testing, which has
a rule in the makefile but is not a prerequisite of the default goal.

Another use of specifying a goal is to run the recipe associated with
a phony target (@pxref{Phony Targets}) or empty target (@pxref{Empty
Targets, ,Empty Target Files to Record Events}).  Many makefiles contain
a phony target named @file{clean} which deletes everything except source
files.  Naturally, this is done only if you request it explicitly with
@w{@samp{make clean}}.  Following is a list of typical phony and empty
target names.  @xref{Standard Targets}, for a detailed list of all the
standard target names which GNU software packages use.

@table @file
@item all
@cindex @code{all} @r{(standard target)}
Make all the top-level targets the makefile knows about.

@item clean
@cindex @code{clean} @r{(standard target)}
Delete all files that are normally created by running @code{make}.

@item mostlyclean
@cindex @code{mostlyclean} @r{(standard target)}
Like @samp{clean}, but may refrain from deleting a few files that people
normally don't want to recompile.  For example, the @samp{mostlyclean}
target for GCC does not delete @file{libgcc.a}, because recompiling it
is rarely necessary and takes a lot of time.

@item distclean
@cindex @code{distclean} @r{(standard target)}
@itemx realclean
@cindex @code{realclean} @r{(standard target)}
@itemx clobber
@cindex @code{clobber} @r{(standard target)}
Any of these targets might be defined to delete @emph{more} files than
@samp{clean} does.  For example, this would delete configuration files
or links that you would normally create as preparation for compilation,
even if the makefile itself cannot create these files.

@item install
@cindex @code{install} @r{(standard target)}
Copy the executable file into a directory that users typically search
for commands; copy any auxiliary files that the executable uses into
the directories where it will look for them.

@item print
@cindex @code{print} @r{(standard target)}
Print listings of the source files that have changed.

@item tar
@cindex @code{tar} @r{(standard target)}
Create a tar file of the source files.

@item shar
@cindex @code{shar} @r{(standard target)}
Create a shell archive (shar file) of the source files.

@item dist
@cindex @code{dist} @r{(standard target)}
Create a distribution file of the source files.  This might
be a tar file, or a shar file, or a compressed version of one of the
above, or even more than one of the above.

@item TAGS
@cindex @code{TAGS} @r{(standard target)}
Update a tags table for this program.

@item check
@cindex @code{check} @r{(standard target)}
@itemx test
@cindex @code{test} @r{(standard target)}
Perform self tests on the program this makefile builds.
@end table

@node Instead of Execution
@section Instead of Executing Recipes
@cindex execution, instead of
@cindex recipes, instead of executing

The makefile tells @code{make} how to tell whether a target is up to date,
and how to update each target.  But updating the targets is not always
what you want.  Certain options specify other activities for @code{make}.

@comment Extra blank lines make it print better.
@table @samp
@item -n
@itemx --just-print
@itemx --dry-run
@itemx --recon
@cindex @code{--just-print}
@cindex @code{--dry-run}
@cindex @code{--recon}
@cindex @code{-n}

``No-op''.  Causes @code{make} to print the recipes that are needed to
make the targets up to date, but not actually execute them.  Note that
some recipes are still executed, even with this flag (@pxref{MAKE
Variable, ,How the @code{MAKE} Variable Works}).  Also any recipes
needed to update included makefiles are still executed
(@pxref{Remaking Makefiles, ,How Makefiles Are Remade}).

@item -t
@itemx --touch
@cindex @code{--touch}
@cindex touching files
@cindex target, touching
@cindex @code{-t}

``Touch''.  Marks targets as up to date without actually changing
them.  In other words, @code{make} pretends to update the targets but
does not really change their contents; instead only their modification
times are updated.

@item -q
@itemx --question
@cindex @code{--question}
@cindex @code{-q}
@cindex question mode

``Question''.  Silently check whether the targets are up to date, but
do not execute recipes; the exit code shows whether any updates are
needed.

@item -W @var{file}
@itemx --what-if=@var{file}
@itemx --assume-new=@var{file}
@itemx --new-file=@var{file}
@cindex @code{--what-if}
@cindex @code{-W}
@cindex @code{--assume-new}
@cindex @code{--new-file}
@cindex what if
@cindex files, assuming new

``What if''.  Each @samp{-W} flag is followed by a file name.  The given
files' modification times are recorded by @code{make} as being the present
time, although the actual modification times remain the same.
You can use the @samp{-W} flag in conjunction with the @samp{-n} flag
to see what would happen if you were to modify specific files.
@end table

With the @samp{-n} flag, @code{make} prints the recipe that it would
normally execute but usually does not execute it.

With the @samp{-t} flag, @code{make} ignores the recipes in the rules
and uses (in effect) the command @code{touch} for each target that needs to
be remade.  The @code{touch} command is also printed, unless @samp{-s} or
@code{.SILENT} is used.  For speed, @code{make} does not actually invoke
the program @code{touch}.  It does the work directly.

With the @samp{-q} flag, @code{make} prints nothing and executes no
recipes, but the exit status code it returns is zero if and only if the
targets to be considered are already up to date.  If the exit status is
one, then some updating needs to be done.  If @code{make} encounters an
error, the exit status is two, so you can distinguish an error from a
target that is not up to date.

It is an error to use more than one of these three flags in the same
invocation of @code{make}.

@cindex +, and recipe execution
The @samp{-n}, @samp{-t}, and @samp{-q} options do not affect recipe
lines that begin with @samp{+} characters or contain the strings
@samp{$(MAKE)} or @samp{$@{MAKE@}}.  Note that only the line containing
the @samp{+} character or the strings @samp{$(MAKE)} or @samp{$@{MAKE@}}
is run regardless of these options.  Other lines in the same rule are
not run unless they too begin with @samp{+} or contain @samp{$(MAKE)} or
@samp{$@{MAKE@}} (@xref{MAKE Variable, ,How the @code{MAKE} Variable Works}.)

@cindex phony targets and recipe execution
The @samp{-t} flag prevents phony targets (@pxref{Phony Targets}) from
being updated, unless there are recipe lines beginning with @samp{+}
or containing @samp{$(MAKE)} or @samp{$@{MAKE@}}.

The @samp{-W} flag provides two features:

@itemize @bullet
@item
If you also use the @samp{-n} or @samp{-q} flag, you can see what
@code{make} would do if you were to modify some files.

@item
Without the @samp{-n} or @samp{-q} flag, when @code{make} is actually
executing recipes, the @samp{-W} flag can direct @code{make} to act as
if some files had been modified, without actually running the recipes
for those files.
@end itemize

Note that the options @samp{-p} and @samp{-v} allow you to obtain other
information about @code{make} or about the makefiles in use
(@pxref{Options Summary, ,Summary of Options}).

@node Avoiding Compilation
@section Avoiding Recompilation of Some Files
@cindex @code{-o}
@cindex @code{--old-file}
@cindex @code{--assume-old}
@cindex files, assuming old
@cindex files, avoiding recompilation of
@cindex recompilation, avoiding

Sometimes you may have changed a source file but you do not want to
recompile all the files that depend on it.  For example, suppose you add
a macro or a declaration to a header file that many other files depend
on.  Being conservative, @code{make} assumes that any change in the
header file requires recompilation of all dependent files, but you know
that they do not need to be recompiled and you would rather not waste
the time waiting for them to compile.

If you anticipate the problem before changing the header file, you can
use the @samp{-t} flag.  This flag tells @code{make} not to run the
recipes in the rules, but rather to mark the target up to date by
changing its last-modification date.  You would follow this procedure:

@enumerate
@item
Use the command @samp{make} to recompile the source files that really
need recompilation, ensuring that the object files are up-to-date
before you begin.

@item
Make the changes in the header files.

@item
Use the command @samp{make -t} to mark all the object files as
up to date.  The next time you run @code{make}, the changes in the
header files will not cause any recompilation.
@end enumerate

If you have already changed the header file at a time when some files
do need recompilation, it is too late to do this.  Instead, you can
use the @w{@samp{-o @var{file}}} flag, which marks a specified file as
``old'' (@pxref{Options Summary, ,Summary of Options}).  This means
that the file itself will not be remade, and nothing else will be
remade on its account.  Follow this procedure:

@enumerate
@item
Recompile the source files that need compilation for reasons independent
of the particular header file, with @samp{make -o @var{headerfile}}.
If several header files are involved, use a separate @samp{-o} option
for each header file.

@item
Touch all the object files with @samp{make -t}.
@end enumerate

@node Overriding
@section Overriding Variables
@cindex overriding variables with arguments
@cindex variables, overriding with arguments
@cindex command line variables
@cindex variables, command line

An argument that contains @samp{=} specifies the value of a variable:
@samp{@var{v}=@var{x}} sets the value of the variable @var{v} to @var{x}.
If you specify a value in this way, all ordinary assignments of the same
variable in the makefile are ignored; we say they have been
@dfn{overridden} by the command line argument.

The most common way to use this facility is to pass extra flags to
compilers.  For example, in a properly written makefile, the variable
@code{CFLAGS} is included in each recipe that runs the C compiler, so a
file @file{foo.c} would be compiled something like this:

@example
cc -c $(CFLAGS) foo.c
@end example

Thus, whatever value you set for @code{CFLAGS} affects each compilation
that occurs.  The makefile probably specifies the usual value for
@code{CFLAGS}, like this:

@example
CFLAGS=-g
@end example

Each time you run @code{make}, you can override this value if you
wish.  For example, if you say @samp{make CFLAGS='-g -O'}, each C
compilation will be done with @samp{cc -c -g -O}.  (This also
illustrates how you can use quoting in the shell to enclose spaces and
other special characters in the value of a variable when you override
it.)

The variable @code{CFLAGS} is only one of many standard variables that
exist just so that you can change them this way.  @xref{Implicit
Variables, , Variables Used by Implicit Rules}, for a complete list.

You can also program the makefile to look at additional variables of your
own, giving the user the ability to control other aspects of how the
makefile works by changing the variables.

When you override a variable with a command line argument, you can
define either a recursively-expanded variable or a simply-expanded
variable.  The examples shown above make a recursively-expanded
variable; to make a simply-expanded variable, write @samp{:=} or
@samp{::=} instead of @samp{=}.  But, unless you want to include a
variable reference or function call in the @emph{value} that you
specify, it makes no difference which kind of variable you create.

There is one way that the makefile can change a variable that you have
overridden.  This is to use the @code{override} directive, which is a line
that looks like this: @samp{override @var{variable} = @var{value}}
(@pxref{Override Directive, ,The @code{override} Directive}).

@node Testing
@section Testing the Compilation of a Program
@cindex testing compilation
@cindex compilation, testing

Normally, when an error happens in executing a shell command, @code{make}
gives up immediately, returning a nonzero status.  No further recipes are
executed for any target.  The error implies that the goal cannot be
correctly remade, and @code{make} reports this as soon as it knows.

When you are compiling a program that you have just changed, this is not
what you want.  Instead, you would rather that @code{make} try compiling
every file that can be tried, to show you as many compilation errors
as possible.

@cindex @code{-k}
@cindex @code{--keep-going}
On these occasions, you should use the @samp{-k} or
@samp{--keep-going} flag.  This tells @code{make} to continue to
consider the other prerequisites of the pending targets, remaking them
if necessary, before it gives up and returns nonzero status.  For
example, after an error in compiling one object file, @samp{make -k}
will continue compiling other object files even though it already
knows that linking them will be impossible.  In addition to continuing
after failed shell commands, @samp{make -k} will continue as much as
possible after discovering that it does not know how to make a target
or prerequisite file.  This will always cause an error message, but
without @samp{-k}, it is a fatal error (@pxref{Options Summary,
,Summary of Options}).

The usual behavior of @code{make} assumes that your purpose is to get the
goals up to date; once @code{make} learns that this is impossible, it might as
well report the failure immediately.  The @samp{-k} flag allows testing as
many of the changes made in the program as possible, perhaps to find several
independent problems so that you can correct them all before the next attempt
to compile.  This is why Emacs' @kbd{M-x compile} command passes the @samp{-k}
flag by default.

@node Warnings
@section Makefile Warnings
@cindex warnings
@cindex enabling warnings

GNU Make can detect some types of incorrect usage in makefiles.  When one of
these incorrect usages is detected, GNU Make can perform one of these actions:

@table @samp
@item ignore
@cindex warning action ignore
@cindex ignore, warning action
Ignore the usage.

@item warn
@cindex warning action warn
@cindex warn, warning action
Show a warning about the usage and continue processing the makefile.

@item error
@cindex warning action error
@cindex error, warning action
Show an error for the usage and immediately stop processing the makefile.
@end table

@noindent
The types of warnings GNU Make can detect are:

@table @samp
@item circular-dep
@findex circular-dep
@cindex warning circular dependency
Finding a loop in the dependency graph (the prerequisites of a target contain
or depend on the target itself).  If the action is not @samp{error}, the
circular reference is dropped from the graph before continuing.  The default
action is @samp{warn}.

@item invalid-ref
@findex invalid-ref
@cindex warning invalid reference
Using an invalid variable name in a variable reference.  The default action is
@samp{warn}.

@item invalid-var
@findex invalid-var
@cindex warning invalid variable
Assigning to an invalid variable name (e.g., a name containing whitespace).
The default action is @samp{warn}.

@item undefined-var
@findex undefined-var
@cindex warning undefined variable
Referencing a variable that has not been defined.  The default action is
@samp{ignore}.  Note the deprecated @code{--warn-undefined-variables} option
sets the action for this warning to @samp{warn}.
@end table

The actions for these warnings can be changed by specifying warning control
options.  Each warning control option consists of either a warning type, or a
warning action, or a warning type and warning action separated by a colon
(@code{:}).  Multiple control options are separated by either whitespace or
commas.

If the control option is just a warning type, then the action associated with
that type is set to @code{warn}.  If the option is just an action, then that
action is applied to all warning types (a ``global action'').

``Global actions'' take precedence over default actions.  Actions associated
with a specific warning type take precedence over ``global actions'' and
default actions.

If multiple control options provide actions for the same warning type, the
last action specified will be used.

There are two ways to specify control options: using the @code{--warn} command
line option, or using the @code{.WARNINGS} variable.

@subsubheading The @code{.WARNINGS} variable
@vindex .WARNINGS
Warning control options provided in the @code{.WARNINGS} variable take effect
as soon as the variable assignment is parsed and will last until this instance
of @code{make} finishes parsing all makefiles.  These settings will not be
passed to recursive invocations of @code{make}.

Note that the value of this variable is expanded immediately, even if the
recursive expansion assignment operator (@code{=}) is used.

Each assignment of @code{.WARNINGS} completely replaces any previous settings.
If you want to preserve the previous settings, use the @code{+=} assignment
operator.

Currently, assigning @code{.WARNINGS} as a target-specific or pattern-specific
variable has no effect.  This may change in the future.

@subsubheading The @code{--warn} option
@cindex @code{--warn}
The @code{--warn} option can be specified on the command line, or by adding it
to the @code{MAKEFLAGS} variable (@pxref{Recursion, ,Recursive Use of
@code{make}}).  Settings added to @code{MAKEFLAGS} take affect after the
assignment is parsed.  This option is passed to sub-makes through the
@code{MAKEFLAGS} variable.

The @code{--warn} option can be provided multiple times: the effects are
cumulative with later options overriding over earlier options.  When GNU Make
provides warning settings to sub-makes, they are all combined into a single
@code{--warn} option in @code{MAKEFLAGS} with a standard order.

Specifying @code{--warn} with no arguments is equivalent to using
@code{--warn=warn}, which sets the action for all warning types to
@samp{warn}.

Any action specified with an @code{--warn} option will take precedence over
actions provided in the makefile with @code{.WARNINGS}.  This means if you use
@code{--warn=error}, for example, all warnings will be treated as errors
regardless of any @code{.WARNINGS} assignments.

@node Temporary Files
@section Temporary Files
@cindex temporary files

In some situations, @code{make} will need to create its own temporary files.
These files must not be disturbed while @code{make} is running, including all
recursively-invoked instances of @code{make}.  All temporary filenames created
by GNU Make will start with the letters @samp{Gm}.

@cindex @code{MAKE_TMPDIR}
If the environment variable @code{MAKE_TMPDIR} is set then all temporary files
created by @code{make} will be placed there.

@cindex @code{TMPDIR}
@cindex @code{TMP}
@cindex @code{TEMP}
If @code{MAKE_TMPDIR} is not set, then the standard location for temporary
files for the current operating system will be used.  For POSIX systems this
will be the location set in the @code{TMPDIR} environment variable, or else
the system's default location (e.g., @file{/tmp}) is used.  On Windows,
first @code{TMP} then @code{TEMP} will be checked, then @code{TMPDIR}, and
finally the system default temporary file location will be used.

Note that this directory must already exist or @code{make} will fail:
@code{make} will not attempt to create it.

These variables @emph{cannot} be set from within a makefile: GNU @code{make}
must have access to this location before it begins reading the makefiles.

@node Options Summary
@section Summary of Options
@cindex options
@cindex flags
@cindex switches

Here is a table of all the options @code{make} understands:

@table @samp
@item --
@cindex @code{--}
This signals the end of options.  Any subsequent command line arguments are
considered to be @code{make} targets or variable assignments, even if they
look like options.

@item -
@cindex @code{-}
GNU Make treats a @code{-} option in a special way: if there is a target
@code{-} defined in the makefile then that target will be built.  If there is
no target @code{-} then this option is ignored.  This historical behavior
supports the (deprecated) sub-@code{make} invocation style @samp{$(MAKE)
-$(MAKEFLAGS)} (do not use this).

@item -b
@cindex @code{-b}
@itemx -m
@cindex @code{-m}
These options are ignored for compatibility with other versions of @code{make}.

@item -B
@cindex @code{-B}
@itemx --always-make
@cindex @code{--always-make}
Consider all targets out-of-date.  GNU @code{make} proceeds to
consider targets and their prerequisites using the normal algorithms;
however, all targets so considered are always remade regardless of the
status of their prerequisites.  To avoid infinite recursion, if
@code{MAKE_RESTARTS} (@pxref{Special Variables, , Other Special
Variables}) is set to a number greater than 0 this option is disabled
when considering whether to remake makefiles (@pxref{Remaking
Makefiles, , How Makefiles Are Remade}).

@item -C @var{dir}
@cindex @code{-C}
@itemx --directory=@var{dir}
@cindex @code{--directory}
Change to directory @var{dir} before reading the makefiles.  If multiple
@samp{-C} options are specified, each is interpreted relative to the
previous one: @samp{-C / -C etc} is equivalent to @samp{-C /etc}.
This is typically used with recursive invocations of @code{make}
(@pxref{Recursion, ,Recursive Use of @code{make}}).

@item -d
@cindex @code{-d}
@c Extra blank line here makes the table look better.

Print debugging information in addition to normal processing.  The
debugging information says which files are being considered for
remaking, which file-times are being compared and with what results,
which files actually need to be remade, which implicit rules are
considered and which are applied---everything interesting about how
@code{make} decides what to do.  The @code{-d} option is equivalent to
@samp{--debug=a} (see below).

@item --debug[=@var{options}]
@cindex @code{--debug}
@c Extra blank line here makes the table look better.

Print debugging information in addition to normal processing.  Various
levels and types of output can be chosen.  With no arguments, print the
``basic'' level of debugging.  Possible arguments are below; only the
first character is considered, and values must be comma- or
space-separated.

@table @code
@item a (@i{all})
All types of debugging output are enabled.  This is equivalent to using
@samp{-d}.

@item b (@i{basic})
Basic debugging prints each target that was found to be out-of-date, and
whether the build was successful or not.

@item v (@i{verbose})
A level above @samp{basic}; includes messages about which makefiles were
parsed, prerequisites that did not need to be rebuilt, etc.  This option
also enables @samp{basic} messages.

@item i (@i{implicit})
Prints messages describing the implicit rule searches for each target.
This option also enables @samp{basic} messages.

@item j (@i{jobs})
Prints messages giving details on the invocation of specific sub-commands.

@item m (@i{makefile})
By default, debug messages are not enabled while trying to remake the
makefiles.  This option enables messages while rebuilding makefiles, too.  The
@samp{all} option enables this option as well.  This option also enables
@samp{basic} messages.

@item p (@i{print})
Prints the recipe to be executed, even when the recipe is normally
silent (due to @code{.SILENT} or @samp{@@}).

@item w (@i{why})
Explains why each target must be remade by showing which prerequisites
are more up to date than the target.

@item n (@i{none})
Disable all debugging currently enabled.  If additional debugging
flags are encountered after this they will still take effect.
@end table

@item -e
@cindex @code{-e}
@itemx --environment-overrides
@cindex @code{--environment-overrides}
Give variables taken from the environment precedence
over variables from makefiles.
@xref{Environment, ,Variables from the Environment}.

@item -E @var{string}
@cindex @code{-E}
@item --eval=@var{string}
@cindex @code{--eval}
@c Extra blank line here makes the table look better.

Evaluate @var{string} as makefile syntax.  This is a command-line
version of the @code{eval} function (@pxref{Eval Function}).  The
evaluation is performed after the default rules and variables have
been defined, but before any makefiles are read.

@item -f @var{file}
@cindex @code{-f}
@itemx --file=@var{file}
@cindex @code{--file}
@itemx --makefile=@var{file}
@cindex @code{--makefile}
Read the file named @var{file} as a makefile.
@xref{Makefiles, ,Writing Makefiles}.

@item -h
@cindex @code{-h}
@itemx --help
@cindex @code{--help}
@c Extra blank line here makes the table look better.

Remind you of the options that @code{make} understands and then exit.

@item -i
@cindex @code{-i}
@itemx --ignore-errors
@cindex @code{--ignore-errors}
Ignore all errors in recipes executed to remake files.
@xref{Errors, ,Errors in Recipes}.

@item -I @var{dir}
@cindex @code{-I}
@itemx --include-dir=@var{dir}
@cindex @code{--include-dir}
Specifies a directory @var{dir} to search for included makefiles.
@xref{Include, ,Including Other Makefiles}.  If several @samp{-I}
options are used to specify several directories, the directories are
searched in the order specified.  If the directory @var{dir} is a
single dash (@code{-}) then any already-specified directories up to
that point (including the default directory paths) will be discarded.
You can examine the current list of directories to be searched via the
@code{.INCLUDE_DIRS} variable.

@item -j [@var{jobs}]
@cindex @code{-j}
@itemx --jobs[=@var{jobs}]
@cindex @code{--jobs}
Specifies the number of recipes (jobs) to run simultaneously.  With no
argument, @code{make} runs as many recipes simultaneously as are allowed by
the dependency graph.  If there is more than one @samp{-j} option, the last
one is effective.  @xref{Parallel, ,Parallel Execution}, for more information
on how recipes are run.  Note that this option is ignored on MS-DOS.

@item --jobserver-style=[@var{style}]
@cindex @code{--jobserver-style}
Chooses the style of jobserver to use.  This option only has effect if
parallel builds are enabled (@pxref{Parallel, ,Parallel Execution}).  On POSIX
systems @var{style} can be one of @code{fifo} (the default) or @code{pipe}.
On Windows the only acceptable @var{style} is @code{sem} (the default).  This
option is useful if you need to use an older version of GNU @code{make}, or a
different tool that requires a specific jobserver style.

@item -k
@cindex @code{-k}
@itemx --keep-going
@cindex @code{--keep-going}
Continue as much as possible after an error.  While the target that
failed, and those that depend on it, cannot be remade, the other
prerequisites of these targets can be processed all the same.
@xref{Testing, ,Testing the Compilation of a Program}.

@item -l [@var{load}]
@cindex @code{-l}
@itemx --load-average[=@var{load}]
@cindex @code{--load-average}
@itemx --max-load[=@var{load}]
@cindex @code{--max-load}
Specifies that no new recipes should be started if there are other
recipes running and the load average is at least @var{load} (a
floating-point number).  With no argument, removes a previous load
limit.  @xref{Parallel, ,Parallel Execution}.

@item -L
@cindex @code{-L}
@itemx --check-symlink-times
@cindex @code{--check-symlink-times}
On systems that support symbolic links, this option causes @code{make}
to consider the timestamps on any symbolic links in addition to the
timestamp on the file referenced by those links.  When this option is
provided, the most recent timestamp among the file and the symbolic
links is taken as the modification time for this target file.

@item -n
@cindex @code{-n}
@itemx --just-print
@cindex @code{--just-print}
@itemx --dry-run
@cindex @code{--dry-run}
@itemx --recon
@cindex @code{--recon}
@c Extra blank line here makes the table look better.

Print the recipe that would be executed, but do not execute it (except
in certain circumstances).
@xref{Instead of Execution, ,Instead of Executing Recipes}.

@item -o @var{file}
@cindex @code{-o}
@itemx --old-file=@var{file}
@cindex @code{--old-file}
@itemx --assume-old=@var{file}
@cindex @code{--assume-old}
Do not remake the file @var{file} even if it is older than its
prerequisites, and do not remake anything on account of changes in
@var{file}.  Essentially the file is treated as very old and its rules
are ignored.  @xref{Avoiding Compilation, ,Avoiding Recompilation of
Some Files}.

@item -O[@var{type}]
@cindex @code{-O}
@itemx --output-sync[=@var{type}]
@cindex @code{--output-sync}
@cindex output during parallel execution
@cindex parallel execution, output during
Ensure that the complete output from each recipe is printed in one
uninterrupted sequence.  This option is only useful when using the
@code{--jobs} option to run multiple recipes simultaneously
(@pxref{Parallel, ,Parallel Execution})  Without this option output
will be displayed as it is generated by the recipes.

With no type or the type @samp{target}, output from the entire recipe
of each target is grouped together.  With the type @samp{line}, output
from each line in the recipe is grouped together.  With the type
@samp{recurse}, the output from an entire recursive make is grouped
together.  With the type @samp{none}, no output synchronization is
performed.  @xref{Parallel Output, ,Output During Parallel Execution}.

@item -p
@cindex @code{-p}
@itemx --print-data-base
@cindex @code{--print-data-base}
@cindex data base of @code{make} rules
@cindex predefined rules and variables, printing
Print the data base (rules and variable values) that results from reading the
makefiles; then execute as usual or as otherwise specified.  This also prints
the version information given by the @samp{-v} switch (see below).  To print
the data base without trying to remake any files, use @w{@samp{make -qp}}.  To
print the data base of predefined rules and variables, use @w{@samp{make -p -f
/dev/null}}.  The data base output contains file name and line number
information for recipe and variable definitions, so it can be a useful
debugging tool in complex environments.

@item --print-targets
@cindex @code{--print-targets}
@cindex print @file{makefile} targets
@cindex targets, printing
Print all the targets defined by reading the makefiles, one target per line,
then exit immediately with success.  No implicit targets are printed.  No
special targets (target names consisting of ``.'' followed by all upper-case
letters) are printed.

No commands are run, including commands that would rebuild makefiles
(@pxref{Remaking Makefiles, ,How Makefiles Are Remade}); if makefiles need to
be rebuilt then the targets listed might be outdated.  Also, @code{make} will
not generate any errors or warnings for missing @code{include} files.

@item -q
@cindex @code{-q}
@itemx --question
@cindex @code{--question}
``Question mode''.  Do not run any recipes, or print anything; just
return an exit status that is zero if the specified targets are already
up to date, one if any remaking is required, or two if an error is
encountered.  @xref{Instead of Execution, ,Instead of Executing
Recipes}.

@item -r
@cindex @code{-r}
@itemx --no-builtin-rules
@cindex @code{--no-builtin-rules}
Eliminate use of the built-in implicit rules (@pxref{Implicit Rules,
,Using Implicit Rules}).  You can still define your own by writing
pattern rules (@pxref{Pattern Rules, ,Defining and Redefining Pattern
Rules}).  The @samp{-r} option also clears out the default list of
suffixes for suffix rules (@pxref{Suffix Rules, ,Old-Fashioned Suffix
Rules}).  But you can still define your own suffixes with a rule for
@code{.SUFFIXES}, and then define your own suffix rules.  Note that only
@emph{rules} are affected by the @code{-r} option; default variables
remain in effect (@pxref{Implicit Variables, ,Variables Used by Implicit
Rules}); see the @samp{-R} option below.

@item -R
@cindex @code{-R}
@itemx --no-builtin-variables
@cindex @code{--no-builtin-variables}
Eliminate use of the built-in rule-specific variables (@pxref{Implicit
Variables, ,Variables Used by Implicit Rules}).  You can still define
your own, of course.  The @samp{-R} option also automatically enables
the @samp{-r} option (see above), since it doesn't make sense to have
implicit rules without any definitions for the variables that they use.

@item -s
@cindex @code{-s}
@itemx --silent
@cindex @code{--silent}
@itemx --quiet
@cindex @code{--quiet}
@c Extra blank line here makes the table look better.

Silent operation; do not print the recipes as they are executed.
@xref{Echoing, ,Recipe Echoing}.

@item -S
@cindex @code{-S}
@itemx --no-keep-going
@cindex @code{--no-keep-going}
@itemx --stop
@cindex @code{--stop}
@c Extra blank line here makes the table look better.

Cancel the effect of the @samp{-k} option.  This is never necessary
except in a recursive @code{make} where @samp{-k} might be inherited
from the top-level @code{make} via @code{MAKEFLAGS}
(@pxref{Recursion, ,Recursive Use of @code{make}})
or if you set @samp{-k} in @code{MAKEFLAGS} in your environment.

@item --shuffle[=@var{mode}]
@cindex @code{--shuffle}
@c Extra blank line here makes the table look better.

This option enables a form of fuzz-testing of prerequisite relationships.
When parallelism is enabled (@samp{-j}) the order in which targets are
built becomes less deterministic.  If prerequisites are not fully declared
in the makefile this can lead to intermittent and hard-to-track-down build
failures.

The @samp{--shuffle} option forces @code{make} to purposefully reorder goals
and prerequisites so target/prerequisite relationships still hold, but
ordering of prerequisites of a given target are reordered as described below.

The order in which prerequisites are listed in automatic variables is not
changed by this option.

The @code{.NOTPARALLEL} pseudo-target disables shuffling for that makefile.
Also any prerequisite list which contains @code{.WAIT} will not be shuffled.
@xref{Parallel Disable, ,Disabling Parallel Execution}.

The @samp{--shuffle=} option accepts these values:

@table @code
@item random
Choose a random seed for the shuffle.  This is the default if no mode is
specified.  The chosen seed is also provided to sub-@code{make} commands.  The
seed is included in error messages so that it can be re-used in future runs to
reproduce the problem or verify that it has been resolved.

@item reverse
Reverse the order of goals and prerequisites, rather than a random shuffle.

@item @var{seed}
Use @samp{random} shuffle initialized with the specified seed value.  The
@var{seed} is an integer.

@item none
Disable shuffling.  This negates any previous @samp{--shuffle} options.
@end table

@item -t
@cindex @code{-t}
@itemx --touch
@cindex @code{--touch}
@c Extra blank line here makes the table look better.

Touch files (mark them up to date without really changing them)
instead of running their recipes.  This is used to pretend that the
recipes were done, in order to fool future invocations of
@code{make}.  @xref{Instead of Execution, ,Instead of Executing Recipes}.

@item --trace
@cindex @code{--trace}
Show tracing information for @code{make} execution.  Using @code{--trace} is
shorthand for @code{--debug=print,why}.

@item -v
@cindex @code{-v}
@itemx --version
@cindex @code{--version}
Print the version of the @code{make} program plus a copyright, a list
of authors, and a notice that there is no warranty; then exit.

@item -w
@cindex @code{-w}
@itemx --print-directory
@cindex @code{--print-directory}
Print a message containing the working directory both before and after
executing the makefile.  This may be useful for tracking down errors
from complicated nests of recursive @code{make} commands.
@xref{Recursion, ,Recursive Use of @code{make}}.  (In practice, you
rarely need to specify this option since @samp{make} does it for you;
see @ref{-w Option, ,The @samp{--print-directory} Option}.)

@item --no-print-directory
@cindex @code{--no-print-directory}
Disable printing of the working directory under @code{-w}.
This option is useful when @code{-w} is turned on automatically,
but you do not want to see the extra messages.
@xref{-w Option, ,The @samp{--print-directory} Option}.

@item -W @var{file}
@cindex @code{-W}
@itemx --what-if=@var{file}
@cindex @code{--what-if}
@itemx --new-file=@var{file}
@cindex @code{--new-file}
@itemx --assume-new=@var{file}
@cindex @code{--assume-new}
Pretend that the target @var{file} has just been modified.  When used
with the @samp{-n} flag, this shows you what would happen if you were
to modify that file.  Without @samp{-n}, it is almost the same as
running a @code{touch} command on the given file before running
@code{make}, except that the modification time is changed only in the
imagination of @code{make}.
@xref{Instead of Execution, ,Instead of Executing Recipes}.


@item --warn[=@var{arg}[,@var{arg}]]
@cindex @code{--warn}
@cindex warnings
Specify the handling of @ref{Warnings, ,Makefile Warnings} detected in
makefiles.

@item --warn-undefined-variables
@cindex @code{--warn-undefined-variables}
@cindex variables, warning for undefined
@cindex undefined variables, warning message
A deprecated name for @code{--warn=undefined-var}.  @xref{Warnings,
,Makefile Warnings}.
@end table

@node Implicit Rules
@chapter Using Implicit Rules
@cindex implicit rule
@cindex rule, implicit

Certain standard ways of remaking target files are used very often.  For
example, one customary way to make an object file is from a C source file
using the C compiler, @code{cc}.

@dfn{Implicit rules} tell @code{make} how to use customary techniques so
that you do not have to specify them in detail when you want to use
them.  For example, there is an implicit rule for C compilation.  File
names determine which implicit rules are run.  For example, C
compilation typically takes a @file{.c} file and makes a @file{.o} file.
So @code{make} applies the implicit rule for C compilation when it sees
this combination of file name endings.

A chain of implicit rules can apply in sequence; for example, @code{make}
will remake a @file{.o} file from a @file{.y} file by way of a @file{.c} file.
@iftex
@xref{Chained Rules, ,Chains of Implicit Rules}.
@end iftex

The built-in implicit rules use several variables in their recipes so
that, by changing the values of the variables, you can change the way the
implicit rule works.  For example, the variable @code{CFLAGS} controls the
flags given to the C compiler by the implicit rule for C compilation.
@iftex
@xref{Implicit Variables, ,Variables Used by Implicit Rules}.
@end iftex

You can define your own implicit rules by writing @dfn{pattern rules}.
@iftex
@xref{Pattern Rules, ,Defining and Redefining Pattern Rules}.
@end iftex

@dfn{Suffix rules} are a more limited way to define implicit rules.
Pattern rules are more general and clearer, but suffix rules are
retained for compatibility.
@iftex
@xref{Suffix Rules, ,Old-Fashioned Suffix Rules}.
@end iftex

@menu
* Using Implicit::              How to use an existing implicit rule
                                  to get the recipes for updating a file.
* Catalogue of Rules::          A list of built-in rules.
* Implicit Variables::          How to change what predefined rules do.
* Chained Rules::               How to use a chain of implicit rules.
* Pattern Rules::               How to define new implicit rules.
* Last Resort::                 How to define a recipe for rules which
                                  cannot find any.
* Suffix Rules::                The old-fashioned style of implicit rule.
* Implicit Rule Search::        The precise algorithm for applying
                                  implicit rules.
@end menu

@node Using Implicit
@section Using Implicit Rules
@cindex implicit rule, how to use
@cindex rule, implicit, how to use

To allow @code{make} to find a customary method for updating a target
file, all you have to do is refrain from specifying recipes yourself.
Either write a rule with no recipe, or don't write a rule at all.
Then @code{make} will figure out which implicit rule to use based on
which kind of source file exists or can be made.

For example, suppose the makefile looks like this:

@example
foo : foo.o bar.o
        cc -o foo foo.o bar.o $(CFLAGS) $(LDFLAGS)
@end example

@noindent
Because you mention @file{foo.o} but do not give a rule for it, @code{make}
will automatically look for an implicit rule that tells how to update it.
This happens whether or not the file @file{foo.o} currently exists.

If an implicit rule is found, it can supply both a recipe and one or
more prerequisites (the source files).  You would want to write a rule
for @file{foo.o} with no recipe if you need to specify additional
prerequisites, such as header files, that the implicit rule cannot
supply.

Each implicit rule has a target pattern and prerequisite patterns.  There may
be many implicit rules with the same target pattern.  For example, numerous
rules make @samp{.o} files: one, from a @samp{.c} file with the C compiler;
another, from a @samp{.p} file with the Pascal compiler; and so on.  The rule
that actually applies is the one whose prerequisites exist or can be made.
So, if you have a file @file{foo.c}, @code{make} will run the C compiler;
otherwise, if you have a file @file{foo.p}, @code{make} will run the Pascal
compiler; and so on.

Of course, when you write the makefile, you know which implicit rule you
want @code{make} to use, and you know it will choose that one because you
know which possible prerequisite files are supposed to exist.
@xref{Catalogue of Rules, ,Catalogue of Built-In Rules},
for a catalogue of all the predefined implicit rules.

Above, we said an implicit rule applies if the required prerequisites ``exist
or can be made''.  A file ``can be made'' if it is mentioned explicitly in
the makefile as a target or a prerequisite, or if an implicit rule can be
recursively found for how to make it.  When an implicit prerequisite is the
result of another implicit rule, we say that @dfn{chaining} is occurring.
@xref{Chained Rules, ,Chains of Implicit Rules}.

In general, @code{make} searches for an implicit rule for each target, and
for each double-colon rule, that has no recipe.  A file that is mentioned
only as a prerequisite is considered a target whose rule specifies nothing,
so implicit rule search happens for it.  @xref{Implicit Rule Search, ,Implicit Rule Search Algorithm}, for the
details of how the search is done.

Note that explicit prerequisites do not influence implicit rule search.
For example, consider this explicit rule:

@example
foo.o: foo.p
@end example

@noindent
The prerequisite on @file{foo.p} does not necessarily mean that
@code{make} will remake @file{foo.o} according to the implicit rule to
make an object file, a @file{.o} file, from a Pascal source file, a
@file{.p} file.  For example, if @file{foo.c} also exists, the implicit
rule to make an object file from a C source file is used instead,
because it appears before the Pascal rule in the list of predefined
implicit rules (@pxref{Catalogue of Rules, , Catalogue of Built-In
Rules}).

If you do not want an implicit rule to be used for a target that has no
recipe, you can give that target an empty recipe by writing a semicolon
(@pxref{Empty Recipes, ,Defining Empty Recipes}).

@node Catalogue of Rules
@section Catalogue of Built-In Rules
@cindex implicit rule, predefined
@cindex rule, implicit, predefined

Here is a catalogue of predefined implicit rules which are always
available unless the makefile explicitly overrides or cancels them.
@xref{Canceling Rules, ,Canceling Implicit Rules}, for information on
canceling or overriding an implicit rule.  The @samp{-r} or
@samp{--no-builtin-rules} option cancels all predefined rules.

This manual only documents the default rules available on POSIX-based
operating systems.  Other operating systems, such as VMS, Windows,
OS/2, etc. may have different sets of default rules.  To see the full
list of default rules and variables available in your version of GNU
@code{make}, run @samp{make -p} in a directory with no makefile.

Not all of these rules will always be defined, even when the @samp{-r}
option is not given.  Many of the predefined implicit rules are
implemented in @code{make} as suffix rules, so which ones will be
defined depends on the @dfn{suffix list} (the list of prerequisites of
the special target @code{.SUFFIXES}).  The default suffix list is:
@code{.out}, @code{.a}, @code{.ln}, @code{.o}, @code{.c}, @code{.cc},
@code{.C}, @code{.cpp}, @code{.p}, @code{.f}, @code{.F}, @code{.m},
@code{.r}, @code{.y}, @code{.l}, @code{.ym}, @code{.lm}, @code{.s},
@code{.S}, @code{.mod}, @code{.sym}, @code{.def}, @code{.h},
@code{.info}, @code{.dvi}, @code{.tex}, @code{.texinfo}, @code{.texi},
@code{.txinfo}, @code{.w}, @code{.ch} @code{.web}, @code{.sh},
@code{.elc}, @code{.el}.  All of the implicit rules described below
whose prerequisites have one of these suffixes are actually suffix
rules.  If you modify the suffix list, the only predefined suffix
rules in effect will be those named by one or two of the suffixes that
are on the list you specify; rules whose suffixes fail to be on the
list are disabled.  @xref{Suffix Rules, ,Old-Fashioned Suffix Rules},
for full details on suffix rules.

@table @asis
@item Compiling C programs
@cindex C, rule to compile
@pindex cc
@pindex gcc
@pindex .o
@pindex .c
@file{@var{n}.o} is made automatically from @file{@var{n}.c} with
a recipe of the form @w{@samp{$(CC) $(CPPFLAGS) $(CFLAGS) -c}}.

@item Compiling C++ programs
@cindex C++, rule to compile
@pindex g++
@pindex .cc
@pindex .cpp
@pindex .C
@file{@var{n}.o} is made automatically from @file{@var{n}.cc},
@file{@var{n}.cpp}, or @file{@var{n}.C} with a recipe of the form
@w{@samp{$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c}}.  We encourage you to use the
suffix @samp{.cc} or @samp{.cpp} for C++ source files instead of @samp{.C} to
better support case-insensitive file systems.

@item Compiling Pascal programs
@cindex Pascal, rule to compile
@pindex pc
@pindex .p
@file{@var{n}.o} is made automatically from @file{@var{n}.p}
with the recipe @samp{$(PC) $(PFLAGS) -c}.

@item Compiling Fortran and Ratfor programs
@cindex Fortran, rule to compile
@cindex Ratfor, rule to compile
@pindex f77
@pindex .f
@pindex .r
@pindex .F
@file{@var{n}.o} is made automatically from @file{@var{n}.r},
@file{@var{n}.F} or @file{@var{n}.f} by running the
Fortran compiler.  The precise recipe used is as follows:

@table @samp
@item .f
@samp{$(FC) $(FFLAGS) -c}.
@item .F
@samp{$(FC) $(FFLAGS) $(CPPFLAGS) -c}.
@item .r
@samp{$(FC) $(FFLAGS) $(RFLAGS) -c}.
@end table

@item Preprocessing Fortran and Ratfor programs
@file{@var{n}.f} is made automatically from @file{@var{n}.r} or
@file{@var{n}.F}.  This rule runs just the preprocessor to convert a
Ratfor or preprocessable Fortran program into a strict Fortran
program.  The precise recipe used is as follows:

@table @samp
@item .F
@samp{$(FC) $(CPPFLAGS) $(FFLAGS) -F}.
@item .r
@samp{$(FC) $(FFLAGS) $(RFLAGS) -F}.
@end table

@item Compiling Modula-2 programs
@cindex Modula-2, rule to compile
@pindex m2c
@pindex .sym
@pindex .def
@pindex .mod
@file{@var{n}.sym} is made from @file{@var{n}.def} with a recipe of the form
@w{@samp{$(M2C) $(M2FLAGS) $(DEFFLAGS)}}.  @file{@var{n}.o} is made from
@file{@var{n}.mod}; the form is: @w{@samp{$(M2C) $(M2FLAGS) $(MODFLAGS)}}.

@need 1200
@item Assembling and preprocessing assembler programs
@cindex assembly, rule to compile
@pindex as
@pindex .s
@file{@var{n}.o} is made automatically from @file{@var{n}.s} by
running the assembler, @code{as}.  The precise recipe is
@samp{$(AS) $(ASFLAGS)}.

@pindex .S
@file{@var{n}.s} is made automatically from @file{@var{n}.S} by
running the C preprocessor, @code{cpp}.  The precise recipe is
@w{@samp{$(CPP) $(CPPFLAGS)}}.

@item Linking a single object file
@cindex linking, predefined rule for
@pindex ld
@pindex .o
@file{@var{n}} is made automatically from @file{@var{n}.o} by running the C
compiler to link the program.  The precise recipe used is @w{@samp{$(CC)
$(LDFLAGS) @var{n}.o $(LOADLIBES) $(LDLIBS)}}.

This rule does the right thing for a simple program with only one
source file.  It will also do the right thing if there are multiple
object files (presumably coming from various other source files), one
of which has a name matching that of the executable file.  Thus,

@example
x: y.o z.o
@end example

@noindent
when @file{x.c}, @file{y.c} and @file{z.c} all exist will execute:

@example
@group
cc -c x.c -o x.o
cc -c y.c -o y.o
cc -c z.c -o z.o
cc x.o y.o z.o -o x
rm -f x.o
rm -f y.o
rm -f z.o
@end group
@end example

@noindent
In more complicated cases, such as when there is no object file whose
name derives from the executable file name, you must write an explicit
recipe for linking.

Each kind of file automatically made into @samp{.o} object files will
be automatically linked by using the compiler (@samp{$(CC)},
@samp{$(FC)} or @samp{$(PC)}; the C compiler @samp{$(CC)} is used to
assemble @samp{.s} files) without the @samp{-c} option.  This could be
done by using the @samp{.o} object files as intermediates, but it is
faster to do the compiling and linking in one step, so that's how it's
done.

@item Yacc for C programs
@pindex yacc
@cindex Yacc, rule to run
@pindex .y
@file{@var{n}.c} is made automatically from @file{@var{n}.y} by
running Yacc with the recipe @samp{$(YACC) $(YFLAGS)}.

@item Lex for C programs
@pindex lex
@cindex Lex, rule to run
@pindex .l
@file{@var{n}.c} is made automatically from @file{@var{n}.l} by
running Lex.  The actual recipe is @samp{$(LEX) $(LFLAGS)}.

@item Lex for Ratfor programs
@file{@var{n}.r} is made automatically from @file{@var{n}.l} by
running Lex.  The actual recipe is @samp{$(LEX) $(LFLAGS)}.

The convention of using the same suffix @samp{.l} for all Lex files
regardless of whether they produce C code or Ratfor code makes it
impossible for @code{make} to determine automatically which of the two
languages you are using in any particular case.  If @code{make} is
called upon to remake an object file from a @samp{.l} file, it must
guess which compiler to use.  It will guess the C compiler, because
that is more common.  If you are using Ratfor, make sure @code{make}
knows this by mentioning @file{@var{n}.r} in the makefile.  Or, if you
are using Ratfor exclusively, with no C files, remove @samp{.c} from
the list of implicit rule suffixes with:

@example
@group
.SUFFIXES:
.SUFFIXES: .o .r .f .l @dots{}
@end group
@end example

@item Making Lint Libraries from C, Yacc, or Lex programs
@pindex lint
@cindex @code{lint}, rule to run
@pindex .ln
@file{@var{n}.ln} is made from @file{@var{n}.c} by running @code{lint}.
The precise recipe is @w{@samp{$(LINT) $(LINTFLAGS) $(CPPFLAGS) -i}}.
The same recipe is used on the C code produced from
@file{@var{n}.y} or @file{@var{n}.l}.

@item @TeX{} and Web
@cindex @TeX{}, rule to run
@cindex Web, rule to run
@pindex tex
@pindex cweave
@pindex weave
@pindex tangle
@pindex ctangle
@pindex .dvi
@pindex .tex
@pindex .web
@pindex .w
@pindex .ch
@file{@var{n}.dvi} is made from @file{@var{n}.tex} with the recipe
@samp{$(TEX)}.  @file{@var{n}.tex} is made from @file{@var{n}.web} with
@samp{$(WEAVE)}, or from @file{@var{n}.w} (and from @file{@var{n}.ch} if
it exists or can be made) with @samp{$(CWEAVE)}.  @file{@var{n}.p} is
made from @file{@var{n}.web} with @samp{$(TANGLE)} and @file{@var{n}.c}
is made from @file{@var{n}.w} (and from @file{@var{n}.ch} if it exists
or can be made) with @samp{$(CTANGLE)}.

@item Texinfo and Info
@cindex Texinfo, rule to format
@cindex Info, rule to format
@pindex texi2dvi
@pindex makeinfo
@pindex .texinfo
@pindex .info
@pindex .texi
@pindex .txinfo
@file{@var{n}.dvi} is made from @file{@var{n}.texinfo},
@file{@var{n}.texi}, or @file{@var{n}.txinfo}, with the recipe
@w{@samp{$(TEXI2DVI) $(TEXI2DVI_FLAGS)}}.  @file{@var{n}.info} is made from
@file{@var{n}.texinfo}, @file{@var{n}.texi}, or @file{@var{n}.txinfo}, with
the recipe @w{@samp{$(MAKEINFO) $(MAKEINFO_FLAGS)}}.

@item RCS
@cindex RCS, rule to extract from
@pindex co
@pindex ,v @r{(RCS file extension)}
Any file @file{@var{n}} is extracted if necessary from an RCS file
named either @file{@var{n},v} or @file{RCS/@var{n},v}.  The precise
recipe used is @w{@samp{$(CO) $(COFLAGS)}}.  @file{@var{n}} will not be
extracted from RCS if it already exists, even if the RCS file is
newer.  The rules for RCS are terminal
(@pxref{Match-Anything Rules, ,Match-Anything Pattern Rules}),
so RCS files cannot be generated from another source; they must
actually exist.

@item SCCS
@cindex SCCS, rule to extract from
@pindex get
@pindex s. @r{(SCCS file prefix)}
Any file @file{@var{n}} is extracted if necessary from an SCCS file
named either @file{s.@var{n}} or @file{SCCS/s.@var{n}}.  The precise
recipe used is @w{@samp{$(GET) $(GFLAGS)}}.  The rules for SCCS are
terminal (@pxref{Match-Anything Rules, ,Match-Anything Pattern Rules}),
so SCCS files cannot be generated from another source; they must
actually exist.

@pindex .sh
For the benefit of SCCS, a file @file{@var{n}} is copied from
@file{@var{n}.sh} and made executable (by everyone).  This is for
shell scripts that are checked into SCCS.  Since RCS preserves the
execution permission of a file, you do not need to use this feature
with RCS.

We recommend that you avoid using SCCS.  RCS is widely held to be
superior, and is also free.  By choosing free software in place of
comparable (or inferior) proprietary software, you support the free
software movement.
@end table

Usually, you want to change only the variables listed in the table
above, which are documented in the following section.

However, the recipes in built-in implicit rules actually use
variables such as @code{COMPILE.c}, @code{LINK.p}, and
@code{PREPROCESS.S}, whose values contain the recipes listed above.

@code{make} follows the convention that the rule to compile a
@file{.@var{x}} source file uses the variable @code{COMPILE.@var{x}}.
Similarly, the rule to produce an executable from a @file{.@var{x}}
file uses @code{LINK.@var{x}}; and the rule to preprocess a
@file{.@var{x}} file uses @code{PREPROCESS.@var{x}}.

@vindex OUTPUT_OPTION
Every rule that produces an object file uses the variable
@code{OUTPUT_OPTION}.  @code{make} defines this variable either to
contain @samp{-o $@@}, or to be empty, depending on a compile-time
option.  You need the @samp{-o} option to ensure that the output goes
into the right file when the source file is in a different directory,
as when using @code{VPATH} (@pxref{Directory Search}).  However,
compilers on some systems do not accept a @samp{-o} switch for object
files.  If you use such a system, and use @code{VPATH}, some
compilations will put their output in the wrong place.
A possible workaround for this problem is to give @code{OUTPUT_OPTION}
the value @w{@samp{; mv $*.o $@@}}.

@node Implicit Variables
@section Variables Used by Implicit Rules
@cindex flags for compilers

The recipes in built-in implicit rules make liberal use of certain
predefined variables.  You can alter the values of these variables in
the makefile, with arguments to @code{make}, or in the environment to
alter how the implicit rules work without redefining the rules
themselves.  You can cancel all variables used by implicit rules with
the @samp{-R} or @samp{--no-builtin-variables} option.

For example, the recipe used to compile a C source file actually says
@samp{$(CC) -c $(CFLAGS) $(CPPFLAGS)}.  The default values of the variables
used are @samp{cc} and nothing, resulting in the command @samp{cc -c}.  By
redefining @samp{CC} to @samp{ncc}, you could cause @samp{ncc} to be
used for all C compilations performed by the implicit rule.  By redefining
@samp{CFLAGS} to be @samp{-g}, you could pass the @samp{-g} option to
each compilation.  @emph{All} implicit rules that do C compilation use
@samp{$(CC)} to get the program name for the compiler and @emph{all}
include @samp{$(CFLAGS)} among the arguments given to the compiler.

The variables used in implicit rules fall into two classes: those that are
names of programs (like @code{CC}) and those that contain arguments for the
programs (like @code{CFLAGS}).  (The ``name of a program'' may also contain
some command arguments, but it must start with an actual executable program
name.)  If a variable value contains more than one argument, separate them
with spaces.

The following tables describe some of the more commonly used built-in
variables.  This list is not exhaustive, and the default values shown here may
not be what @code{make} selects for your environment.  To see the
complete list of predefined variables for your instance of GNU @code{make} you
can run @samp{make -p} in a directory with no makefiles.

Here is a table of some of the more common variables used as names of
programs in built-in rules:

@table @code
@item AR
@vindex AR
Archive-maintaining program; default @samp{ar}.
@pindex ar

@item AS
@vindex AS
Program for compiling assembly files; default @samp{as}.
@pindex as

@item CC
@vindex CC
Program for compiling C programs; default @samp{cc}.
@pindex cc

@item CXX
@vindex CXX
Program for compiling C++ programs; default @samp{g++}.
@pindex g++

@item CPP
@vindex CPP
Program for running the C preprocessor, with results to standard output;
default @samp{$(CC) -E}.

@item FC
@vindex FC
Program for compiling or preprocessing Fortran and Ratfor programs;
default @samp{f77}.
@pindex f77

@item M2C
@vindex M2C
Program to use to compile Modula-2 source code; default @samp{m2c}.
@pindex m2c

@item PC
@vindex PC
Program for compiling Pascal programs; default @samp{pc}.
@pindex pc

@item CO
@vindex CO
Program for extracting a file from RCS; default @samp{co}.
@pindex co

@item GET
@vindex GET
Program for extracting a file from SCCS; default @samp{get}.
@pindex get

@item LEX
@vindex LEX
Program to use to turn Lex grammars into source code; default @samp{lex}.
@pindex lex

@item YACC
@vindex YACC
Program to use to turn Yacc grammars into source code; default @samp{yacc}.
@pindex yacc

@item LINT
@vindex LINT
Program to use to run lint on source code; default @samp{lint}.
@pindex lint

@item MAKEINFO
@vindex MAKEINFO
Program to convert a Texinfo source file into an Info file; default
@samp{makeinfo}.
@pindex makeinfo

@item TEX
@vindex TEX
Program to make @TeX{} @sc{dvi} files from @TeX{} source;
default @samp{tex}.
@pindex tex

@item TEXI2DVI
@vindex TEXI2DVI
Program to make @TeX{} @sc{dvi} files from Texinfo source;
default @samp{texi2dvi}.
@pindex texi2dvi

@item WEAVE
@vindex WEAVE
Program to translate Web into @TeX{}; default @samp{weave}.
@pindex weave

@item CWEAVE
@vindex CWEAVE
Program to translate C Web into @TeX{}; default @samp{cweave}.
@pindex cweave

@item TANGLE
@vindex TANGLE
Program to translate Web into Pascal; default @samp{tangle}.
@pindex tangle

@item CTANGLE
@vindex CTANGLE
Program to translate C Web into C; default @samp{ctangle}.
@pindex ctangle

@item RM
@vindex RM
Command to remove a file; default @samp{rm -f}.
@pindex rm
@end table

Here is a table of variables whose values are additional arguments for the
programs above.  The default values for all of these is the empty
string, unless otherwise noted.

@table @code
@item ARFLAGS
@vindex ARFLAGS
Flags to give the archive-maintaining program; default @samp{rv}.

@item ASFLAGS
@vindex ASFLAGS
Extra flags to give to the assembler (when explicitly
invoked on a @samp{.s} or @samp{.S} file).

@item CFLAGS
@vindex CFLAGS
Extra flags to give to the C compiler.

@item CXXFLAGS
@vindex CXXFLAGS
Extra flags to give to the C++ compiler.

@item COFLAGS
@vindex COFLAGS
Extra flags to give to the RCS @code{co} program.

@item CPPFLAGS
@vindex CPPFLAGS
Extra flags to give to the C preprocessor and programs
that use it (the C and Fortran compilers).

@item FFLAGS
@vindex FFLAGS
Extra flags to give to the Fortran compiler.

@item GFLAGS
@vindex GFLAGS
Extra flags to give to the SCCS @code{get} program.

@item LDFLAGS
@vindex LDFLAGS
Extra flags to give to compilers when they are supposed to invoke the linker,
@samp{ld}, such as @code{-L}.  Libraries (@code{-lfoo}) should be
added to the @code{LDLIBS} variable instead.

@item LDLIBS
@vindex LDLIBS
@vindex LOADLIBES
Library flags or names given to compilers when they are supposed to
invoke the linker, @samp{ld}.  @code{LOADLIBES} is a deprecated (but
still supported) alternative to @code{LDLIBS}.  Non-library linker
flags, such as @code{-L}, should go in the @code{LDFLAGS} variable.

@item LFLAGS
@vindex LFLAGS
Extra flags to give to Lex.

@item YFLAGS
@vindex YFLAGS
Extra flags to give to Yacc.

@item PFLAGS
@vindex PFLAGS
Extra flags to give to the Pascal compiler.

@item RFLAGS
@vindex RFLAGS
Extra flags to give to the Fortran compiler for Ratfor programs.

@item LINTFLAGS
@vindex LINTFLAGS
Extra flags to give to lint.
@end table

@node Chained Rules
@section Chains of Implicit Rules

@cindex chains of rules
@cindex rule, implicit, chains of
Sometimes a file can be made by a sequence of implicit rules.  For example,
a file @file{@var{n}.o} could be made from @file{@var{n}.y} by running
first Yacc and then @code{cc}.  Such a sequence is called a @dfn{chain}.

If the file @file{@var{n}.c} exists, or is mentioned in the makefile, no
special searching is required: @code{make} finds that the object file can
be made by C compilation from @file{@var{n}.c}; later on, when considering
how to make @file{@var{n}.c}, the rule for running Yacc is
used.  Ultimately both @file{@var{n}.c} and @file{@var{n}.o} are
updated.

@cindex intermediate files
@cindex files, intermediate
However, even if @file{@var{n}.c} does not exist and is not mentioned,
@code{make} knows how to envision it as the missing link between
@file{@var{n}.o} and @file{@var{n}.y}!  In this case, @file{@var{n}.c} is
called an @dfn{intermediate file}.  Once @code{make} has decided to use the
intermediate file, it is entered in the data base as if it had been
mentioned in the makefile, along with the implicit rule that says how to
create it.

Intermediate files are remade using their rules just like all other
files.  But intermediate files are treated differently in two ways.

The first difference is what happens if the intermediate file does not
exist.  If an ordinary file @var{b} does not exist, and @code{make}
considers a target that depends on @var{b}, it invariably creates
@var{b} and then updates the target from @var{b}.  But if @var{b} is
an intermediate file, then @code{make} can leave well enough alone:
it won't create @var{b} unless one of its prerequisites is out of
date.  This means the target depending on @var{b} won't be rebuilt
either, unless there is some other reason to update that target: for
example the target doesn't exist or a different prerequisite is newer
than the target.

The second difference is that if @code{make} @emph{does} create @var{b} in
order to update something else, it deletes @var{b} later on after it is no
longer needed.  Therefore, an intermediate file which did not exist before
@code{make} also does not exist after @code{make}.  @code{make} reports the
deletion to you by printing a @samp{rm} command showing which file it is
deleting.

You can explicitly mark a file as intermediate by listing it as a prerequisite
of the special target @code{.INTERMEDIATE}.  This takes effect even if the
file is mentioned explicitly in some other way.

A file cannot be intermediate if it is mentioned in the makefile as a target
or prerequisite, so one way to avoid the deletion of intermediate files is by
adding it as a prerequisite to some target.  However, doing so can cause make
to do extra work when searching pattern rules (@pxref{Implicit Rule Search,
,Implicit Rule Search Algorithm}).

As an alternative, listing a file as a prerequisite of the special target
@code{.NOTINTERMEDIATE} forces it to not be considered intermediate (just as
any other mention of the file will do).  Also, listing the target pattern of a
pattern rule as a prerequisite of @code{.NOTINTERMEDIATE} ensures that no
targets generated using that pattern rule are considered intermediate.

You can disable intermediate files completely in your makefile by
providing @code{.NOTINTERMEDIATE} as a target with no prerequisites:
in that case it applies to every file in the makefile.

@cindex intermediate files, preserving
@cindex preserving intermediate files
@cindex secondary files
If you do not want @code{make} to create a file merely because it does
not already exist, but you also do not want @code{make} to
automatically delete the file, you can mark it as a @dfn{secondary}
file.  To do this, list it as a prerequisite of the special target
@code{.SECONDARY}.  Marking a file as secondary also marks it as
intermediate.

A chain can involve more than two implicit rules.  For example, it is
possible to make a file @file{foo} from @file{RCS/foo.y,v} by running RCS,
Yacc and @code{cc}.  Then both @file{foo.y} and @file{foo.c} are
intermediate files that are deleted at the end.

No single implicit rule can appear more than once in a chain.  This means
that @code{make} will not even consider such a ridiculous thing as making
@file{foo} from @file{foo.o.o} by running the linker twice.  This
constraint has the added benefit of preventing any infinite loop in the
search for an implicit rule chain.

There are some special implicit rules to optimize certain cases that would
otherwise be handled by rule chains.  For example, making @file{foo} from
@file{foo.c} could be handled by compiling and linking with separate
chained rules, using @file{foo.o} as an intermediate file.  But what
actually happens is that a special rule for this case does the compilation
and linking with a single @code{cc} command.  The optimized rule is used in
preference to the step-by-step chain because it comes earlier in the
ordering of rules.

Finally, for performance reasons @code{make} will not consider non-terminal
match-anything rules (i.e., @samp{%:}) when searching for a rule to
build a prerequisite of an implicit rule (@pxref{Match-Anything Rules}).

@node Pattern Rules
@section Defining and Redefining Pattern Rules

You define an implicit rule by writing a @dfn{pattern rule}.  A pattern
rule looks like an ordinary rule, except that its target contains the
character @samp{%} (exactly one of them).  The target is considered a
pattern for matching file names; the @samp{%} can match any nonempty
substring, while other characters match only themselves.  The prerequisites
likewise use @samp{%} to show how their names relate to the target name.

Thus, a pattern rule @samp{%.o : %.c ; @var{recipe}} describes a process for
building any file @file{@var{stem}.o} from another file @file{@var{stem}.c}
using the recipe @var{recipe}.

It is not possible to create a pattern rule that does not have a recipe;
omitting the recipe removes that pattern rule instead (@pxref{Canceling Rules,
,Canceling Implicit Rules}).

Note that expansion using @samp{%} in pattern rules occurs
@strong{after} any variable or function expansions, which take place
when the makefile is read.  @xref{Using Variables, , How to Use
Variables}, and @ref{Functions, ,Functions for Transforming Text}.

@menu
* Pattern Intro::               An introduction to pattern rules.
* Pattern Examples::            Examples of pattern rules.
* Automatic Variables::         How to use automatic variables in the
                                  recipe of implicit rules.
* Pattern Match::               How patterns match.
* Match-Anything Rules::        Precautions you should take prior to
                                  defining rules that can match any
                                  target file whatever.
* Canceling Rules::             How to override or cancel built-in rules.
@end menu

@node Pattern Intro
@subsection Introduction to Pattern Rules
@cindex pattern rule
@cindex rule, pattern

A pattern rule contains the character @samp{%} (exactly one of them)
in the target; otherwise, it looks exactly like an ordinary rule.  The
target is a pattern for matching file names; the @samp{%} matches any
nonempty substring, while other characters match only themselves.
@cindex target pattern, implicit
@cindex @code{%}, in pattern rules

For example, @samp{%.c} as a pattern matches any file name that ends in
@samp{.c}.  @samp{s.%.c} as a pattern matches any file name that starts
with @samp{s.}, ends in @samp{.c} and is at least five characters long.
(There must be at least one character to match the @samp{%}.)  The substring
that the @samp{%} matches is called the @dfn{stem}.

@samp{%} in a prerequisite of a pattern rule stands for the same stem
that was matched by the @samp{%} in the target.  In order for the
pattern rule to apply, its target pattern must match the file name
under consideration and all of its prerequisites (after pattern
substitution) must name files that exist or can be made.  These files
become prerequisites of the target.
@cindex prerequisite pattern, implicit

Thus, a rule of the form

@example
%.o : %.c ; @var{recipe}@dots{}
@end example

@noindent
specifies how to make a file @file{@var{n}.o}, with another file
@file{@var{n}.c} as its prerequisite, provided that @file{@var{n}.c}
exists or can be made.

There may also be prerequisites that do not use @samp{%}; such a prerequisite
attaches to every file made by this pattern rule.  These unvarying
prerequisites are useful occasionally.

A pattern rule need not have any prerequisites that contain @samp{%}, or
in fact any prerequisites at all.  Such a rule is effectively a general
wildcard.  It provides a way to make any file that matches the target
pattern.  @xref{Last Resort}.

More than one pattern rule may match a target.  In this case
@code{make} will choose the ``best fit'' rule.  @xref{Pattern Match,
,How Patterns Match}.

@cindex multiple targets, in pattern rule
@cindex target, multiple in pattern rule
Pattern rules may have more than one target; however, every target must
contain a @code{%} character.  Multiple target patterns in pattern rules are
always treated as grouped targets (@pxref{Multiple Targets, , Multiple Targets
in a Rule}) regardless of whether they use the @code{:} or @code{&:}
separator.

There is one exception: if a pattern target is out of date or does
not exist and the makefile does not need to build it, then it will not cause
the other targets to be considered out of date.  Note that this historical
exception will be removed in future versions of GNU @code{make} and should not
be relied on.  If this situation is detected @code{make} will generate a
warning @emph{pattern recipe did not update peer target}; however, @code{make}
cannot detect all such situations.  Please be sure that your recipe updates
@emph{all} the target patterns when it runs.

@node Pattern Examples
@subsection Pattern Rule Examples

Here are some examples of pattern rules actually predefined in
@code{make}.  First, the rule that compiles @samp{.c} files into @samp{.o}
files:

@example
%.o : %.c
        $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@@
@end example

@noindent
defines a rule that can make any file @file{@var{x}.o} from
@file{@var{x}.c}.  The recipe uses the automatic variables @samp{$@@} and
@samp{$<} to substitute the names of the target file and the source file
in each case where the rule applies (@pxref{Automatic Variables}).

Here is a second built-in rule:

@example
% :: RCS/%,v
        $(CO) $(COFLAGS) $<
@end example

@noindent
defines a rule that can make any file @file{@var{x}} whatsoever from a
corresponding file @file{@var{x},v} in the sub-directory @file{RCS}.  Since
the target is @samp{%}, this rule will apply to any file whatever, provided
the appropriate prerequisite file exists.  The double colon makes the rule
@dfn{terminal}, which means that its prerequisite may not be an intermediate
file (@pxref{Match-Anything Rules, ,Match-Anything Pattern Rules}).

@need 500
This pattern rule has two targets:

@example
@group
%.tab.c %.tab.h: %.y
        bison -d $<
@end group
@end example

@noindent
@c The following paragraph is rewritten to avoid overfull hboxes
This tells @code{make} that the recipe @samp{bison -d @var{x}.y} will
make both @file{@var{x}.tab.c} and @file{@var{x}.tab.h}.  If the file
@file{foo} depends on the files @file{parse.tab.o} and @file{scan.o}
and the file @file{scan.o} depends on the file @file{parse.tab.h},
when @file{parse.y} is changed, the recipe @samp{bison -d parse.y}
will be executed only once, and the prerequisites of both
@file{parse.tab.o} and @file{scan.o} will be satisfied.  (Presumably
the file @file{parse.tab.o} will be recompiled from @file{parse.tab.c}
and the file @file{scan.o} from @file{scan.c}, while @file{foo} is
linked from @file{parse.tab.o}, @file{scan.o}, and its other
prerequisites, and it will execute happily ever after.)

@node Automatic Variables
@subsection Automatic Variables
@cindex automatic variables
@cindex variables, automatic
@cindex variables, and implicit rule

Suppose you are writing a pattern rule to compile a @samp{.c} file into a
@samp{.o} file: how do you write the @samp{cc} command so that it operates
on the right source file name?  You cannot write the name in the recipe,
because the name is different each time the implicit rule is applied.

What you do is use a special feature of @code{make}, the @dfn{automatic
variables}.  These variables have values computed afresh for each rule that
is executed, based on the target and prerequisites of the rule.  In this
example, you would use @samp{$@@} for the object file name and @samp{$<}
for the source file name.

@cindex automatic variables in prerequisites
@cindex prerequisites, and automatic variables
It's very important that you recognize the limited scope in which
automatic variable values are available: they only have values within
the recipe.  In particular, you cannot use them anywhere
within the target list of a rule; they have no value there and will
expand to the empty string.  Also, they cannot be accessed directly
within the prerequisite list of a rule.  A common mistake is
attempting to use @code{$@@} within the prerequisites list; this will
not work.  However, there is a special feature of GNU @code{make},
secondary expansion (@pxref{Secondary Expansion}), which will allow
automatic variable values to be used in prerequisite lists.

Here is a table of automatic variables:

@table @code
@vindex $@@
@vindex @@ @r{(automatic variable)}
@item $@@
The file name of the target of the rule.  If the target is an archive
member, then @samp{$@@} is the name of the archive file.  In a pattern
rule that has multiple targets (@pxref{Pattern Intro, ,Introduction to
Pattern Rules}), @samp{$@@} is the name of whichever target caused the
rule's recipe to be run.

@vindex $%
@vindex % @r{(automatic variable)}
@item $%
The target member name, when the target is an archive member.
@xref{Archives}.  For example, if the target is @file{foo.a(bar.o)} then
@samp{$%} is @file{bar.o} and @samp{$@@} is @file{foo.a}.  @samp{$%} is
empty when the target is not an archive member.

@vindex $<
@vindex < @r{(automatic variable)}
@item $<
The name of the first prerequisite.  If the target got its recipe from
an implicit rule, this will be the first prerequisite added by the
implicit rule (@pxref{Implicit Rules}).

@vindex $?
@vindex ? @r{(automatic variable)}
@item $?
The names of all the prerequisites that are newer than the target, with
spaces between them.  If the target does not exist, all prerequisites
will be included.  For prerequisites which are archive members, only the
named member is used (@pxref{Archives}).

@samp{$?} is useful even in explicit rules when you wish to operate on only
the prerequisites that have changed.  For example, suppose that an archive
named @file{lib} is supposed to contain copies of several object files.
This rule copies just the changed object files into the archive:

@example
@group
lib: foo.o bar.o lose.o win.o
        ar r lib $?
@end group
@end example
@cindex prerequisites, list of changed
@cindex list of changed prerequisites

@vindex $^
@vindex ^ @r{(automatic variable)}
@item $^
The names of all the prerequisites, with spaces between them.  For
prerequisites which are archive members, only the named member is used
(@pxref{Archives}).  A target has only one prerequisite on each other file
it depends on, no matter how many times each file is listed as a
prerequisite.  So if you list a prerequisite more than once for a target,
the value of @code{$^} contains just one copy of the name.  This list
does @strong{not} contain any of the order-only prerequisites; for those
see the @samp{$|} variable, below.
@cindex prerequisites, list of all
@cindex list of all prerequisites

@vindex $+
@vindex + @r{(automatic variable)}
@item $+
This is like @samp{$^}, but prerequisites listed more than once are
duplicated in the order they were listed in the makefile.  This is
primarily useful for use in linking commands where it is meaningful to
repeat library file names in a particular order.

@vindex $|
@vindex | @r{(automatic variable)}
@item $|
The names of all the order-only prerequisites, with spaces between
them.

@vindex $*
@vindex * @r{(automatic variable)}
@item $*
The stem with which an implicit rule matches (@pxref{Pattern Match, ,How
Patterns Match}).  If the target is @file{dir/a.foo.b} and the target
pattern is @file{a.%.b} then the stem is @file{dir/foo}.  The stem is
useful for constructing names of related files.
@cindex stem, variable for

In a static pattern rule, the stem is part of the file name that matched
the @samp{%} in the target pattern.

In an explicit rule, there is no stem; so @samp{$*} cannot be determined
in that way.  Instead, if the target name ends with a recognized suffix
(@pxref{Suffix Rules, ,Old-Fashioned Suffix Rules}), @samp{$*} is set to
the target name minus the suffix.  For example, if the target name is
@samp{foo.c}, then @samp{$*} is set to @samp{foo}, since @samp{.c} is a
suffix.  GNU @code{make} does this bizarre thing only for compatibility
with other implementations of @code{make}.  You should generally avoid
using @samp{$*} except in implicit rules or static pattern rules.

If the target name in an explicit rule does not end with a recognized
suffix, @samp{$*} is set to the empty string for that rule.
@end table

Of the variables listed above, four have values that are single file
names, and three have values that are lists of file names.  These
seven have variants that get just the file's directory name or just
the file name within the directory.  The variant variables' names are
formed by appending @samp{D} or @samp{F}, respectively.  The functions
@code{dir} and @code{notdir} can be used to obtain a similar effect
(@pxref{File Name Functions, , Functions for File Names}).  Note,
however, that the @samp{D} variants all omit the trailing slash which
always appears in the output of the @code{dir} function.  Here is a
table of the variants:

@table @samp
@vindex $(@@D)
@vindex @@D @r{(automatic variable)}
@item $(@@D)
The directory part of the file name of the target, with the trailing
slash removed.  If the value of @samp{$@@} is @file{dir/foo.o} then
@samp{$(@@D)} is @file{dir}.  This value is @file{.} if @samp{$@@} does
not contain a slash.

@vindex $(@@F)
@vindex @@F @r{(automatic variable)}
@item $(@@F)
The file-within-directory part of the file name of the target.  If the
value of @samp{$@@} is @file{dir/foo.o} then @samp{$(@@F)} is
@file{foo.o}.  @samp{$(@@F)} is equivalent to @samp{$(notdir $@@)}.

@vindex $(*D)
@vindex *D @r{(automatic variable)}
@item $(*D)
@vindex $(*F)
@vindex *F @r{(automatic variable)}
@itemx $(*F)
The directory part and the file-within-directory
part of the stem; @file{dir} and @file{foo} in this example.

@vindex $(%D)
@vindex %D @r{(automatic variable)}
@item $(%D)
@vindex $(%F)
@vindex %F @r{(automatic variable)}
@itemx $(%F)
The directory part and the file-within-directory part of the target
archive member name.  This makes sense only for archive member targets
of the form @file{@var{archive}(@var{member})} and is useful only when
@var{member} may contain a directory name.  (@xref{Archive Members,
,Archive Members as Targets}.)

@vindex $(<D)
@vindex <D @r{(automatic variable)}
@item $(<D)
@vindex $(<F)
@vindex <F @r{(automatic variable)}
@itemx $(<F)
The directory part and the file-within-directory
part of the first prerequisite.

@vindex $(^D)
@vindex ^D @r{(automatic variable)}
@item $(^D)
@vindex $(^F)
@vindex ^F @r{(automatic variable)}
@itemx $(^F)
Lists of the directory parts and the file-within-directory
parts of all prerequisites.

@vindex $(+D)
@vindex +D @r{(automatic variable)}
@item $(+D)
@vindex $(+F)
@vindex +F @r{(automatic variable)}
@itemx $(+F)
Lists of the directory parts and the file-within-directory
parts of all prerequisites, including multiple instances of duplicated
prerequisites.

@vindex $(?D)
@vindex ?D @r{(automatic variable)}
@item $(?D)
@vindex $(?F)
@vindex ?F @r{(automatic variable)}
@itemx $(?F)
Lists of the directory parts and the file-within-directory parts of
all prerequisites that are newer than the target.
@end table

Note that we use a special stylistic convention when we talk about these
automatic variables; we write ``the value of @samp{$<}'', rather than
@w{``the variable @code{<}''} as we would write for ordinary variables
such as @code{objects} and @code{CFLAGS}.  We think this convention
looks more natural in this special case.  Please do not assume it has a
deep significance; @samp{$<} refers to the variable named @code{<} just
as @samp{$(CFLAGS)} refers to the variable named @code{CFLAGS}.
You could just as well use @samp{$(<)} in place of @samp{$<}.

@node Pattern Match
@subsection How Patterns Match

@cindex stem
A target pattern is composed of a @samp{%} between a prefix and a suffix,
either or both of which may be empty.  The pattern matches a file name only
if the file name starts with the prefix and ends with the suffix, without
overlap.  The text between the prefix and the suffix is called the
@dfn{stem}.  Thus, when the pattern @samp{%.o} matches the file name
@file{test.o}, the stem is @samp{test}.  The pattern rule prerequisites are
turned into actual file names by substituting the stem for the character
@samp{%}.  Thus, if in the same example one of the prerequisites is written
as @samp{%.c}, it expands to @samp{test.c}.

When the target pattern does not contain a slash (and it usually does
not), directory names in the file names are removed from the file name
before it is compared with the target prefix and suffix.  After the
comparison of the file name to the target pattern, the directory
names, along with the slash that ends them, are added on to the
prerequisite file names generated from the pattern rule's prerequisite
patterns and the file name.  The directories are ignored only for the
purpose of finding an implicit rule to use, not in the application of
that rule.  Thus, @samp{e%t} matches the file name @file{src/eat},
with @samp{src/a} as the stem.  When prerequisites are turned into file
names, the directories from the stem are added at the front, while the
rest of the stem is substituted for the @samp{%}.  The stem
@samp{src/a} with a prerequisite pattern @samp{c%r} gives the file name
@file{src/car}.

@cindex pattern rules, order of
@cindex order of pattern rules
A pattern rule can be used to build a given file only if there is a
target pattern that matches the file name, @emph{and} all
prerequisites in that rule either exist or can be built.  The rules
you write take precedence over those that are built in. Note however,
that a rule which can be satisfied without chaining other implicit
rules (for example, one which has no prerequisites or its
prerequisites already exist or are mentioned) always takes priority
over a rule with prerequisites that must be made by chaining other
implicit rules.

@cindex stem, shortest
It is possible that more than one pattern rule will meet these
criteria.  In that case, @code{make} will choose the rule with the
shortest stem (that is, the pattern that matches most specifically).
If more than one pattern rule has the shortest stem, @code{make} will
choose the first one found in the makefile.

This algorithm results in more specific rules being preferred over
more generic ones; for example:

@example
%.o: %.c
        $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@@

%.o : %.f
        $(COMPILE.F) $(OUTPUT_OPTION) $<

lib/%.o: lib/%.c
        $(CC) -fPIC -c $(CFLAGS) $(CPPFLAGS) $< -o $@@
@end example

Given these rules and asked to build @file{bar.o} where both
@file{bar.c} and @file{bar.f} exist, @code{make} will choose the first
rule and compile @file{bar.c} into @file{bar.o}.  In the same
situation where @file{bar.c} does not exist, then @code{make} will
choose the second rule and compile @file{bar.f} into @file{bar.o}.

If @code{make} is asked to build @file{lib/bar.o} and both
@file{lib/bar.c} and @file{lib/bar.f} exist, then the third rule will
be chosen since the stem for this rule (@samp{bar}) is shorter than
the stem for the first rule (@samp{lib/bar}).  If @file{lib/bar.c}
does not exist then the third rule is not eligible and the second rule
will be used, even though the stem is longer.

@node Match-Anything Rules
@subsection Match-Anything Pattern Rules

@cindex match-anything rule
@cindex terminal rule
When a pattern rule's target is just @samp{%}, it matches any file name
whatever.  We call these rules @dfn{match-anything} rules.  They are very
useful, but it can take a lot of time for @code{make} to think about them,
because it must consider every such rule for each file name listed either
as a target or as a prerequisite.

Suppose the makefile mentions @file{foo.c}.  For this target, @code{make}
would have to consider making it by linking an object file @file{foo.c.o},
or by C compilation-and-linking in one step from @file{foo.c.c}, or by
Pascal compilation-and-linking from @file{foo.c.p}, and many other
possibilities.

We know these possibilities are ridiculous since @file{foo.c} is a C source
file, not an executable.  If @code{make} did consider these possibilities,
it would ultimately reject them, because files such as @file{foo.c.o} and
@file{foo.c.p} would not exist.  But these possibilities are so
numerous that @code{make} would run very slowly if it had to consider
them.

To gain speed, we have put various constraints on the way @code{make}
considers match-anything rules.  There are two different constraints that
can be applied, and each time you define a match-anything rule you must
choose one or the other for that rule.

One choice is to mark the match-anything rule as @dfn{terminal} by defining
it with a double colon.  When a rule is terminal, it does not apply unless
its prerequisites actually exist.  Prerequisites that could be made with
other implicit rules are not good enough.  In other words, no further
chaining is allowed beyond a terminal rule.

For example, the built-in implicit rules for extracting sources from RCS
and SCCS files are terminal; as a result, if the file @file{foo.c,v} does
not exist, @code{make} will not even consider trying to make it as an
intermediate file from @file{foo.c,v.o} or from @file{RCS/SCCS/s.foo.c,v}.
RCS and SCCS files are generally ultimate source files, which should not be
remade from any other files; therefore, @code{make} can save time by not
looking for ways to remake them.

If you do not mark the match-anything rule as terminal, then it is
non-terminal.  A non-terminal match-anything rule cannot apply to a
prerequisite of an implicit rule, or to a file name that indicates a
specific type of data.  A file name indicates a specific type of data
if some non-match-anything implicit rule target matches it.

For example, the file name @file{foo.c} matches the target for the pattern
rule @samp{%.c : %.y} (the rule to run Yacc).  Regardless of whether this
rule is actually applicable (which happens only if there is a file
@file{foo.y}), the fact that its target matches is enough to prevent
consideration of any non-terminal match-anything rules for the file
@file{foo.c}.  Thus, @code{make} will not even consider trying to make
@file{foo.c} as an executable file from @file{foo.c.o}, @file{foo.c.c},
@file{foo.c.p}, etc.

The motivation for this constraint is that non-terminal match-anything
rules are used for making files containing specific types of data (such as
executable files) and a file name with a recognized suffix indicates some
other specific type of data (such as a C source file).

Special built-in dummy pattern rules are provided solely to recognize
certain file names so that non-terminal match-anything rules will not be
considered.  These dummy rules have no prerequisites and no recipes, and
they are ignored for all other purposes.  For example, the built-in
implicit rule

@example
%.p :
@end example

@noindent
exists to make sure that Pascal source files such as @file{foo.p} match a
specific target pattern and thereby prevent time from being wasted looking
for @file{foo.p.o} or @file{foo.p.c}.

Dummy pattern rules such as the one for @samp{%.p} are made for every
suffix listed as valid for use in suffix rules (@pxref{Suffix Rules, ,Old-Fashioned Suffix Rules}).

@node Canceling Rules
@subsection Canceling Implicit Rules

You can override a built-in implicit rule (or one you have defined
yourself) by defining a new pattern rule with the same target and
prerequisites, but a different recipe.  When the new rule is defined, the
built-in one is replaced.  The new rule's position in the sequence of
implicit rules is determined by where you write the new rule.

You can cancel a built-in implicit rule by defining a pattern rule with the
same target and prerequisites, but no recipe.  For example, the following
would cancel the rule that runs the assembler:

@example
%.o : %.s
@end example

@node Last Resort
@section Defining Last-Resort Default Rules
@cindex last-resort default rules
@cindex default rules, last-resort

You can define a last-resort implicit rule by writing a terminal
match-anything pattern rule with no prerequisites (@pxref{Match-Anything
Rules}).  This is just like any other pattern rule; the only thing
special about it is that it will match any target.  So such a rule's
recipe is used for all targets and prerequisites that have no recipe
of their own and for which no other implicit rule applies.

For example, when testing a makefile, you might not care if the source
files contain real data, only that they exist.  Then you might do this:

@example
%::
        touch $@@
@end example

@noindent
to cause all the source files needed (as prerequisites) to be created
automatically.

@findex .DEFAULT
You can instead define a recipe to be used for targets for which there
are no rules at all, even ones which don't specify recipes.  You do
this by writing a rule for the target @code{.DEFAULT}.  Such a rule's
recipe is used for all prerequisites which do not appear as targets in
any explicit rule, and for which no implicit rule applies.  Naturally,
there is no @code{.DEFAULT} rule unless you write one.

If you use @code{.DEFAULT} with no recipe or prerequisites:

@example
.DEFAULT:
@end example

@noindent
the recipe previously stored for @code{.DEFAULT} is cleared.  Then
@code{make} acts as if you had never defined @code{.DEFAULT} at all.

If you do not want a target to get the recipe from a match-anything
pattern rule or @code{.DEFAULT}, but you also do not want any recipe
to be run for the target, you can give it an empty recipe
(@pxref{Empty Recipes, ,Defining Empty Recipes}).

You can use a last-resort rule to override part of another makefile.
@xref{Overriding Makefiles, , Overriding Part of Another Makefile}.

@node Suffix Rules
@section Old-Fashioned Suffix Rules
@cindex old-fashioned suffix rules
@cindex suffix rule
@cindex inference rule

@dfn{Suffix rules} (called @dfn{inference rules} in POSIX) are an
old-fashioned way of defining implicit rules for @code{make}.  Suffix rules
are less powerful and harder to use than pattern rules (@pxref{Pattern Rules,
,Defining and Redefining Pattern Rules}); they are supported by GNU Make
primarily for POSIX compatibility.  They come in two flavors:
@dfn{single-suffix} and @dfn{double-suffix}.

A single-suffix rule is defined by a single suffix, which is the source
suffix.  It matches any file name, and the corresponding implicit prerequisite
name is made by appending the source suffix.  A single-suffix rule whose
source suffix is @samp{.c} is equivalent to the pattern rule @samp{% : %.c}.

A double-suffix rule is defined by a pair of suffixes: the source suffix and
the target suffix.  It matches any file whose name ends with the target
suffix.  The corresponding implicit prerequisite is made by replacing the
target suffix with the source suffix in the file name.  A two-suffix rule
@samp{.c.o} has a source suffix @samp{.c} and a target suffix @samp{.o}, and
is equivalent to the pattern rule @samp{%.o : %.c}.

Suffix rule definitions are recognized by comparing each rule's target
against a defined list of known suffixes.  When @code{make} sees a rule
whose target is a known suffix, this rule is considered a single-suffix
rule.  When @code{make} sees a rule whose target is two known suffixes
concatenated, this rule is taken as a double-suffix rule.

For example, @samp{.c} and @samp{.o} are both on the default list of
known suffixes.  Therefore, if you define a rule whose target is
@samp{.c.o}, @code{make} takes it to be a double-suffix rule with source
suffix @samp{.c} and target suffix @samp{.o}.  Here is the old-fashioned
way to define the rule for compiling a C source file:

@example
.c.o:
        $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@@ $<
@end example

Suffix rules cannot have any prerequisites of their own.  If they have any,
they are treated as normal files with funny names, not as suffix rules.
Thus, the rule:

@example
.c.o: foo.h
        $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@@ $<
@end example

@noindent
tells how to make the file @file{.c.o} from the prerequisite file
@file{foo.h}, and is not at all like the pattern rule:

@example
%.o: %.c foo.h
        $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@@ $<
@end example

@noindent
which tells how to make @samp{.o} files from @samp{.c} files, and makes all
@samp{.o} files using this pattern rule also depend on @file{foo.h}.

Suffix rules with no recipe are also meaningless.  They do not remove
previous rules as do pattern rules with no recipe (@pxref{Canceling
Rules, , Canceling Implicit Rules}).  They simply enter the suffix or
pair of suffixes concatenated as a target in the data base.

The known suffixes are simply the names of the prerequisites of the special
target @code{.SUFFIXES}.  You can add your own suffixes by writing a rule
for @code{.SUFFIXES} that adds more prerequisites, as in:

@example
.SUFFIXES: .hack .win
@end example

@noindent
which adds @samp{.hack} and @samp{.win} to the end of the list of suffixes.

If you wish to eliminate the default known suffixes instead of just adding
to them, write a rule for @code{.SUFFIXES} with no prerequisites.  By
special dispensation, this eliminates all existing prerequisites of
@code{.SUFFIXES}.  You can then write another rule to add the suffixes you
want.  For example,

@example
@group
.SUFFIXES:            # @r{Delete the default suffixes}
.SUFFIXES: .c .o .h   # @r{Define our suffix list}
@end group
@end example

The @samp{-r} or @samp{--no-builtin-rules} flag causes the default
list of suffixes to be empty.

@vindex SUFFIXES
The variable @code{SUFFIXES} is defined to the default list of suffixes
before @code{make} reads any makefiles.  You can change the list of suffixes
with a rule for the special target @code{.SUFFIXES}, but that does not alter
this variable.

@node Implicit Rule Search
@section Implicit Rule Search Algorithm
@cindex implicit rule, search algorithm
@cindex search algorithm, implicit rule

Here is the procedure @code{make} uses for searching for an implicit rule
for a target @var{t}.  This procedure is followed for each double-colon
rule with no recipe, for each target of ordinary rules none of which have
a recipe, and for each prerequisite that is not the target of any rule.  It
is also followed recursively for prerequisites that come from implicit
rules, in the search for a chain of rules.

Suffix rules are not mentioned in this algorithm because suffix rules are
converted to equivalent pattern rules once the makefiles have been read in.

For an archive member target of the form
@samp{@var{archive}(@var{member})}, the following algorithm is run
twice, first using the entire target name @var{t}, and second using
@samp{(@var{member})} as the target @var{t} if the first run found no
rule.

@enumerate
@item
Split @var{t} into a directory part, called @var{d}, and the rest,
called @var{n}.  For example, if @var{t} is @samp{src/foo.o}, then
@var{d} is @samp{src/} and @var{n} is @samp{foo.o}.

@item
Make a list of all the pattern rules one of whose targets matches
@var{t} or @var{n}.  If the target pattern contains a slash, it is
matched against @var{t}; otherwise, against @var{n}.

@item
If any rule in that list is @emph{not} a match-anything rule, or if
@var{t} is a prerequisite of an implicit rule, then remove all
non-terminal match-anything rules from the list.

@item
Remove from the list all rules with no recipe.

@item
For each pattern rule in the list:

@enumerate a
@item
Find the stem @var{s}, which is the nonempty part of @var{t} or @var{n}
matched by the @samp{%} in the target pattern.

@item
Compute the prerequisite names by substituting @var{s} for @samp{%}; if
the target pattern does not contain a slash, append @var{d} to
the front of each prerequisite name.

@item
Test whether all the prerequisites exist or ought to exist.  (If a
file name is mentioned in the makefile as a target or as an explicit
prerequisite of target T, then we say it ought to exist.)

If all prerequisites exist or ought to exist, or there are no prerequisites,
then this rule applies.
@end enumerate

@item
If no pattern rule has been found so far, try harder.
For each pattern rule in the list:

@enumerate a
@item
If the rule is terminal, ignore it and go on to the next rule.

@item
Compute the prerequisite names as before.

@item
Test whether all the prerequisites exist or ought to exist.

@item
For each prerequisite that does not exist, follow this algorithm
recursively to see if the prerequisite can be made by an implicit
rule.

@item
If all prerequisites exist, ought to exist, or can be
made by implicit rules, then this rule applies.
@end enumerate

@item
If no pattern rule has been found then try step 5 and step 6 again with a
modified definition of ``ought to exist'': if a filename is mentioned as a
target or as an explicit prerequisite of @emph{any} target, then it ought to
exist.  This check is only present for backward-compatibility with older
versions of GNU Make: we don't recommend relying on it.

@item
If no implicit rule applies, the rule for @code{.DEFAULT}, if any,
applies.  In that case, give @var{t} the same recipe that
@code{.DEFAULT} has.  Otherwise, there is no recipe for @var{t}.
@end enumerate

Once a rule that applies has been found, for each target pattern of the rule
other than the one that matched @var{t} or @var{n}, the @samp{%} in the
pattern is replaced with @var{s} and the resultant file name is stored until
the recipe to remake the target file @var{t} is executed.  After the recipe is
executed each of these stored file names is entered into the data base, marked
as having been updated, and given the same update status as the file @var{t}.

When the recipe of a pattern rule is executed for @var{t}, the
automatic variables are set corresponding to the target and
prerequisites.  @xref{Automatic Variables}.

@node Archives
@chapter Using @code{make} to Update Archive Files
@cindex archive

@dfn{Archive files} are files containing named sub-files called
@dfn{members}; they are maintained with the program @code{ar} and their
main use is as subroutine libraries for linking.

@menu
* Archive Members::             Archive members as targets.
* Archive Update::              The implicit rule for archive member targets.
* Archive Pitfalls::            Dangers to watch out for when using archives.
* Archive Suffix Rules::        You can write a special kind of suffix rule
                                  for updating archives.
@end menu

@node Archive Members
@section Archive Members as Targets
@cindex archive member targets

An individual member of an archive file can be used as a target or
prerequisite in @code{make}.  You specify the member named @var{member} in
archive file @var{archive} as follows:

@example
@var{archive}(@var{member})
@end example

@noindent
This construct is available only in targets and prerequisites, not in
recipes!  Most programs that you might use in recipes do not support
this syntax and cannot act directly on archive members.  Only
@code{ar} and other programs specifically designed to operate on
archives can do so.  Therefore, valid recipes to update an archive
member target probably must use @code{ar}.  For example, this rule
says to create a member @file{hack.o} in archive @file{foolib} by
copying the file @file{hack.o}:

@example
foolib(hack.o) : hack.o
        ar cr foolib hack.o
@end example

In fact, nearly all archive member targets are updated in just this way
and there is an implicit rule to do it for you.  @strong{Please note:} The
@samp{c} flag to @code{ar} is required if the archive file does not
already exist.

To specify several members in the same archive, you can write all the
member names together between the parentheses.  For example:

@example
foolib(hack.o kludge.o)
@end example

@noindent
is equivalent to:

@example
foolib(hack.o) foolib(kludge.o)
@end example

@cindex wildcard, in archive member
You can also use shell-style wildcards in an archive member reference.
@xref{Wildcards, ,Using Wildcard Characters in File Names}.  For
example, @w{@samp{foolib(*.o)}} expands to all existing members of the
@file{foolib} archive whose names end in @samp{.o}; perhaps
@samp{@w{foolib(hack.o)} @w{foolib(kludge.o)}}.

@node Archive Update
@section Implicit Rule for Archive Member Targets

Recall that a target that looks like @file{@var{a}(@var{m})} stands for the
member named @var{m} in the archive file @var{a}.

When @code{make} looks for an implicit rule for such a target, as a special
feature it considers implicit rules that match @file{(@var{m})}, as well as
those that match the actual target @file{@var{a}(@var{m})}.

This causes one special rule whose target is @file{(%)} to match.  This
rule updates the target @file{@var{a}(@var{m})} by copying the file @var{m}
into the archive.  For example, it will update the archive member target
@file{foo.a(bar.o)} by copying the @emph{file} @file{bar.o} into the
archive @file{foo.a} as a @emph{member} named @file{bar.o}.

When this rule is chained with others, the result is very powerful.
Thus, @samp{make "foo.a(bar.o)"} (the quotes are needed to protect the
@samp{(} and @samp{)} from being interpreted specially by the shell) in
the presence of a file @file{bar.c} is enough to cause the following
recipe to be run, even without a makefile:

@example
cc -c bar.c -o bar.o
ar r foo.a bar.o
rm -f bar.o
@end example

@noindent
Here @code{make} has envisioned the file @file{bar.o} as an intermediate
file.  @xref{Chained Rules, ,Chains of Implicit Rules}.

Implicit rules such as this one are written using the automatic variable
@samp{$%}.  @xref{Automatic Variables}.

An archive member name in an archive cannot contain a directory name, but
it may be useful in a makefile to pretend that it does.  If you write an
archive member target @file{foo.a(dir/file.o)}, @code{make} will perform
automatic updating with this recipe:

@example
ar r foo.a dir/file.o
@end example

@noindent
which has the effect of copying the file @file{dir/file.o} into a member
named @file{file.o}.  In connection with such usage, the automatic variables
@code{%D} and @code{%F} may be useful.

@menu
* Archive Symbols::             How to update archive symbol directories.
@end menu

@node Archive Symbols
@subsection Updating Archive Symbol Directories
@cindex @code{__.SYMDEF}
@cindex updating archive symbol directories
@cindex archive symbol directory updating
@cindex symbol directories, updating archive
@cindex directories, updating archive symbol

An archive file that is used as a library usually contains a special member
named @file{__.SYMDEF} that contains a directory of the external symbol
names defined by all the other members.  After you update any other
members, you need to update @file{__.SYMDEF} so that it will summarize the
other members properly.  This is done by running the @code{ranlib} program:

@example
ranlib @var{archivefile}
@end example

Normally you would put this command in the rule for the archive file,
and make all the members of the archive file prerequisites of that rule.
For example,

@example
libfoo.a: libfoo.a(x.o y.o @dots{})
        ranlib libfoo.a
@end example

@noindent
The effect of this is to update archive members @file{x.o}, @file{y.o},
etc., and then update the symbol directory member @file{__.SYMDEF} by
running @code{ranlib}.  The rules for updating the members are not shown
here; most likely you can omit them and use the implicit rule which copies
files into the archive, as described in the preceding section.

This is not necessary when using the GNU @code{ar} program, which
updates the @file{__.SYMDEF} member automatically.

@node Archive Pitfalls
@section Dangers When Using Archives
@cindex archive, and parallel execution
@cindex parallel execution, and archive update
@cindex archive, and @code{-j}
@cindex @code{-j}, and archive update

The built-in rules for updating archives are incompatible with parallel
builds.  These rules (required by the POSIX standard) add each object file
into the archive as it's compiled.  When parallel builds are enabled this
allows multiple @code{ar} commands to be updating the same archive
simultaneously, which is not supported.

If you want to use parallel builds with archives you can override the default
rules by adding these lines to your makefile:

@example
(%) : % ;
%.a : ; $(AR) $(ARFLAGS) $@@ $?
@end example

The first line changes the rule that updates an individual object in the
archive to do nothing, and the second line changes the default rule for
building an archive to update all the outdated objects (@code{$?}) in one
command.

Of course you will still need to declare the prerequisites of your library
using the archive syntax:

@example
libfoo.a: libfoo.a(x.o y.o @dots{})
@end example

If you prefer to write an explicit rule you can use:

@example
libfoo.a: libfoo.a(x.o y.o @dots{})
        $(AR) $(ARFLAGS) $@@ $?
@end example

@node Archive Suffix Rules
@section Suffix Rules for Archive Files
@cindex suffix rule, for archive
@cindex archive, suffix rule for
@cindex library archive, suffix rule for
@cindex @code{.a} (archives)

You can write a special kind of suffix rule for dealing with archive
files.  @xref{Suffix Rules}, for a full explanation of suffix rules.
Archive suffix rules are obsolete in GNU @code{make}, because pattern
rules for archives are a more general mechanism (@pxref{Archive
Update}).  But they are retained for compatibility with other
@code{make}s.

To write a suffix rule for archives, you simply write a suffix rule
using the target suffix @samp{.a} (the usual suffix for archive files).
For example, here is the old-fashioned suffix rule to update a library
archive from C source files:

@example
@group
.c.a:
        $(CC) $(CFLAGS) $(CPPFLAGS) -c $< -o $*.o
        $(AR) r $@@ $*.o
        $(RM) $*.o
@end group
@end example

@noindent
This works just as if you had written the pattern rule:

@example
@group
(%.o): %.c
        $(CC) $(CFLAGS) $(CPPFLAGS) -c $< -o $*.o
        $(AR) r $@@ $*.o
        $(RM) $*.o
@end group
@end example

In fact, this is just what @code{make} does when it sees a suffix rule
with @samp{.a} as the target suffix.  Any double-suffix rule
@w{@samp{.@var{x}.a}} is converted to a pattern rule with the target
pattern @samp{(%.o)} and a prerequisite pattern of @samp{%.@var{x}}.

Since you might want to use @samp{.a} as the suffix for some other kind
of file, @code{make} also converts archive suffix rules to pattern rules
in the normal way (@pxref{Suffix Rules}).  Thus a double-suffix rule
@w{@samp{.@var{x}.a}} produces two pattern rules: @samp{@w{(%.o):}
@w{%.@var{x}}} and @samp{@w{%.a}: @w{%.@var{x}}}.

@node Extending make
@chapter Extending GNU @code{make}
@cindex make extensions

GNU @code{make} provides many advanced capabilities, including many
useful functions.  However, it does not contain a complete programming
language and so it has limitations.  Sometimes these limitations can be
overcome through use of the @code{shell} function to invoke a separate
program, although this can be inefficient.

In cases where the built-in capabilities of GNU @code{make} are
insufficient to your requirements there are two options for extending
@code{make}.  On systems where it's provided, you can utilize GNU
Guile as an embedded scripting language (@pxref{Guile Integration,,GNU
Guile Integration}).  On systems which support dynamically loadable
objects, you can write your own extension in any language (which can
be compiled into such an object) and load it to provide extended
capabilities (@pxref{load Directive, ,The @code{load} Directive}).

@menu
* Guile Integration::           Using Guile as an embedded scripting language.
* Loading Objects::             Loading dynamic objects as extensions.
@end menu

@node Guile Integration
@section GNU Guile Integration
@cindex Guile
@cindex extensions, Guile

GNU @code{make} may be built with support for GNU Guile as an embedded
extension language.  Guile implements the Scheme language.  A review
of GNU Guile and the Scheme language and its features is beyond the
scope of this manual: see the documentation for GNU Guile and Scheme.

You can determine if @code{make} contains support for Guile by
examining the @code{.FEATURES} variable; it will contain the word
@var{guile} if Guile support is available.

The Guile integration provides one new @code{make} function: @code{guile}.
The @code{guile} function takes one argument which is first expanded
by @code{make} in the normal fashion, then passed to the GNU Guile
evaluator.  The result of the evaluator is converted into a string and
used as the expansion of the @code{guile} function in the makefile.

In addition, GNU @code{make} exposes Guile procedures for use in Guile
scripts.

@menu
* Guile Types::                 Converting Guile types to @code{make} strings.
* Guile Interface::             Invoking @code{make} functions from Guile.
* Guile Example::               Example using Guile in @code{make}.
@end menu

@node Guile Types
@subsection Conversion of Guile Types
@cindex convert guile types
@cindex guile, conversion of types
@cindex types, conversion of

There is only one ``data type'' in @code{make}: a string.  GNU Guile,
on the other hand, provides a rich variety of different data types.
An important aspect of the interface between @code{make} and GNU Guile
is the conversion of Guile data types into @code{make} strings.

This conversion is relevant in two places: when a makefile invokes the
@code{guile} function to evaluate a Guile expression, the result of
that evaluation must be converted into a make string so it can be
further evaluated by @code{make}.  And secondly, when a Guile script
invokes one of the procedures exported by @code{make} the argument
provided to the procedure must be converted into a string.

The conversion of Guile types into @code{make} strings is as below:

@table @code
@item #f
False is converted into the empty string: in @code{make} conditionals
the empty string is considered false.

@item #t
True is converted to the string @samp{#t}: in @code{make} conditionals
any non-empty string is considered true.

@item symbol
@item number
A symbol or number is converted into the string representation of that
symbol or number.

@item character
A printable character is converted to the same character.

@item string
A string containing only printable characters is converted to the same
string.

@item list
A list is converted recursively according to the above rules.  This
implies that any structured list will be flattened (that is, a result
of @samp{'(a b (c d) e)} will be converted to the @code{make} string
@samp{a b c d e}).

@item other
Any other Guile type results in an error.  In future versions of
@code{make}, other Guile types may be converted.

@end table

The translation of @samp{#f} (to the empty string) and @samp{#t} (to
the non-empty string @samp{#t}) is designed to allow you to use Guile
boolean results directly as @code{make} boolean conditions.  For
example:

@example
$(if $(guile (access? "myfile" R_OK)),$(info myfile exists))
@end example

As a consequence of these conversion rules you must consider the
result of your Guile script, as that result will be converted into a
string and parsed by @code{make}.  If there is no natural result for
the script (that is, the script exists solely for its side-effects),
you should add @samp{#f} as the final expression in order to avoid
syntax errors in your makefile.

@node Guile Interface
@subsection Interfaces from Guile to @code{make}
@cindex make interface to guile
@cindex make procedures in guile

In addition to the @code{guile} function available in makefiles,
@code{make} exposes some procedures for use in your Guile scripts.  At
startup @code{make} creates a new Guile module, @code{gnu make}, and
exports these procedures as public interfaces from that module:

@table @code
@item gmk-expand
@findex gmk-expand
This procedure takes a single argument which is converted into a
string.  The string is expanded by @code{make} using normal
@code{make} expansion rules.  The result of the expansion is converted
into a Guile string and provided as the result of the procedure.

@item gmk-eval
@findex gmk-eval
This procedure takes a single argument which is converted into a
string.  The string is evaluated by @code{make} as if it were a
makefile.  This is the same capability available via the @code{eval}
function (@pxref{Eval Function}).  The result of the @code{gmk-eval}
procedure is always the empty string.

Note that @code{gmk-eval} is not quite the same as using
@code{gmk-expand} with the @code{eval} function: in the latter case
the evaluated string will be expanded @emph{twice}; first by
@code{gmk-expand}, then again by the @code{eval} function.

@end table

@node Guile Example
@subsection Example Using Guile in @code{make}
@cindex Guile example
@cindex example using Guile

Here is a very simple example using GNU Guile to manage writing to a
file.  These Guile procedures simply open a file, allow writing to the
file (one string per line), and close the file.  Note that because we
cannot store complex values such as Guile ports in @code{make}
variables, we'll keep the port as a global variable in the Guile
interpreter.

You can create Guile functions easily using @code{define}/@code{endef}
to create a Guile script, then use the @code{guile} function to
internalize it:

@example
@group
define GUILEIO
;; A simple Guile IO library for GNU Make

(define MKPORT #f)

(define (mkopen name mode)
  (set! MKPORT (open-file name mode))
  #f)

(define (mkwrite s)
  (display s MKPORT)
  (newline MKPORT)
  #f)

(define (mkclose)
  (close-port MKPORT)
  #f)

#f
endef

# Internalize the Guile IO functions
$(guile $(GUILEIO))
@end group
@end example

If you have a significant amount of Guile support code, you might
consider keeping it in a different file (e.g., @file{guileio.scm}) and
then loading it in your makefile using the @code{guile} function:

@example
$(guile (load "guileio.scm"))
@end example

An advantage to this method is that when editing @file{guileio.scm},
your editor will understand that this file contains Scheme syntax
rather than makefile syntax.

Now you can use these Guile functions to create files.  Suppose you
need to operate on a very large list, which cannot fit on the command
line, but the utility you're using accepts the list as input as well:

@example
@group
prog: $(PREREQS)
        @@$(guile (mkopen "tmp.out" "w")) \
         $(foreach X,$^,$(guile (mkwrite "$(X)"))) \
         $(guile (mkclose))
        $(LINK) < tmp.out
@end group
@end example

A more comprehensive suite of file manipulation procedures is possible
of course.  You could, for example, maintain multiple output files at
the same time by choosing a symbol for each one and using it as the
key to a hash table, where the value is a port, then returning the
symbol to be stored in a @code{make} variable.

@node Loading Objects
@section Loading Dynamic Objects
@cindex loaded objects
@cindex objects, loaded
@cindex extensions, loading

Many operating systems provide a facility for dynamically loading compiled
objects.  If your system provides this facility, GNU @code{make} can make use
of it to load dynamic objects at runtime, providing new capabilities which may
then be invoked by your makefile.

The @code{load} makefile directive is used to load a dynamic object.  Once the
object is loaded, an initializing function will be invoked to allow the object
to initialize itself and register new facilities with GNU @code{make}.  A
dynamic object might include new @code{make} functions, for example, and the
initializing function would register them with GNU @code{make}'s function
handling system.

@menu
* load Directive::              Loading dynamic objects as extensions.
* Initializing Functions::      How initializing functions are called.
* Remaking Loaded Objects::     How loaded objects get remade.
* Loaded Object API::           Programmatic interface for loaded objects.
* Loaded Object Example::       Example of a loaded object
@end menu

@node load Directive
@subsection The @code{load} Directive
@cindex load directive
@cindex extensions, load directive

Objects are loaded into GNU @code{make} by placing the @code{load}
directive into your makefile.  The syntax of the @code{load} directive
is as follows:

@findex load
@example
load @var{object-file} @dots{}
@end example

or:

@example
load @var{object-file}(@var{symbol-name}) @dots{}
@end example

More than one object file may be loaded with a single @code{load} directive,
and both forms of @code{load} arguments may be used in the same directive.

The file @var{object-file} is dynamically loaded by GNU @code{make}.  If
@var{object-file} does not include a directory path then it is first looked
for in the current directory.  If it is not found there, or a directory path
is included, then system-specific paths will be searched.  If the load fails
for any reason, @code{make} will print a message and exit.

If the load succeeds @code{make} will invoke an initializing function.  If
@var{symbol-name} is provided, it will be used as the name of the initializing
function.

If @var{symbol-name} is not provided, the initializing function name is created
by taking the base file name of @var{object-file}, up to the first character
which is not a valid symbol name character (alphanumerics and underscores are
valid symbol name characters).  To this prefix will be appended the suffix
@code{_gmk_setup}.

For example:

@example
load ../mk_funcs.so
@end example

will load the dynamic object @file{../mk_funcs.so}.  After the object is
loaded, @code{make} will invoke the initializing function (assumed to be
defined by the shared object) @code{mk_funcs_gmk_setup}.

On the other hand:

@example
load ../mk_funcs.so(init_mk_func)
@end example

will load the dynamic object @file{../mk_funcs.so}.  After the object is
loaded, @code{make} will invoke the initializing function @code{init_mk_func}.

Regardless of how many times an object file appears in a @code{load}
directive, it will only be loaded (and its setup function will only be
invoked) once.

@vindex .LOADED
After an object has been successfully loaded, its file name is appended to the
@code{.LOADED} variable.

@findex -load
If you would prefer that failure to load a dynamic object not be reported as
an error, you can use the @code{-load} directive instead of @code{load}.  GNU
@code{make} will not fail and no message will be generated if an object fails
to load.  The failed object is not added to the @code{.LOADED} variable, which
can then be consulted to determine if the load was successful.

@subsubheading Unloading Objects
@cindex unloading objects
@cindex unload function for loaded objects

When GNU Make needs to unload a loaded object, either because it is exiting or
because the loaded object has been rebuilt, it will invoke an unload function.
The unload function name is created by taking the base file name of the object
file, up to the first character which is not a valid symbol name character
(alphanumerics and underscores are valid symbol name characters), then
appending the suffix @code{_gmk_unload}.

If that function exists it will be called using the signature:

@example
void <name>_gmk_unload (void);
@end example

If the function does not exist, it will not be called.

Note that only one unload function may be defined per loaded object,
regardless of how many different setup methods are provided in that loaded
object.  If your loaded object provides multiple setup methods that require
unload support it's up to you to coordinate which setups have been invoked in
the unload function.

@node Initializing Functions
@subsection Initializing Functions
@cindex loaded object initializing function
@cindex initializing function, for loaded objects

The initializing function defined by the loaded object must have this
signature:

@example
int <name> (unsigned int abi_version, const gmk_floc *floc);
@end example

Where @emph{<name>} is described in the previous section.

The @code{abi_version} value will be the value of the @code{GMK_ABI_VERSION}
constant (see the @file{gnumake.h} file) for this GNU Make release.  The
@code{floc} pointer provides the file name and line number of the invocation
of the @code{load} operation.

The initializing function should return an @code{int}, which must be @code{0}
on failure and non-@code{0} on success.  If the return value is @code{-1},
then GNU Make will @emph{not} attempt to rebuild the object file
(@pxref{Remaking Loaded Objects, ,How Loaded Objects Are Remade}).

@node Remaking Loaded Objects
@subsection How Loaded Objects Are Remade
@cindex updating loaded objects
@cindex remaking loaded objects
@cindex loaded objects, remaking of

Loaded objects undergo the same re-make procedure as makefiles
(@pxref{Remaking Makefiles, ,How Makefiles Are Remade}).  If any loaded object
is recreated, then @code{make} will start from scratch and re-read all the
makefiles, and reload the object files again.  It is not necessary for the
loaded object to do anything special to support this.

It's up to the makefile author to provide the rules needed for rebuilding the
loaded object.

@node Loaded Object API
@subsection Loaded Object Interface
@cindex loaded object API
@cindex interface for loaded objects

To be useful, loaded objects must be able to interact with GNU @code{make}.
This interaction includes both interfaces the loaded object provides to
makefiles and also interfaces @code{make} provides to the loaded object to
manipulate @code{make}'s operation.

The interface between loaded objects and @code{make} is defined by the
@file{gnumake.h} C header file.  All loaded objects written in C should
include this header file.  Any loaded object not written in C will need to
implement the interface defined in this header file.

Typically, a loaded object will register one or more new GNU @code{make}
functions using the @code{gmk_add_function} routine from within its setup
function.  The implementations of these @code{make} functions may make use of
the @code{gmk_expand} and @code{gmk_eval} routines to perform their tasks,
then optionally return a string as the result of the function expansion.

@subsubheading Loaded Object Licensing
@cindex loaded object licensing
@cindex plugin_is_GPL_compatible

Every dynamic extension should define the global symbol
@code{plugin_is_GPL_compatible} to assert that it has been licensed under a
GPL-compatible license.  If this symbol does not exist, @code{make} emits a
fatal error and exits when it tries to load your extension.

The declared type of the symbol should be @code{int}. It does not need to be
in any allocated section, though.  The code merely asserts that the symbol
exists in the global scope. Something like this is enough:

@example
int plugin_is_GPL_compatible;
@end example

@subsubheading Data Structures

@table @code
@item gmk_floc
This structure represents a filename/location pair.  It is provided when
defining items, so GNU @code{make} can inform the user where the definition
occurred if necessary.
@end table

@subsubheading Checking Versions
@findex gmk_get_version

The @code{gmk_get_version} allows loaded objects to check which loaded object
API version is supported by GNU Make.  The API version is specified as two
values: the @emph{major} version and the @emph{minor} version.  Note, these
two values are not the same as the version of GNU Make!

The @emph{major} version is incremented when there is a change to the loaded
object ABI, which might cause .

It is called as:

@example
void gmk_get_version (unsigned int *major, unsigned int *minor);
@end example

@table @code
@item major
If not NULL, the major version number is placed here.

@item minor
If not NULL, the minor version number is placed here.
@end table


@subsubheading Registering Functions
@findex gmk_add_function

There is currently one way for makefiles to invoke operations provided by the
loaded object: through the @code{make} function call interface.  A loaded
object can register one or more new functions which may then be invoked from
within the makefile in the same way as any other function.

Use @code{gmk_add_function} to create a new @code{make} function.  Its
arguments are as follows:

@table @code
@item name
The function name.  This is what the makefile should use to invoke the
function.  The name must be between 1 and 255 characters long and it may only
contain alphanumeric, period (@samp{.}), dash (@samp{-}), and underscore
(@samp{_}) characters.  It may not begin with a period.

@item func_ptr
A pointer to a function that @code{make} will invoke when it expands the
function in a makefile.  This function must be defined by the loaded object.

@item min_args
The minimum number of arguments the function will accept.  Must be between 0
and 255.  GNU @code{make} will check this and fail before invoking
@code{func_ptr} if the function was invoked with too few arguments.

@item max_args
The maximum number of arguments the function will accept.  Must be between 0
and 255.  GNU @code{make} will check this and fail before invoking
@code{func_ptr} if the function was invoked with too many arguments.  If the
value is 0, then any number of arguments is accepted.  If the value is greater
than 0, then it must be greater than or equal to @code{min_args}.

@item flags
Flags that specify how this function will operate; the desired flags should be
OR'd together.  If the @code{GMK_FUNC_NOEXPAND} flag is given then the
function arguments will not be expanded before the function is called;
otherwise they will be expanded first.
@end table

@subsubheading Registered Function Interface
@findex gmk_func_ptr

A function registered with @code{make} must match the @code{gmk_func_ptr}
type.  It will be invoked with three parameters: @code{name} (the name of the
function), @code{argc} (the number of arguments to the function), and
@code{argv} (an array of pointers to arguments to the function).  The last
pointer (that is, @code{argv[argc]}) will be null (@code{0}).

The return value of the function is the result of expanding the function.  If
the function expands to nothing the return value may be null.  Otherwise, it
must be a pointer to a string created with @code{gmk_alloc}.  Once the
function returns, @code{make} owns this string and will free it when
appropriate; it cannot be accessed by the loaded object.

@subsubheading GNU @code{make} Facilities

There are some facilities exported by GNU @code{make} for use by loaded
objects.  Typically these would be run from within the setup function and/or
the functions registered via @code{gmk_add_function}, to retrieve or modify
the data @code{make} works with.

@table @code
@item gmk_expand
@findex gmk_expand
This function takes a string and expands it using @code{make} expansion rules.
The result of the expansion is returned in a nil-terminated string buffer.
The caller is responsible for calling @code{gmk_free} with a pointer to the
returned buffer when done.

@item gmk_eval
@findex gmk_eval
This function takes a buffer and evaluates it as a segment of makefile syntax.
This function can be used to define new variables, new rules, etc.  It is
equivalent to using the @code{eval} @code{make} function.
@end table

Note that there is a difference between @code{gmk_eval} and calling
@code{gmk_expand} with a string using the @code{eval} function: in the latter
case the string will be expanded @emph{twice}; once by @code{gmk_expand} and
then again by the @code{eval} function.  Using @code{gmk_eval} the buffer is
only expanded once, at most (as it's read by the @code{make} parser).

@subsubheading Memory Management

Some systems allow for different memory management schemes.  Thus you should
never pass memory that you've allocated directly to any @code{make} function,
nor should you attempt to directly free any memory returned to you by any
@code{make} function.  Instead, use the @code{gmk_alloc} and @code{gmk_free}
functions.

In particular, the string returned to @code{make} by a function registered
using @code{gmk_add_function} @emph{must} be allocated using @code{gmk_alloc},
and the string returned from the @code{make} @code{gmk_expand} function
@emph{must} be freed (when no longer needed) using @code{gmk_free}.

@table @code
@item gmk_alloc
@findex gmk_alloc
Return a pointer to a newly-allocated buffer.  This function will always
return a valid pointer; if not enough memory is available @code{make} will
exit.  @code{gmk_alloc} does not initialize allocated memory.

@item gmk_free
@findex gmk_free
Free a buffer returned to you by @code{make}.  Once the @code{gmk_free}
function returns the string will no longer be valid.  If NULL is passed to
@code{gmk_free}, no operation is performed.
@end table

@node Loaded Object Example
@subsection Example Loaded Object
@cindex loaded object example
@cindex example of loaded objects

Let's suppose we wanted to write a new GNU @code{make} function that would
create a temporary file and return its name.  We would like our function to
take a prefix as an argument.  First we can write the function in a file
@file{mk_temp.c}:

@example
@group
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>

#include <gnumake.h>

int plugin_is_GPL_compatible;

struct tmpfile @{
  struct tmpfile *next;
  char *name;
@};
static struct tmpfile *files = NULL;
@end group

@group
static char *
gen_tmpfile(const char *nm, unsigned int argc, char **argv)
@{
  int fd;

  /* Compute the size of the filename and allocate space for it.  */
  int len = strlen (argv[0]) + 6 + 1;
  char *buf = gmk_alloc (len);

  strcpy (buf, argv[0]);
  strcat (buf, "XXXXXX");

  fd = mkstemp(buf);
  if (fd >= 0)
    @{
      struct tmpfile *new = malloc (sizeof (struct tmpfile));
      new->name = strdup (buf);
      new->next = files;
      files = new;

      /* Don't leak the file descriptor.  */
      close (fd);
      return buf;
    @}

  /* Failure.  */
  fprintf (stderr, "mkstemp(%s) failed: %s\n", buf, strerror (errno));
  gmk_free (buf);
  return NULL;
@}
@end group

@group
int
mk_temp_gmk_setup (unsigned int abi, const gmk_floc *floc)
@{
  printf ("mk_temp abi %u plugin loaded from %s:%lu\n",
          abi, floc->filenm, floc->lineno);
  /* Register the function with make name "mk-temp".  */
  gmk_add_function ("mk-temp", gen_tmpfile, 1, 1, 1);
  return 1;
@}
@end group

@group
void
mk_temp_gmk_close ()
@{
  while (files)
    @{
      struct tmpfile *f = files;
      files = f->next;
      printf ("mk_temp removing %s\n", f->name);
      remove (f->name);
      free (f->name);
      free (f);
    @}
  printf ("mk_temp plugin closed\n");
@}
@end group
@end example

Next, we will write a @file{Makefile} that can build this shared object, load
it, and use it:

@example
@group
all:
        @@echo Temporary file: $(mk-temp tmpfile.)
        @@echo Temporary file: $(mk-temp tmpfile.)

-load mk_temp.so

mk_temp.so: mk_temp.c
        $(CC) -shared -fPIC -o $@@ $<
@end group
@end example

On MS-Windows, due to peculiarities of how shared objects are produced, the
compiler needs to scan the @dfn{import library} produced when building
@code{make}, typically called @file{libgnumake-@var{version}.dll.a}, where
@var{version} is the version of the load object API.  So the recipe to produce
a shared object will look like this on Windows (assuming the API version is
1):

@example
@group
mk_temp.dll: mk_temp.c
        $(CC) -shared -o $@@ $< -lgnumake-1
@end group
@end example

Now when you run @code{make} you'll see something like:

@example
@group
$ make
cc -shared -fPIC -o mk_temp.so mk_temp.c
mk_temp abi 1 plugin loaded from Makefile:5
Temporary file: tmpfile.OYkGMT
Temporary file: tmpfile.sYsJO0
mk_temp removing tmpfile.sYsJO0
mk_temp removing tmpfile.OYkGMT
mk_temp plugin closed
@end group
@end example

@node Integrating make
@chapter Integrating GNU @code{make}
@cindex make integration

GNU @code{make} is often one component in a larger system of tools,
including integrated development environments, compiler toolchains,
and others.  The role of @code{make} is to start commands and
determine whether they succeeded or not: no special integration is
needed to accomplish that.  However, sometimes it is convenient to
bind @code{make} more tightly with other parts of the system, both
higher-level (tools that invoke @code{make}) and lower-level (tools
that @code{make} invokes).

@menu
* Job Slots::                   Share job slots with GNU Make.
* Terminal Output::             Control output to terminals.
@end menu

@node Job Slots
@section Sharing Job Slots with GNU @code{make}
@cindex job slots, sharing
@cindex tools, sharing job slots

GNU @code{make} has the ability to run multiple recipes in parallel
(@pxref{Parallel, ,Parallel Execution}) and to cap the total number of
parallel jobs even across recursive invocations of @code{make}
(@pxref{Options/Recursion, ,Communicating Options to a
Sub-@code{make}}).  Tools that @code{make} invokes which are also able
to run multiple operations in parallel, either using multiple threads
or multiple processes, can be enhanced to participate in GNU
@code{make}'s job management facility to ensure that the total number
of active threads/processes running on the system does not exceed the
maximum number of slots provided to GNU @code{make}. 

@cindex jobserver
GNU @code{make} uses a method called the ``jobserver'' to control the
number of active jobs across recursive invocations.  The actual
implementation of the jobserver varies across different operating
systems, but some fundamental aspects are always true.

@cindex @code{--jobserver-auth}
First, @code{make} will provide information necessary for accessing the
jobserver through the environment to its children, in the @code{MAKEFLAGS}
environment variable.  Tools which want to participate in the jobserver
protocol will need to parse this environment variable and find the word
starting with @code{--jobserver-auth=}.  The value of this option will
describe how to communicate with the jobserver.  The interpretation of this
value is described in the sections below.

Be aware that the @code{MAKEFLAGS} variable may contain multiple instances of
the @code{--jobserver-auth=} option.  Only the @emph{last} instance is
relevant.

Remember that GNU Make does not create a jobserver at all if the user requests
``infinite parallelism'' by using @samp{-j} with no argument limiting the
number of job slots.  In this situation there's no maximum number of job
slots, thus no need for a jobserver, and there will be no
@samp{--jobserver-auth} option in @code{MAKEFLAGS}: every instance of
@code{make} can invoke as many jobs in parallel as it likes.

Second, every command @code{make} starts has one implicit job slot
reserved for it before it starts.  Any tool which wants to participate
in the jobserver protocol should assume it can always run one job
without having to contact the jobserver at all.

Finally, it's critical that tools that participate in the jobserver
protocol return the exact number of slots they obtained from the
jobserver back to the jobserver before they exit, even under error
conditions.  Remember that the implicit job slot should @strong{not}
be returned to the jobserver!  Returning too few slots means that
those slots will be lost for the rest of the build process; returning
too many slots means that extra slots will be available.  The
top-level @code{make} command will print an error message at the end
of the build if it detects an incorrect number of slots available in
the jobserver.

As an example, suppose you are implementing a linker which provides
for multi-threaded operation.  You would like to enhance the linker so
that if it is invoked by GNU @code{make} it can participate in the
jobserver protocol to control how many threads are used during link.
First you will need to modify the linker to determine if the
@code{MAKEFLAGS} environment variable is set.  Next you will need to
parse the value of that variable to determine if the jobserver is
available, and how to access it.  If it is available then you can
access it to obtain job slots controlling how much parallelism your
tool can use.  Once done your tool must return those job slots back to
the jobserver.

@menu
* POSIX Jobserver::             Using the jobserver on POSIX systems.
* Windows Jobserver::           Using the jobserver on Windows systems.
@end menu

@node POSIX Jobserver
@subsection POSIX Jobserver Interaction
@cindex jobserver on POSIX

On POSIX systems the jobserver is implemented in one of two ways: on systems
that support it, GNU @code{make} will create a named pipe and use that for the
jobserver.  In this case the auth option will have the form
@code{--jobserver-auth=fifo:PATH} where @samp{PATH} is the pathname of the
named pipe.  To access the jobserver you should open the named pipe path and
read/write to it as described below.

@cindex @code{--jobserver-style}
If the system doesn't support named pipes, or if the user provided the
@code{--jobserver-style} option and specified @samp{pipe}, then the jobserver
will be implemented as a simple UNIX pipe.  In this case the auth option will
have the form @code{--jobserver-auth=R,W} where @samp{R} and @samp{W} are
non-negative integers representing file descriptors: @samp{R} is the read file
descriptor and @samp{W} is the write file descriptor.  If either or both of
these file descriptors are negative, it means the jobserver is disabled for
this process.

When using a simple pipe, only command lines that @code{make} understands to
be recursive invocations of @code{make} (@pxref{MAKE Variable, ,How the
@code{MAKE} Variable Works}) will have access to the jobserver.  When writing
makefiles you must be sure to mark the command as recursive (most commonly by
prefixing the command line with the @code{+} indicator (@pxref{Recursion,
,Recursive Use of @code{make}}).  Note that the read side of the jobserver
pipe is set to ``blocking'' mode.  This should not be changed.

In both implementations of the jobserver, the pipe will be pre-loaded with one
single-character token for each available job.  To obtain an extra slot you
must read a single character from the jobserver; to release a slot you must
write a single character back into the jobserver.

It's important that when you release the job slot, you write back the same
character you read.  Don't assume that all tokens are the same character;
different characters may have different meanings to GNU @code{make}.  The
order is not important, since @code{make} has no idea in what order jobs will
complete anyway.

There are various error conditions you must consider to ensure your
implementation is robust:

@itemize @bullet
@item
If you have a command-line argument controlling the parallel operation of your
tool, consider whether your tool should detect situations where both the
jobserver and the command-line argument are specified, and how it should
react.

@item
If your tool does not recognize the format of the @code{--jobserver-auth}
string, it should assume the jobserver is using a different style and it
cannot connect.

@item
If your tool determines that the @code{--jobserver-auth} option references a
simple pipe but that the file descriptors specified are closed, this means
that the calling @code{make} process did not think that your tool was a
recursive @code{make} invocation (e.g., the command line was not prefixed with
a @code{+} character).  You should notify your users of this situation.

@item
Your tool should be sure to write back the tokens it read, even under error
conditions.  This includes not only errors in your tool but also outside
influences such as interrupts (@code{SIGINT}), etc.  You may want to install
signal handlers to manage this write-back.

@item
Your tool may also examine the first word of the @code{MAKEFLAGS} variable and
look for the character @code{n}.  If this character is present then
@code{make} was invoked with the @samp{-n} option and your tool may want to
stop without performing any operations.
@end itemize

@node Windows Jobserver
@subsection Windows Jobserver Interaction
@cindex jobserver on Windows

On Windows systems the jobserver is implemented as a named semaphore.
The semaphore will be set with an initial count equal to the number of
available slots; to obtain a slot you must wait on the semaphore (with
or without a timeout).  To release a slot, release the semaphore.

To access the semaphore you must parse the @code{MAKEFLAGS} variable and
look for the argument string @code{--jobserver-auth=NAME} where
@samp{NAME} is the name of the named semaphore.  Use this name with
@code{OpenSemaphore} to create a handle to the semaphore.

@cindex @code{--jobserver-style} for Windows
The only valid style for @code{--jobserver-style} is @samp{sem}.

There are various error conditions you must consider to ensure your
implementation is robust:

@itemize @bullet
@item
Usually you will have a command-line argument controlling the parallel
operation of your tool.  Consider whether your tool should detect
situations where both the jobserver and the command-line argument are
specified, and how it should react.

@item
Your tool should be sure to release the semaphore for the tokens it
read, even under error conditions.  This includes not only errors in
your tool but also outside influences such as interrupts
(@code{SIGINT}), etc.  You may want to install signal handlers to
manage this write-back.
@end itemize

@node Terminal Output
@section Synchronized Terminal Output
@cindex parallel output to terminal
@cindex terminal, output to

Normally GNU @code{make} will invoke all commands with access to the
same standard and error outputs that @code{make} itself was started
with.  A number of tools will detect whether the output is a terminal
or not-a-terminal, and use this information to change the output
style.  For example if the output goes to a terminal the tool may add
control characters that set color, or even change the location of the
cursor.  If the output is not going to a terminal then these special
control characters are not emitted so that they don't corrupt log
files, etc.

The @code{--output-sync} (@pxref{Parallel Output, ,Output During
Parallel Execution}) option will defeat the terminal detection.  When
output synchronization is enabled GNU @code{make} arranges for all
command output to be written to a file, so that its output can be
written as a block without interference from other commands.  This
means that all tools invoked by @code{make} will believe that their
output is not going to be displayed on a terminal, even when it will
be (because @code{make} will display it there after the command is
completed).

In order to facilitate tools which would like to determine whether or
not their output will be displayed on a terminal, GNU @code{make} will
set the @code{MAKE_TERMOUT} and @code{MAKE_TERMERR} environment
variables before invoking any commands.  Tools which would like to
determine whether standard or error output (respectively) will be
displayed on a terminal can check these environment variables to
determine if they exist and contain a non-empty value.  If so the tool
can assume that the output will (eventually) be displayed on a
terminal.  If the variables are not set or have an empty value, then
the tool should fall back to its normal methods of detecting whether
output is going to a terminal or not.

The content of the variables can be parsed to determine the type of
terminal which will be used to display the output.

Similarly, environments which invoke @code{make} and would like to
capture the output and eventually display it on a terminal (or some
display which can interpret terminal control characters) can set these
variables before invoking @code{make}.  GNU @code{make} will not
modify these environment variables if they already exist when it
starts.

@node Features
@chapter Features of GNU @code{make}
@cindex features of GNU @code{make}
@cindex portability
@cindex compatibility

Here is a summary of the features of GNU @code{make}, for comparison
with and credit to other versions of @code{make}.  We consider the
features of @code{make} in 4.2 BSD systems as a baseline.  If you are
concerned with writing portable makefiles, you should not use the
features of @code{make} listed here, nor the ones in @ref{Missing}.

Many features come from the version of @code{make} in System V.

@itemize @bullet
@item
The @code{VPATH} variable and its special meaning.
@xref{Directory Search, , Searching Directories for Prerequisites}.
This feature exists in System V @code{make}, but is undocumented.
It is documented in 4.3 BSD @code{make} (which says it mimics System V's
@code{VPATH} feature).

@item
Included makefiles.  @xref{Include, ,Including Other Makefiles}.
Allowing multiple files to be included with a single directive is a GNU
extension.

@item
Variables are read from and communicated via the environment.
@xref{Environment, ,Variables from the Environment}.

@item
Options passed through the variable @code{MAKEFLAGS} to recursive
invocations of @code{make}.
@xref{Options/Recursion, ,Communicating Options to a Sub-@code{make}}.

@item
The automatic variable @code{$%} is set to the member name
in an archive reference.  @xref{Automatic Variables}.

@item
The automatic variables @code{$@@}, @code{$*}, @code{$<}, @code{$%},
and @code{$?} have corresponding forms like @code{$(@@F)} and
@code{$(@@D)}.  We have generalized this to @code{$^} as an obvious
extension.  @xref{Automatic Variables}.

@item
Substitution variable references.
@xref{Reference, ,Basics of Variable References}.

@item
The command line options @samp{-b} and @samp{-m}, accepted and
ignored.  In System V @code{make}, these options actually do something.

@item
Execution of recursive commands to run @code{make} via the variable
@code{MAKE} even if @samp{-n}, @samp{-q} or @samp{-t} is specified.
@xref{Recursion, ,Recursive Use of @code{make}}.

@item
Support for suffix @samp{.a} in suffix rules.  @xref{Archive Suffix
Rules}.  This feature is obsolete in GNU @code{make}, because the
general feature of rule chaining (@pxref{Chained Rules, ,Chains of
Implicit Rules}) allows one pattern rule for installing members in an
archive (@pxref{Archive Update}) to be sufficient.

@item
The arrangement of lines and backslash/newline combinations in
recipes is retained when the recipes are printed, so they appear as
they do in the makefile, except for the stripping of initial
whitespace.
@end itemize

The following features were inspired by various other versions of
@code{make}.  In some cases it is unclear exactly which versions inspired
which others.

@itemize @bullet
@item
Pattern rules using @samp{%}.
This has been implemented in several versions of @code{make}.
We're not sure who invented it first, but it's been spread around a bit.
@xref{Pattern Rules, ,Defining and Redefining Pattern Rules}.

@item
Rule chaining and implicit intermediate files.
This was implemented by Stu Feldman in his version of @code{make}
for AT&T Eighth Edition Research Unix, and later by Andrew Hume of
AT&T Bell Labs in his @code{mk} program (where he terms it
``transitive closure'').  We do not really know if
we got this from either of them or thought it up ourselves at the
same time.  @xref{Chained Rules, ,Chains of Implicit Rules}.

@item
The automatic variable @code{$^} containing a list of all prerequisites
of the current target.  We did not invent this, but we have no idea who
did.  @xref{Automatic Variables}.  The automatic variable
@code{$+} is a simple extension of @code{$^}.

@item
The ``what if'' flag (@samp{-W} in GNU @code{make}) was (as far as we know)
invented by Andrew Hume in @code{mk}.
@xref{Instead of Execution, ,Instead of Executing Recipes}.

@item
The concept of doing several things at once (parallelism) exists in
many incarnations of @code{make} and similar programs, though not in the
System V or BSD implementations.  @xref{Execution, ,Recipe Execution}.

@item
A number of different build tools that support parallelism also
support collecting output and displaying as a single block.
@xref{Parallel Output, ,Output During Parallel Execution}.

@item
Modified variable references using pattern substitution come from
SunOS 4.  @xref{Reference, ,Basics of Variable References}.
This functionality was provided in GNU @code{make} by the
@code{patsubst} function before the alternate syntax was implemented
for compatibility with SunOS 4.  It is not altogether clear who
inspired whom, since GNU @code{make} had @code{patsubst} before SunOS
4 was released.

@item
The special significance of @samp{+} characters preceding recipe lines
(@pxref{Instead of Execution, ,Instead of Executing Recipes}) is
mandated by @cite{IEEE Standard 1003.1-2024} (POSIX).

@item
The @samp{+=} syntax to append to the value of a variable comes from SunOS
4 @code{make}.  @xref{Appending Assignment, , Appending More Text to Variables}.

@item
The syntax @w{@samp{@var{archive}(@var{mem1} @var{mem2}@dots{})}} to list
multiple members in a single archive file comes from SunOS 4 @code{make}.
@xref{Archive Members}.

@item
The @code{-include} directive to include makefiles with no error for a
nonexistent file comes from SunOS 4 @code{make}.  (But note that SunOS 4
@code{make} does not allow multiple makefiles to be specified in one
@code{-include} directive.)  The same feature appears with the name
@code{sinclude} in SGI @code{make} and perhaps others.

@item
The @code{!=} shell assignment operator exists in many BSD of
@code{make} and is purposefully implemented here to behave identically
to those implementations.

@item
Various build management tools are implemented using scripting
languages such as Perl or Python and thus provide a natural embedded
scripting language, similar to GNU @code{make}'s integration of GNU
Guile.
@end itemize

The remaining features are inventions new in GNU @code{make}:

@itemize @bullet
@item
Use the @samp{-v} or @samp{--version} option to print version and
copyright information.

@item
Use the @samp{-h} or @samp{--help} option to summarize the options to
@code{make}.

@item
Simply-expanded variables.  @xref{Flavors, ,The Two Flavors of Variables}.

@item
Pass command line variable assignments automatically through the
variable @code{MAKE} to recursive @code{make} invocations.
@xref{Recursion, ,Recursive Use of @code{make}}.

@item
Use the @samp{-C} or @samp{--directory} command option to change
directory.  @xref{Options Summary, ,Summary of Options}.

@item
Make verbatim variable definitions with @code{define}.
@xref{Multi-Line, ,Defining Multi-Line Variables}.

@item
Declare phony targets with the special target @code{.PHONY}.

Andrew Hume of AT&T Bell Labs implemented a similar feature with a
different syntax in his @code{mk} program.  This seems to be a case of
parallel discovery.  @xref{Phony Targets, ,Phony Targets}.

@item
Manipulate text by calling functions.
@xref{Functions, ,Functions for Transforming Text}.

@item
Use the @samp{-o} or @samp{--old-file}
option to pretend a file's modification-time is old.
@xref{Avoiding Compilation, ,Avoiding Recompilation of Some Files}.

@item
Conditional execution.

This feature has been implemented numerous times in various versions
of @code{make}; it seems a natural extension derived from the features
of the C preprocessor and similar macro languages and is not a
revolutionary concept.  @xref{Conditionals, ,Conditional Parts of Makefiles}.

@item
Specify a search path for included makefiles.
@xref{Include, ,Including Other Makefiles}.

@item
Specify extra makefiles to read with an environment variable.
@xref{MAKEFILES Variable, ,The Variable @code{MAKEFILES}}.

@item
Strip leading sequences of @samp{./} from file names, so that
@file{./@var{file}} and @file{@var{file}} are considered to be the
same file.

@item
Use a special search method for library prerequisites written in the
form @samp{-l@var{name}}.
@xref{Libraries/Search, ,Directory Search for Link Libraries}.

@item
Allow suffixes for suffix rules
(@pxref{Suffix Rules, ,Old-Fashioned Suffix Rules}) to contain any
characters.  In other versions of @code{make}, they must begin with
@samp{.} and not contain any @samp{/} characters.

@item
Keep track of the current level of @code{make} recursion using the
variable @code{MAKELEVEL}.  @xref{Recursion, ,Recursive Use of @code{make}}.

@item
Provide any goals given on the command line in the variable
@code{MAKECMDGOALS}.  @xref{Goals, ,Arguments to Specify the Goals}.

@item
Specify static pattern rules.  @xref{Static Pattern, ,Static Pattern Rules}.

@item
Provide selective @code{vpath} search.
@xref{Directory Search, ,Searching Directories for Prerequisites}.

@item
Provide computed variable references.
@xref{Reference, ,Basics of Variable References}.

@item
Update makefiles.  @xref{Remaking Makefiles, ,How Makefiles Are Remade}.
System V @code{make} has a very, very limited form of this
functionality in that it will check out SCCS files for makefiles.

@item
Various new built-in implicit rules.
@xref{Catalogue of Rules, ,Catalogue of Built-In Rules}.

@item
Load dynamic objects which can modify the behavior of @code{make}.
@xref{Loading Objects, ,Loading Dynamic Objects}.
@end itemize

@node Missing
@chapter Incompatibilities and Missing Features
@cindex incompatibilities
@cindex missing features
@cindex features, missing

The @code{make} programs in various other systems support a few features
that are not implemented in GNU @code{make}.  The POSIX standard
(@cite{IEEE Standard 1003.1-2024}) which specifies @code{make} does not
require any of these features.

@itemize @bullet
@item
A target of the form @samp{@var{file}((@var{entry}))} stands for a member
of archive file @var{file}.  The member is chosen, not by name, but by
being an object file which defines the linker symbol @var{entry}.

This feature was not put into GNU @code{make} because of the
non-modularity of putting knowledge into @code{make} of the internal
format of archive file symbol tables.
@xref{Archive Symbols, ,Updating Archive Symbol Directories}.

@item
Suffixes (used in suffix rules) that end with the character @samp{~}
have a special meaning to System V @code{make};
they refer to the SCCS file that corresponds
to the file one would get without the @samp{~}.  For example, the
suffix rule @samp{.c~.o} would make the file @file{@var{n}.o} from
the SCCS file @file{s.@var{n}.c}.  For complete coverage, a whole
series of such suffix rules is required.
@xref{Suffix Rules, ,Old-Fashioned Suffix Rules}.

In GNU @code{make}, this entire series of cases is handled by two
pattern rules for extraction from SCCS, in combination with the
general feature of rule chaining.
@xref{Chained Rules, ,Chains of Implicit Rules}.

@item
In System V and 4.3 BSD @code{make}, files found by @code{VPATH}
search (@pxref{Directory Search, ,Searching Directories for
Prerequisites}) have their names changed inside recipes.  We feel it
is much cleaner to always use automatic variables and thus make this
feature unnecessary.

@item
In some Unix @code{make}s, the automatic variable @code{$*} appearing in
the prerequisites of a rule has the amazingly strange ``feature'' of
expanding to the full name of the @emph{target of that rule}.  We cannot
imagine what went on in the minds of Unix @code{make} developers to do
this; it is utterly inconsistent with the normal definition of @code{$*}.
@vindex * @r{(automatic variable), unsupported bizarre usage}

@item
In some Unix @code{make}s, implicit rule search (@pxref{Implicit
Rules, ,Using Implicit Rules}) is apparently done for @emph{all}
targets, not just those without recipes.  This means you can
do:

@example
@group
foo.o:
        cc -c foo.c
@end group
@end example

@noindent
and Unix @code{make} will intuit that @file{foo.o} depends on
@file{foo.c}.

We feel that such usage is broken.  The prerequisite properties of
@code{make} are well-defined (for GNU @code{make}, at least),
and doing such a thing simply does not fit the model.

@item
GNU @code{make} does not include any built-in implicit rules for
compiling or preprocessing EFL programs.  If we hear of anyone who is
using EFL, we will gladly add them.

@item
It appears that in SVR4 @code{make}, a suffix rule can be specified
with no recipe, and it is treated as if it had an empty recipe
(@pxref{Empty Recipes}).  For example:

@example
.c.a:
@end example

@noindent
will override the built-in @file{.c.a} suffix rule.

We feel that it is cleaner for a rule without a recipe to always simply
add to the prerequisite list for the target.  The above example can be
easily rewritten to get the desired behavior in GNU @code{make}:

@example
.c.a: ;
@end example

@item
Some versions of @code{make} invoke the shell with the @samp{-e} flag,
except under @samp{-k} (@pxref{Testing, ,Testing the Compilation of a
Program}).  The @samp{-e} flag tells the shell to exit as soon as any
program it runs returns a nonzero status.  We feel it is cleaner to
write each line of the recipe to stand on its own and not require this
special treatment.
@end itemize

@comment The makefile standards are in a separate file that is also
@comment included by standards.texi.
@include make-stds.texi

@node Quick Reference
@appendix Quick Reference

This appendix summarizes the directives, text manipulation functions,
and special variables which GNU @code{make} understands.
@xref{Special Targets}, @ref{Catalogue of Rules, ,Catalogue of Built-In Rules},
and @ref{Options Summary, ,Summary of Options},
for other summaries.

@menu
* Makefile Directives::         All makefile directives.
* Makefile Functions::          All makefile built-in functions.
* Automatic Variable Reference::  All automatic variables for recipes.
* Special Variable Reference::  All special variables for makefiles.
@end menu

@node Makefile Directives
@appendixsec Makefile Directives Reference

Here is a summary of the directives GNU @code{make} recognizes:

@table @code
@item define @var{variable}
@itemx define @var{variable} =
@itemx define @var{variable} :=
@itemx define @var{variable} ::=
@itemx define @var{variable} :::=
@itemx define @var{variable} !=
@itemx define @var{variable} ?=
@itemx define @var{variable} ?:=
@itemx define @var{variable} ?::=
@itemx define @var{variable} ?:::=
@itemx define @var{variable} ?!=
@itemx define @var{variable} +=
@itemx endef
Define multi-line variables.@*
@xref{Multi-Line}.

@item undefine @var{variable}
Undefining variables.@*
@xref{Undefine Directive}.

@item ifdef @var{variable}
@itemx ifndef @var{variable}
@itemx ifeq (@var{a},@var{b})
@itemx ifeq "@var{a}" "@var{b}"
@itemx ifeq '@var{a}' '@var{b}'
@itemx ifneq (@var{a},@var{b})
@itemx ifneq "@var{a}" "@var{b}"
@itemx ifneq '@var{a}' '@var{b}'
@itemx else
@itemx endif
Conditionally evaluate part of the makefile.@*
@xref{Conditionals}.

@item include @var{file}
@itemx -include @var{file}
@itemx sinclude @var{file}
Include another makefile.@*
@xref{Include, ,Including Other Makefiles}.

@item override @var{variable-assignment}
Define a variable, overriding any previous definition, even one from
the command line.@*
@xref{Override Directive, ,The @code{override} Directive}.

@item export
Tell @code{make} to export all variables to child processes by default.@*
@xref{Variables/Recursion, , Communicating Variables to a Sub-@code{make}}.

@item export @var{variable}
@itemx export @var{variable-assignment}
@itemx unexport @var{variable}
Tell @code{make} whether or not to export a particular variable to child
processes.@*
@xref{Variables/Recursion, , Communicating Variables to a Sub-@code{make}}.

@item private @var{variable-assignment}
Do not allow this variable assignment to be inherited by prerequisites.@*
@xref{Suppressing Inheritance}.

@item vpath @var{pattern} @var{path}
Specify a search path for files matching a @samp{%} pattern.@*
@xref{Selective Search, , The @code{vpath} Directive}.

@item vpath @var{pattern}
Remove all search paths previously specified for @var{pattern}.@*
@xref{Selective Search, , The @code{vpath} Directive}.

@item vpath
Remove all search paths previously specified in any @code{vpath} directive.@*
@xref{Selective Search, , The @code{vpath} Directive}.
@end table

@node Makefile Functions
@appendixsec Makefile Functions Reference

Here is a summary of the built-in functions (@pxref{Functions}):

@table @code
@item $(abspath @var{names}@dots{})
For each file name in @var{names}, expand to an absolute name that does not
contain any @code{.} or @code{..} components, but preserves symlinks.@*
@xref{File Name Functions, ,Functions for File Names}.

@item $(addprefix @var{prefix},@var{names}@dots{})
Prepend @var{prefix} to each word in @var{names}.@*
@xref{File Name Functions, ,Functions for File Names}.

@item $(addsuffix @var{suffix},@var{names}@dots{})
Append @var{suffix} to each word in @var{names}.@*
@xref{File Name Functions, ,Functions for File Names}.

@item $(and @var{condition1}[,@var{condition2}[,@var{condition3}@dots{}]])
Evaluate each condition @var{conditionN} one at a time; if any
expansion results in the empty string substitute the empty string.  If
all expansions result in a non-empty string, substitute the expansion
of the last @var{condition}.@*
@xref{Conditional Functions, ,Functions for Conditionals}.

@item $(basename @var{names}@dots{})
Extract the base name (name without suffix) of each file name.@*
@xref{File Name Functions, ,Functions for File Names}.

@item $(call @var{var},@var{param},@dots{})
Evaluate the variable @var{var} replacing any references to @code{$(1)},
@code{$(2)} with the first, second, etc.@: @var{param} values.@*
@xref{Call Function, ,The @code{call} Function}.

@item $(dir @var{names}@dots{})
Extract the directory part of each file name.@*
@xref{File Name Functions, ,Functions for File Names}.

@item $(error @var{text}@dots{})
When this function is expanded, @code{make} prints @var{text} to standard
error, then @code{make} exits with a failure code.@*
@xref{Make Control Functions, ,Functions That Control Make}.

@item $(eval @var{text})
Evaluate @var{text} then read the results as makefile commands.
Expands to the empty string.@*
@xref{Eval Function, ,The @code{eval} Function}.

@item $(file @var{op} @var{filename},@var{text})
Expand the arguments, then open the file @var{filename} using mode
@var{op} and write @var{text} to that file.@*
@xref{File Function, ,The @code{file} Function}.

@item $(filter @var{pattern}@dots{},@var{text})
Select words in @var{text} that match one of the @var{pattern} words.@*
@xref{Text Functions, , Functions for String Substitution and Analysis}.

@item $(filter-out @var{pattern}@dots{},@var{text})
Select words in @var{text} that @emph{do not} match any of the @var{pattern}
words.@*
@xref{Text Functions, , Functions for String Substitution and Analysis}.

@item $(findstring @var{find},@var{text})
Locate @var{find} in @var{text}.@*
@xref{Text Functions, , Functions for String Substitution and Analysis}.

@item $(firstword @var{names}@dots{})
Extract the first word of @var{names}.@*
@xref{Text Functions, , Functions for String Substitution and Analysis}.

@item $(flavor @var{variable})
Return a string describing the flavor of the @code{make} variable
@var{variable}.@*
@xref{Flavor Function, , The @code{flavor} Function}.

@item $(foreach @var{var},@var{words},@var{text})
Evaluate @var{text} with @var{var} bound to each word in @var{words},
and concatenate the results.@*
@xref{Foreach Function, ,The @code{foreach} Function}.

@item $(if @var{condition},@var{then-part}[,@var{else-part}])
Evaluate the condition @var{condition}; if it's non-empty substitute
the expansion of the @var{then-part} otherwise substitute the
expansion of the @var{else-part}.@*
@xref{Conditional Functions, ,Functions for Conditionals}.

@item $(info @var{text}@dots{})
When this function is expanded, @code{make} prints @var{text} to standard
output and the function expands to the empty string.@*
@xref{Make Control Functions, ,Functions That Control Make}.

@item $(intcmp @var{lhs},@var{rhs}[,@var{lt-part}[,@var{eq-part}[,@var{gt-part}]]])
Compare @var{lhs} and @var{rhs} numerically; substitute the expansion of
@var{lt-part}, @var{eq-part}, or @var{gt-part} depending on whether the
left-hand side is less-than, equal-to, or greater-than the right-hand side,
respectively.@*
@xref{Conditional Functions, ,Functions for Conditionals}.

@item $(join @var{list1},@var{list2})
Join two parallel lists of words.@*
@xref{File Name Functions, ,Functions for File Names}.

@item $(lastword @var{names}@dots{})
Extract the last word of @var{names}.@*
@xref{Text Functions, , Functions for String Substitution and Analysis}.

@item $(let @var{var} [@var{var} ...],@var{words},@var{text})
Evaluate @var{text} with the @var{var}s bound to the words in
@var{words}.@*
@xref{Let Function, ,The @code{let} Function}.

@item $(notdir @var{names}@dots{})
Extract the non-directory part of each file name.@*
@xref{File Name Functions, ,Functions for File Names}.

@item $(or @var{condition1}[,@var{condition2}[,@var{condition3}@dots{}]])
Evaluate each condition @var{conditionN} one at a time; substitute the
first non-empty expansion.  If all expansions are empty, substitute
the empty string.@*
@xref{Conditional Functions, ,Functions for Conditionals}.

@item $(origin @var{variable})
Return a string describing how the @code{make} variable @var{variable} was
defined.@*
@xref{Origin Function, , The @code{origin} Function}.

@item $(patsubst @var{pattern},@var{replacement},@var{text})
Replace words matching @var{pattern} with @var{replacement} in @var{text}.@*
@xref{Text Functions, , Functions for String Substitution and Analysis}.

@item $(realpath @var{names}@dots{})
For each file name in @var{names}, expand to an absolute name that does not
contain any @code{.}, @code{..}, nor symlinks.@*
@xref{File Name Functions, ,Functions for File Names}.

@item $(shell @var{command})
Execute a shell command and expand to its standard output.@*
@xref{Shell Function, , The @code{shell} Function}.

@item $(sort @var{list})
Sort the words in @var{list} lexicographically, removing duplicates.@*
@xref{Text Functions, , Functions for String Substitution and Analysis}.

@item $(strip @var{string})
Remove excess whitespace characters from @var{string}.@*
@xref{Text Functions, , Functions for String Substitution and Analysis}.

@item $(subst @var{from},@var{to},@var{text})
Replace @var{from} with @var{to} in @var{text}.@*
@xref{Text Functions, , Functions for String Substitution and Analysis}.

@item $(suffix @var{names}@dots{})
Extract the suffix (the last @samp{.} and following characters) of each file
name.@*
@xref{File Name Functions, ,Functions for File Names}.

@item $(value @var{var})
Evaluates to the contents of the variable @var{var}, with no expansion
performed on it.@*
@xref{Value Function, ,The @code{value} Function}.

@item $(warning @var{text}@dots{})
When this function is expanded, @code{make} prints @var{text} to standard
error, prefixed with the current filename and line number.@*
@xref{Make Control Functions, ,Functions That Control Make}.

@item $(wildcard @var{pattern}@dots{})
Find file names matching a shell file name pattern (@emph{not} a @samp{%}
pattern).@*
@xref{Wildcard Function, ,The Function @code{wildcard}}.

@item $(word @var{n},@var{text})
Extract the @var{n}th word (one-origin) of @var{text}.@*
@xref{Text Functions, , Functions for String Substitution and Analysis}.

@item $(wordlist @var{s},@var{e},@var{text})
Returns the list of words in @var{text} from @var{s} to @var{e}.@*
@xref{Text Functions, , Functions for String Substitution and Analysis}.

@item $(words @var{text})
Count the number of words in @var{text}.@*
@xref{Text Functions, , Functions for String Substitution and Analysis}.
@end table

@node Automatic Variable Reference
@appendixsec Automatic Variable Reference

Here is a summary of the automatic variables.  Remember automatic variables
@emph{only} have values @emph{inside} a recipe.  @xref{Automatic Variables},
for full information.

@table @code
@item $@@
The file name of the target.

@item $%
The target member name, when the target is an archive member.

@item $<
The name of the first prerequisite.

@item $?
The names of all the prerequisites that are
newer than the target, with spaces between them.
For prerequisites which are archive members, only
the named member is used (@pxref{Archives}).

@item $^
@itemx $+
The names of all the prerequisites, with spaces between them.  For
prerequisites which are archive members, only the named member is used
(@pxref{Archives}).  The value of @code{$^} omits duplicate
prerequisites, while @code{$+} retains them and preserves their order.

@item $*
The stem with which an implicit rule matches
(@pxref{Pattern Match, ,How Patterns Match}).

@item $(@@D)
@itemx $(@@F)
The directory part and the file-within-directory part of @code{$@@}.

@item $(*D)
@itemx $(*F)
The directory part and the file-within-directory part of @code{$*}.

@item $(%D)
@itemx $(%F)
The directory part and the file-within-directory part of @code{$%}.

@item $(<D)
@itemx $(<F)
The directory part and the file-within-directory part of @code{$<}.

@item $(^D)
@itemx $(^F)
The directory part and the file-within-directory part of @code{$^}.

@item $(+D)
@itemx $(+F)
The directory part and the file-within-directory part of @code{$+}.

@item $(?D)
@itemx $(?F)
The directory part and the file-within-directory part of @code{$?}.
@end table

@node Special Variable Reference
@appendixsec Special Variable Reference

These variables are used specially by GNU @code{make}:

@table @code
@item CURDIR
Set to the absolute pathname of the current working directory (after
all @code{-C} options are processed, if any).  Setting this variable
has no effect on the operation of @code{make}.@*
@xref{Recursion, ,Recursive Use of @code{make}}.

@item GNUMAKEFLAGS
Other flags parsed by @code{make}.  You can set this in the environment or a
makefile to set @code{make} command-line flags.  This variable is only needed
if you'd like to set GNU @code{make}-specific flags in a POSIX-compliant
makefile.  This variable will be seen by GNU @code{make} and ignored by other
@code{make} implementations.  It's not needed if you only use GNU @code{make};
just use @code{MAKEFLAGS} directly.@* @xref{Options/Recursion, ,Communicating
Options to a Sub-@code{make}}.

@item .LIBPATTERNS
Defines the naming of the libraries @code{make} searches for, and their
order.@*
@xref{Libraries/Search, ,Directory Search for Link Libraries}.

@item MAKE
The name with which @code{make} was invoked.  Using this variable in
recipes has special meaning.@*
@xref{MAKE Variable, ,How the @code{MAKE} Variable Works}.

@item MAKECMDGOALS
The targets given to @code{make} on the command line.  Setting this
variable has no effect on the operation of @code{make}.@*
@xref{Goals, ,Arguments to Specify the Goals}.

@item MAKEFILES
Makefiles to be read on every invocation of @code{make}.@*
@xref{MAKEFILES Variable, ,The Variable @code{MAKEFILES}}.

@item MAKEFLAGS
The flags given to @code{make}.  You can set this in the environment or
a makefile to set flags.@*
@xref{Options/Recursion, ,Communicating Options to a Sub-@code{make}}.

Don't use @code{MAKEFLAGS} explicitly in a recipe line: its contents may not
be quoted correctly for use in the shell.  Always allow recursive @code{make}
invocations to obtain these values through the environment from its parent.

@item MAKELEVEL
The number of levels of recursion (sub-@code{make}s).@*
@xref{Variables/Recursion}.

@item MAKESHELL
On MS-DOS only, the name of the command interpreter that is to be used
by @code{make}.  This value takes precedence over the value of
@code{SHELL}.@*
@xref{Choosing the Shell}.

@item MAKE_VERSION
The built-in variable @samp{MAKE_VERSION} expands to the version
number of the GNU @code{make} program.
@vindex MAKE_VERSION

@item MAKE_HOST
The built-in variable @samp{MAKE_HOST} expands to a string
representing the host that GNU @code{make} was built to run on.
@vindex MAKE_HOST

@item SHELL
The name of the system default command interpreter, usually @file{/bin/sh}.
You can set @code{SHELL} in the makefile to change the shell used to run
recipes.  @xref{Execution, ,Recipe Execution}.  The @code{SHELL}
variable is handled specially when importing from and exporting to the
environment.@*
@xref{Choosing the Shell}.

@item SUFFIXES
The default list of suffixes before @code{make} reads any makefiles.@*
@xref{Suffix Rules, ,Old-Fashioned Suffix Rules}.

@item VPATH
Directory search path for files not found in the current directory.@*
@xref{General Search, , @code{VPATH} Search Path for All Prerequisites}.
@end table

@node Error Messages
@comment  node-name,  next,  previous,  up
@appendix Errors Generated by Make

Here is a list of the more common errors you might see generated by
@code{make}, and some information about what they mean and how to fix
them.

Sometimes @code{make} errors are not fatal, especially in the presence
of a @code{-} prefix on a recipe line, or the @code{-k} command line
option.  Errors that are fatal are prefixed with the string
@code{***}.

Error messages are all either prefixed with the name of the program
(usually @samp{make}), or, if the error is found in a makefile, the name
of the file and line number containing the problem.

In the table below, these common prefixes are left off.

@table @samp

@item [@var{foo}] Error @var{NN}
@itemx [@var{foo}] @var{signal description}
These errors are not really @code{make} errors at all.  They mean that a
program that @code{make} invoked as part of a recipe returned a
non-0 error code (@samp{Error @var{NN}}), which @code{make} interprets
as failure, or it exited in some other abnormal fashion (with a
signal of some type).  @xref{Errors, ,Errors in Recipes}.

If no @code{***} is attached to the message, then the sub-process failed
but the rule in the makefile was prefixed with the @code{-} special
character, so @code{make} ignored the error.

@item missing separator.  Stop.
@itemx missing separator (did you mean TAB instead of 8 spaces?).  Stop.
This means that @code{make} could not understand much of anything
about the makefile line it just read.  GNU @code{make} looks for
various separators (@code{:}, @code{=}, recipe prefix characters,
etc.) to indicate what kind of line it's parsing.  This message means
it couldn't find a valid one.

One of the most common reasons for this message is that you (or
perhaps your oh-so-helpful editor, as is the case with many MS-Windows
editors) have attempted to indent your recipe lines with spaces
instead of a tab character.  In this case, @code{make} will use the
second form of the error above.  Remember that every line in the
recipe must begin with a tab character (unless you set
@code{.RECIPEPREFIX}; @pxref{Special Variables}).  Eight spaces do not
count.  @xref{Rule Syntax}.

@item recipe commences before first target.  Stop.
@itemx missing rule before recipe.  Stop.
This means the first thing in the makefile seems to be part of a
recipe: it begins with a recipe prefix character and doesn't appear to
be a legal @code{make} directive (such as a variable assignment).
Recipes must always be associated with a target.

The second form is generated if the line has a semicolon as the first
non-whitespace character; @code{make} interprets this to mean you left
out the "target: prerequisite" section of a rule.  @xref{Rule Syntax}.

@item No rule to make target `@var{xxx}'.
@itemx No rule to make target `@var{xxx}', needed by `@var{yyy}'.
This means that @code{make} decided it needed to build a target, but
then couldn't find any instructions in the makefile on how to do that,
either explicit or implicit (including in the default rules database).

If you want that file to be built, you will need to add a rule to your
makefile describing how that target can be built.  Other possible
sources of this problem are typos in the makefile (if that file name is
wrong) or a corrupted source tree (if that file is not supposed to be
built, but rather only a prerequisite).

@item No targets specified and no makefile found.  Stop.
@itemx No targets.  Stop.
The former means that you didn't provide any targets to be built on the
command line, and @code{make} couldn't find any makefiles to read in.
The latter means that some makefile was found, but it didn't contain any
default goal and none was given on the command line.  GNU @code{make}
has nothing to do in these situations.
@xref{Makefile Arguments, ,Arguments to Specify the Makefile}.

@item Makefile `@var{xxx}' was not found.
@itemx Included makefile `@var{xxx}' was not found.
A makefile specified on the command line (first form) or included
(second form) was not found.

@item warning: overriding recipe for target `@var{xxx}'
@itemx warning: ignoring old recipe for target `@var{xxx}'
GNU @code{make} allows only one recipe to be specified per target
(except for double-colon rules).  If you give a recipe for a target
which already has been defined to have one, this warning is issued and
the second recipe will overwrite the first.  @xref{Multiple Rules,
,Multiple Rules for One Target}.

@item Circular @var{xxx} <- @var{yyy} dependency dropped.
This means that @code{make} detected a loop in the dependency graph:
after tracing the prerequisite @var{yyy} of target @var{xxx}, and its
prerequisites, etc., one of them depended on @var{xxx} again.

@item Recursive variable `@var{xxx}' references itself (eventually).  Stop.
This means you've defined a normal (recursive) @code{make} variable
@var{xxx} that, when it's expanded, will refer to itself (@var{xxx}).
This is not allowed; either use simply-expanded variables (@samp{:=}
or @samp{::=}) or use the append operator (@samp{+=}).  @xref{Using
Variables, ,How to Use Variables}.

@item Unterminated variable reference.  Stop.
This means you forgot to provide the proper closing parenthesis
or brace in your variable or function reference.

@item insufficient arguments to function `@var{xxx}'.  Stop.
This means you haven't provided the requisite number of arguments for
this function.  See the documentation of the function for a description
of its arguments.  @xref{Functions, ,Functions for Transforming Text}.

@item missing target pattern.  Stop.
@itemx multiple target patterns.  Stop.
@itemx target pattern contains no `%'.  Stop.
@itemx mixed implicit and static pattern rules.  Stop.
These errors are generated for malformed static pattern rules
(@pxref{Static Usage, ,Syntax of Static Pattern Rules}).  The first
means the target-pattern part of the rule is empty; the second means
there are multiple pattern characters (@code{%}) in the target-pattern
part; the third means there are no pattern characters in the
target-pattern part; and the fourth means that all three parts of the
static pattern rule contain pattern characters (@code{%})--the first
part should not contain pattern characters.

If you see these errors and you aren't trying to create a static
pattern rule, check the value of any variables in your target and
prerequisite lists to be sure they do not contain colons.

@item warning: -jN forced in submake: disabling jobserver mode.
This warning and the next are generated if @code{make} detects error
conditions related to parallel processing on systems where
sub-@code{make}s can communicate (@pxref{Options/Recursion,
,Communicating Options to a Sub-@code{make}}).  This warning is
generated if a recursive invocation of a @code{make} process is forced
to have @samp{-j@var{N}} in its argument list (where @var{N} is greater
than one).  This could happen, for example, if you set the @code{MAKE}
environment variable to @samp{make -j2}.  In this case, the
sub-@code{make} doesn't communicate with other @code{make} processes and
will simply pretend it has two jobs of its own.

@item warning: jobserver unavailable: using -j1.  Add `+' to parent make rule.
In order for @code{make} processes to communicate, the parent will pass
information to the child.  Since this could result in problems if the
child process isn't actually a @code{make}, the parent will only do this
if it thinks the child is a @code{make}.  The parent uses the normal
algorithms to determine this (@pxref{MAKE Variable, ,How the @code{MAKE}
Variable Works}).  If the makefile is constructed such that the parent
doesn't know the child is a @code{make} process, then the child will
receive only part of the information necessary.  In this case, the child
will generate this warning message and proceed with its build in a
sequential manner.

@item warning: ignoring prerequisites on suffix rule definition
According to POSIX, a suffix rule cannot contain prerequisites.  If a rule
that could be a suffix rule has prerequisites it is interpreted as a simple
explicit rule, with an odd target name.  This requirement is obeyed when
POSIX-conforming mode is enabled (the @code{.POSIX} target is defined).  In
versions of GNU @code{make} prior to 4.3, no warning was emitted and a
suffix rule was created, however all prerequisites were ignored and were not
part of the suffix rule.  Starting with GNU @code{make} 4.3 the behavior is
the same, and in addition this warning is generated.  In a future version
the POSIX-conforming behavior will be the only behavior: no rule with a
prerequisite can be a suffix rule and this warning will be removed.

@end table

@node Troubleshooting
@appendix Troubleshooting Make and Makefiles

Troubleshooting @code{make} and makefiles can be tricky.  There are two
reasons: first, makefiles are not procedural programs and many users are used
to procedural languages and scripts.  Second, makefiles consist of two
different syntaxes in one file: makefile syntax, that @code{make} reads, and
shell syntax, which is sent to a shell program for parsing and execution.

If you have problems with GNU Make, first consider the type of problem you are
having.  Problems will generally be in one of these categories:

@itemize @bullet
@item
A syntax or other error was reported when @code{make} attempted to parse your
makefiles.

@item
A command that @code{make} invoked failed (exited with a non-0 exit code).

@item
The command that @code{make} invoked was not the one you expected.

@item
@code{make} was not able to find a rule to build a target.

@item
@code{make} rebuilds a target that you didn't think was out of date.

@item
Or, @code{make} did not rebuild a target that you expected it to build.
@end itemize

The strategies for troubleshooting differ for different types of problems.
For issues related to how makefiles are parsed, strategies include:

@itemize @bullet
@item
Using the @samp{-p} option to show the makefile database, after evaluation
(@pxref{Options Summary, ,Summary of Options}).

@item
Using the @code{info} or @code{warning} functions to understand how elements
of the makefile are expanded (@pxref{Make Control Functions, ,Functions That
Control Make}).
@end itemize

For issues related to how rules are applied, strategies include:

@itemize @bullet
@item
Using the @samp{-n} or @samp{--trace} options to show the commands that
@code{make} ran, and to explain which rules @code{make} invokes and why
(@pxref{Options Summary, ,Summary of Options}).

@item
Using the @samp{--debug=v,i} or full @samp{-d} options to show how @code{make}
is determining which recipes should be used, or why targets do not need to be
rebuilt (@pxref{Options Summary, ,Summary of Options}).
@end itemize

@menu
* Parse Error::                 Syntax errors when parsing makefiles.
* Command Failure::             Recipe commands exit with error codes.
* Wrong Rule::                  @code{make} chooses the wrong rule.
* No Rule Found::               No rule was found to build a target.
* Extra Rebuilds::              Targets are rebuilt unnecessarily.
* Missing Rebuilds::            Out-of-date targets are not rebuilt.
* Troubleshooting Strategies::  Strategies used for troubleshooting issues.
@end menu

@node Parse Error
@appendixsec Errors When Parsing Makefiles

This type of error is the simplest to resolve.  The error output you will see
will have a format like this:

@example
Makefile:10: *** missing separator.  Stop.
@end example

This message gives you all the information you need to address the error: it
gives the name of the makefile (here @samp{Makefile}) and the line number
(here @samp{10}) in that makefile where GNU Make's parser failed.  Following
that is a description of the error.  Further explanations of these error
messages can be found in @ref{Error Messages, ,Errors Generated by Make}.

@node Command Failure
@appendixsec Errors Reported by Commands

If GNU Make parses the makefiles correctly and runs a command to rebuild a
target, it expects that command to exit with an error code of @samp{0} (for
success).  Any other exit code will be reported by @code{make} as a failure
and will generate an error message with this form:

@example
make: *** [Makefile:10: target] Error 2
@end example

All the information you need to find that command are given: the name of the
makefile (here @samp{Makefile}) and line number (here @samp{10}) of the
command make invoked, the target (here @samp{target}) that make was trying to
build, and finally the exit code of the command (here @samp{2}).

The precise meaning of the error code is different for different commands: you
must consult the documentation of the command that failed to interpret it.
However there are some reasons for error codes:

@table @samp
@item 2
If the shell fails to parse your command (invalid shell syntax) it will exit
with a code of @samp{2}.  On Windows this can also mean that the command was
not found.

@item 127
If the command you wanted to run was not found, the shell will exit with a
code of @samp{127}.
@end table

To troubleshoot these errors (@pxref{Troubleshooting Strategies, ,Strategies
for Troubleshooting}), use the @samp{-n} or @samp{--trace} options so you can
see the complete command line being invoked.

Than, examine the output of the command that @code{make} invoked to determine
what went wrong and why: this output will appear @emph{before} the above error
message.  The error may be due to an incorrect command line in which case the
error is in the way your command was written in the makefile, but it's far
more likely to be a problem with something outside of the makefile (for
example, a syntax error in the code you are trying to compile).

@node Wrong Rule
@appendixsec Choosing the Wrong Rule

If @code{make} seems to be invoking a different command than the one you
intended, it could be that the wrong rule is being chosen.

To troubleshoot these errors (@pxref{Troubleshooting Strategies, ,Strategies
for Troubleshooting}), add the @samp{--trace} option to the @code{make}
command line.  This shows the rule that was chosen.

You can also use the @samp{--debug=v,i} or the full @samp{-d} option to
determine how @code{make} decided to use that rule.

@node No Rule Found
@appendixsec No Rule to Build A Target

If @code{make} cannot locate a rule to build a target that you requested,
either via the command line or as a prerequisite of another target, it shows
an error such as:

@example
make: *** No rule to make target 'aprogram'.  Stop.
@end example

If the makefile doesn't provide a rule for this target, you can add one.  If
there is a rule which you intended @code{make} to use to build this target and
it wasn't used, the most common reasons for this are:

@itemize @bullet
@item
The target was misspelled.  You should consider following the @dfn{DRY}
principle (Don't Repeat Yourself) by assigning file names (targets and
prerequisites) to makefile variables and using those variables rather than
retyping the file names.

@item
The target is in a different directory.  @code{make} considers @samp{target}
and @samp{dir/target} (for example) to be different targets.  If you are using
rules that create files outside of the current working directory, be sure you
correctly prefix them with their directories everywhere that they appear in
the makefile.

@item
A pattern rule didn't match because one of its prerequisites cannot be built.
Pattern rules will only be used when @strong{all} prerequisites can be
satisfied: either they exist already or @code{make} can find a way to build
them.  If any prerequisite cannot be created, then the pattern does not match
and @code{make} will continue looking for another matching pattern.  If no
matching pattern can be found, then @code{make} will fail.
@end itemize

To troubleshoot these issues (@pxref{Troubleshooting Strategies, ,Strategies
for Troubleshooting}), run @code{make} with the @samp{--debug=v,i} option, or
the full @samp{-d} option, and examine the detailed output.

If the definition of the rule in your makefile is complicated, you can use the
@samp{-p} option to ask make to print its internal database of rules to ensure
they are correct, or as a last resort add invocations of the @code{info} or
@code{warning} functions to show what steps @code{make} is taking during
evaluation.

@node Extra Rebuilds
@appendixsec Unwanted Rebuilding of Targets

If @code{make} is rebuilding a target which you feel is already up to date and
doesn't need to be rebuilt, there can be a number of reasons:

@itemize @bullet
@item
The recipe does not update the target.  A makefile rule is a promise to
@code{make} that if it invokes the recipe, the target will be updated.  The
file which @code{make} expects to be updated is placed in the @samp{$@@}
variable.  If the recipe doesn't update this file, and @emph{exactly} this
file, then the next time @code{make} is invoked it will try to re-build that
target again.

@item
A prerequisite is marked as phony (@pxref{Special Targets, ,Special Built-in
Target Names}).  All phony targets are always considered out of date, and so
any targets depending on them are also out of date.

@item
A directory is used as a prerequisite.  Directories are not treated specially
by @code{make}: if their modification time is newer than the target then the
target is considered out of date and rebuilt.  Since directory modification
times are changed whenever a file is created, deleted, or renamed in that
directory, it means targets depending on the directory will be considered out
of date whenever a file is created, deleted, or renamed in that directory.

@item
Something is deleting the target file.  Of course if a file does not exist it
is always considered out of date (see the first item above).  If something in
your environment, either inside the @samp{makefile} or outside if it, is
deleting the target file then @code{make} will always rebuild it.

@item
The target is created with a ``too-old'' modification time.  If the recipe
creates the target with a modification time in the past, then it may still be
out of date with respect to its prerequisites.  This could happen if, for
example, you are extracting files from an archive or copying them from another
location and the tool used to do the extraction or copying preserves the
original file's modification time.
@end itemize

To troubleshoot these issues (@pxref{Troubleshooting Strategies, ,Strategies
for Troubleshooting}), use the @samp{--trace} option to understand why
@code{make} decides to build a target and see the full command used.

@node Missing Rebuilds
@appendixsec Out-of-Date Targets Not Rebuilt

The opposite of the previous problem is @code{make} not rebuilding targets
that you think should be rebuilt.  Some reasons for this might be:

@itemize @bullet
@item
The target is not being considered.  Unless the command line specifies
otherwise, @code{make} will only consider the first target in the makefile and
its prerequisites.  If the target you expected to be built is not one of
these, then @code{make} won't build it.

@item
A different file is being built instead.  Be sure that your target is the file
you want to be built (including any directory prefix!) and that the recipe
will create the file listed as @samp{$@@}.  Also consider the directory that
@code{make} is running in and whether relative pathnames are the ones you
expect.
@end itemize

To troubleshoot these issues (@pxref{Troubleshooting Strategies, ,Strategies
for Troubleshooting}), use the @samp{-n} or @samp{--trace} option to see what
command is being invoked.

You can also use the @samp{--debug=v,i} option or the full @samp{-d} option to
obtain a complete description of everything @code{make} considered and why it
decided to build or not build every target.

@node Troubleshooting Strategies
@appendixsec Strategies for Troubleshooting

The strategies for troubleshooting differ for different types of problems.

@subheading Show the makefile database

Use the @samp{-p} option to show the makefile database, after evaluation
(@pxref{Options Summary, ,Summary of Options}).  This allows you to see how
@code{make} has evaluated the variables and recipes in your makefile.

The @samp{-p} option asks GNU Make to print its internal database of rules,
after all makefiles have been parsed.  You might see output such as:

@example
program: prereq.o
#  Implicit rule search has not been done.
#  Modification time never checked.
#  File has not been updated.
#  recipe to execute (from 'Makefile', line 10):
        $(CC) -o $@@ $^ $(LDFLAGS) $(LDLIBS)
@end example

Here we can see the target (here @samp{program}) and prerequisites (here
@samp{prereq.o}) after variable expansion, as well as the location of the
definition of the recipe for this rule (here, in @samp{Makefile} at line
@samp{10}).

@subheading Add @code{info} or @code{warning} calls

The @code{info} or @code{warning} function can help to understand how elements
of the makefile are expanded (@pxref{Make Control Functions, ,Functions That
Control Make}).

Although their use requires modifying the makefile, these functions are a
powerful tool for troubleshooting complex makefiles such as those using
@code{eval} and @code{call} functions to dynamically generate rules.  For
example if your makefile contains:

@example
$(foreach T,$(TARGET),$(eval $(call makerule,$T,$($T_PREREQ))))
@end example

@noindent
then duplicating this line and replacing @code{eval} with @code{info} will
show exactly what content @code{make} will be evaluating:

@example
$(foreach T,$(TARGET),$(info $(call makerule,$T,$($T_PREREQ))))
@end example

The @code{warning} function provides the same output as @code{info}, except
that each output line is prefixed with the filename and line number where the
@code{warning} function is expanded, and that the output goes to the standard
error file descriptor and not to standard output.

@subheading Use the @samp{--trace} or @samp{-n} option

The @samp{-n} option will show you which commands would be invoked to bring
targets up to date, without actually invoking them.  Commands are displayed
even if the @samp{@@} prefix or the @code{.SILENT} target was specified
(@pxref{Options Summary, ,Summary of Options}).

Use the @samp{--trace} option on the @code{make} command line to explain which
rules @code{make} invokes and why (@pxref{Options Summary, ,Summary of
Options}).  This option also overrides @samp{@@} recipe prefixes and the
@code{.SILENT} special target and shows the expanded command line.

Adding this will result in output like this for each target which is rebuilt:

@example
Makefile:10: update target 'program' due to: target does not exist
@end example

@noindent
or:

@example
Makefile:10: update target 'program' due to: prereq.o
@end example

This shows the filename of the makefile (here @samp{Makefile}) and line number
(here @samp{10}), along with the name of the target (here @samp{program}) and
the reason why it was rebuilt: in the first example because the target does
not exist, and in the second example because there were prerequisites (here,
@samp{prereq.o}) that were newer than the target.  All newer prerequisites
would be listed here.

After this information, the expanded command line make invoked will be shown,
just as it is passed to the shell.  You should examine it carefully to
determine whether it's correct, and compare it to the pre-expansion recipe in
the makefile to see what aspect of the recipe might be incorrect.

@subheading Use the @samp{--debug=v,i} or @samp{-d} option

Use the @samp{--debug=v,i}, or the full @samp{-d}, options to show how
@code{make} is determining which recipes should be used, or why targets do not
need to be rebuilt (@pxref{Options Summary, ,Summary of Options}).

The amount of output generated by these options can be daunting, but
redirecting the output to a file then searching it for the target you are
concerned with will show you exactly what steps GNU Make took when considering
this target, and why it decided to build, or not build, that target.

If the issue is that @code{make} decides @emph{not} to rebuild the target when
you think it should, this may be your only recourse since @samp{--trace} only
shows why targets are considered out of date.

@node Complex Makefile
@appendix Complex Makefile Example

Here is the makefile for the GNU @code{tar} program.  This is a
moderately complex makefile.  The first line uses a @code{#!} setting
to allow the makefile to be executed directly.

Because it is the first target, the default goal is @samp{all}.  An
interesting feature of this makefile is that @file{testpad.h} is a
source file automatically created by the @code{testpad} program,
itself compiled from @file{testpad.c}.

If you type @samp{make} or @samp{make all}, then @code{make} creates
the @file{tar} executable, the @file{rmt} daemon that provides
remote tape access, and the @file{tar.info} Info file.

If you type @samp{make install}, then @code{make} not only creates
@file{tar}, @file{rmt}, and @file{tar.info}, but also installs
them.

If you type @samp{make clean}, then @code{make} removes the @samp{.o}
files, and the @file{tar}, @file{rmt}, @file{testpad},
@file{testpad.h}, and @file{core} files.

If you type @samp{make distclean}, then @code{make} not only removes
the same files as does @samp{make clean} but also the
@file{TAGS}, @file{Makefile}, and @file{config.status} files.
(Although it is not evident, this makefile (and
@file{config.status}) is generated by the user with the
@code{configure} program, which is provided in the @code{tar}
distribution, but is not shown here.)

If you type @samp{make realclean}, then @code{make} removes the same
files as does @samp{make distclean} and also removes the Info files
generated from @file{tar.texinfo}.

In addition, there are targets @code{shar} and @code{dist} that create
distribution kits.

@example
@group
#!/usr/bin/make -f
# Generated automatically from Makefile.in by configure.
# Un*x Makefile for GNU tar program.
# Copyright (C) 1991 Free Software Foundation, Inc.
@end group

@group
# This program is free software; you can redistribute
# it and/or modify it under the terms of the GNU
# General Public License @dots{}
@dots{}
@dots{}
@end group

SHELL = /bin/sh

#### Start of system configuration section. ####

srcdir = .

@group
# If you use gcc, you should either run the
# fixincludes script that comes with it or else use
# gcc with the -traditional option.  Otherwise ioctl
# calls will be compiled incorrectly on some systems.
CC = gcc -O
YACC = bison -y
INSTALL = /usr/local/bin/install -c
INSTALLDATA = /usr/local/bin/install -c -m 644
@end group

# Things you might add to DEFS:
# -DSTDC_HEADERS        If you have ANSI C headers and
#                       libraries.
# -DPOSIX               If you have POSIX.1 headers and
#                       libraries.
# -DBSD42               If you have sys/dir.h (unless
#                       you use -DPOSIX), sys/file.h,
#                       and st_blocks in `struct stat'.
# -DUSG                 If you have System V/ANSI C
#                       string and memory functions
#                       and headers, sys/sysmacros.h,
#                       fcntl.h, getcwd, no valloc,
#                       and ndir.h (unless
#                       you use -DDIRENT).
# -DNO_MEMORY_H         If USG or STDC_HEADERS but do not
#                       include memory.h.
# -DDIRENT              If USG and you have dirent.h
#                       instead of ndir.h.
# -DSIGTYPE=int         If your signal handlers
#                       return int, not void.
# -DNO_MTIO             If you lack sys/mtio.h
#                       (magtape ioctls).
# -DNO_REMOTE           If you do not have a remote shell
#                       or rexec.
# -DUSE_REXEC           To use rexec for remote tape
#                       operations instead of
#                       forking rsh or remsh.
# -DVPRINTF_MISSING     If you lack vprintf function
#                       (but have _doprnt).
# -DDOPRNT_MISSING      If you lack _doprnt function.
#                       Also need to define
#                       -DVPRINTF_MISSING.
# -DFTIME_MISSING       If you lack ftime system call.
# -DSTRSTR_MISSING      If you lack strstr function.
# -DVALLOC_MISSING      If you lack valloc function.
# -DMKDIR_MISSING       If you lack mkdir and
#                       rmdir system calls.
# -DRENAME_MISSING      If you lack rename system call.
# -DFTRUNCATE_MISSING   If you lack ftruncate
#                       system call.
# -DV7                  On Version 7 Unix (not
#                       tested in a long time).
# -DEMUL_OPEN3          If you lack a 3-argument version
#                       of open, and want to emulate it
#                       with system calls you do have.
# -DNO_OPEN3            If you lack the 3-argument open
#                       and want to disable the tar -k
#                       option instead of emulating open.
# -DXENIX               If you have sys/inode.h
#                       and need it 94 to be included.

DEFS =  -DSIGTYPE=int -DDIRENT -DSTRSTR_MISSING \
        -DVPRINTF_MISSING -DBSD42
# Set this to rtapelib.o unless you defined NO_REMOTE,
# in which case make it empty.
RTAPELIB = rtapelib.o
LIBS =
DEF_AR_FILE = /dev/rmt8
DEFBLOCKING = 20

@group
CDEBUG = -g
CFLAGS = $(CDEBUG) -I. -I$(srcdir) $(DEFS) \
        -DDEF_AR_FILE=\"$(DEF_AR_FILE)\" \
        -DDEFBLOCKING=$(DEFBLOCKING)
LDFLAGS = -g
@end group

@group
prefix = /usr/local
# Prefix for each installed program,
# normally empty or `g'.
binprefix =

# The directory to install tar in.
bindir = $(prefix)/bin

# The directory to install the info files in.
infodir = $(prefix)/info
@end group

#### End of system configuration section. ####

@group
SRCS_C  = tar.c create.c extract.c buffer.c   \
          getoldopt.c update.c gnu.c mangle.c \
          version.c list.c names.c diffarch.c \
          port.c wildmat.c getopt.c getopt1.c \
          regex.c
SRCS_Y  = getdate.y
SRCS    = $(SRCS_C) $(SRCS_Y)
OBJS    = $(SRCS_C:.c=.o) $(SRCS_Y:.y=.o) $(RTAPELIB)
@end group
@group
AUX =   README COPYING ChangeLog Makefile.in  \
        makefile.pc configure configure.in \
        tar.texinfo tar.info* texinfo.tex \
        tar.h port.h open3.h getopt.h regex.h \
        rmt.h rmt.c rtapelib.c alloca.c \
        msd_dir.h msd_dir.c tcexparg.c \
        level-0 level-1 backup-specs testpad.c
@end group

.PHONY: all
all:    tar rmt tar.info

@group
tar:    $(OBJS)
        $(CC) $(LDFLAGS) -o $@@ $(OBJS) $(LIBS)
@end group

@group
rmt:    rmt.c
        $(CC) $(CFLAGS) $(LDFLAGS) -o $@@ rmt.c
@end group

@group
tar.info: tar.texinfo
        makeinfo tar.texinfo
@end group

@group
.PHONY: install
install: all
        $(INSTALL) tar $(bindir)/$(binprefix)tar
        -test ! -f rmt || $(INSTALL) rmt /etc/rmt
        $(INSTALLDATA) $(srcdir)/tar.info* $(infodir)
@end group

@group
$(OBJS): tar.h port.h testpad.h
regex.o buffer.o tar.o: regex.h
# getdate.y has 8 shift/reduce conflicts.
@end group

@group
testpad.h: testpad
        ./testpad
@end group

@group
testpad: testpad.o
        $(CC) -o $@@ testpad.o
@end group

@group
TAGS:   $(SRCS)
        etags $(SRCS)
@end group

@group
.PHONY: clean
clean:
        rm -f *.o tar rmt testpad testpad.h core
@end group

@group
.PHONY: distclean
distclean: clean
        rm -f TAGS Makefile config.status
@end group

@group
.PHONY: realclean
realclean: distclean
        rm -f tar.info*
@end group

@group
.PHONY: shar
shar: $(SRCS) $(AUX)
        shar $(SRCS) $(AUX) | compress \
          > tar-`sed -e '/version_string/!d' \
                     -e 's/[^0-9.]*\([0-9.]*\).*/\1/' \
                     -e q
                     version.c`.shar.Z
@end group

@group
.PHONY: dist
dist: $(SRCS) $(AUX)
        echo tar-`sed \
             -e '/version_string/!d' \
             -e 's/[^0-9.]*\([0-9.]*\).*/\1/' \
             -e q
             version.c` > .fname
        -rm -rf `cat .fname`
        mkdir `cat .fname`
        ln $(SRCS) $(AUX) `cat .fname`
        tar chZf `cat .fname`.tar.Z `cat .fname`
        -rm -rf `cat .fname` .fname
@end group

@group
tar.zoo: $(SRCS) $(AUX)
        -rm -rf tmp.dir
        -mkdir tmp.dir
        -rm tar.zoo
        for X in $(SRCS) $(AUX) ; do \
            echo $$X ; \
            sed 's/$$/^M/' $$X \
            > tmp.dir/$$X ; done
        cd tmp.dir ; zoo aM ../tar.zoo *
        -rm -rf tmp.dir
@end group
@end example

@node GNU Free Documentation License
@appendix GNU Free Documentation License
@cindex FDL, GNU Free Documentation License
@include fdl.texi

@node Concept Index
@unnumbered Index of Concepts

@printindex cp

@node Name Index
@unnumbered Index of Functions, Variables, & Directives

@printindex fn

@bye

@c Local Variables:
@c eval: (setq fill-column 78)
@c End: