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* Standards: (standards). GNU coding standards.
GNU Coding Standards Copyright (C) 1992, 1993, 1994, 1995, 1996,
1997, 1998 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Free Software Foundation.

File:, Node: Top, Next: Preface, Prev: (dir), Up: (dir)
Last updated March 13, 1998.
* Menu:
* Preface:: About the GNU Coding Standards
* Intellectual Property:: Keeping Free Software Free
* Design Advice:: General Program Design
* Program Behavior:: Program Behavior for All Programs
* Writing C:: Making The Best Use of C
* Documentation:: Documenting Programs
* Managing Releases:: The Release Process

File:, Node: Preface, Next: Intellectual Property, Prev: Top, Up: Top
About the GNU Coding Standards
The GNU Coding Standards were written by Richard Stallman and other
GNU Project volunteers. Their purpose is to make the GNU system clean,
consistent, and easy to install. This document can also be read as a
guide to writing portable, robust and reliable programs. It focuses on
programs written in C, but many of the rules and principles are useful
even if you write in another programming language. The rules often
state reasons for writing in a certain way.
Corrections or suggestions for this document should be sent to
<>. If you make a suggestion, please include a suggested
new wording for it; our time is limited. We prefer a context diff to
the `standards.texi' or `make-stds.texi' files, but if you don't have
those files, please mail your suggestion anyway.
This release of the GNU Coding Standards was last updated March 13,

File:, Node: Intellectual Property, Next: Design Advice, Prev: Preface, Up: Top
Keeping Free Software Free
This node discusses how you can make sure that GNU software remains
* Menu:
* Reading Non-Free Code:: Referring to Proprietary Programs
* Contributions:: Accepting Contributions

File:, Node: Reading Non-Free Code, Next: Contributions, Up: Intellectual Property
Referring to Proprietary Programs
Don't in any circumstances refer to Unix source code for or during
your work on GNU! (Or to any other proprietary programs.)
If you have a vague recollection of the internals of a Unix program,
this does not absolutely mean you can't write an imitation of it, but
do try to organize the imitation internally along different lines,
because this is likely to make the details of the Unix version
irrelevant and dissimilar to your results.
For example, Unix utilities were generally optimized to minimize
memory use; if you go for speed instead, your program will be very
different. You could keep the entire input file in core and scan it
there instead of using stdio. Use a smarter algorithm discovered more
recently than the Unix program. Eliminate use of temporary files. Do
it in one pass instead of two (we did this in the assembler).
Or, on the contrary, emphasize simplicity instead of speed. For some
applications, the speed of today's computers makes simpler algorithms
Or go for generality. For example, Unix programs often have static
tables or fixed-size strings, which make for arbitrary limits; use
dynamic allocation instead. Make sure your program handles NULs and
other funny characters in the input files. Add a programming language
for extensibility and write part of the program in that language.
Or turn some parts of the program into independently usable
libraries. Or use a simple garbage collector instead of tracking
precisely when to free memory, or use a new GNU facility such as

File:, Node: Contributions, Prev: Reading Non-Free Code, Up: Intellectual Property
Accepting Contributions
If someone else sends you a piece of code to add to the program you
are working on, we need legal papers to use it--the same sort of legal
papers we will need to get from you. _Each_ significant contributor to
a program must sign some sort of legal papers in order for us to have
clear title to the program. The main author alone is not enough.
So, before adding in any contributions from other people, please tell
us, so we can arrange to get the papers. Then wait until we tell you
that we have received the signed papers, before you actually use the
This applies both before you release the program and afterward. If
you receive diffs to fix a bug, and they make significant changes, we
need legal papers for that change.
This also applies to comments and documentation files. For copyright
law, comments and code are just text. Copyright applies to all kinds of
text, so we need legal papers for all kinds.
You don't need papers for changes of a few lines here or there, since
they are not significant for copyright purposes. Also, you don't need
papers if all you get from the suggestion is some ideas, not actual code
which you use. For example, if you write a different solution to the
problem, you don't need to get papers.
We know this is frustrating; it's frustrating for us as well. But if
you don't wait, you are going out on a limb--for example, what if the
contributor's employer won't sign a disclaimer? You might have to take
that code out again!
The very worst thing is if you forget to tell us about the other
contributor. We could be very embarrassed in court some day as a
We have more detailed advice for maintainers of programs; if you have
reached the stage of actually maintaining a program for GNU (whether
released or not), please ask us for a copy.

File:, Node: Design Advice, Next: Program Behavior, Prev: Intellectual Property, Up: Top
General Program Design
This node discusses some of the issues you should take into account
when designing your program.
* Menu:
* Compatibility:: Compatibility with other implementations
* Using Extensions:: Using non-standard features
* ANSI C:: Using ANSI C features
* Source Language:: Using languages other than C

File:, Node: Compatibility, Next: Using Extensions, Up: Design Advice
Compatibility with Other Implementations
With occasional exceptions, utility programs and libraries for GNU
should be upward compatible with those in Berkeley Unix, and upward
compatible with ANSI C if ANSI C specifies their behavior, and upward
compatible with POSIX if POSIX specifies their behavior.
When these standards conflict, it is useful to offer compatibility
modes for each of them.
ANSI C and POSIX prohibit many kinds of extensions. Feel free to
make the extensions anyway, and include a `--ansi', `--posix', or
`--compatible' option to turn them off. However, if the extension has
a significant chance of breaking any real programs or scripts, then it
is not really upward compatible. Try to redesign its interface.
Many GNU programs suppress extensions that conflict with POSIX if the
environment variable `POSIXLY_CORRECT' is defined (even if it is
defined with a null value). Please make your program recognize this
variable if appropriate.
When a feature is used only by users (not by programs or command
files), and it is done poorly in Unix, feel free to replace it
completely with something totally different and better. (For example,
`vi' is replaced with Emacs.) But it is nice to offer a compatible
feature as well. (There is a free `vi' clone, so we offer it.)
Additional useful features not in Berkeley Unix are welcome.

File:, Node: Using Extensions, Next: ANSI C, Prev: Compatibility, Up: Design Advice
Using Non-standard Features
Many GNU facilities that already exist support a number of convenient
extensions over the comparable Unix facilities. Whether to use these
extensions in implementing your program is a difficult question.
On the one hand, using the extensions can make a cleaner program.
On the other hand, people will not be able to build the program unless
the other GNU tools are available. This might cause the program to
work on fewer kinds of machines.
With some extensions, it might be easy to provide both alternatives.
For example, you can define functions with a "keyword" `INLINE' and
define that as a macro to expand into either `inline' or nothing,
depending on the compiler.
In general, perhaps it is best not to use the extensions if you can
straightforwardly do without them, but to use the extensions if they
are a big improvement.
An exception to this rule are the large, established programs (such
as Emacs) which run on a great variety of systems. Such programs would
be broken by use of GNU extensions.
Another exception is for programs that are used as part of
compilation: anything that must be compiled with other compilers in
order to bootstrap the GNU compilation facilities. If these require
the GNU compiler, then no one can compile them without having them
installed already. That would be no good.

File:, Node: ANSI C, Next: Source Language, Prev: Using Extensions, Up: Design Advice
ANSI C and pre-ANSI C
Do not ever use the "trigraph" feature of ANSI C.
ANSI C is widespread enough now that it is ok to write new programs
that use ANSI C features (and therefore will not work in non-ANSI
compilers). And if a program is already written in ANSI C, there's no
need to convert it to support non-ANSI compilers.
However, it is easy to support non-ANSI compilers in most programs,
so you might still consider doing so when you write a program. Instead
of writing function definitions in ANSI prototype form,
foo (int x, int y)
write the definition in pre-ANSI style like this,
foo (x, y)
int x, y;
and use a separate declaration to specify the argument prototype:
int foo (int, int);
You need such a declaration anyway, in a header file, to get the
benefit of ANSI C prototypes in all the files where the function is
called. And once you have it, you lose nothing by writing the function
definition in the pre-ANSI style.
If you don't know non-ANSI C, there's no need to learn it; just
write in ANSI C.

File:, Node: Source Language, Prev: ANSI C, Up: Design Advice
Using Languages Other Than C
Using a language other than C is like using a non-standard feature:
it will cause trouble for users. Even if GCC supports the other
language, users may find it inconvenient to have to install the
compiler for that other language in order to build your program. For
example, if you write your program in C++, people will have to install
the C++ compiler in order to compile your program. Thus, it is better
if you write in C.
But there are three situations when there is no disadvantage in using
some other language:
* It is okay to use another language if your program contains an
interpreter for that language.
For example, if your program links with GUILE, it is ok to write
part of the program in Scheme or another language supported by
* It is okay to use another language in a tool specifically intended
for use with that language.
This is okay because the only people who want to build the tool
will be those who have installed the other language anyway.
* If an application is of interest to a narrow community, then
perhaps it's not important if the application is inconvenient to
C has one other advantage over C++ and other compiled languages: more
people know C, so more people will find it easy to read and modify the
program if it is written in C.

File:, Node: Program Behavior, Next: Writing C, Prev: Design Advice, Up: Top
Program Behavior for All Programs
This node describes how to write robust software. It also describes
general standards for error messages, the command line interface, and
how libraries should behave.
* Menu:
* Semantics:: Writing robust programs
* Libraries:: Library behavior
* Errors:: Formatting error messages
* User Interfaces:: Standards for command line interfaces
* Option Table:: Table of long options.
* Memory Usage:: When and how to care about memory needs

File:, Node: Semantics, Next: Libraries, Up: Program Behavior
Writing Robust Programs
Avoid arbitrary limits on the length or number of _any_ data
structure, including file names, lines, files, and symbols, by
allocating all data structures dynamically. In most Unix utilities,
"long lines are silently truncated". This is not acceptable in a GNU
Utilities reading files should not drop NUL characters, or any other
nonprinting characters _including those with codes above 0177_. The
only sensible exceptions would be utilities specifically intended for
interface to certain types of printers that can't handle those
Check every system call for an error return, unless you know you
wish to ignore errors. Include the system error text (from `perror' or
equivalent) in _every_ error message resulting from a failing system
call, as well as the name of the file if any and the name of the
utility. Just "cannot open foo.c" or "stat failed" is not sufficient.
Check every call to `malloc' or `realloc' to see if it returned
zero. Check `realloc' even if you are making the block smaller; in a
system that rounds block sizes to a power of 2, `realloc' may get a
different block if you ask for less space.
In Unix, `realloc' can destroy the storage block if it returns zero.
GNU `realloc' does not have this bug: if it fails, the original block
is unchanged. Feel free to assume the bug is fixed. If you wish to
run your program on Unix, and wish to avoid lossage in this case, you
can use the GNU `malloc'.
You must expect `free' to alter the contents of the block that was
freed. Anything you want to fetch from the block, you must fetch before
calling `free'.
If `malloc' fails in a noninteractive program, make that a fatal
error. In an interactive program (one that reads commands from the
user), it is better to abort the command and return to the command
reader loop. This allows the user to kill other processes to free up
virtual memory, and then try the command again.
Use `getopt_long' to decode arguments, unless the argument syntax
makes this unreasonable.
When static storage is to be written in during program execution, use
explicit C code to initialize it. Reserve C initialized declarations
for data that will not be changed.
Try to avoid low-level interfaces to obscure Unix data structures
(such as file directories, utmp, or the layout of kernel memory), since
these are less likely to work compatibly. If you need to find all the
files in a directory, use `readdir' or some other high-level interface.
These will be supported compatibly by GNU.
By default, the GNU system will provide the signal handling
functions of BSD and of POSIX. So GNU software should be written to use
In error checks that detect "impossible" conditions, just abort.
There is usually no point in printing any message. These checks
indicate the existence of bugs. Whoever wants to fix the bugs will have
to read the source code and run a debugger. So explain the problem with
comments in the source. The relevant data will be in variables, which
are easy to examine with the debugger, so there is no point moving them
Do not use a count of errors as the exit status for a program.
_That does not work_, because exit status values are limited to 8 bits
(0 through 255). A single run of the program might have 256 errors; if
you try to return 256 as the exit status, the parent process will see 0
as the status, and it will appear that the program succeeded.
If you make temporary files, check the `TMPDIR' environment
variable; if that variable is defined, use the specified directory
instead of `/tmp'.

File:, Node: Libraries, Next: Errors, Prev: Semantics, Up: Program Behavior
Library Behavior
Try to make library functions reentrant. If they need to do dynamic
storage allocation, at least try to avoid any nonreentrancy aside from
that of `malloc' itself.
Here are certain name conventions for libraries, to avoid name
Choose a name prefix for the library, more than two characters long.
All external function and variable names should start with this prefix.
In addition, there should only be one of these in any given library
member. This usually means putting each one in a separate source file.
An exception can be made when two external symbols are always used
together, so that no reasonable program could use one without the
other; then they can both go in the same file.
External symbols that are not documented entry points for the user
should have names beginning with `_'. They should also contain the
chosen name prefix for the library, to prevent collisions with other
libraries. These can go in the same files with user entry points if
you like.
Static functions and variables can be used as you like and need not
fit any naming convention.

File:, Node: Errors, Next: User Interfaces, Prev: Libraries, Up: Program Behavior
Formatting Error Messages
Error messages from compilers should look like this:
Error messages from other noninteractive programs should look like
when there is an appropriate source file, or like this:
when there is no relevant source file.
In an interactive program (one that is reading commands from a
terminal), it is better not to include the program name in an error
message. The place to indicate which program is running is in the
prompt or with the screen layout. (When the same program runs with
input from a source other than a terminal, it is not interactive and
would do best to print error messages using the noninteractive style.)
The string MESSAGE should not begin with a capital letter when it
follows a program name and/or file name. Also, it should not end with
a period.
Error messages from interactive programs, and other messages such as
usage messages, should start with a capital letter. But they should not
end with a period.

File:, Node: User Interfaces, Next: Option Table, Prev: Errors, Up: Program Behavior
Standards for Command Line Interfaces
Please don't make the behavior of a utility depend on the name used
to invoke it. It is useful sometimes to make a link to a utility with
a different name, and that should not change what it does.
Instead, use a run time option or a compilation switch or both to
select among the alternate behaviors.
Likewise, please don't make the behavior of the program depend on the
type of output device it is used with. Device independence is an
important principle of the system's design; do not compromise it merely
to save someone from typing an option now and then.
If you think one behavior is most useful when the output is to a
terminal, and another is most useful when the output is a file or a
pipe, then it is usually best to make the default behavior the one that
is useful with output to a terminal, and have an option for the other
Compatibility requires certain programs to depend on the type of
output device. It would be disastrous if `ls' or `sh' did not do so in
the way all users expect. In some of these cases, we supplement the
program with a preferred alternate version that does not depend on the
output device type. For example, we provide a `dir' program much like
`ls' except that its default output format is always multi-column
It is a good idea to follow the POSIX guidelines for the
command-line options of a program. The easiest way to do this is to use
`getopt' to parse them. Note that the GNU version of `getopt' will
normally permit options anywhere among the arguments unless the special
argument `--' is used. This is not what POSIX specifies; it is a GNU
Please define long-named options that are equivalent to the
single-letter Unix-style options. We hope to make GNU more user
friendly this way. This is easy to do with the GNU function
One of the advantages of long-named options is that they can be
consistent from program to program. For example, users should be able
to expect the "verbose" option of any GNU program which has one, to be
spelled precisely `--verbose'. To achieve this uniformity, look at the
table of common long-option names when you choose the option names for
your program (*note Option Table::).
It is usually a good idea for file names given as ordinary arguments
to be input files only; any output files would be specified using
options (preferably `-o' or `--output'). Even if you allow an output
file name as an ordinary argument for compatibility, try to provide an
option as another way to specify it. This will lead to more consistency
among GNU utilities, and fewer idiosyncracies for users to remember.
All programs should support two standard options: `--version' and
This option should direct the program to information about its
name, version, origin and legal status, all on standard output,
and then exit successfully. Other options and arguments should be
ignored once this is seen, and the program should not perform its
normal function.
The first line is meant to be easy for a program to parse; the
version number proper starts after the last space. In addition,
it contains the canonical name for this program, in this format:
GNU Emacs 19.30
The program's name should be a constant string; _don't_ compute it
from `argv[0]'. The idea is to state the standard or canonical
name for the program, not its file name. There are other ways to
find out the precise file name where a command is found in `PATH'.
If the program is a subsidiary part of a larger package, mention
the package name in parentheses, like this:
emacsserver (GNU Emacs) 19.30
If the package has a version number which is different from this
program's version number, you can mention the package version
number just before the close-parenthesis.
If you *need* to mention the version numbers of libraries which
are distributed separately from the package which contains this
program, you can do so by printing an additional line of version
info for each library you want to mention. Use the same format
for these lines as for the first line.
Please do not mention all of the libraries that the program uses
"just for completeness"--that would produce a lot of unhelpful
clutter. Please mention library version numbers only if you find
in practice that they are very important to you in debugging.
The following line, after the version number line or lines, should
be a copyright notice. If more than one copyright notice is
called for, put each on a separate line.
Next should follow a brief statement that the program is free
software, and that users are free to copy and change it on certain
conditions. If the program is covered by the GNU GPL, say so
here. Also mention that there is no warranty, to the extent
permitted by law.
It is ok to finish the output with a list of the major authors of
the program, as a way of giving credit.
Here's an example of output that follows these rules:
GNU Emacs 19.34.5
Copyright (C) 1996 Free Software Foundation, Inc.
GNU Emacs comes with NO WARRANTY,
to the extent permitted by law.
You may redistribute copies of GNU Emacs
under the terms of the GNU General Public License.
For more information about these matters,
see the files named COPYING.
You should adapt this to your program, of course, filling in the
proper year, copyright holder, name of program, and the references
to distribution terms, and changing the rest of the wording as
This copyright notice only needs to mention the most recent year in
which changes were made--there's no need to list the years for
previous versions' changes. You don't have to mention the name of
the program in these notices, if that is inconvenient, since it
appeared in the first line.
This option should output brief documentation for how to invoke the
program, on standard output, then exit successfully. Other
options and arguments should be ignored once this is seen, and the
program should not perform its normal function.
Near the end of the `--help' option's output there should be a line
that says where to mail bug reports. It should have this format:
Report bugs to MAILING-ADDRESS.

File:, Node: Option Table, Next: Memory Usage, Prev: User Interfaces, Up: Program Behavior
Table of Long Options
Here is a table of long options used by GNU programs. It is surely
incomplete, but we aim to list all the options that a new program might
want to be compatible with. If you use names not already in the table,
please send <> a list of them, with their meanings, so we
can update the table.
`-N' in `tar'.
`-a' in `du', `ls', `nm', `stty', `uname', and `unexpand'.
`-a' in `diff'.
`-A' in `ls'.
`-a' in `etags', `tee', `time'; `-r' in `tar'.
`-a' in `cp'.
`-n' in `shar'.
`-l' in `m4'.
`-a' in `diff'.
`-v' in `gawk'.
`-W' in Make.
`-o' in Make.
`-a' in `recode'.
`-a' in `wdiff'.
`-A' in `ptx'.
`-n' in `wdiff'.
`-B' in `ctags'.
`-f' in `shar'.
Used in GDB.
Used in GDB.
`-b' in `tac'.
`-b' in `cpio' and `diff'.
`-b' in `shar'.
Used in `cpio' and `tar'.
`-b' in `head' and `tail'.
`-b' in `ptx'.
Used in various programs to make output shorter.
`-c' in `head', `split', and `tail'.
`-C' in `etags'.
`-A' in `tar'.
Used in various programs to specify the directory to use.
`-c' in `chgrp' and `chown'.
`-F' in `ls'.
`-c' in `recode'.
`-c' in `su'; `-x' in GDB.
`-d' in `tar'.
Used in `gawk'.
`-Z' in `tar' and `shar'.
`-A' in `tar'.
`-w' in `tar'.
Used in `diff'.
`-W copyleft' in `gawk'.
`-C' in `ptx', `recode', and `wdiff'; `-W copyright' in `gawk'.
Used in GDB.
`-q' in `who'.
`-l' in `du'.
Used in `tar' and `cpio'.
`-c' in `shar'.
`-x' in `ctags'.
`-d' in `touch'.
`-d' in Make and `m4'; `-t' in Bison.
`-D' in `m4'.
`-d' in Bison and `ctags'.
`-D' in `tar'.
`-L' in `chgrp', `chown', `cpio', `du', `ls', and `tar'.
`-D' in `du'.
`-d' in `recode'.
`-d' in `look'.
`-d' in `tar'.
`-n' in `csplit'.
Specify the directory to use, in various programs. In `ls', it
means to show directories themselves rather than their contents.
In `rm' and `ln', it means to not treat links to directories
`-x' in `strip'.
`-X' in `strip'.
`-n' in Make.
`-e' in `diff'.
`-z' in `csplit'.
`-x' in `wdiff'.
`-z' in `wdiff'.
`-N' in `diff'.
`-e' in Make.
`-e' in `xargs'.
Used in GDB.
Used in `makeinfo'.
`-o' in `m4'.
`-b' in `ls'.
`-X' in `tar'.
Used in GDB.
`-x' in `xargs'.
`-e' in `unshar'.
`-t' in `diff'.
`-e' in `sed'.
`-g' in `nm'.
`-i' in `cpio'; `-x' in `tar'.
`-f' in `finger'.
`-f' in `su'.
`-E' in `m4'.
`-f' in `info', `gawk', Make, `mt', and `tar'; `-n' in `sed'; `-r'
in `touch'.
`-F' in `gawk'.
`-b' in Bison.
`-F' in `ls'.
`-T' in `tar'.
Used in `makeinfo'.
`-F' in `ptx'.
`-y' in Bison.
`-f' in `tail'.
Used in `makeinfo'.
`-f' in `cp', `ln', `mv', and `rm'.
`-F' in `shar'.
Used in `ls', `time', and `ptx'.
`-F' in `m4'.
Used in GDB.
`-g' in `ptx'.
`-x' in `tar'.
`-i' in `ul'.
`-g' in `recode'.
`-g' in `install'.
`-z' in `tar' and `shar'.
`-H' in `m4'.
`-h' in `objdump' and `recode'
`-H' in `who'.
Used to ask for brief usage information.
`-d' in `shar'.
`-q' in `ls'.
`-u' in `who'.
`-D' in `diff'.
`-I' in `ls'; `-x' in `recode'.
`-w' in `diff'.
`-B' in `ls'.
`-B' in `diff'.
`-f' in `look' and `ptx'; `-i' in `diff' and `wdiff'.
`-i' in Make.
`-i' in `ptx'.
`-I' in `etags'.
`-f' in Oleo.
`-i' in `tee'.
`-I' in `diff'.
`-b' in `diff'.
`-i' in `tar'.
`-i' in `etags'; `-I' in `m4'.
`-I' in Make.
`-G' in `tar'.
`-i', `-l', and `-m' in Finger.
`-i' in `expand'.
`-T' in `diff'.
`-i' in `ls'.
`-i' in `cp', `ln', `mv', `rm'; `-e' in `m4'; `-p' in `xargs';
`-w' in `tar'.
`-p' in `shar'.
`-j' in Make.
`-n' in Make.
`-k' in Make.
`-k' in `csplit'.
`-k' in `du' and `ls'.
`-l' in `etags'.
`-l' in `wdiff'.
`-g' in `shar'.
`-C' in `split'.
Used in `split', `head', and `tail'.
`-l' in `cpio'.
Used in `gawk'.
`-t' in `cpio'; `-l' in `recode'.
`-t' in `tar'.
`-N' in `ls'.
`-l' in Make.
Used in `su'.
No listing of which programs already use this; someone should
check to see if any actually do, and tell <>.
`-M' in `ptx'.
`-m' in `hello' and `uname'.
`-d' in `cpio'.
`-f' in Make.
Used in GDB.
`-n' in `xargs'.
`-n' in `xargs'.
`-l' in `xargs'.
`-l' in Make.
`-P' in `xargs'.
`-T' in `who'.
`-T' in `who'.
`-d' in `diff'.
`-M' in `shar'.
`-m' in `install', `mkdir', and `mkfifo'.
`-m' in `tar'.
`-M' in `tar'.
`-a' in Bison.
`-L' in `m4'.
`-a' in `shar'.
`-W' in Make.
`-r' in Make.
`-w' in `shar'.
`-x' in `shar'.
`-3' in `wdiff'.
`-c' in `touch'.
`-D' in `etags'.
`-1' in `wdiff'.
`-d' in `cp'.
`-2' in `wdiff'.
`-S' in Make.
`-l' in Bison.
`-P' in `shar'.
`-e' in `gprof'.
`-R' in `etags'.
`-p' in `nm'.
Used in `makeinfo'.
`-a' in `gprof'.
`-E' in `gprof'.
`-m' in `shar'.
Used in `makeinfo'.
Used in `emacsclient'.
Used in various programs to inhibit warnings.
`-n' in `info'.
`-n' in `uname'.
`-f' in `cpio'.
`-n' in `objdump'.
`-0' in `xargs'.
`-n' in `cat'.
`-b' in `cat'.
`-n' in `nm'.
`-n' in `cpio' and `ls'.
Used in GDB.
`-o' in `tar'.
`-o' in Make.
`-l' in `tar', `cp', and `du'.
`-o' in `ptx'.
`-f' in `gprof'.
`-F' in `gprof'.
In various programs, specify the output file name.
`-o' in `shar'.
`-o' in `rm'.
`-c' in `unshar'.
`-o' in `install'.
`-l' in `diff'.
Used in `makeinfo'.
`-p' in `mkdir' and `rmdir'.
`-p' in `ul'.
`-p' in `cpio'.
`-P' in `finger'.
`-c' in `cpio' and `tar'.
Used in `gawk'.
`-P' in `m4'.
`-f' in `csplit'.
Used in `tar' and `cp'.
`-p' in `su'.
`-m' in `cpio'.
`-s' in `tar'.
`-p' in `tar'.
`-l' in `diff'.
`-L' in `cmp'.
`-p' in Make.
`-w' in Make.
`-o' in `nm'.
`-s' in `nm'.
`-p' in `wdiff'.
`-p' in `ed'.
`-X' in `shar'.
`-q' in Make.
Used in many programs to inhibit the usual output. *Note:* every
program accepting `--quiet' should accept `--silent' as a synonym.
`-Q' in `shar'
`-Q' in `ls'.
`-n' in `diff'.
Used in `gawk'.
`-B' in `tar'.
Used in GDB.
`-n' in Make.
`-R' in `tar'.
Used in `chgrp', `chown', `cp', `ls', `diff', and `rm'.
Used in `makeinfo'.
`-r' in `ptx'.
`-r' in `tac' and `etags'.
`-r' in `uname'.
`-R' in `m4'.
`-r' in `objdump'.
`-r' in `cpio'.
`-i' in `xargs'.
`-s' in `diff'.
`-a' in `cpio'.
`-r' in `ls' and `nm'.
`-f' in `diff'.
`-R' in `ptx'.
`-s' in `tar'.
`-p' in `tar'.
`-g' in `stty'.
Used in GDB.
`-S' in `ptx'.
`-S' in `du'.
`-s' in `tac'.
Used by `recode' to chose files or pipes for sequencing passes.
`-s' in `su'.
`-A' in `cat'.
`-p' in `diff'.
`-E' in `cat'.
`-F' in `diff'.
`-T' in `cat'.
Used in many programs to inhibit the usual output. *Note:* every
program accepting `--silent' should accept `--quiet' as a synonym.
`-s' in `ls'.
Used in `ls'.
`-W source' in `gawk'.
`-S' in `tar'.
`-H' in `diff'.
`-E' in `unshar'.
`-L' in `shar'.
`-s' in `cat'.
`-w' in `wdiff'.
`-y' in `wdiff'.
Used in `tar' and `diff' to specify which file within a directory
to start processing with.
`-s' in `wdiff'.
`-S' in `shar'.
`-S' in Make.
`-s' in `recode'.
`-s' in `install'.
`-s' in `strip'.
`-S' in `strip'.
`-s' in `shar'.
`-S' in `cp', `ln', `mv'.
`-b' in `csplit'.
`-s' in `gprof'.
`-s' in `du'.
`-s' in `ln'.
Used in GDB and `objdump'.
`-s' in `m4'.
`-s' in `uname'.
`-t' in `expand' and `unexpand'.
`-T' in `ls'.
`-T' in `tput' and `ul'. `-t' in `wdiff'.
`-a' in `diff'.
`-T' in `shar'.
Used in `ls' and `touch'.
`-O' in `tar'.
`-c' in `du'.
`-t' in Make, `ranlib', and `recode'.
`-t' in `m4'.
`-t' in `hello'; `-W traditional' in `gawk'; `-G' in `ed', `m4',
and `ptx'.
Used in GDB.
`-t' in `ctags'.
`-T' in `ctags'.
`-t' in `ptx'.
`-z' in `tar'.
`-u' in `cpio'.
`-U' in `m4'.
`-u' in `nm'.
`-u' in `cp', `ctags', `mv', `tar'.
Used in `gawk'; same as `--help'.
`-B' in `shar'.
`-V' in `shar'.
Print more information about progress. Many programs support this.
`-W' in `tar'.
Print the version number.
`-V' in `cp', `ln', `mv'.
`-v' in `ctags'.
`-V' in `tar'.
`-W' in Make.
`-l' in `shar'.
`-w' in `ls' and `ptx'.
`-W' in `ptx'.
`-T' in `who'.
`-z' in `gprof'.

File:, Node: Memory Usage, Prev: Option Table, Up: Program Behavior
Memory Usage
If it typically uses just a few meg of memory, don't bother making
any effort to reduce memory usage. For example, if it is impractical
for other reasons to operate on files more than a few meg long, it is
reasonable to read entire input files into core to operate on them.
However, for programs such as `cat' or `tail', that can usefully
operate on very large files, it is important to avoid using a technique
that would artificially limit the size of files it can handle. If a
program works by lines and could be applied to arbitrary user-supplied
input files, it should keep only a line in memory, because this is not
very hard and users will want to be able to operate on input files that
are bigger than will fit in core all at once.
If your program creates complicated data structures, just make them
in core and give a fatal error if `malloc' returns zero.

File:, Node: Writing C, Next: Documentation, Prev: Program Behavior, Up: Top
Making The Best Use of C
This node provides advice on how best to use the C language when
writing GNU software.
* Menu:
* Formatting:: Formatting Your Source Code
* Comments:: Commenting Your Work
* Syntactic Conventions:: Clean Use of C Constructs
* Names:: Naming Variables and Functions
* System Portability:: Portability between different operating systems
* CPU Portability:: Supporting the range of CPU types
* System Functions:: Portability and ``standard'' library functions
* Internationalization:: Techniques for internationalization
* Mmap:: How you can safely use `mmap'.

File:, Node: Formatting, Next: Comments, Up: Writing C
Formatting Your Source Code
It is important to put the open-brace that starts the body of a C
function in column zero, and avoid putting any other open-brace or
open-parenthesis or open-bracket in column zero. Several tools look
for open-braces in column zero to find the beginnings of C functions.
These tools will not work on code not formatted that way.
It is also important for function definitions to start the name of
the function in column zero. This helps people to search for function
definitions, and may also help certain tools recognize them. Thus, the
proper format is this:
static char *
concat (s1, s2) /* Name starts in column zero here */
char *s1, *s2;
{ /* Open brace in column zero here */
or, if you want to use ANSI C, format the definition like this:
static char *
concat (char *s1, char *s2)
In ANSI C, if the arguments don't fit nicely on one line, split it
like this:
lots_of_args (int an_integer, long a_long, short a_short,
double a_double, float a_float)
For the body of the function, we prefer code formatted like this:
if (x < foo (y, z))
haha = bar[4] + 5;
while (z)
haha += foo (z, z);
return ++x + bar ();
We find it easier to read a program when it has spaces before the
open-parentheses and after the commas. Especially after the commas.
When you split an expression into multiple lines, split it before an
operator, not after one. Here is the right way:
if (foo_this_is_long && bar > win (x, y, z)
&& remaining_condition)
Try to avoid having two operators of different precedence at the same
level of indentation. For example, don't write this:
mode = (inmode[j] == VOIDmode
|| GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])
? outmode[j] : inmode[j]);
Instead, use extra parentheses so that the indentation shows the
mode = ((inmode[j] == VOIDmode
|| (GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])))
? outmode[j] : inmode[j]);
Insert extra parentheses so that Emacs will indent the code properly.
For example, the following indentation looks nice if you do it by hand,
but Emacs would mess it up:
v = rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
+ rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000;
But adding a set of parentheses solves the problem:
v = (rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
+ rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000);
Format do-while statements like this:
a = foo (a);
while (a > 0);
Please use formfeed characters (control-L) to divide the program into
pages at logical places (but not within a function). It does not matter
just how long the pages are, since they do not have to fit on a printed
page. The formfeeds should appear alone on lines by themselves.

File:, Node: Comments, Next: Syntactic Conventions, Prev: Formatting, Up: Writing C
Commenting Your Work
Every program should start with a comment saying briefly what it is
for. Example: `fmt - filter for simple filling of text'.
Please write the comments in a GNU program in English, because
English is the one language that nearly all programmers in all
countries can read. If you do not write English well, please write
comments in English as well as you can, then ask other people to help
rewrite them. If you can't write comments in English, please find
someone to work with you and translate your comments into English.
Please put a comment on each function saying what the function does,
what sorts of arguments it gets, and what the possible values of
arguments mean and are used for. It is not necessary to duplicate in
words the meaning of the C argument declarations, if a C type is being
used in its customary fashion. If there is anything nonstandard about
its use (such as an argument of type `char *' which is really the
address of the second character of a string, not the first), or any
possible values that would not work the way one would expect (such as,
that strings containing newlines are not guaranteed to work), be sure
to say so.
Also explain the significance of the return value, if there is one.
Please put two spaces after the end of a sentence in your comments,
so that the Emacs sentence commands will work. Also, please write
complete sentences and capitalize the first word. If a lower-case
identifier comes at the beginning of a sentence, don't capitalize it!
Changing the spelling makes it a different identifier. If you don't
like starting a sentence with a lower case letter, write the sentence
differently (e.g., "The identifier lower-case is ...").
The comment on a function is much clearer if you use the argument
names to speak about the argument values. The variable name itself
should be lower case, but write it in upper case when you are speaking
about the value rather than the variable itself. Thus, "the inode
number NODE_NUM" rather than "an inode".
There is usually no purpose in restating the name of the function in
the comment before it, because the reader can see that for himself.
There might be an exception when the comment is so long that the
function itself would be off the bottom of the screen.
There should be a comment on each static variable as well, like this:
/* Nonzero means truncate lines in the display;
zero means continue them. */
int truncate_lines;
Every `#endif' should have a comment, except in the case of short
conditionals (just a few lines) that are not nested. The comment should
state the condition of the conditional that is ending, _including its
sense_. `#else' should have a comment describing the condition _and
sense_ of the code that follows. For example:
#ifdef foo
#else /* not foo */
#endif /* not foo */
#ifdef foo
#endif /* foo */
but, by contrast, write the comments this way for a `#ifndef':
#ifndef foo
#else /* foo */
#endif /* foo */
#ifndef foo
#endif /* not foo */

File:, Node: Syntactic Conventions, Next: Names, Prev: Comments, Up: Writing C
Clean Use of C Constructs
Please explicitly declare all arguments to functions. Don't omit
them just because they are `int's.
Declarations of external functions and functions to appear later in
the source file should all go in one place near the beginning of the
file (somewhere before the first function definition in the file), or
else should go in a header file. Don't put `extern' declarations inside
It used to be common practice to use the same local variables (with
names like `tem') over and over for different values within one
function. Instead of doing this, it is better declare a separate local
variable for each distinct purpose, and give it a name which is
meaningful. This not only makes programs easier to understand, it also
facilitates optimization by good compilers. You can also move the
declaration of each local variable into the smallest scope that includes
all its uses. This makes the program even cleaner.
Don't use local variables or parameters that shadow global
Don't declare multiple variables in one declaration that spans lines.
Start a new declaration on each line, instead. For example, instead of
int foo,
write either this:
int foo, bar;
or this:
int foo;
int bar;
(If they are global variables, each should have a comment preceding it
When you have an `if'-`else' statement nested in another `if'
statement, always put braces around the `if'-`else'. Thus, never write
like this:
if (foo)
if (bar)
win ();
lose ();
always like this:
if (foo)
if (bar)
win ();
lose ();
If you have an `if' statement nested inside of an `else' statement,
either write `else if' on one line, like this,
if (foo)
else if (bar)
with its `then'-part indented like the preceding `then'-part, or write
the nested `if' within braces like this:
if (foo)
if (bar)
Don't declare both a structure tag and variables or typedefs in the
same declaration. Instead, declare the structure tag separately and
then use it to declare the variables or typedefs.
Try to avoid assignments inside `if'-conditions. For example, don't
write this:
if ((foo = (char *) malloc (sizeof *foo)) == 0)
fatal ("virtual memory exhausted");
instead, write this:
foo = (char *) malloc (sizeof *foo);
if (foo == 0)
fatal ("virtual memory exhausted");
Don't make the program ugly to placate `lint'. Please don't insert
any casts to `void'. Zero without a cast is perfectly fine as a null
pointer constant, except when calling a varargs function.

File:, Node: Names, Next: System Portability, Prev: Syntactic Conventions, Up: Writing C
Naming Variables and Functions
The names of global variables and functions in a program serve as
comments of a sort. So don't choose terse names--instead, look for
names that give useful information about the meaning of the variable or
function. In a GNU program, names should be English, like other
Local variable names can be shorter, because they are used only
within one context, where (presumably) comments explain their purpose.
Please use underscores to separate words in a name, so that the Emacs
word commands can be useful within them. Stick to lower case; reserve
upper case for macros and `enum' constants, and for name-prefixes that
follow a uniform convention.
For example, you should use names like `ignore_space_change_flag';
don't use names like `iCantReadThis'.
Variables that indicate whether command-line options have been
specified should be named after the meaning of the option, not after
the option-letter. A comment should state both the exact meaning of
the option and its letter. For example,
/* Ignore changes in horizontal whitespace (-b). */
int ignore_space_change_flag;
When you want to define names with constant integer values, use
`enum' rather than `#define'. GDB knows about enumeration constants.
Use file names of 14 characters or less, to avoid creating gratuitous
problems on older System V systems. You can use the program `doschk'
to test for this. `doschk' also tests for potential name conflicts if
the files were loaded onto an MS-DOS file system--something you may or
may not care about.

File:, Node: System Portability, Next: CPU Portability, Prev: Names, Up: Writing C
Portability between System Types
In the Unix world, "portability" refers to porting to different Unix
versions. For a GNU program, this kind of portability is desirable, but
not paramount.
The primary purpose of GNU software is to run on top of the GNU
kernel, compiled with the GNU C compiler, on various types of CPU. The
amount and kinds of variation among GNU systems on different CPUs will
be comparable to the variation among Linux-based GNU systems or among
BSD systems today. So the kinds of portability that are absolutely
necessary are quite limited.
But many users do run GNU software on non-GNU Unix or Unix-like
systems. So supporting a variety of Unix-like systems is desirable,
although not paramount.
The easiest way to achieve portability to most Unix-like systems is
to use Autoconf. It's unlikely that your program needs to know more
information about the host platform than Autoconf can provide, simply
because most of the programs that need such knowledge have already been
Avoid using the format of semi-internal data bases (e.g.,
directories) when there is a higher-level alternative (`readdir').
As for systems that are not like Unix, such as MSDOS, Windows, the
Macintosh, VMS, and MVS, supporting them is usually so much work that it
is better if you don't.
The planned GNU kernel is not finished yet, but you can tell which
facilities it will provide by looking at the GNU C Library Manual. The
GNU kernel is based on Mach, so the features of Mach will also be
available. However, if you use Mach features, you'll probably have
trouble debugging your program today.

File:, Node: CPU Portability, Next: System Functions, Prev: System Portability, Up: Writing C
Portability between CPUs
Even GNU systems will differ because of differences among CPU
types--for example, difference in byte ordering and alignment
requirements. It is absolutely essential to handle these differences.
However, don't make any effort to cater to the possibility that an
`int' will be less than 32 bits. We don't support 16-bit machines in
Don't assume that the address of an `int' object is also the address
of its least-significant byte. This is false on big-endian machines.
Thus, don't make the following mistake:
int c;
while ((c = getchar()) != EOF)
write(file_descriptor, &c, 1);
When calling functions, you need not worry about the difference
between pointers of various types, or between pointers and integers.
On most machines, there's no difference anyway. As for the few
machines where there is a difference, all of them support ANSI C, so
you can use prototypes (conditionalized to be active only in ANSI C) to
make the code work on those systems.
In certain cases, it is ok to pass integer and pointer arguments
indiscriminately to the same function, and use no prototype on any
system. For example, many GNU programs have error-reporting functions
that pass their arguments along to `printf' and friends:
error (s, a1, a2, a3)
char *s;
int a1, a2, a3;
fprintf (stderr, "error: ");
fprintf (stderr, s, a1, a2, a3);
In practice, this works on all machines, and it is much simpler than any
"correct" alternative. Be sure _not_ to use a prototype for such
However, avoid casting pointers to integers unless you really need
to. These assumptions really reduce portability, and in most programs
they are easy to avoid. In the cases where casting pointers to
integers is essential--such as, a Lisp interpreter which stores type
information as well as an address in one word--it is ok to do so, but
you'll have to make explicit provisions to handle different word sizes.

File:, Node: System Functions, Next: Internationalization, Prev: CPU Portability, Up: Writing C
Calling System Functions
C implementations differ substantially. ANSI C reduces but does not
eliminate the incompatibilities; meanwhile, many users wish to compile
GNU software with pre-ANSI compilers. This chapter gives
recommendations for how to use the more or less standard C library
functions to avoid unnecessary loss of portability.
* Don't use the value of `sprintf'. It returns the number of
characters written on some systems, but not on all systems.
* `main' should be declared to return type `int'. It should
terminate either by calling `exit' or by returning the integer
status code; make sure it cannot ever return an undefined value.
* Don't declare system functions explicitly.
Almost any declaration for a system function is wrong on some
system. To minimize conflicts, leave it to the system header
files to declare system functions. If the headers don't declare a
function, let it remain undeclared.
While it may seem unclean to use a function without declaring it,
in practice this works fine for most system library functions on
the systems where this really happens; thus, the disadvantage is
only theoretical. By contrast, actual declarations have
frequently caused actual conflicts.
* If you must declare a system function, don't specify the argument
types. Use an old-style declaration, not an ANSI prototype. The
more you specify about the function, the more likely a conflict.
* In particular, don't unconditionally declare `malloc' or `realloc'.
Most GNU programs use those functions just once, in functions
conventionally named `xmalloc' and `xrealloc'. These functions
call `malloc' and `realloc', respectively, and check the results.
Because `xmalloc' and `xrealloc' are defined in your program, you
can declare them in other files without any risk of type conflict.
On most systems, `int' is the same length as a pointer; thus, the
calls to `malloc' and `realloc' work fine. For the few
exceptional systems (mostly 64-bit machines), you can use
*conditionalized* declarations of `malloc' and `realloc'--or put
these declarations in configuration files specific to those
* The string functions require special treatment. Some Unix systems
have a header file `string.h'; others have `strings.h'. Neither
file name is portable. There are two things you can do: use
Autoconf to figure out which file to include, or don't include
either file.
* If you don't include either strings file, you can't get
declarations for the string functions from the header file in the
usual way.
That causes less of a problem than you might think. The newer ANSI
string functions should be avoided anyway because many systems
still don't support them. The string functions you can use are
strcpy strncpy strcat strncat
strlen strcmp strncmp
strchr strrchr
The copy and concatenate functions work fine without a declaration
as long as you don't use their values. Using their values without
a declaration fails on systems where the width of a pointer
differs from the width of `int', and perhaps in other cases. It
is trivial to avoid using their values, so do that.
The compare functions and `strlen' work fine without a declaration
on most systems, possibly all the ones that GNU software runs on.
You may find it necessary to declare them *conditionally* on a few
The search functions must be declared to return `char *'. Luckily,
there is no variation in the data type they return. But there is
variation in their names. Some systems give these functions the
names `index' and `rindex'; other systems use the names `strchr'
and `strrchr'. Some systems support both pairs of names, but
neither pair works on all systems.
You should pick a single pair of names and use it throughout your
program. (Nowadays, it is better to choose `strchr' and `strrchr'
for new programs, since those are the standard ANSI names.)
Declare both of those names as functions returning `char *'. On
systems which don't support those names, define them as macros in
terms of the other pair. For example, here is what to put at the
beginning of your file (or in a header) if you want to use the
names `strchr' and `strrchr' throughout:
#define strchr index
#define strrchr rindex
char *strchr ();
char *strrchr ();
Here we assume that `HAVE_STRCHR' and `HAVE_STRRCHR' are macros
defined in systems where the corresponding functions exist. One way to
get them properly defined is to use Autoconf.

File:, Node: Internationalization, Next: Mmap, Prev: System Functions, Up: Writing C
GNU has a library called GNU gettext that makes it easy to translate
the messages in a program into various languages. You should use this
library in every program. Use English for the messages as they appear
in the program, and let gettext provide the way to translate them into
other languages.
Using GNU gettext involves putting a call to the `gettext' macro
around each string that might need translation--like this:
printf (gettext ("Processing file `%s'..."));
This permits GNU gettext to replace the string `"Processing file
`%s'..."' with a translated version.
Once a program uses gettext, please make a point of writing calls to
`gettext' when you add new strings that call for translation.
Using GNU gettext in a package involves specifying a "text domain
name" for the package. The text domain name is used to separate the
translations for this package from the translations for other packages.
Normally, the text domain name should be the same as the name of the
package--for example, `fileutils' for the GNU file utilities.
To enable gettext to work well, avoid writing code that makes
assumptions about the structure of words or sentences. When you want
the precise text of a sentence to vary depending on the data, use two or
more alternative string constants each containing a complete sentences,
rather than inserting conditionalized words or phrases into a single
sentence framework.
Here is an example of what not to do:
printf ("%d file%s processed", nfiles,
nfiles != 1 ? "s" : "");
The problem with that example is that it assumes that plurals are made
by adding `s'. If you apply gettext to the format string, like this,
printf (gettext ("%d file%s processed"), nfiles,
nfiles != 1 ? "s" : "");
the message can use different words, but it will still be forced to use
`s' for the plural. Here is a better way:
printf ((nfiles != 1 ? "%d files processed"
: "%d file processed"),
This way, you can apply gettext to each of the two strings
printf ((nfiles != 1 ? gettext ("%d files processed")
: gettext ("%d file processed")),
This can be any method of forming the plural of the word for "file", and
also handles languages that require agreement in the word for
A similar problem appears at the level of sentence structure with
this code:
printf ("# Implicit rule search has%s been done.\n",
f->tried_implicit ? "" : " not");
Adding `gettext' calls to this code cannot give correct results for all
languages, because negation in some languages requires adding words at
more than one place in the sentence. By contrast, adding `gettext'
calls does the job straightfowardly if the code starts out like this:
printf (f->tried_implicit
? "# Implicit rule search has been done.\n",
: "# Implicit rule search has not been done.\n");

File:, Node: Mmap, Prev: Internationalization, Up: Writing C
Don't assume that `mmap' either works on all files or fails for all
files. It may work on some files and fail on others.
The proper way to use `mmap' is to try it on the specific file for
which you want to use it--and if `mmap' doesn't work, fall back on
doing the job in another way using `read' and `write'.
The reason this precaution is needed is that the GNU kernel (the
HURD) provides a user-extensible file system, in which there can be many
different kinds of "ordinary files." Many of them support `mmap', but
some do not. It is important to make programs handle all these kinds
of files.

File:, Node: Documentation, Next: Managing Releases, Prev: Writing C, Up: Top
Documenting Programs
* Menu:
* GNU Manuals:: Writing proper manuals.
* Manual Structure Details:: Specific structure conventions.
* NEWS File:: NEWS files supplement manuals.
* Change Logs:: Recording Changes
* Man Pages:: Man pages are secondary.
* Reading other Manuals:: How far you can go in learning
from other manuals.

File:, Node: GNU Manuals, Next: Manual Structure Details, Up: Documentation
GNU Manuals
The preferred way to document part of the GNU system is to write a
manual in the Texinfo formatting language. See the Texinfo manual,
either the hardcopy, or the on-line version available through `info' or
the Emacs Info subsystem (`C-h i').
Programmers often find it most natural to structure the documentation
following the structure of the implementation, which they know. But
this structure is not necessarily good for explaining how to use the
program; it may be irrelevant and confusing for a user.
At every level, from the sentences in a paragraph to the grouping of
topics into separate manuals, the right way to structure documentation
is according to the concepts and questions that a user will have in mind
when reading it. Sometimes this structure of ideas matches the
structure of the implementation of the software being documented--but
often they are different. Often the most important part of learning to
write good documentation is learning to notice when you are structuring
the documentation like the implementation, and think about better
For example, each program in the GNU system probably ought to be
documented in one manual; but this does not mean each program should
have its own manual. That would be following the structure of the
implementation, rather than the structure that helps the user
Instead, each manual should cover a coherent _topic_. For example,
instead of a manual for `diff' and a manual for `diff3', we have one
manual for "comparison of files" which covers both of those programs,
as well as `cmp'. By documenting these programs together, we can make
the whole subject clearer.
The manual which discusses a program should document all of the
program's command-line options and all of its commands. It should give
examples of their use. But don't organize the manual as a list of
features. Instead, organize it logically, by subtopics. Address the
questions that a user will ask when thinking about the job that the
program does.
In general, a GNU manual should serve both as tutorial and reference.
It should be set up for convenient access to each topic through Info,
and for reading straight through (appendixes aside). A GNU manual
should give a good introduction to a beginner reading through from the
start, and should also provide all the details that hackers want.
That is not as hard as it first sounds. Arrange each chapter as a
logical breakdown of its topic, but order the sections, and write their
text, so that reading the chapter straight through makes sense. Do
likewise when structuring the book into chapters, and when structuring a
section into paragraphs. The watchword is, _at each point, address the
most fundamental and important issue raised by the preceding text._
If necessary, add extra chapters at the beginning of the manual which
are purely tutorial and cover the basics of the subject. These provide
the framework for a beginner to understand the rest of the manual. The
Bison manual provides a good example of how to do this.
Don't use Unix man pages as a model for how to write GNU
documentation; most of them are terse, badly structured, and give
inadequate explanation of the underlying concepts. (There are, of
course exceptions.) Also Unix man pages use a particular format which
is different from what we use in GNU manuals.
Please do not use the term "pathname" that is used in Unix
documentation; use "file name" (two words) instead. We use the term
"path" only for search paths, which are lists of file names.
Please do not use the term "illegal" to refer to erroneous input to a
computer program. Please use "invalid" for this, and reserve the term
"illegal" for violations of law.

File:, Node: Manual Structure Details, Next: NEWS File, Prev: GNU Manuals, Up: Documentation
Manual Structure Details
The title page of the manual should state the version of the
programs or packages documented in the manual. The Top node of the
manual should also contain this information. If the manual is changing
more frequently than or independent of the program, also state a version
number for the manual in both of these places.
Each program documented in the manual should should have a node named
`PROGRAM Invocation' or `Invoking PROGRAM'. This node (together with
its subnodes, if any) should describe the program's command line
arguments and how to run it (the sort of information people would look
in a man page for). Start with an `@example' containing a template for
all the options and arguments that the program uses.
Alternatively, put a menu item in some menu whose item name fits one
of the above patterns. This identifies the node which that item points
to as the node for this purpose, regardless of the node's actual name.
There will be automatic features for specifying a program name and
quickly reading just this part of its manual.
If one manual describes several programs, it should have such a node
for each program described.

File:, Node: NEWS File, Next: Change Logs, Prev: Manual Structure Details, Up: Documentation
The NEWS File
In addition to its manual, the package should have a file named
`NEWS' which contains a list of user-visible changes worth mentioning.
In each new release, add items to the front of the file and identify
the version they pertain to. Don't discard old items; leave them in
the file after the newer items. This way, a user upgrading from any
previous version can see what is new.
If the `NEWS' file gets very long, move some of the older items into
a file named `ONEWS' and put a note at the end referring the user to
that file.

File:, Node: Change Logs, Next: Man Pages, Prev: NEWS File, Up: Documentation
Change Logs
Keep a change log to describe all the changes made to program source
files. The purpose of this is so that people investigating bugs in the
future will know about the changes that might have introduced the bug.
Often a new bug can be found by looking at what was recently changed.
More importantly, change logs can help you eliminate conceptual
inconsistencies between different parts of a program, by giving you a
history of how the conflicting concepts arose and who they came from.
* Menu:
* Change Log Concepts::
* Style of Change Logs::
* Simple Changes::
* Conditional Changes::

File:, Node: Change Log Concepts, Next: Style of Change Logs, Up: Change Logs
Change Log Concepts
You can think of the change log as a conceptual "undo list" which
explains how earlier versions were different from the current version.
People can see the current version; they don't need the change log to
tell them what is in it. What they want from a change log is a clear
explanation of how the earlier version differed.
The change log file is normally called `ChangeLog' and covers an
entire directory. Each directory can have its own change log, or a
directory can use the change log of its parent directory-it's up to you.
Another alternative is to record change log information with a
version control system such as RCS or CVS. This can be converted
automatically to a `ChangeLog' file.
There's no need to describe the full purpose of the changes or how
they work together. If you think that a change calls for explanation,
you're probably right. Please do explain it--but please put the
explanation in comments in the code, where people will see it whenever
they see the code. For example, "New function" is enough for the
change log when you add a function, because there should be a comment
before the function definition to explain what it does.
However, sometimes it is useful to write one line to describe the
overall purpose of a batch of changes.
The easiest way to add an entry to `ChangeLog' is with the Emacs
command `M-x add-change-log-entry'. An entry should have an asterisk,
the name of the changed file, and then in parentheses the name of the
changed functions, variables or whatever, followed by a colon. Then
describe the changes you made to that function or variable.

File:, Node: Style of Change Logs, Next: Simple Changes, Prev: Change Log Concepts, Up: Change Logs
Style of Change Logs
Here are some examples of change log entries:
* register.el (insert-register): Return nil.
(jump-to-register): Likewise.
* sort.el (sort-subr): Return nil.
* tex-mode.el (tex-bibtex-file, tex-file, tex-region):
Restart the tex shell if process is gone or stopped.
(tex-shell-running): New function.
* expr.c (store_one_arg): Round size up for move_block_to_reg.
(expand_call): Round up when emitting USE insns.
* stmt.c (assign_parms): Round size up for move_block_from_reg.
It's important to name the changed function or variable in full.
Don't abbreviate function or variable names, and don't combine them.
Subsequent maintainers will often search for a function name to find all
the change log entries that pertain to it; if you abbreviate the name,
they won't find it when they search.
For example, some people are tempted to abbreviate groups of function
names by writing `* register.el ({insert,jump-to}-register)'; this is
not a good idea, since searching for `jump-to-register' or
`insert-register' would not find that entry.
Separate unrelated change log entries with blank lines. When two
entries represent parts of the same change, so that they work together,
then don't put blank lines between them. Then you can omit the file
name and the asterisk when successive entries are in the same file.

File:, Node: Simple Changes, Next: Conditional Changes, Prev: Style of Change Logs, Up: Change Logs
Simple Changes
Certain simple kinds of changes don't need much detail in the change
When you change the calling sequence of a function in a simple
fashion, and you change all the callers of the function, there is no
need to make individual entries for all the callers that you changed.
Just write in the entry for the function being called, "All callers
* keyboard.c (Fcommand_execute): New arg SPECIAL.
All callers changed.
When you change just comments or doc strings, it is enough to write
an entry for the file, without mentioning the functions. Just "Doc
fixes" is enough for the change log.
There's no need to make change log entries for documentation files.
This is because documentation is not susceptible to bugs that are hard
to fix. Documentation does not consist of parts that must interact in a
precisely engineered fashion. To correct an error, you need not know
the history of the erroneous passage; it is enough to compare what the
documentation says with the way the program actually works.

File:, Node: Conditional Changes, Prev: Simple Changes, Up: Change Logs
Conditional Changes
C programs often contain compile-time `#if' conditionals. Many
changes are conditional; sometimes you add a new definition which is
entirely contained in a conditional. It is very useful to indicate in
the change log the conditions for which the change applies.
Our convention for indicating conditional changes is to use square
brackets around the name of the condition.
Here is a simple example, describing a change which is conditional
but does not have a function or entity name associated with it:
* xterm.c [SOLARIS2]: Include string.h.
Here is an entry describing a new definition which is entirely
conditional. This new definition for the macro `FRAME_WINDOW_P' is
used only when `HAVE_X_WINDOWS' is defined:
* frame.h [HAVE_X_WINDOWS] (FRAME_WINDOW_P): Macro defined.
Here is an entry for a change within the function `init_display',
whose definition as a whole is unconditional, but the changes themselves
are contained in a `#ifdef HAVE_LIBNCURSES' conditional:
* dispnew.c (init_display) [HAVE_LIBNCURSES]: If X, call tgetent.
Here is an entry for a change that takes affect only when a certain
macro is _not_ defined:
(gethostname) [!HAVE_SOCKETS]: Replace with winsock version.

File:, Node: Man Pages, Next: Reading other Manuals, Prev: Change Logs, Up: Documentation
Man Pages
In the GNU project, man pages are secondary. It is not necessary or
expected for every GNU program to have a man page, but some of them do.
It's your choice whether to include a man page in your program.
When you make this decision, consider that supporting a man page
requires continual effort each time the program is changed. The time
you spend on the man page is time taken away from more useful work.
For a simple program which changes little, updating the man page may
be a small job. Then there is little reason not to include a man page,
if you have one.
For a large program that changes a great deal, updating a man page
may be a substantial burden. If a user offers to donate a man page,
you may find this gift costly to accept. It may be better to refuse
the man page unless the same person agrees to take full responsibility
for maintaining it--so that you can wash your hands of it entirely. If
this volunteer later ceases to do the job, then don't feel obliged to
pick it up yourself; it may be better to withdraw the man page from the
distribution until someone else agrees to update it.
When a program changes only a little, you may feel that the
discrepancies are small enough that the man page remains useful without
updating. If so, put a prominent note near the beginning of the man
page explaining that you don't maintain it and that the Texinfo manual
is more authoritative. The note should say how to access the Texinfo

File:, Node: Reading other Manuals, Prev: Man Pages, Up: Documentation
Reading other Manuals
There may be non-free books or documentation files that describe the
program you are documenting.
It is ok to use these documents for reference, just as the author of
a new algebra textbook can read other books on algebra. A large portion
of any non-fiction book consists of facts, in this case facts about how
a certain program works, and these facts are necessarily the same for
everyone who writes about the subject. But be careful not to copy your
outline structure, wording, tables or examples from preexisting non-free
documentation. Copying from free documentation may be ok; please check
with the FSF about the individual case.

File:, Node: Managing Releases, Prev: Documentation, Up: Top
The Release Process
Making a release is more than just bundling up your source files in a
tar file and putting it up for FTP. You should set up your software so
that it can be configured to run on a variety of systems. Your Makefile
should conform to the GNU standards described below, and your directory
layout should also conform to the standards discussed below. Doing so
makes it easy to include your package into the larger framework of all
GNU software.
* Menu:
* Configuration:: How Configuration Should Work
* Makefile Conventions:: Makefile Conventions
* Releases:: Making Releases

File:, Node: Configuration, Next: Makefile Conventions, Up: Managing Releases
How Configuration Should Work
Each GNU distribution should come with a shell script named
`configure'. This script is given arguments which describe the kind of
machine and system you want to compile the program for.
The `configure' script must record the configuration options so that
they affect compilation.
One way to do this is to make a link from a standard name such as
`config.h' to the proper configuration file for the chosen system. If
you use this technique, the distribution should _not_ contain a file
named `config.h'. This is so that people won't be able to build the
program without configuring it first.
Another thing that `configure' can do is to edit the Makefile. If
you do this, the distribution should _not_ contain a file named
`Makefile'. Instead, it should include a file `' which
contains the input used for editing. Once again, this is so that people
won't be able to build the program without configuring it first.
If `configure' does write the `Makefile', then `Makefile' should
have a target named `Makefile' which causes `configure' to be rerun,
setting up the same configuration that was set up last time. The files
that `configure' reads should be listed as dependencies of `Makefile'.
All the files which are output from the `configure' script should
have comments at the beginning explaining that they were generated
automatically using `configure'. This is so that users won't think of
trying to edit them by hand.
The `configure' script should write a file named `config.status'
which describes which configuration options were specified when the
program was last configured. This file should be a shell script which,
if run, will recreate the same configuration.
The `configure' script should accept an option of the form
`--srcdir=DIRNAME' to specify the directory where sources are found (if
it is not the current directory). This makes it possible to build the
program in a separate directory, so that the actual source directory is
not modified.
If the user does not specify `--srcdir', then `configure' should
check both `.' and `..' to see if it can find the sources. If it finds
the sources in one of these places, it should use them from there.
Otherwise, it should report that it cannot find the sources, and should
exit with nonzero status.
Usually the easy way to support `--srcdir' is by editing a
definition of `VPATH' into the Makefile. Some rules may need to refer
explicitly to the specified source directory. To make this possible,
`configure' can add to the Makefile a variable named `srcdir' whose
value is precisely the specified directory.
The `configure' script should also take an argument which specifies
the type of system to build the program for. This argument should look
like this:
For example, a Sun 3 might be `m68k-sun-sunos4.1'.
The `configure' script needs to be able to decode all plausible
alternatives for how to describe a machine. Thus, `sun3-sunos4.1'
would be a valid alias. For many programs, `vax-dec-ultrix' would be
an alias for `vax-dec-bsd', simply because the differences between
Ultrix and BSD are rarely noticeable, but a few programs might need to
distinguish them.
There is a shell script called `config.sub' that you can use as a
subroutine to validate system types and canonicalize aliases.
Other options are permitted to specify in more detail the software
or hardware present on the machine, and include or exclude optional
parts of the package:
Configure the package to build and install an optional user-level
facility called FEATURE. This allows users to choose which
optional features to include. Giving an optional PARAMETER of
`no' should omit FEATURE, if it is built by default.
No `--enable' option should *ever* cause one feature to replace
another. No `--enable' option should ever substitute one useful
behavior for another useful behavior. The only proper use for
`--enable' is for questions of whether to build part of the program
or exclude it.
The package PACKAGE will be installed, so configure this package
to work with PACKAGE.
Possible values of PACKAGE include `gnu-as' (or `gas'), `gnu-ld',
`gnu-libc', `gdb', `x', and `x-toolkit'.
Do not use a `--with' option to specify the file name to use to
find certain files. That is outside the scope of what `--with'
options are for.
The target machine has no floating point processor.
The target machine assembler is GAS, the GNU assembler. This is
obsolete; users should use `--with-gnu-as' instead.
The target machine has the X Window System installed. This is
obsolete; users should use `--with-x' instead.
All `configure' scripts should accept all of these "detail" options,
whether or not they make any difference to the particular package at
hand. In particular, they should accept any option that starts with
`--with-' or `--enable-'. This is so users will be able to configure
an entire GNU source tree at once with a single set of options.
You will note that the categories `--with-' and `--enable-' are
narrow: they *do not* provide a place for any sort of option you might
think of. That is deliberate. We want to limit the possible
configuration options in GNU software. We do not want GNU programs to
have idiosyncratic configuration options.
Packages that perform part of the compilation process may support
cross-compilation. In such a case, the host and target machines for
the program may be different. The `configure' script should normally
treat the specified type of system as both the host and the target,
thus producing a program which works for the same type of machine that
it runs on.
The way to build a cross-compiler, cross-assembler, or what have
you, is to specify the option `--host=HOSTTYPE' when running
`configure'. This specifies the host system without changing the type
of target system. The syntax for HOSTTYPE is the same as described
Bootstrapping a cross-compiler requires compiling it on a machine
other than the host it will run on. Compilation packages accept a
configuration option `--build=HOSTTYPE' for specifying the
configuration on which you will compile them, in case that is different
from the host.
Programs for which cross-operation is not meaningful need not accept
the `--host' option, because configuring an entire operating system for
cross-operation is not a meaningful thing.
Some programs have ways of configuring themselves automatically. If
your program is set up to do this, your `configure' script can simply
ignore most of its arguments.

File:, Node: Makefile Conventions, Next: Releases, Prev: Configuration, Up: Managing Releases
Makefile Conventions
This node describes conventions for writing the Makefiles for GNU
* Menu:
* Makefile Basics:: General Conventions for Makefiles
* Utilities in Makefiles:: Utilities in Makefiles
* Command Variables:: Variables for Specifying Commands
* Directory Variables:: Variables for Installation Directories
* Standard Targets:: Standard Targets for Users
* Install Command Categories:: Three categories of commands in the `install'
rule: normal, pre-install and post-install.

File:, Node: Makefile Basics, Next: Utilities in Makefiles, Up: Makefile Conventions
General Conventions for Makefiles
Every Makefile should contain this line:
SHELL = /bin/sh
to avoid trouble on systems where the `SHELL' variable might be
inherited from the environment. (This is never a problem with GNU
Different `make' programs have incompatible suffix lists and
implicit rules, and this sometimes creates confusion or misbehavior. So
it is a good idea to set the suffix list explicitly using only the
suffixes you need in the particular Makefile, like this:
.SUFFIXES: .c .o
The first line clears out the suffix list, the second introduces all
suffixes which may be subject to implicit rules in this Makefile.
Don't assume that `.' is in the path for command execution. When
you need to run programs that are a part of your package during the
make, please make sure that it uses `./' if the program is built as
part of the make or `$(srcdir)/' if the file is an unchanging part of
the source code. Without one of these prefixes, the current search
path is used.
The distinction between `./' (the "build directory") and
`$(srcdir)/' (the "source directory") is important because users can
build in a separate directory using the `--srcdir' option to
`configure'. A rule of the form:
foo.1 : sedscript
sed -e sedscript > foo.1
will fail when the build directory is not the source directory, because
`' and `sedscript' are in the the source directory.
When using GNU `make', relying on `VPATH' to find the source file
will work in the case where there is a single dependency file, since
the `make' automatic variable `$<' will represent the source file
wherever it is. (Many versions of `make' set `$<' only in implicit
rules.) A Makefile target like
foo.o : bar.c
$(CC) -I. -I$(srcdir) $(CFLAGS) -c bar.c -o foo.o
should instead be written as
foo.o : bar.c
$(CC) -I. -I$(srcdir) $(CFLAGS) -c $< -o $@
in order to allow `VPATH' to work correctly. When the target has
multiple dependencies, using an explicit `$(srcdir)' is the easiest way
to make the rule work well. For example, the target above for `foo.1'
is best written as:
foo.1 : sedscript
sed -e $(srcdir)/sedscript $(srcdir)/ > $@
GNU distributions usually contain some files which are not source
files--for example, Info files, and the output from Autoconf, Automake,
Bison or Flex. Since these files normally appear in the source
directory, they should always appear in the source directory, not in the
build directory. So Makefile rules to update them should put the
updated files in the source directory.
However, if a file does not appear in the distribution, then the
Makefile should not put it in the source directory, because building a
program in ordinary circumstances should not modify the source directory
in any way.
Try to make the build and installation targets, at least (and all
their subtargets) work correctly with a parallel `make'.

File:, Node: Utilities in Makefiles, Next: Command Variables, Prev: Makefile Basics, Up: Makefile Conventions
Utilities in Makefiles
Write the Makefile commands (and any shell scripts, such as
`configure') to run in `sh', not in `csh'. Don't use any special
features of `ksh' or `bash'.
The `configure' script and the Makefile rules for building and
installation should not use any utilities directly except these:
cat cmp cp diff echo egrep expr false grep install-info
ln ls mkdir mv pwd rm rmdir sed sleep sort tar test touch true
The compression program `gzip' can be used in the `dist' rule.
Stick to the generally supported options for these programs. For
example, don't use `mkdir -p', convenient as it may be, because most
systems don't support it.
It is a good idea to avoid creating symbolic links in makefiles,
since a few systems don't support them.
The Makefile rules for building and installation can also use
compilers and related programs, but should do so via `make' variables
so that the user can substitute alternatives. Here are some of the
programs we mean:
ar bison cc flex install ld ldconfig lex
make makeinfo ranlib texi2dvi yacc
Use the following `make' variables to run those programs:
When you use `ranlib' or `ldconfig', you should make sure nothing
bad happens if the system does not have the program in question.
Arrange to ignore an error from that command, and print a message before
the command to tell the user that failure of this command does not mean
a problem. (The Autoconf `AC_PROG_RANLIB' macro can help with this.)
If you use symbolic links, you should implement a fallback for
systems that don't have symbolic links.
Additional utilities that can be used via Make variables are:
chgrp chmod chown mknod
It is ok to use other utilities in Makefile portions (or scripts)
intended only for particular systems where you know those utilities

File:, Node: Command Variables, Next: Directory Variables, Prev: Utilities in Makefiles, Up: Makefile Conventions
Variables for Specifying Commands
Makefiles should provide variables for overriding certain commands,
options, and so on.
In particular, you should run most utility programs via variables.
Thus, if you use Bison, have a variable named `BISON' whose default
value is set with `BISON = bison', and refer to it with `$(BISON)'
whenever you need to use Bison.
File management utilities such as `ln', `rm', `mv', and so on, need
not be referred to through variables in this way, since users don't
need to replace them with other programs.
Each program-name variable should come with an options variable that
is used to supply options to the program. Append `FLAGS' to the
program-name variable name to get the options variable name--for
example, `BISONFLAGS'. (The names `CFLAGS' for the C compiler,
`YFLAGS' for yacc, and `LFLAGS' for lex, are exceptions to this rule,
but we keep them because they are standard.) Use `CPPFLAGS' in any
compilation command that runs the preprocessor, and use `LDFLAGS' in
any compilation command that does linking as well as in any direct use
of `ld'.
If there are C compiler options that _must_ be used for proper
compilation of certain files, do not include them in `CFLAGS'. Users
expect to be able to specify `CFLAGS' freely themselves. Instead,
arrange to pass the necessary options to the C compiler independently
of `CFLAGS', by writing them explicitly in the compilation commands or
by defining an implicit rule, like this:
Do include the `-g' option in `CFLAGS', because that is not
_required_ for proper compilation. You can consider it a default that
is only recommended. If the package is set up so that it is compiled
with GCC by default, then you might as well include `-O' in the default
value of `CFLAGS' as well.
Put `CFLAGS' last in the compilation command, after other variables
containing compiler options, so the user can use `CFLAGS' to override
the others.
`CFLAGS' should be used in every invocation of the C compiler, both
those which do compilation and those which do linking.
Every Makefile should define the variable `INSTALL', which is the
basic command for installing a file into the system.
Every Makefile should also define the variables `INSTALL_PROGRAM'
and `INSTALL_DATA'. (The default for each of these should be
`$(INSTALL)'.) Then it should use those variables as the commands for
actual installation, for executables and nonexecutables respectively.
Use these variables as follows:
$(INSTALL_PROGRAM) foo $(bindir)/foo
$(INSTALL_DATA) libfoo.a $(libdir)/libfoo.a
Optionally, you may prepend the value of `DESTDIR' to the target
filename. Doing this allows the installer to create a snapshot of the
installation to be copied onto the real target filesystem later. Do not
set the value of `DESTDIR' in your Makefile, and do not include it in
any installed files. With support for `DESTDIR', the above examples
$(INSTALL_PROGRAM) foo $(DESTDIR)$(bindir)/foo
$(INSTALL_DATA) libfoo.a $(DESTDIR)$(libdir)/libfoo.a
Always use a file name, not a directory name, as the second argument of
the installation commands. Use a separate command for each file to be

File:, Node: Directory Variables, Next: Standard Targets, Prev: Command Variables, Up: Makefile Conventions
Variables for Installation Directories
Installation directories should always be named by variables, so it
is easy to install in a nonstandard place. The standard names for these
variables are described below. They are based on a standard filesystem
layout; variants of it are used in SVR4, 4.4BSD, Linux, Ultrix v4, and
other modern operating systems.
These two variables set the root for the installation. All the other
installation directories should be subdirectories of one of these two,
and nothing should be directly installed into these two directories.
A prefix used in constructing the default values of the variables
listed below. The default value of `prefix' should be
`/usr/local'. When building the complete GNU system, the prefix
will be empty and `/usr' will be a symbolic link to `/'. (If you
are using Autoconf, write it as `@prefix@'.)
Running `make install' with a different value of `prefix' from the
one used to build the program should NOT recompile the program.
A prefix used in constructing the default values of some of the
variables listed below. The default value of `exec_prefix' should
be `$(prefix)'. (If you are using Autoconf, write it as
Generally, `$(exec_prefix)' is used for directories that contain
machine-specific files (such as executables and subroutine
libraries), while `$(prefix)' is used directly for other
Running `make install' with a different value of `exec_prefix'
from the one used to build the program should NOT recompile the
Executable programs are installed in one of the following
The directory for installing executable programs that users can
run. This should normally be `/usr/local/bin', but write it as
`$(exec_prefix)/bin'. (If you are using Autoconf, write it as
The directory for installing executable programs that can be run
from the shell, but are only generally useful to system
administrators. This should normally be `/usr/local/sbin', but
write it as `$(exec_prefix)/sbin'. (If you are using Autoconf,
write it as `@sbindir@'.)
The directory for installing executable programs to be run by other
programs rather than by users. This directory should normally be
`/usr/local/libexec', but write it as `$(exec_prefix)/libexec'.
(If you are using Autoconf, write it as `@libexecdir@'.)
Data files used by the program during its execution are divided into
categories in two ways.
* Some files are normally modified by programs; others are never
normally modified (though users may edit some of these).
* Some files are architecture-independent and can be shared by all
machines at a site; some are architecture-dependent and can be
shared only by machines of the same kind and operating system;