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\input texinfo @c -*-texinfo-*-
@c %**start of header
@c @setfilename
@c @setfilename
@c To produce the full manual, use the "" setfilename, and
@c make sure the following do NOT begin with '@c' (and the @clear lines DO)
@set USING
@c To produce a user-only manual, use the "" setfilename, and
@c make sure the following does NOT begin with '@c':
@c @clear INTERNALS
@c To produce a porter-only manual, use the "" setfilename,
@c and make sure the following does NOT begin with '@c':
@c @clear USING
@c (For FSF printing, turn on smallbook, comment out finalout below;
@c that is all that is needed.)
@c 6/27/96 FSF DO wants smallbook fmt for 1st bound edition.
@c @smallbook
@c i also commented out the finalout command, so if there *are* any
@c overfulls, you'll (hopefully) see the rectangle in the right hand
@c margin. -mew 15june93
@c @finalout
@c NOTE: checks/things to do:
@c -have bob do a search in all seven files for "mew" (ideally --mew,
@c but i may have forgotten the occasional "--"..).
@c Just checked... all have `--'! Bob 22Jul96
@c Use this to search: grep -n '\-\-mew' *.texi
@c -item/itemx, text after all (sub/sub)section titles, etc..
@c -consider putting the lists of options on pp 17--> etc in columns or
@c some such.
@c -spellcheck
@c -continuity of phrasing; ie, bit-field vs bitfield in rtl.texi
@c -overfulls. do a search for "mew" in the files, and you will see
@c overfulls that i noted but could not deal with.
@c -have to add text: beginning of chapter 8
@c anything else? --mew 10feb93
@ifset USING
@settitle Using and Porting GNU CC
@end ifset
@end ifset
@c seems reasonable to assume at least one of INTERNALS or USING is set...
@ifclear INTERNALS
@settitle Using GNU CC
@end ifclear
@ifclear USING
@settitle Porting GNU CC
@end ifclear
@syncodeindex fn cp
@syncodeindex vr cp
@c %**end of header
@c Use with @@smallbook.
@c Cause even numbered pages to be printed on the left hand side of
@c the page and odd numbered pages to be printed on the right hand
@c side of the page. Using this, you can print on both sides of a
@c sheet of paper and have the text on the same part of the sheet.
@c The text on right hand pages is pushed towards the right hand
@c margin and the text on left hand pages is pushed toward the left
@c hand margin.
@c (To provide the reverse effect, set bindingoffset to -0.75in.)
@c @tex
@c \global\bindingoffset=0.75in
@c \global\normaloffset =0.75in
@c @end tex
@dircategory Programming
* gcc: (gcc). The GNU C compiler.
@end direntry
@ifset USING
This file documents the use and the internals of the GNU compiler.
@end ifset
@end ifset
@ifclear USING
This file documents the internals of the GNU compiler.
@end ifclear
@ifclear INTERNALS
This file documents the use of the GNU compiler.
@end ifclear
Published by the Free Software Foundation
59 Temple Place - Suite 330
Boston, MA 02111-1307 USA
Copyright (C) 1988, 1989, 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 process this file through Tex and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).
@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that the
sections entitled ``GNU General Public License'' and ``Funding for Free
Software'' are included exactly as in the original, and 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 the sections entitled ``GNU General Public License'' and
``Funding for Free Software'', and this permission notice, may be
included in translations approved by the Free Software Foundation
instead of in the original English.
@end ifinfo
@setchapternewpage odd
@c @finalout
@ifset USING
@center @titlefont{Using and Porting GNU CC}
@end ifset
@end ifset
@ifclear INTERNALS
@title Using GNU CC
@end ifclear
@ifclear USING
@title Porting GNU CC
@end ifclear
@sp 2
@center Richard M. Stallman
@sp 3
@center Last updated 16 March 1998
@sp 1
@c The version number appears five times more in this file.
@center for egcs-1.1.1
@vskip 0pt plus 1filll
Copyright @copyright{} 1988, 89, 92, 93, 94, 95, 96, 98 Free Software Foundation, Inc.
@sp 2
For EGCS Version 1.0@*
@sp 1
Published by the Free Software Foundation @*
59 Temple Place - Suite 330@*
Boston, MA 02111-1307, USA@*
Last printed April, 1998.@*
Printed copies are available for $50 each.@*
ISBN 1-882114-37-X
@sp 1
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 also that the
sections entitled ``GNU General Public License'' and ``Funding for Free
Software'' are included exactly as in the original, and 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 the sections entitled ``GNU General Public License'' and
``Funding for Free Software'', and this permission notice, may be
included in translations approved by the Free Software Foundation
instead of in the original English.
@end titlepage
@node Top, G++ and GCC,, (DIR)
@top Introduction
@cindex introduction
@ifset USING
This manual documents how to run, install and port the GNU
compiler, as well as its new features and incompatibilities, and how to
report bugs. It corresponds to EGCS version 1.1.1.
@end ifset
@end ifset
@ifclear INTERNALS
This manual documents how to run and install the GNU compiler,
as well as its new features and incompatibilities, and how to report
bugs. It corresponds to EGCS version 1.1.1.
@end ifclear
@ifclear USING
This manual documents how to port the GNU compiler,
as well as its new features and incompatibilities, and how to report
bugs. It corresponds to EGCS version 1.1.1.
@end ifclear
@end ifinfo
@ifset USING
* G++ and GCC:: You can compile C or C++ programs.
* Invoking GCC:: Command options supported by @samp{gcc}.
* Installation:: How to configure, compile and install GNU CC.
* C Extensions:: GNU extensions to the C language family.
* C++ Extensions:: GNU extensions to the C++ language.
* Gcov:: gcov: a GNU CC test coverage program.
* Trouble:: If you have trouble installing GNU CC.
* Bugs:: How, why and where to report bugs.
* Service:: How to find suppliers of support for GNU CC.
* Contributing:: How to contribute to testing and developing GNU CC.
* VMS:: Using GNU CC on VMS.
@end ifset
* Portability:: Goals of GNU CC's portability features.
* Interface:: Function-call interface of GNU CC output.
* Passes:: Order of passes, what they do, and what each file is for.
* RTL:: The intermediate representation that most passes work on.
* Machine Desc:: How to write machine description instruction patterns.
* Target Macros:: How to write the machine description C macros.
* Config:: Writing the @file{xm-@var{machine}.h} file.
* Fragments:: Writing the @file{t-@var{target}} and @file{x-@var{host}} files.
@end ifset
* Funding:: How to help assure funding for free software.
* GNU/Linux:: Linux and the GNU Project
* Copying:: GNU General Public License says
how you can copy and share GNU CC.
* Contributors:: People who have contributed to GNU CC.
* Index:: Index of concepts and symbol names.
@end menu
@ifset USING
@node G++ and GCC
@chapter Compile C, C++, or Objective C
@cindex Objective C
The C, C++, and Objective C versions of the compiler are integrated; the
GNU C compiler can compile programs written in C, C++, or Objective C.
@cindex GCC
``GCC'' is a common shorthand term for the GNU C compiler. This is both
the most general name for the compiler, and the name used when the
emphasis is on compiling C programs.
@cindex C++
@cindex G++
When referring to C++ compilation, it is usual to call the compiler
``G++''. Since there is only one compiler, it is also accurate to call
it ``GCC'' no matter what the language context; however, the term
``G++'' is more useful when the emphasis is on compiling C++ programs.
We use the name ``GNU CC'' to refer to the compilation system as a
whole, and more specifically to the language-independent part of the
compiler. For example, we refer to the optimization options as
affecting the behavior of ``GNU CC'' or sometimes just ``the compiler''.
Front ends for other languages, such as Ada 9X, Fortran, Modula-3, and
Pascal, are under development. These front-ends, like that for C++, are
built in subdirectories of GNU CC and link to it. The result is an
integrated compiler that can compile programs written in C, C++,
Objective C, or any of the languages for which you have installed front
In this manual, we only discuss the options for the C, Objective-C, and
C++ compilers and those of the GNU CC core. Consult the documentation
of the other front ends for the options to use when compiling programs
written in other languages.
@cindex compiler compared to C++ preprocessor
@cindex intermediate C version, nonexistent
@cindex C intermediate output, nonexistent
G++ is a @emph{compiler}, not merely a preprocessor. G++ builds object
code directly from your C++ program source. There is no intermediate C
version of the program. (By contrast, for example, some other
implementations use a program that generates a C program from your C++
source.) Avoiding an intermediate C representation of the program means
that you get better object code, and better debugging information. The
GNU debugger, GDB, works with this information in the object code to
give you comprehensive C++ source-level editing capabilities
(@pxref{C,,C and C++,, Debugging with GDB}).
@c FIXME! Someone who knows something about Objective C ought to put in
@c a paragraph or two about it here, and move the index entry down when
@c there is more to point to than the general mention in the 1st par.
@include invoke.texi
@include install.texi
@include extend.texi
@include gcov.texi
@node Trouble
@chapter Known Causes of Trouble with GNU CC
@cindex bugs, known
@cindex installation trouble
@cindex known causes of trouble
This section describes known problems that affect users of GNU CC. Most
of these are not GNU CC bugs per se---if they were, we would fix them.
But the result for a user may be like the result of a bug.
Some of these problems are due to bugs in other software, some are
missing features that are too much work to add, and some are places
where people's opinions differ as to what is best.
* Actual Bugs:: Bugs we will fix later.
* Installation Problems:: Problems that manifest when you install GNU CC.
* Cross-Compiler Problems:: Common problems of cross compiling with GNU CC.
* Interoperation:: Problems using GNU CC with other compilers,
and with certain linkers, assemblers and debuggers.
* External Bugs:: Problems compiling certain programs.
* Incompatibilities:: GNU CC is incompatible with traditional C.
* Fixed Headers:: GNU C uses corrected versions of system header files.
This is necessary, but doesn't always work smoothly.
* Standard Libraries:: GNU C uses the system C library, which might not be
compliant with the ISO/ANSI C standard.
* Disappointments:: Regrettable things we can't change, but not quite bugs.
* C++ Misunderstandings:: Common misunderstandings with GNU C++.
* Protoize Caveats:: Things to watch out for when using @code{protoize}.
* Non-bugs:: Things we think are right, but some others disagree.
* Warnings and Errors:: Which problems in your code get warnings,
and which get errors.
@end menu
@node Actual Bugs
@section Actual Bugs We Haven't Fixed Yet
@itemize @bullet
The @code{fixincludes} script interacts badly with automounters; if the
directory of system header files is automounted, it tends to be
unmounted while @code{fixincludes} is running. This would seem to be a
bug in the automounter. We don't know any good way to work around it.
The @code{fixproto} script will sometimes add prototypes for the
@code{sigsetjmp} and @code{siglongjmp} functions that reference the
@code{jmp_buf} type before that type is defined. To work around this,
edit the offending file and place the typedef in front of the
There are several obscure case of mis-using struct, union, and
enum tags that are not detected as errors by the compiler.
When @samp{-pedantic-errors} is specified, GNU C will incorrectly give
an error message when a function name is specified in an expression
involving the comma operator.
Loop unrolling doesn't work properly for certain C++ programs. This is
a bug in the C++ front end. It sometimes emits incorrect debug info, and
the loop unrolling code is unable to recover from this error.
@end itemize
@node Installation Problems
@section Installation Problems
This is a list of problems (and some apparent problems which don't
really mean anything is wrong) that show up during installation of GNU
@itemize @bullet
On certain systems, defining certain environment variables such as
@code{CC} can interfere with the functioning of @code{make}.
If you encounter seemingly strange errors when trying to build the
compiler in a directory other than the source directory, it could be
because you have previously configured the compiler in the source
directory. Make sure you have done all the necessary preparations.
@xref{Other Dir}.
If you build GNU CC on a BSD system using a directory stored in a System
V file system, problems may occur in running @code{fixincludes} if the
System V file system doesn't support symbolic links. These problems
result in a failure to fix the declaration of @code{size_t} in
@file{sys/types.h}. If you find that @code{size_t} is a signed type and
that type mismatches occur, this could be the cause.
The solution is not to use such a directory for building GNU CC.
In previous versions of GNU CC, the @code{gcc} driver program looked for
@code{as} and @code{ld} in various places; for example, in files
beginning with @file{/usr/local/lib/gcc-}. GNU CC version 2 looks for
them in the directory
Thus, to use a version of @code{as} or @code{ld} that is not the system
default, for example @code{gas} or GNU @code{ld}, you must put them in
that directory (or make links to them from that directory).
Some commands executed when making the compiler may fail (return a
non-zero status) and be ignored by @code{make}. These failures, which
are often due to files that were not found, are expected, and can safely
be ignored.
It is normal to have warnings in compiling certain files about
unreachable code and about enumeration type clashes. These files' names
begin with @samp{insn-}. Also, @file{real.c} may get some warnings that
you can ignore.
Sometimes @code{make} recompiles parts of the compiler when installing
the compiler. In one case, this was traced down to a bug in
@code{make}. Either ignore the problem or switch to GNU Make.
If you have installed a program known as purify, you may find that it
causes errors while linking @code{enquire}, which is part of building
GNU CC. The fix is to get rid of the file @code{real-ld} which purify
installs---so that GNU CC won't try to use it.
On GNU/Linux SLS 1.01, there is a problem with @file{libc.a}: it does not
contain the obstack functions. However, GNU CC assumes that the obstack
functions are in @file{libc.a} when it is the GNU C library. To work
around this problem, change the @code{__GNU_LIBRARY__} conditional
around line 31 to @samp{#if 1}.
On some 386 systems, building the compiler never finishes because
@code{enquire} hangs due to a hardware problem in the motherboard---it
reports floating point exceptions to the kernel incorrectly. You can
install GNU CC except for @file{float.h} by patching out the command to
run @code{enquire}. You may also be able to fix the problem for real by
getting a replacement motherboard. This problem was observed in
Revision E of the Micronics motherboard, and is fixed in Revision F.
It has also been observed in the MYLEX MXA-33 motherboard.
If you encounter this problem, you may also want to consider removing
the FPU from the socket during the compilation. Alternatively, if you
are running SCO Unix, you can reboot and force the FPU to be ignored.
To do this, type @samp{hd(40)unix auto ignorefpu}.
On some 386 systems, GNU CC crashes trying to compile @file{enquire.c}.
This happens on machines that don't have a 387 FPU chip. On 386
machines, the system kernel is supposed to emulate the 387 when you
don't have one. The crash is due to a bug in the emulator.
One of these systems is the Unix from Interactive Systems: 386/ix.
On this system, an alternate emulator is provided, and it does work.
To use it, execute this command as super-user:
ln /etc/emulator.rel1 /etc/emulator
@end example
and then reboot the system. (The default emulator file remains present
under the name @file{emulator.dflt}.)
Try using @file{/etc/emulator.att}, if you have such a problem on the
SCO system.
Another system which has this problem is Esix. We don't know whether it
has an alternate emulator that works.
On NetBSD 0.8, a similar problem manifests itself as these error messages:
enquire.c: In function `fprop':
enquire.c:2328: floating overflow
@end example
On SCO systems, when compiling GNU CC with the system's compiler,
do not use @samp{-O}. Some versions of the system's compiler miscompile
GNU CC with @samp{-O}.
@cindex @code{genflags}, crash on Sun 4
Sometimes on a Sun 4 you may observe a crash in the program
@code{genflags} or @code{genoutput} while building GNU CC. This is said to
be due to a bug in @code{sh}. You can probably get around it by running
@code{genflags} or @code{genoutput} manually and then retrying the
On Solaris 2, executables of GNU CC version 2.0.2 are commonly
available, but they have a bug that shows up when compiling current
versions of GNU CC: undefined symbol errors occur during assembly if you
use @samp{-g}.
The solution is to compile the current version of GNU CC without
@samp{-g}. That makes a working compiler which you can use to recompile
with @samp{-g}.
Solaris 2 comes with a number of optional OS packages. Some of these
packages are needed to use GNU CC fully. If you did not install all
optional packages when installing Solaris, you will need to verify that
the packages that GNU CC needs are installed.
To check whether an optional package is installed, use
the @code{pkginfo} command. To add an optional package, use the
@code{pkgadd} command. For further details, see the Solaris
For Solaris 2.0 and 2.1, GNU CC needs six packages: @samp{SUNWarc},
@samp{SUNWbtool}, @samp{SUNWesu}, @samp{SUNWhea}, @samp{SUNWlibm}, and
For Solaris 2.2, GNU CC needs an additional seventh package: @samp{SUNWsprot}.
On Solaris 2, trying to use the linker and other tools in
@file{/usr/ucb} to install GNU CC has been observed to cause trouble.
For example, the linker may hang indefinitely. The fix is to remove
@file{/usr/ucb} from your @code{PATH}.
If you use the 1.31 version of the MIPS assembler (such as was shipped
with Ultrix 3.1), you will need to use the -fno-delayed-branch switch
when optimizing floating point code. Otherwise, the assembler will
complain when the GCC compiler fills a branch delay slot with a
floating point instruction, such as @code{add.d}.
If on a MIPS system you get an error message saying ``does not have gp
sections for all it's [sic] sectons [sic]'', don't worry about it. This
happens whenever you use GAS with the MIPS linker, but there is not
really anything wrong, and it is okay to use the output file. You can
stop such warnings by installing the GNU linker.
It would be nice to extend GAS to produce the gp tables, but they are
optional, and there should not be a warning about their absence.
In Ultrix 4.0 on the MIPS machine, @file{stdio.h} does not work with GNU
CC at all unless it has been fixed with @code{fixincludes}. This causes
problems in building GNU CC. Once GNU CC is installed, the problems go
To work around this problem, when making the stage 1 compiler, specify
this option to Make:
GCC_FOR_TARGET="./xgcc -B./ -I./include"
@end example
When making stage 2 and stage 3, specify this option:
CFLAGS="-g -I./include"
@end example
Users have reported some problems with version 2.0 of the MIPS
compiler tools that were shipped with Ultrix 4.1. Version 2.10
which came with Ultrix 4.2 seems to work fine.
Users have also reported some problems with version 2.20 of the
MIPS compiler tools that were shipped with RISC/os 4.x. The earlier
version 2.11 seems to work fine.
Some versions of the MIPS linker will issue an assertion failure
when linking code that uses @code{alloca} against shared
libraries on RISC-OS 5.0, and DEC's OSF/1 systems. This is a bug
in the linker, that is supposed to be fixed in future revisions.
To protect against this, GNU CC passes @samp{-non_shared} to the
linker unless you pass an explicit @samp{-shared} or
@samp{-call_shared} switch.
On System V release 3, you may get this error message
while linking:
ld fatal: failed to write symbol name @var{something}
in strings table for file @var{whatever}
@end smallexample
This probably indicates that the disk is full or your ULIMIT won't allow
the file to be as large as it needs to be.
This problem can also result because the kernel parameter @code{MAXUMEM}
is too small. If so, you must regenerate the kernel and make the value
much larger. The default value is reported to be 1024; a value of 32768
is said to work. Smaller values may also work.
On System V, if you get an error like this,
/usr/local/lib/bison.simple: In function `yyparse':
/usr/local/lib/bison.simple:625: virtual memory exhausted
@end example
that too indicates a problem with disk space, ULIMIT, or @code{MAXUMEM}.
Current GNU CC versions probably do not work on version 2 of the NeXT
operating system.
On NeXTStep 3.0, the Objective C compiler does not work, due,
apparently, to a kernel bug that it happens to trigger. This problem
does not happen on 3.1.
On the Tower models 4@var{n}0 and 6@var{n}0, by default a process is not
allowed to have more than one megabyte of memory. GNU CC cannot compile
itself (or many other programs) with @samp{-O} in that much memory.
To solve this problem, reconfigure the kernel adding the following line
to the configuration file:
MAXUMEM = 4096
@end smallexample
On HP 9000 series 300 or 400 running HP-UX release 8.0, there is a bug
in the assembler that must be fixed before GNU CC can be built. This
bug manifests itself during the first stage of compilation, while
building @file{libgcc2.a}:
cc1: warning: `-g' option not supported on this version of GCC
cc1: warning: `-g1' option not supported on this version of GCC
./xgcc: Internal compiler error: program as got fatal signal 11
@end smallexample
A patched version of the assembler is available by anonymous ftp from
@code{} as the file
@file{archive/cph/hpux-8.0-assembler}. If you have HP software support,
the patch can also be obtained directly from HP, as described in the
following note:
This is the patched assembler, to patch SR#1653-010439, where the
assembler aborts on floating point constants.
The bug is not really in the assembler, but in the shared library
version of the function ``cvtnum(3c)''. The bug on ``cvtnum(3c)'' is
SR#4701-078451. Anyway, the attached assembler uses the archive
library version of ``cvtnum(3c)'' and thus does not exhibit the bug.
@end quotation
This patch is also known as PHCO_4484.
On HP-UX version 8.05, but not on 8.07 or more recent versions,
the @code{fixproto} shell script triggers a bug in the system shell.
If you encounter this problem, upgrade your operating system or
use BASH (the GNU shell) to run @code{fixproto}.
Some versions of the Pyramid C compiler are reported to be unable to
compile GNU CC. You must use an older version of GNU CC for
bootstrapping. One indication of this problem is if you get a crash
when GNU CC compiles the function @code{muldi3} in file @file{libgcc2.c}.
You may be able to succeed by getting GNU CC version 1, installing it,
and using it to compile GNU CC version 2. The bug in the Pyramid C
compiler does not seem to affect GNU CC version 1.
There may be similar problems on System V Release 3.1 on 386 systems.
On the Intel Paragon (an i860 machine), if you are using operating
system version 1.0, you will get warnings or errors about redefinition
of @code{va_arg} when you build GNU CC.
If this happens, then you need to link most programs with the library
@file{iclib.a}. You must also modify @file{stdio.h} as follows: before
the lines
#if defined(__i860__) && !defined(_VA_LIST)
#include <va_list.h>
@end example
insert the line
#if __PGC__
@end example
and after the lines
extern int vprintf(const char *, va_list );
extern int vsprintf(char *, const char *, va_list );
@end example
insert the line
#endif /* __PGC__ */
@end example
These problems don't exist in operating system version 1.1.
On the Altos 3068, programs compiled with GNU CC won't work unless you
fix a kernel bug. This happens using system versions V.2.2 1.0gT1 and
V.2.2 1.0e and perhaps later versions as well. See the file
You will get several sorts of compilation and linking errors on the
we32k if you don't follow the special instructions. @xref{Configurations}.
A bug in the HP-UX 8.05 (and earlier) shell will cause the fixproto
program to report an error of the form:
./fixproto: sh internal 1K buffer overflow
@end example
To fix this, change the first line of the fixproto script to look like:
@end example
@end itemize
@node Cross-Compiler Problems
@section Cross-Compiler Problems
You may run into problems with cross compilation on certain machines,
for several reasons.
@itemize @bullet
Cross compilation can run into trouble for certain machines because
some target machines' assemblers require floating point numbers to be
written as @emph{integer} constants in certain contexts.
The compiler writes these integer constants by examining the floating
point value as an integer and printing that integer, because this is
simple to write and independent of the details of the floating point
representation. But this does not work if the compiler is running on
a different machine with an incompatible floating point format, or
even a different byte-ordering.
In addition, correct constant folding of floating point values
requires representing them in the target machine's format.
(The C standard does not quite require this, but in practice
it is the only way to win.)
It is now possible to overcome these problems by defining macros such
as @code{REAL_VALUE_TYPE}. But doing so is a substantial amount of
work for each target machine.
@end ifset
@ifclear INTERNALS
@xref{Cross-compilation,,Cross Compilation and Floating Point Format,, Using and Porting GCC}.
@end ifclear
At present, the program @file{mips-tfile} which adds debug
support to object files on MIPS systems does not work in a cross
compile environment.
@end itemize
@node Interoperation
@section Interoperation
This section lists various difficulties encountered in using GNU C or
GNU C++ together with other compilers or with the assemblers, linkers,
libraries and debuggers on certain systems.
@itemize @bullet
Objective C does not work on the RS/6000.
GNU C++ does not do name mangling in the same way as other C++
compilers. This means that object files compiled with one compiler
cannot be used with another.
This effect is intentional, to protect you from more subtle problems.
Compilers differ as to many internal details of C++ implementation,
including: how class instances are laid out, how multiple inheritance is
implemented, and how virtual function calls are handled. If the name
encoding were made the same, your programs would link against libraries
provided from other compilers---but the programs would then crash when
run. Incompatible libraries are then detected at link time, rather than
at run time.
Older GDB versions sometimes fail to read the output of GNU CC version
2. If you have trouble, get GDB version 4.4 or later.
@cindex DBX
DBX rejects some files produced by GNU CC, though it accepts similar
constructs in output from PCC. Until someone can supply a coherent
description of what is valid DBX input and what is not, there is
nothing I can do about these problems. You are on your own.
The GNU assembler (GAS) does not support PIC. To generate PIC code, you
must use some other assembler, such as @file{/bin/as}.
On some BSD systems, including some versions of Ultrix, use of profiling
causes static variable destructors (currently used only in C++) not to
be run.
Use of @samp{-I/usr/include} may cause trouble.
Many systems come with header files that won't work with GNU CC unless
corrected by @code{fixincludes}. The corrected header files go in a new
directory; GNU CC searches this directory before @file{/usr/include}.
If you use @samp{-I/usr/include}, this tells GNU CC to search
@file{/usr/include} earlier on, before the corrected headers. The
result is that you get the uncorrected header files.
Instead, you should use these options (when compiling C programs):
-I/usr/local/lib/gcc-lib/@var{target}/@var{version}/include -I/usr/include
@end smallexample
For C++ programs, GNU CC also uses a special directory that defines C++
interfaces to standard C subroutines. This directory is meant to be
searched @emph{before} other standard include directories, so that it
takes precedence. If you are compiling C++ programs and specifying
include directories explicitly, use this option first, then the two
options above:
@end example
@cindex @code{vfork}, for the Sun-4
There is a bug in @code{vfork} on the Sun-4 which causes the registers
of the child process to clobber those of the parent. Because of this,
programs that call @code{vfork} are likely to lose when compiled
optimized with GNU CC when the child code alters registers which contain
C variables in the parent. This affects variables which are live in the
parent across the call to @code{vfork}.
If you encounter this, you can work around the problem by declaring
variables @code{volatile} in the function that calls @code{vfork}, until
the problem goes away, or by not declaring them @code{register} and not
using @samp{-O} for those source files.
@end ignore
On some SGI systems, when you use @samp{-lgl_s} as an option,
it gets translated magically to @samp{-lgl_s -lX11_s -lc_s}.
Naturally, this does not happen when you use GNU CC.
You must specify all three options explicitly.
On a Sparc, GNU CC aligns all values of type @code{double} on an 8-byte
boundary, and it expects every @code{double} to be so aligned. The Sun
compiler usually gives @code{double} values 8-byte alignment, with one
exception: function arguments of type @code{double} may not be aligned.
As a result, if a function compiled with Sun CC takes the address of an
argument of type @code{double} and passes this pointer of type
@code{double *} to a function compiled with GNU CC, dereferencing the
pointer may cause a fatal signal.
One way to solve this problem is to compile your entire program with GNU
CC. Another solution is to modify the function that is compiled with
Sun CC to copy the argument into a local variable; local variables
are always properly aligned. A third solution is to modify the function
that uses the pointer to dereference it via the following function
@code{access_double} instead of directly with @samp{*}:
inline double
access_double (double *unaligned_ptr)
union d2i @{ double d; int i[2]; @};
union d2i *p = (union d2i *) unaligned_ptr;
union d2i u;
u.i[0] = p->i[0];
u.i[1] = p->i[1];
return u.d;
@end smallexample
Storing into the pointer can be done likewise with the same union.
On Solaris, the @code{malloc} function in the @file{libmalloc.a} library
may allocate memory that is only 4 byte aligned. Since GNU CC on the
Sparc assumes that doubles are 8 byte aligned, this may result in a
fatal signal if doubles are stored in memory allocated by the
@file{libmalloc.a} library.
The solution is to not use the @file{libmalloc.a} library. Use instead
@code{malloc} and related functions from @file{libc.a}; they do not have
this problem.
Sun forgot to include a static version of @file{libdl.a} with some
versions of SunOS (mainly 4.1). This results in undefined symbols when
linking static binaries (that is, if you use @samp{-static}). If you
see undefined symbols @code{_dlclose}, @code{_dlsym} or @code{_dlopen}
when linking, compile and link against the file
@file{mit/util/misc/dlsym.c} from the MIT version of X windows.
The 128-bit long double format that the Sparc port supports currently
works by using the architecturally defined quad-word floating point
instructions. Since there is no hardware that supports these
instructions they must be emulated by the operating system. Long
doubles do not work in Sun OS versions 4.0.3 and earlier, because the
kernel emulator uses an obsolete and incompatible format. Long doubles
do not work in Sun OS version 4.1.1 due to a problem in a Sun library.
Long doubles do work on Sun OS versions 4.1.2 and higher, but GNU CC
does not enable them by default. Long doubles appear to work in Sun OS
5.x (Solaris 2.x).
On HP-UX version 9.01 on the HP PA, the HP compiler @code{cc} does not
compile GNU CC correctly. We do not yet know why. However, GNU CC
compiled on earlier HP-UX versions works properly on HP-UX 9.01 and can
compile itself properly on 9.01.
On the HP PA machine, ADB sometimes fails to work on functions compiled
with GNU CC. Specifically, it fails to work on functions that use
@code{alloca} or variable-size arrays. This is because GNU CC doesn't
generate HP-UX unwind descriptors for such functions. It may even be
impossible to generate them.
Debugging (@samp{-g}) is not supported on the HP PA machine, unless you use
the preliminary GNU tools (@pxref{Installation}).
Taking the address of a label may generate errors from the HP-UX
PA assembler. GAS for the PA does not have this problem.
Using floating point parameters for indirect calls to static functions
will not work when using the HP assembler. There simply is no way for GCC
to specify what registers hold arguments for static functions when using
the HP assembler. GAS for the PA does not have this problem.
In extremely rare cases involving some very large functions you may
receive errors from the HP linker complaining about an out of bounds
unconditional branch offset. This used to occur more often in previous
versions of GNU CC, but is now exceptionally rare. If you should run
into it, you can work around by making your function smaller.
GNU CC compiled code sometimes emits warnings from the HP-UX assembler of
the form:
(warning) Use of GR3 when
frame >= 8192 may cause conflict.
@end smallexample
These warnings are harmless and can be safely ignored.
The current version of the assembler (@file{/bin/as}) for the RS/6000
has certain problems that prevent the @samp{-g} option in GCC from
working. Note that @file{} uses @samp{-g} by default when
compiling @file{libgcc2.c}.
IBM has produced a fixed version of the assembler. The upgraded
assembler unfortunately was not included in any of the AIX 3.2 update
PTF releases (3.2.2, 3.2.3, or 3.2.3e). Users of AIX 3.1 should request
PTF U403044 from IBM and users of AIX 3.2 should request PTF U416277.
See the file @file{README.RS6000} for more details on these updates.
You can test for the presense of a fixed assembler by using the
as -u < /dev/null
@end smallexample
If the command exits normally, the assembler fix already is installed.
If the assembler complains that "-u" is an unknown flag, you need to
order the fix.
On the IBM RS/6000, compiling code of the form
extern int foo;
@dots{} foo @dots{}
static int foo;
@end smallexample
will cause the linker to report an undefined symbol @code{foo}.
Although this behavior differs from most other systems, it is not a
bug because redefining an @code{extern} variable as @code{static}
is undefined in ANSI C.
AIX on the RS/6000 provides support (NLS) for environments outside of
the United States. Compilers and assemblers use NLS to support
locale-specific representations of various objects including
floating-point numbers ("." vs "," for separating decimal fractions).
There have been problems reported where the library linked with GCC does
not produce the same floating-point formats that the assembler accepts.
If you have this problem, set the LANG environment variable to "C" or
Even if you specify @samp{-fdollars-in-identifiers},
you cannot successfully use @samp{$} in identifiers on the RS/6000 due
to a restriction in the IBM assembler. GAS supports these
On the RS/6000, XLC version will miscompile @file{jump.c}. XLC
version or later fixes this problem. You can obtain XLC-
by requesting PTF 421749 from IBM.
There is an assembler bug in versions of DG/UX prior to that
occurs when the @samp{fldcr} instruction is used. GNU CC uses
@samp{fldcr} on the 88100 to serialize volatile memory references. Use
the option @samp{-mno-serialize-volatile} if your version of the
assembler has this bug.
On VMS, GAS versions 1.38.1 and earlier may cause spurious warning
messages from the linker. These warning messages complain of mismatched
psect attributes. You can ignore them. @xref{VMS Install}.
On NewsOS version 3, if you include both of the files @file{stddef.h}
and @file{sys/types.h}, you get an error because there are two typedefs
of @code{size_t}. You should change @file{sys/types.h} by adding these
lines around the definition of @code{size_t}:
#ifndef _SIZE_T
#define _SIZE_T
@var{actual typedef here}
@end smallexample
@cindex Alliant
On the Alliant, the system's own convention for returning structures
and unions is unusual, and is not compatible with GNU CC no matter
what options are used.
@cindex RT PC
@cindex IBM RT PC
On the IBM RT PC, the MetaWare HighC compiler (hc) uses a different
convention for structure and union returning. Use the option
@samp{-mhc-struct-return} to tell GNU CC to use a convention compatible
with it.
@cindex Vax calling convention
@cindex Ultrix calling convention
On Ultrix, the Fortran compiler expects registers 2 through 5 to be saved
by function calls. However, the C compiler uses conventions compatible
with BSD Unix: registers 2 through 5 may be clobbered by function calls.
GNU CC uses the same convention as the Ultrix C compiler. You can use
these options to produce code compatible with the Fortran compiler:
-fcall-saved-r2 -fcall-saved-r3 -fcall-saved-r4 -fcall-saved-r5
@end smallexample
On the WE32k, you may find that programs compiled with GNU CC do not
work with the standard shared C library. You may need to link with
the ordinary C compiler. If you do so, you must specify the following
-L/usr/local/lib/gcc-lib/we32k-att-sysv/2.8.1 -lgcc -lc_s
@end smallexample
The first specifies where to find the library @file{libgcc.a}
specified with the @samp{-lgcc} option.
GNU CC does linking by invoking @code{ld}, just as @code{cc} does, and
there is no reason why it @emph{should} matter which compilation program
you use to invoke @code{ld}. If someone tracks this problem down,
it can probably be fixed easily.
On the Alpha, you may get assembler errors about invalid syntax as a
result of floating point constants. This is due to a bug in the C
library functions @code{ecvt}, @code{fcvt} and @code{gcvt}. Given valid
floating point numbers, they sometimes print @samp{NaN}.
On Irix 4.0.5F (and perhaps in some other versions), an assembler bug
sometimes reorders instructions incorrectly when optimization is turned
on. If you think this may be happening to you, try using the GNU
assembler; GAS version 2.1 supports ECOFF on Irix.
Or use the @samp{-noasmopt} option when you compile GNU CC with itself,
and then again when you compile your program. (This is a temporary
kludge to turn off assembler optimization on Irix.) If this proves to
be what you need, edit the assembler spec in the file @file{specs} so
that it unconditionally passes @samp{-O0} to the assembler, and never
passes @samp{-O2} or @samp{-O3}.
@end itemize
@node External Bugs
@section Problems Compiling Certain Programs
@c prevent bad page break with this line
Certain programs have problems compiling.
@itemize @bullet
Parse errors may occur compiling X11 on a Decstation running Ultrix 4.2
because of problems in DEC's versions of the X11 header files
@file{X11/Xlib.h} and @file{X11/Xutil.h}. People recommend adding
@samp{-I/usr/include/mit} to use the MIT versions of the header files,
using the @samp{-traditional} switch to turn off ANSI C, or fixing the
header files by adding this:
#ifdef __STDC__
#define NeedFunctionPrototypes 0
@end example
If you have trouble compiling Perl on a SunOS 4 system, it may be
because Perl specifies @samp{-I/usr/ucbinclude}. This accesses the
unfixed header files. Perl specifies the options
-traditional -Dvolatile=__volatile__
-I/usr/include/sun -I/usr/ucbinclude
@end example
most of which are unnecessary with GCC 2.4.5 and newer versions. You
can make a properly working Perl by setting @code{ccflags} to
@samp{-fwritable-strings} (implied by the @samp{-traditional} in the
original options) and @code{cppflags} to empty in @file{}, then
typing @samp{./doSH; make depend; make}.
On various 386 Unix systems derived from System V, including SCO, ISC,
and ESIX, you may get error messages about running out of virtual memory
while compiling certain programs.
You can prevent this problem by linking GNU CC with the GNU malloc
(which thus replaces the malloc that comes with the system). GNU malloc
is available as a separate package, and also in the file
@file{src/gmalloc.c} in the GNU Emacs 19 distribution.
If you have installed GNU malloc as a separate library package, use this
option when you relink GNU CC:
@end example
Alternatively, if you have compiled @file{gmalloc.c} from Emacs 19, copy
the object file to @file{gmalloc.o} and use this option when you relink
@end example
@end itemize
@node Incompatibilities
@section Incompatibilities of GNU CC
@cindex incompatibilities of GNU CC
There are several noteworthy incompatibilities between GNU C and most
existing (non-ANSI) versions of C. The @samp{-traditional} option
eliminates many of these incompatibilities, @emph{but not all}, by
telling GNU C to behave like the other C compilers.
@itemize @bullet
@cindex string constants
@cindex read-only strings
@cindex shared strings
GNU CC normally makes string constants read-only. If several
identical-looking string constants are used, GNU CC stores only one
copy of the string.
@cindex @code{mktemp}, and constant strings
One consequence is that you cannot call @code{mktemp} with a string
constant argument. The function @code{mktemp} always alters the
string its argument points to.
@cindex @code{sscanf}, and constant strings
@cindex @code{fscanf}, and constant strings
@cindex @code{scanf}, and constant strings
Another consequence is that @code{sscanf} does not work on some systems
when passed a string constant as its format control string or input.
This is because @code{sscanf} incorrectly tries to write into the string
constant. Likewise @code{fscanf} and @code{scanf}.
The best solution to these problems is to change the program to use
@code{char}-array variables with initialization strings for these
purposes instead of string constants. But if this is not possible,
you can use the @samp{-fwritable-strings} flag, which directs GNU CC
to handle string constants the same way most C compilers do.
@samp{-traditional} also has this effect, among others.
@code{-2147483648} is positive.
This is because 2147483648 cannot fit in the type @code{int}, so
(following the ANSI C rules) its data type is @code{unsigned long int}.
Negating this value yields 2147483648 again.
GNU CC does not substitute macro arguments when they appear inside of
string constants. For example, the following macro in GNU CC
#define foo(a) "a"
@end example
will produce output @code{"a"} regardless of what the argument @var{a} is.
The @samp{-traditional} option directs GNU CC to handle such cases
(among others) in the old-fashioned (non-ANSI) fashion.
@cindex @code{setjmp} incompatibilities
@cindex @code{longjmp} incompatibilities
When you use @code{setjmp} and @code{longjmp}, the only automatic
variables guaranteed to remain valid are those declared
@code{volatile}. This is a consequence of automatic register
allocation. Consider this function:
jmp_buf j;
foo ()
int a, b;
a = fun1 ();
if (setjmp (j))
return a;
a = fun2 ();
/* @r{@code{longjmp (j)} may occur in @code{fun3}.} */
return a + fun3 ();
@end example
Here @code{a} may or may not be restored to its first value when the
@code{longjmp} occurs. If @code{a} is allocated in a register, then
its first value is restored; otherwise, it keeps the last value stored
in it.
If you use the @samp{-W} option with the @samp{-O} option, you will
get a warning when GNU CC thinks such a problem might be possible.
The @samp{-traditional} option directs GNU C to put variables in
the stack by default, rather than in registers, in functions that
call @code{setjmp}. This results in the behavior found in
traditional C compilers.
Programs that use preprocessing directives in the middle of macro
arguments do not work with GNU CC. For example, a program like this
will not work:
foobar (
#define luser
@end example
ANSI C does not permit such a construct. It would make sense to support
it when @samp{-traditional} is used, but it is too much work to
@cindex external declaration scope
@cindex scope of external declarations
@cindex declaration scope
Declarations of external variables and functions within a block apply
only to the block containing the declaration. In other words, they
have the same scope as any other declaration in the same place.
In some other C compilers, a @code{extern} declaration affects all the
rest of the file even if it happens within a block.
The @samp{-traditional} option directs GNU C to treat all @code{extern}
declarations as global, like traditional compilers.
In traditional C, you can combine @code{long}, etc., with a typedef name,
as shown here:
typedef int foo;
typedef long foo bar;
@end example
In ANSI C, this is not allowed: @code{long} and other type modifiers
require an explicit @code{int}. Because this criterion is expressed
by Bison grammar rules rather than C code, the @samp{-traditional}
flag cannot alter it.
@cindex typedef names as function parameters
PCC allows typedef names to be used as function parameters. The
difficulty described immediately above applies here too.
@cindex whitespace
PCC allows whitespace in the middle of compound assignment operators
such as @samp{+=}. GNU CC, following the ANSI standard, does not
allow this. The difficulty described immediately above applies here
@cindex apostrophes
@cindex '
GNU CC complains about unterminated character constants inside of
preprocessing conditionals that fail. Some programs have English
comments enclosed in conditionals that are guaranteed to fail; if these
comments contain apostrophes, GNU CC will probably report an error. For
example, this code would produce an error:
#if 0
You can't expect this to work.
@end example
The best solution to such a problem is to put the text into an actual
C comment delimited by @samp{/*@dots{}*/}. However,
@samp{-traditional} suppresses these error messages.
Many user programs contain the declaration @samp{long time ();}. In the
past, the system header files on many systems did not actually declare
@code{time}, so it did not matter what type your program declared it to
return. But in systems with ANSI C headers, @code{time} is declared to
return @code{time_t}, and if that is not the same as @code{long}, then
@samp{long time ();} is erroneous.
The solution is to change your program to use @code{time_t} as the return
type of @code{time}.
@cindex @code{float} as function value type
When compiling functions that return @code{float}, PCC converts it to
a double. GNU CC actually returns a @code{float}. If you are concerned
with PCC compatibility, you should declare your functions to return
@code{double}; you might as well say what you mean.
@cindex structures
@cindex unions
When compiling functions that return structures or unions, GNU CC
output code normally uses a method different from that used on most
versions of Unix. As a result, code compiled with GNU CC cannot call
a structure-returning function compiled with PCC, and vice versa.
The method used by GNU CC is as follows: a structure or union which is
1, 2, 4 or 8 bytes long is returned like a scalar. A structure or union
with any other size is stored into an address supplied by the caller
(usually in a special, fixed register, but on some machines it is passed
on the stack). The machine-description macros @code{STRUCT_VALUE} and
@code{STRUCT_INCOMING_VALUE} tell GNU CC where to pass this address.
By contrast, PCC on most target machines returns structures and unions
of any size by copying the data into an area of static storage, and then
returning the address of that storage as if it were a pointer value.
The caller must copy the data from that memory area to the place where
the value is wanted. GNU CC does not use this method because it is
slower and nonreentrant.
On some newer machines, PCC uses a reentrant convention for all
structure and union returning. GNU CC on most of these machines uses a
compatible convention when returning structures and unions in memory,
but still returns small structures and unions in registers.
You can tell GNU CC to use a compatible convention for all structure and
union returning with the option @samp{-fpcc-struct-return}.
@cindex preprocessing tokens
@cindex preprocessing numbers
GNU C complains about program fragments such as @samp{0x74ae-0x4000}
which appear to be two hexadecimal constants separated by the minus
operator. Actually, this string is a single @dfn{preprocessing token}.
Each such token must correspond to one token in C. Since this does not,
GNU C prints an error message. Although it may appear obvious that what
is meant is an operator and two values, the ANSI C standard specifically
requires that this be treated as erroneous.
A @dfn{preprocessing token} is a @dfn{preprocessing number} if it
begins with a digit and is followed by letters, underscores, digits,
periods and @samp{e+}, @samp{e-}, @samp{E+}, or @samp{E-} character
To make the above program fragment valid, place whitespace in front of
the minus sign. This whitespace will end the preprocessing number.
@end itemize
@node Fixed Headers
@section Fixed Header Files
GNU CC needs to install corrected versions of some system header files.
This is because most target systems have some header files that won't
work with GNU CC unless they are changed. Some have bugs, some are
incompatible with ANSI C, and some depend on special features of other
Installing GNU CC automatically creates and installs the fixed header
files, by running a program called @code{fixincludes} (or for certain
targets an alternative such as @code{fixinc.svr4}). Normally, you
don't need to pay attention to this. But there are cases where it
doesn't do the right thing automatically.
@itemize @bullet
If you update the system's header files, such as by installing a new
system version, the fixed header files of GNU CC are not automatically
updated. The easiest way to update them is to reinstall GNU CC. (If
you want to be clever, look in the makefile and you can find a
On some systems, in particular SunOS 4, header file directories contain
machine-specific symbolic links in certain places. This makes it
possible to share most of the header files among hosts running the
same version of SunOS 4 on different machine models.
The programs that fix the header files do not understand this special
way of using symbolic links; therefore, the directory of fixed header
files is good only for the machine model used to build it.
In SunOS 4, only programs that look inside the kernel will notice the
difference between machine models. Therefore, for most purposes, you
need not be concerned about this.
It is possible to make separate sets of fixed header files for the
different machine models, and arrange a structure of symbolic links so
as to use the proper set, but you'll have to do this by hand.
On Lynxos, GNU CC by default does not fix the header files. This is
because bugs in the shell cause the @code{fixincludes} script to fail.
This means you will encounter problems due to bugs in the system header
files. It may be no comfort that they aren't GNU CC's fault, but it
does mean that there's nothing for us to do about them.
@end itemize
@node Standard Libraries
@section Standard Libraries
GNU CC by itself attempts to be what the ISO/ANSI C standard calls a
@dfn{conforming freestanding implementation}. This means all ANSI
C language features are available, as well as the contents of
@file{float.h}, @file{limits.h}, @file{stdarg.h}, and
@file{stddef.h}. The rest of the C library is supplied by the
vendor of the operating system. If that C library doesn't conform to
the C standards, then your programs might get warnings (especially when
using @samp{-Wall}) that you don't expect.
For example, the @code{sprintf} function on SunOS 4.1.3 returns
@code{char *} while the C standard says that @code{sprintf} returns an
@code{int}. The @code{fixincludes} program could make the prototype for
this function match the Standard, but that would be wrong, since the
function will still return @code{char *}.
If you need a Standard compliant library, then you need to find one, as
GNU CC does not provide one. The GNU C library (called @code{glibc})
has been ported to a number of operating systems, and provides ANSI/ISO,
POSIX, BSD and SystemV compatibility. You could also ask your operating
system vendor if newer libraries are available.
@node Disappointments
@section Disappointments and Misunderstandings
These problems are perhaps regrettable, but we don't know any practical
way around them.
@itemize @bullet
Certain local variables aren't recognized by debuggers when you compile
with optimization.
This occurs because sometimes GNU CC optimizes the variable out of
existence. There is no way to tell the debugger how to compute the
value such a variable ``would have had'', and it is not clear that would
be desirable anyway. So GNU CC simply does not mention the eliminated
variable when it writes debugging information.
You have to expect a certain amount of disagreement between the
executable and your source code, when you use optimization.
@cindex conflicting types
@cindex scope of declaration
Users often think it is a bug when GNU CC reports an error for code
like this:
int foo (struct mumble *);
struct mumble @{ @dots{} @};
int foo (struct mumble *x)
@{ @dots{} @}
@end example
This code really is erroneous, because the scope of @code{struct
mumble} in the prototype is limited to the argument list containing it.
It does not refer to the @code{struct mumble} defined with file scope
immediately below---they are two unrelated types with similar names in
different scopes.
But in the definition of @code{foo}, the file-scope type is used
because that is available to be inherited. Thus, the definition and
the prototype do not match, and you get an error.
This behavior may seem silly, but it's what the ANSI standard specifies.
It is easy enough for you to make your code work by moving the
definition of @code{struct mumble} above the prototype. It's not worth
being incompatible with ANSI C just to avoid an error for the example
shown above.
Accesses to bitfields even in volatile objects works by accessing larger
objects, such as a byte or a word. You cannot rely on what size of
object is accessed in order to read or write the bitfield; it may even
vary for a given bitfield according to the precise usage.
If you care about controlling the amount of memory that is accessed, use
volatile but do not use bitfields.
GNU CC comes with shell scripts to fix certain known problems in system
header files. They install corrected copies of various header files in
a special directory where only GNU CC will normally look for them. The
scripts adapt to various systems by searching all the system header
files for the problem cases that we know about.
If new system header files are installed, nothing automatically arranges
to update the corrected header files. You will have to reinstall GNU CC
to fix the new header files. More specifically, go to the build
directory and delete the files @file{stmp-fixinc} and
@file{stmp-headers}, and the subdirectory @code{include}; then do
@samp{make install} again.
@cindex floating point precision
On 68000 and x86 systems, for instance, you can get paradoxical results
if you test the precise values of floating point numbers. For example,
you can find that a floating point value which is not a NaN is not equal
to itself. This results from the fact that the floating point registers
hold a few more bits of precision than fit in a @code{double} in memory.
Compiled code moves values between memory and floating point registers
at its convenience, and moving them into memory truncates them.
You can partially avoid this problem by using the @samp{-ffloat-store}
option (@pxref{Optimize Options}).
On the MIPS, variable argument functions using @file{varargs.h}
cannot have a floating point value for the first argument. The
reason for this is that in the absence of a prototype in scope,
if the first argument is a floating point, it is passed in a
floating point register, rather than an integer register.
If the code is rewritten to use the ANSI standard @file{stdarg.h}
method of variable arguments, and the prototype is in scope at
the time of the call, everything will work fine.
On the H8/300 and H8/300H, variable argument functions must be
implemented using the ANSI standard @file{stdarg.h} method of
variable arguments. Furthermore, calls to functions using @file{stdarg.h}
variable arguments must have a prototype for the called function
in scope at the time of the call.
@end itemize
@node C++ Misunderstandings
@section Common Misunderstandings with GNU C++
@cindex misunderstandings in C++
@cindex surprises in C++
@cindex C++ misunderstandings
C++ is a complex language and an evolving one, and its standard definition
(the ANSI C++ draft standard) is also evolving. As a result,
your C++ compiler may occasionally surprise you, even when its behavior is
correct. This section discusses some areas that frequently give rise to
questions of this sort.
* Static Definitions:: Static member declarations are not definitions
* Temporaries:: Temporaries may vanish before you expect
@end menu
@node Static Definitions
@subsection Declare @emph{and} Define Static Members
@cindex C++ static data, declaring and defining
@cindex static data in C++, declaring and defining
@cindex declaring static data in C++
@cindex defining static data in C++
When a class has static data members, it is not enough to @emph{declare}
the static member; you must also @emph{define} it. For example:
class Foo
void method();
static int bar;
@end example
This declaration only establishes that the class @code{Foo} has an
@code{int} named @code{Foo::bar}, and a member function named
@code{Foo::method}. But you still need to define @emph{both}
@code{method} and @code{bar} elsewhere. According to the draft ANSI
standard, you must supply an initializer in one (and only one) source
file, such as:
int Foo::bar = 0;
@end example
Other C++ compilers may not correctly implement the standard behavior.
As a result, when you switch to @code{g++} from one of these compilers,
you may discover that a program that appeared to work correctly in fact
does not conform to the standard: @code{g++} reports as undefined
symbols any static data members that lack definitions.
@node Temporaries
@subsection Temporaries May Vanish Before You Expect
@cindex temporaries, lifetime of
@cindex portions of temporary objects, pointers to
It is dangerous to use pointers or references to @emph{portions} of a
temporary object. The compiler may very well delete the object before
you expect it to, leaving a pointer to garbage. The most common place
where this problem crops up is in classes like the libg++
@code{String} class, that define a conversion function to type
@code{char *} or @code{const char *}. However, any class that returns
a pointer to some internal structure is potentially subject to this
For example, a program may use a function @code{strfunc} that returns
@code{String} objects, and another function @code{charfunc} that
operates on pointers to @code{char}:
String strfunc ();
void charfunc (const char *);
@end example
In this situation, it may seem natural to write @w{@samp{charfunc
(strfunc ());}} based on the knowledge that class @code{String} has an
explicit conversion to @code{char} pointers. However, what really
happens is akin to @samp{charfunc (@w{strfunc ()}.@w{convert ()});},
where the @code{convert} method is a function to do the same data
conversion normally performed by a cast. Since the last use of the
temporary @code{String} object is the call to the conversion function,
the compiler may delete that object before actually calling
@code{charfunc}. The compiler has no way of knowing that deleting the
@code{String} object will invalidate the pointer. The pointer then
points to garbage, so that by the time @code{charfunc} is called, it
gets an invalid argument.
Code like this may run successfully under some other compilers,
especially those that delete temporaries relatively late. However, the
GNU C++ behavior is also standard-conforming, so if your program depends
on late destruction of temporaries it is not portable.
If you think this is surprising, you should be aware that the ANSI C++
committee continues to debate the lifetime-of-temporaries problem.
For now, at least, the safe way to write such code is to give the
temporary a name, which forces it to remain until the end of the scope of
the name. For example:
String& tmp = strfunc ();
charfunc (tmp);
@end example
@node Protoize Caveats
@section Caveats of using @code{protoize}
The conversion programs @code{protoize} and @code{unprotoize} can
sometimes change a source file in a way that won't work unless you
rearrange it.
@itemize @bullet
@code{protoize} can insert references to a type name or type tag before
the definition, or in a file where they are not defined.
If this happens, compiler error messages should show you where the new
references are, so fixing the file by hand is straightforward.
There are some C constructs which @code{protoize} cannot figure out.
For example, it can't determine argument types for declaring a
pointer-to-function variable; this you must do by hand. @code{protoize}
inserts a comment containing @samp{???} each time it finds such a
variable; so you can find all such variables by searching for this
string. ANSI C does not require declaring the argument types of
pointer-to-function types.
Using @code{unprotoize} can easily introduce bugs. If the program
relied on prototypes to bring about conversion of arguments, these
conversions will not take place in the program without prototypes.
One case in which you can be sure @code{unprotoize} is safe is when
you are removing prototypes that were made with @code{protoize}; if
the program worked before without any prototypes, it will work again
without them.
You can find all the places where this problem might occur by compiling
the program with the @samp{-Wconversion} option. It prints a warning
whenever an argument is converted.
Both conversion programs can be confused if there are macro calls in and
around the text to be converted. In other words, the standard syntax
for a declaration or definition must not result from expanding a macro.
This problem is inherent in the design of C and cannot be fixed. If
only a few functions have confusing macro calls, you can easily convert
them manually.
@code{protoize} cannot get the argument types for a function whose
definition was not actually compiled due to preprocessing conditionals.
When this happens, @code{protoize} changes nothing in regard to such
a function. @code{protoize} tries to detect such instances and warn
about them.
You can generally work around this problem by using @code{protoize} step
by step, each time specifying a different set of @samp{-D} options for
compilation, until all of the functions have been converted. There is
no automatic way to verify that you have got them all, however.
Confusion may result if there is an occasion to convert a function
declaration or definition in a region of source code where there is more
than one formal parameter list present. Thus, attempts to convert code
containing multiple (conditionally compiled) versions of a single
function header (in the same vicinity) may not produce the desired (or
expected) results.
If you plan on converting source files which contain such code, it is
recommended that you first make sure that each conditionally compiled
region of source code which contains an alternative function header also
contains at least one additional follower token (past the final right
parenthesis of the function header). This should circumvent the
@code{unprotoize} can become confused when trying to convert a function
definition or declaration which contains a declaration for a
pointer-to-function formal argument which has the same name as the
function being defined or declared. We recommand you avoid such choices
of formal parameter names.
You might also want to correct some of the indentation by hand and break
long lines. (The conversion programs don't write lines longer than
eighty characters in any case.)
@end itemize
@node Non-bugs
@section Certain Changes We Don't Want to Make
This section lists changes that people frequently request, but which
we do not make because we think GNU CC is better without them.
@itemize @bullet
Checking the number and type of arguments to a function which has an
old-fashioned definition and no prototype.
Such a feature would work only occasionally---only for calls that appear
in the same file as the called function, following the definition. The
only way to check all calls reliably is to add a prototype for the
function. But adding a prototype eliminates the motivation for this
feature. So the feature is not worthwhile.
Warning about using an expression whose type is signed as a shift count.
Shift count operands are probably signed more often than unsigned.
Warning about this would cause far more annoyance than good.
Warning about assigning a signed value to an unsigned variable.
Such assignments must be very common; warning about them would cause
more annoyance than good.
Warning about unreachable code.
It's very common to have unreachable code in machine-generated
programs. For example, this happens normally in some files of GNU C
Warning when a non-void function value is ignored.
Coming as I do from a Lisp background, I balk at the idea that there is
something dangerous about discarding a value. There are functions that
return values which some callers may find useful; it makes no sense to
clutter the program with a cast to @code{void} whenever the value isn't
Assuming (for optimization) that the address of an external symbol is
never zero.
This assumption is false on certain systems when @samp{#pragma weak} is
Making @samp{-fshort-enums} the default.
This would cause storage layout to be incompatible with most other C
compilers. And it doesn't seem very important, given that you can get
the same result in other ways. The case where it matters most is when
the enumeration-valued object is inside a structure, and in that case
you can specify a field width explicitly.
Making bitfields unsigned by default on particular machines where ``the
ABI standard'' says to do so.
The ANSI C standard leaves it up to the implementation whether a bitfield
declared plain @code{int} is signed or not. This in effect creates two
alternative dialects of C.
The GNU C compiler supports both dialects; you can specify the signed
dialect with @samp{-fsigned-bitfields} and the unsigned dialect with
@samp{-funsigned-bitfields}. However, this leaves open the question of
which dialect to use by default.
Currently, the preferred dialect makes plain bitfields signed, because
this is simplest. Since @code{int} is the same as @code{signed int} in
every other context, it is cleanest for them to be the same in bitfields
as well.
Some computer manufacturers have published Application Binary Interface
standards which specify that plain bitfields should be unsigned. It is
a mistake, however, to say anything about this issue in an ABI. This is
because the handling of plain bitfields distinguishes two dialects of C.
Both dialects are meaningful on every type of machine. Whether a
particular object file was compiled using signed bitfields or unsigned
is of no concern to other object files, even if they access the same
bitfields in the same data structures.
A given program is written in one or the other of these two dialects.
The program stands a chance to work on most any machine if it is
compiled with the proper dialect. It is unlikely to work at all if
compiled with the wrong dialect.
Many users appreciate the GNU C compiler because it provides an
environment that is uniform across machines. These users would be
inconvenienced if the compiler treated plain bitfields differently on
certain machines.
Occasionally users write programs intended only for a particular machine
type. On these occasions, the users would benefit if the GNU C compiler
were to support by default the same dialect as the other compilers on
that machine. But such applications are rare. And users writing a
program to run on more than one type of machine cannot possibly benefit
from this kind of compatibility.
This is why GNU CC does and will treat plain bitfields in the same
fashion on all types of machines (by default).
There are some arguments for making bitfields unsigned by default on all
machines. If, for example, this becomes a universal de facto standard,
it would make sense for GNU CC to go along with it. This is something
to be considered in the future.
(Of course, users strongly concerned about portability should indicate
explicitly in each bitfield whether it is signed or not. In this way,
they write programs which have the same meaning in both C dialects.)
Undefining @code{__STDC__} when @samp{-ansi} is not used.
Currently, GNU CC defines @code{__STDC__} as long as you don't use
@samp{-traditional}. This provides good results in practice.
Programmers normally use conditionals on @code{__STDC__} to ask whether
it is safe to use certain features of ANSI C, such as function
prototypes or ANSI token concatenation. Since plain @samp{gcc} supports
all the features of ANSI C, the correct answer to these questions is
Some users try to use @code{__STDC__} to check for the availability of
certain library facilities. This is actually incorrect usage in an ANSI
C program, because the ANSI C standard says that a conforming
freestanding implementation should define @code{__STDC__} even though it
does not have the library facilities. @samp{gcc -ansi -pedantic} is a
conforming freestanding implementation, and it is therefore required to
define @code{__STDC__}, even though it does not come with an ANSI C
Sometimes people say that defining @code{__STDC__} in a compiler that
does not completely conform to the ANSI C standard somehow violates the
standard. This is illogical. The standard is a standard for compilers
that claim to support ANSI C, such as @samp{gcc -ansi}---not for other
compilers such as plain @samp{gcc}. Whatever the ANSI C standard says
is relevant to the design of plain @samp{gcc} without @samp{-ansi} only
for pragmatic reasons, not as a requirement.
GNU CC normally defines @code{__STDC__} to be 1, and in addition
defines @code{__STRICT_ANSI__} if you specify the @samp{-ansi} option.
On some hosts, system include files use a different convention, where
@code{__STDC__} is normally 0, but is 1 if the user specifies strict
conformance to the C Standard. GNU CC follows the host convention when
processing system include files, but when processing user files it follows
the usual GNU C convention.
Undefining @code{__STDC__} in C++.
Programs written to compile with C++-to-C translators get the
value of @code{__STDC__} that goes with the C compiler that is
subsequently used. These programs must test @code{__STDC__}
to determine what kind of C preprocessor that compiler uses:
whether they should concatenate tokens in the ANSI C fashion
or in the traditional fashion.
These programs work properly with GNU C++ if @code{__STDC__} is defined.
They would not work otherwise.
In addition, many header files are written to provide prototypes in ANSI
C but not in traditional C. Many of these header files can work without
change in C++ provided @code{__STDC__} is defined. If @code{__STDC__}
is not defined, they will all fail, and will all need to be changed to
test explicitly for C++ as well.
Deleting ``empty'' loops.
GNU CC does not delete ``empty'' loops because the most likely reason
you would put one in a program is to have a delay. Deleting them will
not make real programs run any faster, so it would be pointless.
It would be different if optimization of a nonempty loop could produce
an empty one. But this generally can't happen.
Making side effects happen in the same order as in some other compiler.
@cindex side effects, order of evaluation
@cindex order of evaluation, side effects
It is never safe to depend on the order of evaluation of side effects.
For example, a function call like this may very well behave differently
from one compiler to another:
void func (int, int);
int i = 2;
func (i++, i++);
@end example
There is no guarantee (in either the C or the C++ standard language
definitions) that the increments will be evaluated in any particular
order. Either increment might happen first. @code{func} might get the
arguments @samp{2, 3}, or it might get @samp{3, 2}, or even @samp{2, 2}.
Not allowing structures with volatile fields in registers.
Strictly speaking, there is no prohibition in the ANSI C standard
against allowing structures with volatile fields in registers, but
it does not seem to make any sense and is probably not what you wanted
to do. So the compiler will give an error message in this case.
@end itemize
@node Warnings and Errors
@section Warning Messages and Error Messages
@cindex error messages
@cindex warnings vs errors
@cindex messages, warning and error
The GNU compiler can produce two kinds of diagnostics: errors and
warnings. Each kind has a different purpose:
@itemize @w{}
@emph{Errors} report problems that make it impossible to compile your
program. GNU CC reports errors with the source file name and line
number where the problem is apparent.
@emph{Warnings} report other unusual conditions in your code that
@emph{may} indicate a problem, although compilation can (and does)
proceed. Warning messages also report the source file name and line
number, but include the text @samp{warning:} to distinguish them
from error messages.
@end itemize
Warnings may indicate danger points where you should check to make sure
that your program really does what you intend; or the use of obsolete
features; or the use of nonstandard features of GNU C or C++. Many
warnings are issued only if you ask for them, with one of the @samp{-W}
options (for instance, @samp{-Wall} requests a variety of useful
GNU CC always tries to compile your program if possible; it never
gratuitously rejects a program whose meaning is clear merely because
(for instance) it fails to conform to a standard. In some cases,
however, the C and C++ standards specify that certain extensions are
forbidden, and a diagnostic @emph{must} be issued by a conforming
compiler. The @samp{-pedantic} option tells GNU CC to issue warnings in
such cases; @samp{-pedantic-errors} says to make them errors instead.
This does not mean that @emph{all} non-ANSI constructs get warnings
or errors.
@xref{Warning Options,,Options to Request or Suppress Warnings}, for
more detail on these and related command-line options.
@node Bugs
@chapter Reporting Bugs
@cindex bugs
@cindex reporting bugs
Your bug reports play an essential role in making GNU CC reliable.
When you encounter a problem, the first thing to do is to see if it is
already known. @xref{Trouble}. If it isn't known, then you should
report the problem.
Reporting a bug may help you by bringing a solution to your problem, or
it may not. (If it does not, look in the service directory; see
@ref{Service}.) In any case, the principal function of a bug report is
to help the entire community by making the next version of GNU CC work
better. Bug reports are your contribution to the maintenance of GNU CC.
Since the maintainers are very overloaded, we cannot respond to every
bug report. However, if the bug has not been fixed, we are likely to
send you a patch and ask you to tell us whether it works.
In order for a bug report to serve its purpose, you must include the
information that makes for fixing the bug.
* Criteria: Bug Criteria. Have you really found a bug?
* Where: Bug Lists. Where to send your bug report.
* Reporting: Bug Reporting. How to report a bug effectively.
* Patches: Sending Patches. How to send a patch for GNU CC.
* Known: Trouble. Known problems.
* Help: Service. Where to ask for help.
@end menu
@node Bug Criteria
@section Have You Found a Bug?
@cindex bug criteria
If you are not sure whether you have found a bug, here are some guidelines:
@itemize @bullet
@cindex fatal signal
@cindex core dump
If the compiler gets a fatal signal, for any input whatever, that is a
compiler bug. Reliable compilers never crash.
@cindex invalid assembly code
@cindex assembly code, invalid
If the compiler produces invalid assembly code, for any input whatever
(except an @code{asm} statement), that is a compiler bug, unless the
compiler reports errors (not just warnings) which would ordinarily
prevent the assembler from being run.
@cindex undefined behavior
@cindex undefined function value
@cindex increment operators
If the compiler produces valid assembly code that does not correctly
execute the input source code, that is a compiler bug.
However, you must double-check to make sure, because you may have run
into an incompatibility between GNU C and traditional C
(@pxref{Incompatibilities}). These incompatibilities might be considered
bugs, but they are inescapable consequences of valuable features.
Or you may have a program whose behavior is undefined, which happened
by chance to give the desired results with another C or C++ compiler.
For example, in many nonoptimizing compilers, you can write @samp{x;}
at the end of a function instead of @samp{return x;}, with the same
results. But the value of the function is undefined if @code{return}
is omitted; it is not a bug when GNU CC produces different results.
Problems often result from expressions with two increment operators,
as in @code{f (*p++, *p++)}. Your previous compiler might have
interpreted that expression the way you intended; GNU CC might
interpret it another way. Neither compiler is wrong. The bug is
in your code.
After you have localized the error to a single source line, it should
be easy to check for these things. If your program is correct and
well defined, you have found a compiler bug.
If the compiler produces an error message for valid input, that is a
compiler bug.
@cindex invalid input
If the compiler does not produce an error message for invalid input,
that is a compiler bug. However, you should note that your idea of
``invalid input'' might be my idea of ``an extension'' or ``support
for traditional practice''.
If you are an experienced user of C or C++ compilers, your suggestions
for improvement of GNU CC or GNU C++ are welcome in any case.
@end itemize
@node Bug Lists
@section Where to Report Bugs
@cindex bug report mailing lists
Send bug reports for GNU C to @samp{}.
Send bug reports for GNU C++ and the C++ runtime libraries to
Often people think of posting bug reports to the newsgroup instead of
mailing them. This appears to work, but it has one problem which can be
crucial: a newsgroup posting does not contain a mail path back to the
sender. Thus, if maintainers need more information, they may be unable
to reach you. For this reason, you should always send bug reports by
mail to the proper mailing list.
As a last resort, send bug reports on paper to:
GNU Compiler Bugs
Free Software Foundation
59 Temple Place - Suite 330
Boston, MA 02111-1307, USA
@end example
@node Bug Reporting
@section How to Report Bugs
@cindex compiler bugs, reporting
The fundamental principle of reporting bugs usefully is this:
@strong{report all the facts}. If you are not sure whether to state a
fact or leave it out, state it!
Often people omit facts because they think they know what causes the
problem and they conclude that some details don't matter. Thus, you might
assume that the name of the variable you use in an example does not matter.
Well, probably it doesn't, but one cannot be sure. Perhaps the bug is a
stray memory reference which happens to fetch from the location where that
name is stored in memory; perhaps, if the name were different, the contents
of that location would fool the compiler into doing the right thing despite
the bug. Play it safe and give a specific, complete example. That is the
easiest thing for you to do, and the most helpful.
Keep in mind that the purpose of a bug report is to enable someone to
fix the bug if it is not known. It isn't very important what happens if
the bug is already known. Therefore, always write your bug reports on
the assumption that the bug is not known.
Sometimes people give a few sketchy facts and ask, ``Does this ring a
bell?'' This cannot help us fix a bug, so it is basically useless. We
respond by asking for enough details to enable us to investigate.
You might as well expedite matters by sending them to begin with.
Try to make your bug report self-contained. If we have to ask you for
more information, it is best if you include all the previous information
in your response, as well as the information that was missing.
Please report each bug in a separate message. This makes it easier for
us to track which bugs have been fixed and to forward your bugs reports
to the appropriate maintainer.
Do not compress and encode any part of your bug report using programs
such as @file{uuencode}. If you do so it will slow down the processing
of your bug. If you must submit multiple large files, use @file{shar},
which allows us to read your message without having to run any
decompression programs.
To enable someone to investigate the bug, you should include all these
@itemize @bullet
The version of GNU CC. You can get this by running it with the
@samp{-v} option.
Without this, we won't know whether there is any point in looking for
the bug in the current version of GNU CC.
A complete input file that will reproduce the bug. If the bug is in the
C preprocessor, send a source file and any header files that it
requires. If the bug is in the compiler proper (@file{cc1}), run your
source file through the C preprocessor by doing @samp{gcc -E
@var{sourcefile} > @var{outfile}}, then include the contents of
@var{outfile} in the bug report. (When you do this, use the same
@samp{-I}, @samp{-D} or @samp{-U} options that you used in actual
A single statement is not enough of an example. In order to compile it,
it must be embedded in a complete file of compiler input; and the bug
might depend on the details of how this is done.
Without a real example one can compile, all anyone can do about your bug
report is wish you luck. It would be futile to try to guess how to
provoke the bug. For example, bugs in register allocation and reloading
frequently depend on every little detail of the function they happen in.
Even if the input file that fails comes from a GNU program, you should
still send the complete test case. Don't ask the GNU CC maintainers to
do the extra work of obtaining the program in question---they are all
overworked as it is. Also, the problem may depend on what is in the
header files on your system; it is unreliable for the GNU CC maintainers
to try the problem with the header files available to them. By sending
CPP output, you can eliminate this source of uncertainty and save us
a certain percentage of wild goose chases.
The command arguments you gave GNU CC or GNU C++ to compile that example
and observe the bug. For example, did you use @samp{-O}? To guarantee
you won't omit something important, list all the options.
If we were to try to guess the arguments, we would probably guess wrong
and then we would not encounter the bug.
The type of machine you are using, and the operating system name and
version number.
The operands you gave to the @code{configure} command when you installed
the compiler.
A complete list of any modifications you have made to the compiler
source. (We don't promise to investigate the bug unless it happens in
an unmodified compiler. But if you've made modifications and don't tell
us, then you are sending us on a wild goose chase.)
Be precise about these changes. A description in English is not
enough---send a context diff for them.
Adding files of your own (such as a machine description for a machine we
don't support) is a modification of the compiler source.
Details of any other deviations from the standard procedure for installing
A description of what behavior you observe that you believe is
incorrect. For example, ``The compiler gets a fatal signal,'' or,
``The assembler instruction at line 208 in the output is incorrect.''
Of course, if the bug is that the compiler gets a fatal signal, then one
can't miss it. But if the bug is incorrect output, the maintainer might
not notice unless it is glaringly wrong. None of us has time to study
all the assembler code from a 50-line C program just on the chance that
one instruction might be wrong. We need @emph{you} to do this part!
Even if the problem you experience is a fatal signal, you should still
say so explicitly. Suppose something strange is going on, such as, your
copy of the compiler is out of synch, or you have encountered a bug in
the C library on your system. (This has happened!) Your copy might
crash and the copy here would not. If you @i{said} to expect a crash,
then when the compiler here fails to crash, we would know that the bug
was not happening. If you don't say to expect a crash, then we would
not know whether the bug was happening. We would not be able to draw
any conclusion from our observations.
If the problem is a diagnostic when compiling GNU CC with some other
compiler, say whether it is a warning or an error.
Often the observed symptom is incorrect output when your program is run.
Sad to say, this is not enough information unless the program is short
and simple. None of us has time to study a large program to figure out
how it would work if compiled correctly, much less which line of it was
compiled wrong. So you will have to do that. Tell us which source line
it is, and what incorrect result happens when that line is executed. A
person who understands the program can find this as easily as finding a
bug in the program itself.
If you send examples of assembler code output from GNU CC or GNU C++,
please use @samp{-g} when you make them. The debugging information
includes source line numbers which are essential for correlating the
output with the input.
If you wish to mention something in the GNU CC source, refer to it by
context, not by line number.
The line numbers in the development sources don't match those in your
sources. Your line numbers would convey no useful information to the
Additional information from a debugger might enable someone to find a
problem on a machine which he does not have available. However, you
need to think when you collect this information if you want it to have
any chance of being useful.
@cindex backtrace for bug reports
For example, many people send just a backtrace, but that is never
useful by itself. A simple backtrace with arguments conveys little
about GNU CC because the compiler is largely data-driven; the same
functions are called over and over for different RTL insns, doing
different things depending on the details of the insn.
Most of the arguments listed in the backtrace are useless because they
are pointers to RTL list structure. The numeric values of the
pointers, which the debugger prints in the backtrace, have no
significance whatever; all that matters is the contents of the objects
they point to (and most of the contents are other such pointers).
In addition, most compiler passes consist of one or more loops that
scan the RTL insn sequence. The most vital piece of information about
such a loop---which insn it has reached---is usually in a local variable,
not in an argument.
@findex debug_rtx
What you need to provide in addition to a backtrace are the values of
the local variables for several stack frames up. When a local
variable or an argument is an RTX, first print its value and then use
the GDB command @code{pr} to print the RTL expression that it points
to. (If GDB doesn't run on your machine, use your debugger to call
the function @code{debug_rtx} with the RTX as an argument.) In
general, whenever a variable is a pointer, its value is no use
without the data it points to.
@end itemize
Here are some things that are not necessary:
@itemize @bullet
A description of the envelope of the bug.
Often people who encounter a bug spend a lot of time investigating
which changes to the input file will make the bug go away and which
changes will not affect it.
This is often time consuming and not very useful, because the way we
will find the bug is by running a single example under the debugger with
breakpoints, not by pure deduction from a series of examples. You might
as well save your time for something else.
Of course, if you can find a simpler example to report @emph{instead} of
the original one, that is a convenience. Errors in the output will be
easier to spot, running under the debugger will take less time, etc.
Most GNU CC bugs involve just one function, so the most straightforward
way to simplify an example is to delete all the function definitions
except the one where the bug occurs. Those earlier in the file may be
replaced by external declarations if the crucial function depends on
them. (Exception: inline functions may affect compilation of functions
defined later in the file.)
However, simplification is not vital; if you don't want to do this,
report the bug anyway and send the entire test case you used.
In particular, some people insert conditionals @samp{#ifdef BUG} around
a statement which, if removed, makes the bug not happen. These are just
clutter; we won't pay any attention to them anyway. Besides, you should
send us cpp output, and that can't have conditionals.
A patch for the bug.
A patch for the bug is useful if it is a good one. But don't omit the
necessary information, such as the test case, on the assumption that a
patch is all we need. We might see problems with your patch and decide
to fix the problem another way, or we might not understand it at all.
Sometimes with a program as complicated as GNU CC it is very hard to
construct an example that will make the program follow a certain path
through the code. If you don't send the example, we won't be able to
construct one, so we won't be able to verify that the bug is fixed.
And if we can't understand what bug you are trying to fix, or why your
patch should be an improvement, we won't install it. A test case will
help us to understand.
@xref{Sending Patches}, for guidelines on how to make it easy for us to
understand and install your patches.
A guess about what the bug is or what it depends on.
Such guesses are usually wrong. Even I can't guess right about such
things without first using the debugger to find the facts.
A core dump file.
We have no way of examining a core dump for your type of machine
unless we have an identical system---and if we do have one,
we should be able to reproduce the crash ourselves.
@end itemize
@node Sending Patches,, Bug Reporting, Bugs
@section Sending Patches for GNU CC
If you would like to write bug fixes or improvements for the GNU C
compiler, that is very helpful. Send suggested fixes to the bug report
mailing list, @code{}.
Please follow these guidelines so we can study your patches efficiently.
If you don't follow these guidelines, your information might still be
useful, but using it will take extra work. Maintaining GNU C is a lot
of work in the best of circumstances, and we can't keep up unless you do
your best to help.
@itemize @bullet
Send an explanation with your changes of what problem they fix or what
improvement they bring about. For a bug fix, just include a copy of the
bug report, and explain why the change fixes the bug.
(Referring to a bug report is not as good as including it, because then
we will have to look it up, and we have probably already deleted it if
we've already fixed the bug.)
Always include a proper bug report for the problem you think you have
fixed. We need to convince ourselves that the change is right before
installing it. Even if it is right, we might have trouble judging it if
we don't have a way to reproduce the problem.
Include all the comments that are appropriate to help people reading the
source in the future understand why this change was needed.
Don't mix together changes made for different reasons.
Send them @emph{individually}.
If you make two changes for separate reasons, then we might not want to
install them both. We might want to install just one. If you send them
all jumbled together in a single set of diffs, we have to do extra work
to disentangle them---to figure out which parts of the change serve
which purpose. If we don't have time for this, we might have to ignore
your changes entirely.
If you send each change as soon as you have written it, with its own
explanation, then the two changes never get tangled up, and we can
consider each one properly without any extra work to disentangle them.
Ideally, each change you send should be impossible to subdivide into
parts that we might want to consider separately, because each of its
parts gets its motivation from the other parts.
Send each change as soon as that change is finished. Sometimes people
think they are helping us by accumulating many changes to send them all
together. As explained above, this is absolutely the worst thing you
could do.
Since you should send each change separately, you might as well send it
right away. That gives us the option of installing it immediately if it
is important.
Use @samp{diff -c} to make your diffs. Diffs without context are hard
for us to install reliably. More than that, they make it hard for us to
study the diffs to decide whether we want to install them. Unidiff
format is better than contextless diffs, but not as easy to read as
@samp{-c} format.
If you have GNU diff, use @samp{diff -cp}, which shows the name of the
function that each change occurs in.
Write the change log entries for your changes. We get lots of changes,
and we don't have time to do all the change log writing ourselves.
Read the @file{ChangeLog} file to see what sorts of information to put
in, and to learn the style that we use. The purpose of the change log
is to show people where to find what was changed. So you need to be
specific about what functions you changed; in large functions, it's
often helpful to indicate where within the function the change was.
On the other hand, once you have shown people where to find the change,
you need not explain its purpose. Thus, if you add a new function, all
you need to say about it is that it is new. If you feel that the
purpose needs explaining, it probably does---but the explanation will be
much more useful if you put it in comments in the code.
If you would like your name to appear in the header line for who made
the change, send us the header line.
When you write the fix, keep in mind that we can't install a change that
would break other systems.
People often suggest fixing a problem by changing machine-independent
files such as @file{toplev.c} to do something special that a particular
system needs. Sometimes it is totally obvious that such changes would
break GNU CC for almost all users. We can't possibly make a change like
that. At best it might tell us how to write another patch that would
solve the problem acceptably.
Sometimes people send fixes that @emph{might} be an improvement in
general---but it is hard to be sure of this. It's hard to install
such changes because we have to study them very carefully. Of course,
a good explanation of the reasoning by which you concluded the change
was correct can help convince us.
The safest changes are changes to the configuration files for a
particular machine. These are safe because they can't create new bugs
on other machines.
Please help us keep up with the workload by designing the patch in a
form that is good to install.
@end itemize
@node Service
@chapter How To Get Help with GNU CC
If you need help installing, using or changing GNU CC, there are two
ways to find it:
@itemize @bullet
Send a message to a suitable network mailing list. First try
@code{}, and if that brings no response, try
Look in the service directory for someone who might help you for a fee.
The service directory is found in the file named @file{SERVICE} in the
GNU CC distribution.
@end itemize
@node Contributing
@chapter Contributing to GNU CC Development
If you would like to help pretest GNU CC releases to assure they work
well, or if you would like to work on improving GNU CC, please contact
the maintainers at @code{}. A pretester should
be willing to try to investigate bugs as well as report them.
If you'd like to work on improvements, please ask for suggested projects
or suggest your own ideas. If you have already written an improvement,
please tell us about it. If you have not yet started work, it is useful
to contact @code{} before you start; the
maintainers may be able to suggest ways to make your extension fit in
better with the rest of GNU CC and with other development plans.
@node VMS
@chapter Using GNU CC on VMS
@c prevent bad page break with this line
Here is how to use GNU CC on VMS.
* Include Files and VMS:: Where the preprocessor looks for the include files.
* Global Declarations:: How to do globaldef, globalref and globalvalue with
* VMS Misc:: Misc information.
@end menu
@node Include Files and VMS
@section Include Files and VMS
@cindex include files and VMS
@cindex VMS and include files
@cindex header files and VMS
Due to the differences between the filesystems of Unix and VMS, GNU CC
attempts to translate file names in @samp{#include} into names that VMS
will understand. The basic strategy is to prepend a prefix to the
specification of the include file, convert the whole filename to a VMS
filename, and then try to open the file. GNU CC tries various prefixes
one by one until one of them succeeds:
The first prefix is the @samp{GNU_CC_INCLUDE:} logical name: this is
where GNU C header files are traditionally stored. If you wish to store
header files in non-standard locations, then you can assign the logical
@samp{GNU_CC_INCLUDE} to be a search list, where each element of the
list is suitable for use with a rooted logical.
The next prefix tried is @samp{SYS$SYSROOT:[SYSLIB.]}. This is where
VAX-C header files are traditionally stored.
If the include file specification by itself is a valid VMS filename, the
preprocessor then uses this name with no prefix in an attempt to open
the include file.
If the file specification is not a valid VMS filename (i.e. does not
contain a device or a directory specifier, and contains a @samp{/}
character), the preprocessor tries to convert it from Unix syntax to
VMS syntax.
Conversion works like this: the first directory name becomes a device,
and the rest of the directories are converted into VMS-format directory
names. For example, the name @file{X11/foobar.h} is
translated to @file{X11:[000000]foobar.h} or @file{X11:foobar.h},
whichever one can be opened. This strategy allows you to assign a
logical name to point to the actual location of the header files.
If none of these strategies succeeds, the @samp{#include} fails.
@end enumerate
Include directives of the form:
#include foobar
@end example
are a common source of incompatibility between VAX-C and GNU CC. VAX-C
treats this much like a standard @code{#include <foobar.h>} directive.
That is incompatible with the ANSI C behavior implemented by GNU CC: to
expand the name @code{foobar} as a macro. Macro expansion should
eventually yield one of the two standard formats for @code{#include}:
#include "@var{file}"
#include <@var{file}>
@end example
If you have this problem, the best solution is to modify the source to
convert the @code{#include} directives to one of the two standard forms.
That will work with either compiler. If you want a quick and dirty fix,
define the file names as macros with the proper expansion, like this:
#define stdio <stdio.h>
@end example
This will work, as long as the name doesn't conflict with anything else
in the program.
Another source of incompatibility is that VAX-C assumes that:
#include "foobar"
@end example
is actually asking for the file @file{foobar.h}. GNU CC does not
make this assumption, and instead takes what you ask for literally;
it tries to read the file @file{foobar}. The best way to avoid this
problem is to always specify the desired file extension in your include
GNU CC for VMS is distributed with a set of include files that is
sufficient to compile most general purpose programs. Even though the
GNU CC distribution does not contain header files to define constants
and structures for some VMS system-specific functions, there is no
reason why you cannot use GNU CC with any of these functions. You first
may have to generate or create header files, either by using the public
domain utility @code{UNSDL} (which can be found on a DECUS tape), or by
extracting the relevant modules from one of the system macro libraries,
and using an editor to construct a C header file.
A @code{#include} file name cannot contain a DECNET node name. The
preprocessor reports an I/O error if you attempt to use a node name,
whether explicitly, or implicitly via a logical name.
@node Global Declarations
@section Global Declarations and VMS
GNU CC does not provide the @code{globalref}, @code{globaldef} and
@code{globalvalue} keywords of VAX-C. You can get the same effect with
an obscure feature of GAS, the GNU assembler. (This requires GAS
version 1.39 or later.) The following macros allow you to use this
feature in a fairly natural way:
#ifdef __GNUC__
asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME)
asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) \
const TYPE NAME[1] \
asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME)
const TYPE NAME[1] \
asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME) \
= @{VALUE@}
globalref TYPE NAME
globaldef TYPE NAME = VALUE
globalvalue TYPE NAME = VALUE
globalvalue TYPE NAME
@end smallexample
(The @code{_$$PsectAttributes_GLOBALSYMBOL} prefix at the start of the
name is removed by the assembler, after it has modified the attributes
of the symbol). These macros are provided in the VMS binaries
distribution in a header file @file{GNU_HACKS.H}. An example of the
usage is:
GLOBALREF (int, ijk);
GLOBALDEF (int, jkl, 0);
@end example
The macros @code{GLOBALREF} and @code{GLOBALDEF} cannot be used
straightforwardly for arrays, since there is no way to insert the array
dimension into the declaration at the right place. However, you can
declare an array with these macros if you first define a typedef for the
array type, like this:
typedef int intvector[10];
GLOBALREF (intvector, foo);
@end example
Array and structure initializers will also break the macros; you can
define the initializer to be a macro of its own, or you can expand the
@code{GLOBALDEF} macro by hand. You may find a case where you wish to
use the @code{GLOBALDEF} macro with a large array, but you are not
interested in explicitly initializing each element of the array. In
such cases you can use an initializer like: @code{@{0,@}}, which will
initialize the entire array to @code{0}.
A shortcoming of this implementation is that a variable declared with
@code{GLOBALVALUEREF} or @code{GLOBALVALUEDEF} is always an array. For
example, the declaration:
@end example
declares the variable @code{ijk} as an array of type @code{int [1]}.
This is done because a globalvalue is actually a constant; its ``value''
is what the linker would normally consider an address. That is not how
an integer value works in C, but it is how an array works. So treating
the symbol as an array name gives consistent results---with the
exception that the value seems to have the wrong type. @strong{Don't
try to access an element of the array.} It doesn't have any elements.
The array ``address'' may not be the address of actual storage.
The fact that the symbol is an array may lead to warnings where the
variable is used. Insert type casts to avoid the warnings. Here is an
example; it takes advantage of the ANSI C feature allowing macros that
expand to use the same name as the macro itself.
GLOBALVALUEREF (int, ss$_normal);
GLOBALVALUEDEF (int, xyzzy,123);
#ifdef __GNUC__
#define ss$_normal ((int) ss$_normal)
#define xyzzy ((int) xyzzy)
@end example
Don't use @code{globaldef} or @code{globalref} with a variable whose
type is an enumeration type; this is not implemented. Instead, make the
variable an integer, and use a @code{globalvaluedef} for each of the
enumeration values. An example of this would be:
#ifdef __GNUC__
GLOBALDEF (int, color, 0);
enum globaldef color @{RED, BLUE, GREEN = 3@};
@end example
@node VMS Misc
@section Other VMS Issues
@cindex exit status and VMS
@cindex return value of @code{main}
@cindex @code{main} and the exit status
GNU CC automatically arranges for @code{main} to return 1 by default if
you fail to specify an explicit return value. This will be interpreted
by VMS as a status code indicating a normal successful completion.
Version 1 of GNU CC did not provide this default.
GNU CC on VMS works only with the GNU assembler, GAS. You need version
1.37 or later of GAS in order to produce value debugging information for
the VMS debugger. Use the ordinary VMS linker with the object files
produced by GAS.
@cindex shared VMS run time system
@cindex @file{VAXCRTL}
Under previous versions of GNU CC, the generated code would occasionally
give strange results when linked to the sharable @file{VAXCRTL} library.
Now this should work.
A caveat for use of @code{const} global variables: the @code{const}
modifier must be specified in every external declaration of the variable
in all of the source files that use that variable. Otherwise the linker
will issue warnings about conflicting attributes for the variable. Your
program will still work despite the warnings, but the variable will be
placed in writable storage.
@cindex name augmentation
@cindex case sensitivity and VMS
@cindex VMS and case sensitivity
Although the VMS linker does distinguish between upper and lower case
letters in global symbols, most VMS compilers convert all such symbols
into upper case and most run-time library routines also have upper case
names. To be able to reliably call such routines, GNU CC (by means of
the assembler GAS) converts global symbols into upper case like other
VMS compilers. However, since the usual practice in C is to distinguish
case, GNU CC (via GAS) tries to preserve usual C behavior by augmenting
each name that is not all lower case. This means truncating the name
to at most 23 characters and then adding more characters at the end
which encode the case pattern of those 23. Names which contain at
least one dollar sign are an exception; they are converted directly into
upper case without augmentation.
Name augmentation yields bad results for programs that use precompiled
libraries (such as Xlib) which were generated by another compiler. You
can use the compiler option @samp{/NOCASE_HACK} to inhibit augmentation;
it makes external C functions and variables case-independent as is usual
on VMS. Alternatively, you could write all references to the functions
and variables in such libraries using lower case; this will work on VMS,
but is not portable to other systems. The compiler option @samp{/NAMES}
also provides control over global name handling.
Function and variable names are handled somewhat differently with GNU
C++. The GNU C++ compiler performs @dfn{name mangling} on function
names, which means that it adds information to the function name to
describe the data types of the arguments that the function takes. One
result of this is that the name of a function can become very long.
Since the VMS linker only recognizes the first 31 characters in a name,
special action is taken to ensure that each function and variable has a
unique name that can be represented in 31 characters.
If the name (plus a name augmentation, if required) is less than 32
characters in length, then no special action is performed. If the name
is longer than 31 characters, the assembler (GAS) will generate a
hash string based upon the function name, truncate the function name to
23 characters, and append the hash string to the truncated name. If the
@samp{/VERBOSE} compiler option is used, the assembler will print both
the full and truncated names of each symbol that is truncated.
The @samp{/NOCASE_HACK} compiler option should not be used when you are
compiling programs that use libg++. libg++ has several instances of
objects (i.e. @code{Filebuf} and @code{filebuf}) which become
indistinguishable in a case-insensitive environment. This leads to
cases where you need to inhibit augmentation selectively (if you were
using libg++ and Xlib in the same program, for example). There is no
special feature for doing this, but you can get the result by defining a
macro for each mixed case symbol for which you wish to inhibit
augmentation. The macro should expand into the lower case equivalent of
itself. For example:
#define StuDlyCapS studlycaps
@end example
These macro definitions can be placed in a header file to minimize the
number of changes to your source code.
@end ifset
@node Portability
@chapter GNU CC and Portability
@cindex portability
@cindex GNU CC and portability
The main goal of GNU CC was to make a good, fast compiler for machines in
the class that the GNU system aims to run on: 32-bit machines that address
8-bit bytes and have several general registers. Elegance, theoretical
power and simplicity are only secondary.
GNU CC gets most of the information about the target machine from a machine
description which gives an algebraic formula for each of the machine's
instructions. This is a very clean way to describe the target. But when
the compiler needs information that is difficult to express in this
fashion, I have not hesitated to define an ad-hoc parameter to the machine
description. The purpose of portability is to reduce the total work needed
on the compiler; it was not of interest for its own sake.
@cindex endianness
@cindex autoincrement addressing, availability
@findex abort
GNU CC does not contain machine dependent code, but it does contain code
that depends on machine parameters such as endianness (whether the most
significant byte has the highest or lowest address of the bytes in a word)
and the availability of autoincrement addressing. In the RTL-generation
pass, it is often necessary to have multiple strategies for generating code
for a particular kind of syntax tree, strategies that are usable for different
combinations of parameters. Often I have not tried to address all possible
cases, but only the common ones or only the ones that I have encountered.
As a result, a new target may require additional strategies. You will know
if this happens because the compiler will call @code{abort}. Fortunately,
the new strategies can be added in a machine-independent fashion, and will
affect only the target machines that need them.
@end ifset
@node Interface
@chapter Interfacing to GNU CC Output
@cindex interfacing to GNU CC output
@cindex run-time conventions
@cindex function call conventions
@cindex conventions, run-time
GNU CC is normally configured to use the same function calling convention
normally in use on the target system. This is done with the
machine-description macros described (@pxref{Target Macros}).
@cindex unions, returning
@cindex structures, returning
@cindex returning structures and unions
However, returning of structure and union values is done differently on
some target machines. As a result, functions compiled with PCC
returning such types cannot be called from code compiled with GNU CC,
and vice versa. This does not cause trouble often because few Unix
library routines return structures or unions.
GNU CC code returns structures and unions that are 1, 2, 4 or 8 bytes
long in the same registers used for @code{int} or @code{double} return
values. (GNU CC typically allocates variables of such types in
registers also.) Structures and unions of other sizes are returned by
storing them into an address passed by the caller (usually in a
register). The machine-description macros @code{STRUCT_VALUE} and
@code{STRUCT_INCOMING_VALUE} tell GNU CC where to pass this address.
By contrast, PCC on most target machines returns structures and unions
of any size by copying the data into an area of static storage, and then
returning the address of that storage as if it were a pointer value.
The caller must copy the data from that memory area to the place where
the value is wanted. This is slower than the method used by GNU CC, and
fails to be reentrant.
On some target machines, such as RISC machines and the 80386, the
standard system convention is to pass to the subroutine the address of
where to return the value. On these machines, GNU CC has been
configured to be compatible with the standard compiler, when this method
is used. It may not be compatible for structures of 1, 2, 4 or 8 bytes.
@cindex argument passing
@cindex passing arguments
GNU CC uses the system's standard convention for passing arguments. On
some machines, the first few arguments are passed in registers; in
others, all are passed on the stack. It would be possible to use
registers for argument passing on any machine, and this would probably
result in a significant speedup. But the result would be complete
incompatibility with code that follows the standard convention. So this
change is practical only if you are switching to GNU CC as the sole C
compiler for the system. We may implement register argument passing on
certain machines once we have a complete GNU system so that we can
compile the libraries with GNU CC.
On some machines (particularly the Sparc), certain types of arguments
are passed ``by invisible reference''. This means that the value is
stored in memory, and the address of the memory location is passed to
the subroutine.
@cindex @code{longjmp} and automatic variables
If you use @code{longjmp}, beware of automatic variables. ANSI C says that
automatic variables that are not declared @code{volatile} have undefined
values after a @code{longjmp}. And this is all GNU CC promises to do,
because it is very difficult to restore register variables correctly, and
one of GNU CC's features is that it can put variables in registers without
your asking it to.
If you want a variable to be unaltered by @code{longjmp}, and you don't
want to write @code{volatile} because old C compilers don't accept it,
just take the address of the variable. If a variable's address is ever
taken, even if just to compute it and ignore it, then the variable cannot
go in a register:
int careful;
@end example
@cindex arithmetic libraries
@cindex math libraries
Code compiled with GNU CC may call certain library routines. Most of
them handle arithmetic for which there are no instructions. This
includes multiply and divide on some machines, and floating point
operations on any machine for which floating point support is disabled
with @samp{-msoft-float}. Some standard parts of the C library, such as
@code{bcopy} or @code{memcpy}, are also called automatically. The usual
function call interface is used for calling the library routines.
These library routines should be defined in the library @file{libgcc.a},
which GNU CC automatically searches whenever it links a program. On
machines that have multiply and divide instructions, if hardware
floating point is in use, normally @file{libgcc.a} is not needed, but it
is searched just in case.
Each arithmetic function is defined in @file{libgcc1.c} to use the
corresponding C arithmetic operator. As long as the file is compiled
with another C compiler, which supports all the C arithmetic operators,
this file will work portably. However, @file{libgcc1.c} does not work if
compiled with GNU CC, because each arithmetic function would compile
into a call to itself!
@end ifset
@node Passes
@chapter Passes and Files of the Compiler
@cindex passes and files of the compiler
@cindex files and passes of the compiler
@cindex compiler passes and files
@cindex top level of compiler
The overall control structure of the compiler is in @file{toplev.c}. This
file is responsible for initialization, decoding arguments, opening and
closing files, and sequencing the passes.
@cindex parsing pass
The parsing pass is invoked only once, to parse the entire input. The RTL
intermediate code for a function is generated as the function is parsed, a
statement at a time. Each statement is read in as a syntax tree and then
converted to RTL; then the storage for the tree for the statement is
reclaimed. Storage for types (and the expressions for their sizes),
declarations, and a representation of the binding contours and how they nest,
remain until the function is finished being compiled; these are all needed
to output the debugging information.
@findex rest_of_compilation
@findex rest_of_decl_compilation
Each time the parsing pass reads a complete function definition or
top-level declaration, it calls either the function
@code{rest_of_compilation}, or the function
@code{rest_of_decl_compilation} in @file{toplev.c}, which are
responsible for all further processing necessary, ending with output of
the assembler language. All other compiler passes run, in sequence,
within @code{rest_of_compilation}. When that function returns from
compiling a function definition, the storage used for that function
definition's compilation is entirely freed, unless it is an inline
@ifset USING
(@pxref{Inline,,An Inline Function is As Fast As a Macro}).
@end ifset
@ifclear USING
(@pxref{Inline,,An Inline Function is As Fast As a Macro,gcc.texi,Using GCC}).
@end ifclear
Here is a list of all the passes of the compiler and their source files.
Also included is a description of where debugging dumps can be requested
with @samp{-d} options.
@itemize @bullet
Parsing. This pass reads the entire text of a function definition,
constructing partial syntax trees. This and RTL generation are no longer
truly separate passes (formerly they were), but it is easier to think
of them as separate.
The tree representation does not entirely follow C syntax, because it is
intended to support other languages as well.
Language-specific data type analysis is also done in this pass, and every
tree node that represents an expression has a data type attached.
Variables are represented as declaration nodes.
@cindex constant folding
@cindex arithmetic simplifications
@cindex simplifications, arithmetic
Constant folding and some arithmetic simplifications are also done
during this pass.
The language-independent source files for parsing are
@file{stor-layout.c}, @file{fold-const.c}, and @file{tree.c}.
There are also header files @file{tree.h} and @file{tree.def}
which define the format of the tree representation.@refill
@c Avoiding overfull is tricky here.
The source files to parse C are
and @file{c-lang.c}
along with header files
@file{c-lex.h}, and
The source files for parsing C++ are @file{cp-parse.y},
@file{cp-cvt.c}, @file{cp-decl.c}, @file{cp-decl2.c},
@file{cp-dem.c}, @file{cp-except.c},@*
@file{cp-expr.c}, @file{cp-init.c}, @file{cp-lex.c},
@file{cp-method.c}, @file{cp-ptree.c},@*
@file{cp-search.c}, @file{cp-tree.c}, @file{cp-type2.c}, and
@file{cp-typeck.c}, along with header files @file{cp-tree.def},
@file{cp-tree.h}, and @file{cp-decl.h}.
The special source files for parsing Objective C are
@file{objc-parse.y}, @file{objc-actions.c}, @file{objc-tree.def}, and
@file{objc-actions.h}. Certain C-specific files are used for this as
The file @file{c-common.c} is also used for all of the above languages.
@cindex RTL generation
RTL generation. This is the conversion of syntax tree into RTL code.
It is actually done statement-by-statement during parsing, but for
most purposes it can be thought of as a separate pass.
@cindex target-parameter-dependent code
This is where the bulk of target-parameter-dependent code is found,
since often it is necessary for strategies to apply only when certain
standard kinds of instructions are available. The purpose of named
instruction patterns is to provide this information to the RTL
generation pass.
@cindex tail recursion optimization
Optimization is done in this pass for @code{if}-conditions that are
comparisons, boolean operations or conditional expressions. Tail
recursion is detected at this time also. Decisions are made about how
best to arrange loops and how to output @code{switch} statements.
@c Avoiding overfull is tricky here.
The source files for RTL generation include
and @file{emit-rtl.c}.
Also, the file
@file{insn-emit.c}, generated from the machine description by the
program @code{genemit}, is used in this pass. The header file
@file{expr.h} is used for communication within this pass.@refill
@findex genflags
@findex gencodes
The header files @file{insn-flags.h} and @file{insn-codes.h},
generated from the machine description by the programs @code{genflags}
and @code{gencodes}, tell this pass which standard names are available
for use and which patterns correspond to them.@refill
Aside from debugging information output, none of the following passes
refers to the tree structure representation of the function (only
part of which is saved).
@cindex inline, automatic
The decision of whether the function can and should be expanded inline
in its subsequent callers is made at the end of rtl generation. The
function must meet certain criteria, currently related to the size of
the function and the types and number of parameters it has. Note that
this function may contain loops, recursive calls to itself
(tail-recursive functions can be inlined!), gotos, in short, all
constructs supported by GNU CC. The file @file{integrate.c} contains
the code to save a function's rtl for later inlining and to inline that
rtl when the function is called. The header file @file{integrate.h}
is also used for this purpose.
The option @samp{-dr} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.rtl} to
the input file name.
@cindex jump optimization
@cindex unreachable code
@cindex dead code
Jump optimization. This pass simplifies jumps to the following
instruction, jumps across jumps, and jumps to jumps. It deletes
unreferenced labels and unreachable code, except that unreachable code
that contains a loop is not recognized as unreachable in this pass.
(Such loops are deleted later in the basic block analysis.) It also
converts some code originally written with jumps into sequences of
instructions that directly set values from the results of comparisons,
if the machine has such instructions.
Jump optimization is performed two or three times. The first time is
immediately following RTL generation. The second time is after CSE,
but only if CSE says repeated jump optimization is needed. The
last time is right before the final pass. That time, cross-jumping
and deletion of no-op move instructions are done together with the
optimizations described above.
The source file of this pass is @file{jump.c}.
The option @samp{-dj} causes a debugging dump of the RTL code after
this pass is run for the first time. This dump file's name is made by
appending @samp{.jump} to the input file name.
@cindex register use analysis
Register scan. This pass finds the first and last use of each
register, as a guide for common subexpression elimination. Its source
is in @file{regclass.c}.
@cindex jump threading
Jump threading. This pass detects a condition jump that branches to an
identical or inverse test. Such jumps can be @samp{threaded} through
the second conditional test. The source code for this pass is in
@file{jump.c}. This optimization is only performed if
@samp{-fthread-jumps} is enabled.
@cindex common subexpression elimination
@cindex constant propagation
Common subexpression elimination. This pass also does constant
propagation. Its source file is @file{cse.c}. If constant
propagation causes conditional jumps to become unconditional or to
become no-ops, jump optimization is run again when CSE is finished.
The option @samp{-ds} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.cse} to
the input file name.
@cindex global common subexpression elimination
@cindex constant propagation
@cindex copy propagation
Global common subexpression elimination. This pass performs GCSE
using Morel-Renvoise Partial Redundancy Elimination, with the exception
that it does not try to move invariants out of loops - that is left to
the loop optimization pass. This pass also performs global constant
and copy propagation.
The source file for this pass is gcse.c.
The option @samp{-dG} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.gcse} to
the input file name.
@cindex loop optimization
@cindex code motion
@cindex strength-reduction
Loop optimization. This pass moves constant expressions out of loops,
and optionally does strength-reduction and loop unrolling as well.
Its source files are @file{loop.c} and @file{unroll.c}, plus the header
@file{loop.h} used for communication between them. Loop unrolling uses
some functions in @file{integrate.c} and the header @file{integrate.h}.
The option @samp{-dL} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.loop} to
the input file name.
If @samp{-frerun-cse-after-loop} was enabled, a second common
subexpression elimination pass is performed after the loop optimization
pass. Jump threading is also done again at this time if it was specified.
The option @samp{-dt} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.cse2} to
the input file name.
@cindex register allocation, stupid
@cindex stupid register allocation
Stupid register allocation is performed at this point in a
nonoptimizing compilation. It does a little data flow analysis as
well. When stupid register allocation is in use, the next pass
executed is the reloading pass; the others in between are skipped.
The source file is @file{stupid.c}.
@cindex data flow analysis
@cindex analysis, data flow
@cindex basic blocks
Data flow analysis (@file{flow.c}). This pass divides the program
into basic blocks (and in the process deletes unreachable loops); then
it computes which pseudo-registers are live at each point in the
program, and makes the first instruction that uses a value point at
the instruction that computed the value.
@cindex autoincrement/decrement analysis
This pass also deletes computations whose results are never used, and
combines memory references with add or subtract instructions to make
autoincrement or autodecrement addressing.
The option @samp{-df} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.flow} to
the input file name. If stupid register allocation is in use, this
dump file reflects the full results of such allocation.
@cindex instruction combination
Instruction combination (@file{combine.c}). This pass attempts to
combine groups of two or three instructions that are related by data
flow into single instructions. It combines the RTL expressions for
the instructions by substitution, simplifies the result using algebra,
and then attempts to match the result against the machine description.
The option @samp{-dc} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.combine}
to the input file name.
@cindex instruction scheduling
@cindex scheduling, instruction
Instruction scheduling (@file{sched.c}). This pass looks for
instructions whose output will not be available by the time that it is
used in subsequent instructions. (Memory loads and floating point
instructions often have this behavior on RISC machines). It re-orders
instructions within a basic block to try to separate the definition and
use of items that otherwise would cause pipeline stalls.
Instruction scheduling is performed twice. The first time is immediately
after instruction combination and the second is immediately after reload.
The option @samp{-dS} causes a debugging dump of the RTL code after this
pass is run for the first time. The dump file's name is made by
appending @samp{.sched} to the input file name.
@cindex register class preference pass
Register class preferencing. The RTL code is scanned to find out
which register class is best for each pseudo register. The source
file is @file{regclass.c}.
@cindex register allocation
@cindex local register allocation
Local register allocation (@file{local-alloc.c}). This pass allocates
hard registers to pseudo registers that are used only within one basic
block. Because the basic block is linear, it can use fast and
powerful techniques to do a very good job.
The option @samp{-dl} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.lreg} to
the input file name.
@cindex global register allocation
Global register allocation (@file{global.c}). This pass
allocates hard registers for the remaining pseudo registers (those
whose life spans are not contained in one basic block).
@cindex reloading
Reloading. This pass renumbers pseudo registers with the hardware
registers numbers they were allocated. Pseudo registers that did not
get hard registers are replaced with stack slots. Then it finds
instructions that are invalid because a value has failed to end up in
a register, or has ended up in a register of the wrong kind. It fixes
up these instructions by reloading the problematical values
temporarily into registers. Additional instructions are generated to
do the copying.
The reload pass also optionally eliminates the frame pointer and inserts
instructions to save and restore call-clobbered registers around calls.
Source files are @file{reload.c} and @file{reload1.c}, plus the header
@file{reload.h} used for communication between them.
The option @samp{-dg} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.greg} to
the input file name.
@cindex instruction scheduling
@cindex scheduling, instruction
Instruction scheduling is repeated here to try to avoid pipeline stalls
due to memory loads generated for spilled pseudo registers.
The option @samp{-dR} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.sched2}
to the input file name.
@cindex cross-jumping
@cindex no-op move instructions
Jump optimization is repeated, this time including cross-jumping
and deletion of no-op move instructions.
The option @samp{-dJ} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.jump2}
to the input file name.
@cindex delayed branch scheduling
@cindex scheduling, delayed branch
Delayed branch scheduling. This optional pass attempts to find
instructions that can go into the delay slots of other instructions,
usually jumps and calls. The source file name is @file{reorg.c}.
The option @samp{-dd} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.dbr}
to the input file name.
@cindex register-to-stack conversion
Conversion from usage of some hard registers to usage of a register
stack may be done at this point. Currently, this is supported only
for the floating-point registers of the Intel 80387 coprocessor. The
source file name is @file{reg-stack.c}.
The options @samp{-dk} causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending @samp{.stack}
to the input file name.
@cindex final pass
@cindex peephole optimization
Final. This pass outputs the assembler code for the function. It is
also responsible for identifying spurious test and compare
instructions. Machine-specific peephole optimizations are performed
at the same time. The function entry and exit sequences are generated
directly as assembler code in this pass; they never exist as RTL.
The source files are @file{final.c} plus @file{insn-output.c}; the
latter is generated automatically from the machine description by the
tool @file{genoutput}. The header file @file{conditions.h} is used
for communication between these files.
@cindex debugging information generation
Debugging information output. This is run after final because it must
output the stack slot offsets for pseudo registers that did not get
hard registers. Source files are @file{dbxout.c} for DBX symbol table
format, @file{sdbout.c} for SDB symbol table format, and
@file{dwarfout.c} for DWARF symbol table format.
@end itemize
Some additional files are used by all or many passes:
@itemize @bullet
Every pass uses @file{machmode.def} and @file{machmode.h} which define
the machine modes.
Several passes use @file{real.h}, which defines the default
representation of floating point constants and how to operate on them.
All the passes that work with RTL use the header files @file{rtl.h}
and @file{rtl.def}, and subroutines in file @file{rtl.c}. The tools
@code{gen*} also use these files to read and work with the machine
description RTL.
@findex genconfig
Several passes refer to the header file @file{insn-config.h} which
contains a few parameters (C macro definitions) generated
automatically from the machine description RTL by the tool
@cindex instruction recognizer
Several passes use the instruction recognizer, which consists of
@file{recog.c} and @file{recog.h}, plus the files @file{insn-recog.c}
and @file{insn-extract.c} that are generated automatically from the
machine description by the tools @file{genrecog} and
Several passes use the header files @file{regs.h} which defines the
information recorded about pseudo register usage, and @file{basic-block.h}
which defines the information recorded about basic blocks.
@file{hard-reg-set.h} defines the type @code{HARD_REG_SET}, a bit-vector
with a bit for each hard register, and some macros to manipulate it.
This type is just @code{int} if the machine has few enough hard registers;
otherwise it is an array of @code{int} and some of the macros expand
into loops.
Several passes use instruction attributes. A definition of the
attributes defined for a particular machine is in file
@file{insn-attr.h}, which is generated from the machine description by
the program @file{genattr}. The file @file{insn-attrtab.c} contains
subroutines to obtain the attribute values for insns. It is generated
from the machine description by the program @file{genattrtab}.@refill
@end itemize
@end ifset
@include rtl.texi
@include md.texi
@include tm.texi
@end ifset
@node Config
@chapter The Configuration File
@cindex configuration file
@cindex @file{xm-@var{machine}.h}
The configuration file @file{xm-@var{machine}.h} contains macro
definitions that describe the machine and system on which the compiler
is running, unlike the definitions in @file{@var{machine}.h}, which
describe the machine for which the compiler is producing output. Most
of the values in @file{xm-@var{machine}.h} are actually the same on all
machines that GNU CC runs on, so large parts of all configuration files
are identical. But there are some macros that vary:
@table @code
@findex USG
@item USG
Define this macro if the host system is System V.
@findex VMS
@item VMS
Define this macro if the host system is VMS.
A C expression for the status code to be returned when the compiler
exits after serious errors.
A C expression for the status code to be returned when the compiler
exits without serious errors.
Defined if the host machine stores words of multi-word values in
big-endian order. (GNU CC does not depend on the host byte ordering
within a word.)
Define this macro to be 1 if the host machine stores @code{DFmode},
@code{XFmode} or @code{TFmode} floating point numbers in memory with the
word containing the sign bit at the lowest address; otherwise, define it
to be zero.
This macro need not be defined if the ordering is the same as for
multi-word integers.
A numeric code distinguishing the floating point format for the host
machine. See @code{TARGET_FLOAT_FORMAT} in @ref{Storage Layout} for the
alternatives and default.
A C expression for the number of bits in @code{char} on the host
A C expression for the number of bits in @code{short} on the host
A C expression for the number of bits in @code{int} on the host
A C expression for the number of bits in @code{long} on the host
Define this macro to indicate that the host compiler only supports
@code{int} bit fields, rather than other integral types, including
@code{enum}, as do most C compilers.
A C expression for the size of ordinary obstack chunks.
If you don't define this, a usually-reasonable default is used.
The function used to allocate obstack chunks.
If you don't define this, @code{xmalloc} is used.
The function used to free obstack chunks.
If you don't define this, @code{free} is used.
@findex USE_C_ALLOCA
Define this macro to indicate that the compiler is running with the
@code{alloca} implemented in C. This version of @code{alloca} can be
found in the file @file{alloca.c}; to use it, you must also alter the
@file{Makefile} variable @code{ALLOCA}. (This is done automatically
for the systems on which we know it is needed.)
If you do define this macro, you should probably do it as follows:
#ifndef __GNUC__
#define USE_C_ALLOCA
#define alloca __builtin_alloca
@end example
so that when the compiler is compiled with GNU CC it uses the more
efficient built-in @code{alloca} function.
Define this macro to indicate that the host compiler does not properly
handle converting a function value to a pointer-to-function when it is
used in an expression.
Define this macro to enable support for multibyte characters in the
input to GNU CC. This requires that the host system support the ANSI C
library functions for converting multibyte characters to wide
@findex POSIX
@item POSIX
Define this if your system is POSIX.1 compliant.
Define this if your system @emph{does not} provide the variable
@vindex sys_siglist
Some systems do provide this variable, but with a different name such
as @code{_sys_siglist}. On these systems, you can define
@code{sys_siglist} as a macro which expands into the name actually
Autoconf normally defines @code{SYS_SIGLIST_DECLARED} when it finds a
declaration of @code{sys_siglist} in the system header files.
However, when you define @code{sys_siglist} to a different name
autoconf will not automatically define @code{SYS_SIGLIST_DECLARED}.
Therefore, if you define @code{sys_siglist}, you should also define
Define this to be 1 if you know that the host compiler supports
prototypes, even if it doesn't define __STDC__, or define
it to be 0 if you do not want any prototypes used in compiling
GNU CC. If @samp{USE_PROTOTYPES} is not defined, it will be
determined automatically whether your compiler supports
prototypes by checking if @samp{__STDC__} is defined.
Define this if you wish suppression of prototypes generated from
the machine description file, but to use other prototypes within
GNU CC. If @samp{USE_PROTOTYPES} is defined to be 0, or the
host compiler does not support prototypes, this macro has no
Define this if you wish to generate prototypes for the
@code{gen_call} or @code{gen_call_value} functions generated from
the machine description file. If @samp{USE_PROTOTYPES} is
defined to be 0, or the host compiler does not support
prototypes, or @samp{NO_MD_PROTOTYPES} is defined, this macro has
no effect. As soon as all of the machine descriptions are
modified to have the appropriate number of arguments, this macro
will be removed.
Define this macro to be a C character constant representing the
character used to separate components in paths. The default value is
the colon character
If your system uses some character other than slash to separate
directory names within a file specification, define this macro to be a C
character constant specifying that character. When GNU CC displays file
names, the character you specify will be used. GNU CC will test for
both slash and the character you specify when parsing filenames.
Define this macro to be a C string representing the suffix for object
files on your machine. If you do not define this macro, GNU CC will use
@samp{.o} as the suffix for object files.
Define this macro to be a C string representing the suffix for executable
files on your machine. If you do not define this macro, GNU CC will use
the null string as the suffix for object files.
If defined, @code{collect2} will scan the individual object files
specified on its command line and create an export list for the linker.
Define this macro for systems like AIX, where the linker discards
object files that are not referenced from @code{main} and uses export
@end table
@findex bzero
@findex bcmp
In addition, configuration files for system V define @code{bcopy},
@code{bzero} and @code{bcmp} as aliases. Some files define @code{alloca}
as a macro when compiled with GNU CC, in order to take advantage of the
benefit of GNU CC's built-in @code{alloca}.
@node Fragments
@chapter Makefile Fragments
@cindex makefile fragment
When you configure GNU CC using the @file{configure} script
(@pxref{Installation}), it will construct the file @file{Makefile} from
the template file @file{}. When it does this, it will
incorporate makefile fragment files from the @file{config} directory,
named @file{t-@var{target}} and @file{x-@var{host}}. If these files do
not exist, it means nothing needs to be added for a given target or
* Target Fragment:: Writing the @file{t-@var{target}} file.
* Host Fragment:: Writing the @file{x-@var{host}} file.
@end menu
@node Target Fragment
@section The Target Makefile Fragment
@cindex target makefile fragment
@cindex @file{t-@var{target}}
The target makefile fragment, @file{t-@var{target}}, defines special
target dependent variables and targets used in the @file{Makefile}:
@table @code
@findex LIBGCC1
@item LIBGCC1
The rule to use to build @file{libgcc1.a}.
If your target does not need to use the functions in @file{libgcc1.a},
set this to empty.
The rule to use to build @file{libgcc1.a} when building a cross
compiler. If your target does not need to use the functions in
@file{libgcc1.a}, set this to empty. @xref{Cross Runtime}.
Compiler flags to use when compiling @file{libgcc2.c}.
A list of source file names to be compiled or assembled and inserted
into @file{libgcc.a}.
Special flags used when compiling @file{crtstuff.c}.
Special flags used when compiling @file{crtstuff.c} for shared
linking. Used if you use @file{crtbeginS.o} and @file{crtendS.o}
in @code{EXTRA-PARTS}.
For some targets, invoking GNU CC in different ways produces objects
that can not be linked together. For example, for some targets GNU CC
produces both big and little endian code. For these targets, you must
arrange for multiple versions of @file{libgcc.a} to be compiled, one for
each set of incompatible options. When GNU CC invokes the linker, it
arranges to link in the right version of @file{libgcc.a}, based on
the command line options used.
The @code{MULTILIB_OPTIONS} macro lists the set of options for which
special versions of @file{libgcc.a} must be built. Write options that
are mutually incompatible side by side, separated by a slash. Write
options that may be used together separated by a space. The build
procedure will build all combinations of compatible options.
For example, if you set @code{MULTILIB_OPTIONS} to @samp{m68000/m68020
msoft-float}, @file{Makefile} will build special versions of
@file{libgcc.a} using the following sets of options: @samp{-m68000},
@samp{-m68020}, @samp{-msoft-float}, @samp{-m68000 -msoft-float}, and
@samp{-m68020 -msoft-float}.
If @code{MULTILIB_OPTIONS} is used, this variable specifies the
directory names that should be used to hold the various libraries.
Write one element in @code{MULTILIB_DIRNAMES} for each element in
@code{MULTILIB_OPTIONS}. If @code{MULTILIB_DIRNAMES} is not used, the
default value will be @code{MULTILIB_OPTIONS}, with all slashes treated
as spaces.
For example, if @code{MULTILIB_OPTIONS} is set to @samp{m68000/m68020
msoft-float}, then the default value of @code{MULTILIB_DIRNAMES} is
@samp{m68000 m68020 msoft-float}. You may specify a different value if
you desire a different set of directory names.
Sometimes the same option may be written in two different ways. If an
option is listed in @code{MULTILIB_OPTIONS}, GNU CC needs to know about
any synonyms. In that case, set @code{MULTILIB_MATCHES} to a list of
items of the form @samp{option=option} to describe all relevant
synonyms. For example, @samp{m68000=mc68000 m68020=mc68020}.
Sometimes when there are multiple sets of @code{MULTILIB_OPTIONS} being
specified, there are combinations that should not be built. In that
case, set @code{MULTILIB_EXCEPTIONS} to be all of the switch exceptions
in shell case syntax that should not be built.
For example, in the PowerPC embedded ABI support, it was not desirable
to build libraries that compiled with the @samp{-mcall-aixdesc} option
and either of the @samp{-mcall-aixdesc} or @samp{-mlittle} options at
the same time, and therefore @code{MULTILIB_EXCEPTIONS} is set to
@code{*mrelocatable/*mcall-aixdesc* *mlittle/*mcall-aixdesc*}.
Sometimes it is desirable that when building multiple versions of
@file{libgcc.a} certain options should always be passed on to the
compiler. In that case, set @code{MULTILIB_EXTRA_OPTS} to be the list
of options to be used for all builds.
@end table
@node Host Fragment
@section The Host Makefile Fragment
@cindex host makefile fragment
@cindex @file{x-@var{host}}
The host makefile fragment, @file{x-@var{host}}, defines special host
dependent variables and targets used in the @file{Makefile}:
@table @code
@findex CC
@item CC
The compiler to use when building the first stage.
@findex CLIB
@item CLIB
Additional host libraries to link with.
@findex OLDCC
@item OLDCC
The compiler to use when building @file{libgcc1.a} for a native
@findex OLDAR
@item OLDAR
The version of @code{ar} to use when building @file{libgcc1.a} for a native
@findex INSTALL
The install program to use.
@end table
@node Funding
@unnumbered Funding Free Software
If you want to have more free software a few years from now, it makes
sense for you to help encourage people to contribute funds for its
development. The most effective approach known is to encourage
commercial redistributors to donate.
Users of free software systems can boost the pace of development by
encouraging for-a-fee distributors to donate part of their selling price
to free software developers---the Free Software Foundation, and others.
The way to convince distributors to do this is to demand it and expect
it from them. So when you compare distributors, judge them partly by
how much they give to free software development. Show distributors
they must compete to be the one who gives the most.
To make this approach work, you must insist on numbers that you can
compare, such as, ``We will donate ten dollars to the Frobnitz project
for each disk sold.'' Don't be satisfied with a vague promise, such as
``A portion of the profits are donated,'' since it doesn't give a basis
for comparison.
Even a precise fraction ``of the profits from this disk'' is not very
meaningful, since creative accounting and unrelated business decisions
can greatly alter what fraction of the sales price counts as profit.
If the price you pay is $50, ten percent of the profit is probably
less than a dollar; it might be a few cents, or nothing at all.
Some redistributors do development work themselves. This is useful too;
but to keep everyone honest, you need to inquire how much they do, and
what kind. Some kinds of development make much more long-term
difference than others. For example, maintaining a separate version of
a program contributes very little; maintaining the standard version of a
program for the whole community contributes much. Easy new ports
contribute little, since someone else would surely do them; difficult
ports such as adding a new CPU to the GNU C compiler contribute more;
major new features or packages contribute the most.
By establishing the idea that supporting further development is ``the
proper thing to do'' when distributing free software for a fee, we can
assure a steady flow of resources into making more free software.
Copyright (C) 1994 Free Software Foundation, Inc.
Verbatim copying and redistribution of this section is permitted
without royalty; alteration is not permitted.
@end display
@node GNU/Linux
@unnumbered Linux and the GNU Project
Many computer users run a modified version of the GNU system every
day, without realizing it. Through a peculiar turn of events, the
version of GNU which is widely used today is more often known as
``Linux'', and many users are not aware of the extent of its
connection with the GNU Project.
There really is a Linux; it is a kernel, and these people are using
it. But you can't use a kernel by itself; a kernel is useful only as
part of a whole system. The system in which Linux is typically used
is a modified variant of the GNU system---in other words, a Linux-based
GNU system.
Many users are not fully aware of the distinction between the kernel,
which is Linux, and the whole system, which they also call ``Linux''.
The ambiguous use of the name doesn't promote understanding.
Programmers generally know that Linux is a kernel. But since they
have generally heard the whole system called ``Linux'' as well, they
often envisage a history which fits that name. For example, many
believe that once Linus Torvalds finished writing the kernel, his
friends looked around for other free software, and for no particular
reason most everything necessary to make a Unix-like system was
already available.
What they found was no accident---it was the GNU system. The available
free software added up to a complete system because the GNU Project
had been working since 1984 to make one. The GNU Manifesto
had set forth the goal of developing a free Unix-like system, called
GNU. By the time Linux was written, the system was almost finished.
Most free software projects have the goal of developing a particular
program for a particular job. For example, Linus Torvalds set out to
write a Unix-like kernel (Linux); Donald Knuth set out to write a text
formatter (TeX); Bob Scheifler set out to develop a window system (X
Windows). It's natural to measure the contribution of this kind of
project by specific programs that came from the project.
If we tried to measure the GNU Project's contribution in this way,
what would we conclude? One CD-ROM vendor found that in their ``Linux
distribution'', GNU software was the largest single contingent, around
28% of the total source code, and this included some of the essential
major components without which there could be no system. Linux itself
was about 3%. So if you were going to pick a name for the system
based on who wrote the programs in the system, the most appropriate
single choice would be ``GNU''.
But we don't think that is the right way to consider the question.
The GNU Project was not, is not, a project to develop specific
software packages. It was not a project to develop a C compiler,
although we did. It was not a project to develop a text editor,
although we developed one. The GNU Project's aim was to develop
@emph{a complete free Unix-like system}.
Many people have made major contributions to the free software in the
system, and they all deserve credit. But the reason it is @emph{a
system}---and not just a collection of useful programs---is because the
GNU Project set out to make it one. We wrote the programs that were
needed to make a @emph{complete} free system. We wrote essential but
unexciting major components, such as the assembler and linker, because
you can't have a system without them. A complete system needs more
than just programming tools, so we wrote other components as well,
such as the Bourne Again SHell, the PostScript interpreter
Ghostscript, and the GNU C library.
By the early 90s we had put together the whole system aside from the
kernel (and we were also working on a kernel, the GNU Hurd, which runs
on top of Mach). Developing this kernel has been a lot harder than we
expected, and we are still working on finishing it.
Fortunately, you don't have to wait for it, because Linux is working
now. When Linus Torvalds wrote Linux, he filled the last major gap.
People could then put Linux together with the GNU system to make a
complete free system: a Linux-based GNU system (or GNU/Linux system,
for short).
Putting them together sounds simple, but it was not a trivial job.
The GNU C library (called glibc for short) needed substantial changes.
Integrating a complete system as a distribution that would work ``out
of the box'' was a big job, too. It required addressing the issue of
how to install and boot the system---a problem we had not tackled,
because we hadn't yet reached that point. The people who developed
the various system distributions made a substantial contribution.
The GNU Project supports GNU/Linux systems as well as @emph{the}
GNU system---even with funds. We funded the rewriting of the
Linux-related extensions to the GNU C library, so that now they are
well integrated, and the newest GNU/Linux systems use the current
library release with no changes. We also funded an early stage of the
development of Debian GNU/Linux.
We use Linux-based GNU systems today for most of our work, and we hope
you use them too. But please don't confuse the public by using the
name ``Linux'' ambiguously. Linux is the kernel, one of the essential
major components of the system. The system as a whole is more or less
the GNU system.
@node Copying
@center Version 2, June 1991
Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc.
59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
@end display
@unnumberedsec Preamble
The licenses for most software are designed to take away your
freedom to share and change it. By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change free
software---to make sure the software is free for all its users. This
General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit to
using it. (Some other Free Software Foundation software is covered by
the GNU Library General Public License instead.) You can apply it to
your programs, too.
When we speak of free software, we are referring to freedom, not
price. Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
this service if you wish), that you receive source code or can get it
if you want it, that you can change the software or use pieces of it
in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid
anyone to deny you these rights or to ask you to surrender the rights.
These restrictions translate to certain responsibilities for you if you
distribute copies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether
gratis or for a fee, you must give the recipients all the rights that
you have. You must make sure that they, too, receive or can get the
source code. And you must show them these terms so they know their
We protect your rights with two steps: (1) copyright the software, and
(2) offer you this license which gives you legal permission to copy,
distribute and/or modify the software.
Also, for each author's protection and ours, we want to make certain
that everyone understands that there is no warranty for this free
software. If the software is modified by someone else and passed on, we
want its recipients to know that what they have is not the original, so
that any problems introduced by others will not reflect on the original
authors' reputations.
Finally, any free program is threatened constantly by software
patents. We wish to avoid the danger that redistributors of a free
program will individually obtain patent licenses, in effect making the
program proprietary. To prevent this, we have made it clear that any
patent must be licensed for everyone's free use or not licensed at all.
The precise terms and conditions for copying, distribution and
modification follow.
@end iftex
@end ifinfo
@enumerate 0
This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License. The ``Program'', below,
refers to any such program or work, and a ``work based on the Program''
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another
language. (Hereinafter, translation is included without limitation in
the term ``modification''.) Each licensee is addressed as ``you''.
Activities other than copying, distribution and modification are not
covered by this License; they are outside its scope. The act of
running the Program is not restricted, and the output from the Program
is covered only if its contents constitute a work based on the
Program (independent of having been made by running the Program).
Whether that is true depends on what the Program does.
You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any warranty;
and give any other recipients of the Program a copy of this License
along with the Program.
You may charge a fee for the physical act of transferring a copy, and
you may at your option offer warranty protection in exchange for a fee.
You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:
@enumerate a
You must cause the modified files to carry prominent notices
stating that you changed the files and the date of any change.