GCC(1) GNU Tools GCC(1)NAME
gcc, g++ - GNU project C and C++ Compiler (gcc-2.95)
SYNOPSISgcc [ option | filename ]...
g++ [ option | filename ]...
WARNING
The information in this man page is an extract from the full documenta‐
tion of the GNU C compiler, and is limited to the meaning of the
options.
This man page is not kept up to date except when volunteers want to
maintain it. If you find a discrepancy between the man page and the
software, please check the Info file, which is the authoritative docu‐
mentation.
If we find that the things in this man page that are out of date cause
significant confusion or complaints, we will stop distributing the man
page. The alternative, updating the man page when we update the Info
file, is impossible because the rest of the work of maintaining GNU CC
leaves us no time for that. The GNU project regards man pages as obso‐
lete and should not let them take time away from other things.
For complete and current documentation, refer to the Info file `gcc' or
the manual Using and Porting GNU CC (for version 2.0). Both are made
from the Texinfo source file gcc.texinfo.
DESCRIPTION
The C and C++ compilers are integrated. Both process input files
through one or more of four stages: preprocessing, compilation, assem‐
bly, and linking. Source filename suffixes identify the source lan‐
guage, but which name you use for the compiler governs default assump‐
tions:
gcc assumes preprocessed (.i) files are C and assumes C style link‐
ing.
g++ assumes preprocessed (.i) files are C++ and assumes C++ style
linking.
Suffixes of source file names indicate the language and kind of pro‐
cessing to be done:
.c C source; preprocess, compile, assemble
.C C++ source; preprocess, compile, assemble
.cc C++ source; preprocess, compile, assemble
.cxx C++ source; preprocess, compile, assemble
.m Objective-C source; preprocess, compile, assemble
.i preprocessed C; compile, assemble
.ii preprocessed C++; compile, assemble
.s Assembler source; assemble
.S Assembler source; preprocess, assemble
.h Preprocessor file; not usually named on command line
Files with other suffixes are passed to the linker. Common cases
include:
.o Object file
.a Archive file
Linking is always the last stage unless you use one of the -c, -S, or
-E options to avoid it (or unless compilation errors stop the whole
process). For the link stage, all .o files corresponding to source
files, -l libraries, unrecognized filenames (including named .o object
files and .a archives) are passed to the linker in command-line order.
OPTIONS
Options must be separate: `-dr' is quite different from `-d -r '.
Most `-f' and `-W' options have two contrary forms: -fname and
-fno-name (or -Wname and -Wno-name). Only the non-default forms are
shown here.
Here is a summary of all the options, grouped by type. Explanations
are in the following sections.
Overall Options
-c -S -E -o file -pipe -v -x language
Language Options
-ansi -fall-virtual -fcond-mismatch -fdollars-in-identifiers
-fenum-int-equiv -fexternal-templates -fno-asm -fno-builtin
-fhosted -fno-hosted -ffreestanding -fno-freestanding
-fno-strict-prototype -fsigned-bitfields -fsigned-char
-fthis-is-variable -funsigned-bitfields -funsigned-char
-fwritable-strings -traditional -traditional-cpp -trigraphs
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -W -Wall
-Waggregate-return -Wcast-align -Wcast-qual -Wchar-subscript
-Wcomment -Wconversion -Wenum-clash -Werror -Wformat
-Wid-clash-len -Wimplicit -Wimplicit-int
-Wimplicit-function-declaration -Winline -Wlong-long -Wmain
-Wmissing-prototypes -Wmissing-declarations -Wnested-externs
-Wno-import -Wparentheses -Wpointer-arith -Wredundant-decls
-Wreturn-type -Wshadow -Wstrict-prototypes -Wswitch
-Wtemplate-debugging -Wtraditional -Wtrigraphs -Wuninitialized
-Wunused -Wwrite-strings
Debugging Options
-a -dletters -fpretend-float -g -glevel -gcoff -gxcoff -gxcoff+
-gdwarf -gdwarf+ -gstabs -gstabs+ -ggdb -p -pg -save-temps
-print-file-name=library -print-libgcc-file-name
-print-prog-name=program
Optimization Options
-fcaller-saves -fcse-follow-jumps -fcse-skip-blocks
-fdelayed-branch -felide-constructors -fexpensive-optimizations
-ffast-math -ffloat-store -fforce-addr -fforce-mem
-finline-functions -fkeep-inline-functions -fmemoize-lookups
-fno-default-inline -fno-defer-pop -fno-function-cse -fno-inline
-fno-peephole -fomit-frame-pointer -frerun-cse-after-loop
-fschedule-insns -fschedule-insns2 -fstrength-reduce
-fthread-jumps -funroll-all-loops -funroll-loops -O -O2 -O3
Preprocessor Options
-Aassertion -C -dD -dM -dN -Dmacro[=defn] -E -H -idirafter dir
-include file -imacros file -iprefix file -iwithprefix dir -M
-MD -MM -MMD -nostdinc -P -Umacro -undef
Assembler Option
-Wa,option
Linker Options
-llibrary -nostartfiles -nostdlib -static -shared -symbolic
-Xlinker option -Wl,option -u symbol
Directory Options
-Bprefix -Idir -I- -Ldir
Target Options
-b machine -V version
Configuration Dependent Options
M680x0 Options
-m68000 -m68020 -m68020-40 -m68030 -m68040 -m68881 -mbitfield
-mc68000 -mc68020 -mfpa -mnobitfield -mrtd -mshort -msoft-float
VAX Options
-mg -mgnu -munix
SPARC Options
-mepilogue -mfpu -mhard-float -mno-fpu -mno-epilogue
-msoft-float -msparclite -mv8 -msupersparc -mcypress
Convex Options
-margcount -mc1 -mc2 -mnoargcount
AMD29K Options
-m29000 -m29050 -mbw -mdw -mkernel-registers -mlarge -mnbw
-mnodw -msmall -mstack-check -muser-registers
M88K Options
-m88000 -m88100 -m88110 -mbig-pic -mcheck-zero-division
-mhandle-large-shift -midentify-revision
-mno-check-zero-division -mno-ocs-debug-info
-mno-ocs-frame-position -mno-optimize-arg-area
-mno-serialize-volatile -mno-underscores -mocs-debug-info
-mocs-frame-position -moptimize-arg-area -mserialize-volatile
-mshort-data-num -msvr3 -msvr4 -mtrap-large-shift
-muse-div-instruction -mversion-03.00 -mwarn-passed-structs
RS6000 Options
-mfp-in-toc -mno-fop-in-toc
RT Options
-mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs
-mfull-fp-blocks -mhc-struct-return -min-line-mul
-mminimum-fp-blocks -mnohc-struct-return
MIPS Options
-mcpu=cpu type -mips2 -mips3 -mint64 -mlong64 -mlonglong128
-mmips-as -mgas -mrnames -mno-rnames -mgpopt -mno-gpopt -mstats
-mno-stats -mmemcpy -mno-memcpy -mno-mips-tfile -mmips-tfile
-msoft-float -mhard-float -mabicalls -mno-abicalls -mhalf-pic
-mno-half-pic -G num -nocpp
i386 Options
-m486 -mno-486 -msoft-float -mno-fp-ret-in-387
HPPA Options
-mpa-risc-1-0 -mpa-risc-1-1 -mkernel -mshared-libs
-mno-shared-libs -mlong-calls -mdisable-fpregs
-mdisable-indexing -mtrailing-colon
i960 Options
-mcpu-type -mnumerics -msoft-float -mleaf-procedures
-mno-leaf-procedures -mtail-call -mno-tail-call -mcomplex-addr
-mno-complex-addr -mcode-align -mno-code-align -mic-compat
-mic2.0-compat -mic3.0-compat -masm-compat -mintel-asm
-mstrict-align -mno-strict-align -mold-align -mno-old-align
DEC Alpha Options
-mfp-regs -mno-fp-regs -mno-soft-float -msoft-float
System V Options
-G -Qy -Qn -YP,paths -Ym,dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg
-finhibit-size-directive -fnonnull-objects -fno-common
-fno-ident -fno-gnu-linker -fpcc-struct-return -fpic -fPIC
-freg-struct-return -fshared-data -fshort-enums -fshort-double
-fvolatile -fvolatile-global -fverbose-asm
OVERALL OPTIONS-x language
Specify explicitly the language for the following input files
(rather than choosing a default based on the file name suffix) .
This option applies to all following input files until the next
`-x' option. Possible values of language are `c', `objec‐
tive-c', `c-header', `c++', `cpp-output', `assembler', and `as‐
sembler-with-cpp'.
-x none
Turn off any specification of a language, so that subsequent
files are handled according to their file name suffixes (as they
are if `-x' has not been used at all).
If you want only some of the four stages (preprocess, compile, assem‐
ble, link), you can use `-x' (or filename suffixes) to tell gcc where
to start, and one of the options `-c', `-S', or `-E' to say where gcc
is to stop. Note that some combinations (for example, `-x cpp-output
-E') instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The com‐
piler output is an object file corresponding to each source
file.
By default, GCC makes the object file name for a source file by
replacing the suffix `.c', `.i', `.s', etc., with `.o'. Use -o
to select another name.
GCC ignores any unrecognized input files (those that do not re‐
quire compilation or assembly) with the -c option.
-S Stop after the stage of compilation proper; do not assemble.
The output is an assembler code file for each non-assembler in‐
put file specified.
By default, GCC makes the assembler file name for a source file
by replacing the suffix `.c', `.i', etc., with `.s'. Use -o to
select another name.
GCC ignores any input files that don't require compilation.
-E Stop after the preprocessing stage; do not run the compiler
proper. The output is preprocessed source code, which is sent
to the standard output.
GCC ignores input files which don't require preprocessing.
-o file
Place output in file file. This applies regardless to whatever
sort of output GCC is producing, whether it be an executable
file, an object file, an assembler file or preprocessed C code.
Since only one output file can be specified, it does not make
sense to use `-o' when compiling more than one input file, un‐
less you are producing an executable file as output.
If you do not specify `-o', the default is to put an executable
file in `a.out', the object file for `source.suffix' in
`source.o', its assembler file in `source.s', and all prepro‐
cessed C source on standard output.
-v Print (on standard error output) the commands executed to run
the stages of compilation. Also print the version number of the
compiler driver program and of the preprocessor and the compiler
proper.
-pipe Use pipes rather than temporary files for communication between
the various stages of compilation. This fails to work on some
systems where the assembler cannot read from a pipe; but the GNU
assembler has no trouble.
LANGUAGE OPTIONS
The following options control the dialect of C that the compiler ac‐
cepts:
-ansi Support all ANSI standard C programs.
This turns off certain features of GNU C that are incompatible
with ANSI C, such as the asm, inline and typeof keywords, and
predefined macros such as unix and vax that identify the type of
system you are using. It also enables the undesirable and
rarely used ANSI trigraph feature, and disallows `$' as part of
identifiers.
The alternate keywords __asm__, __extension__, __inline__ and
__typeof__ continue to work despite `-ansi'. You would not want
to use them in an ANSI C program, of course, but it is useful to
put them in header files that might be included in compilations
done with `-ansi'. Alternate predefined macros such as __unix__
and __vax__ are also available, with or without `-ansi'.
The `-ansi' option does not cause non-ANSI programs to be re‐
jected gratuitously. For that, `-pedantic' is required in addi‐
tion to `-ansi'.
The preprocessor predefines a macro __STRICT_ANSI__ when you use
the `-ansi' option. Some header files may notice this macro and
refrain from declaring certain functions or defining certain
macros that the ANSI standard doesn't call for; this is to avoid
interfering with any programs that might use these names for
other things.
-fno-asm
Do not recognize asm, inline or typeof as a keyword. These
words may then be used as identifiers. You can use __asm__,
__inline__ and __typeof__ instead. `-ansi' implies `-fno-asm'.
-fno-builtin
Don't recognize built-in functions that do not begin with two
leading underscores. Currently, the functions affected include
_exit, abort, abs, alloca, cos, exit, fabs, labs, memcmp, mem‐
cpy, sin, sqrt, strcmp, strcpy, and strlen.
The `-ansi' option prevents alloca and _exit from being builtin
functions.
-fhosted
Compile for a hosted environment; this implies the `-fbuiltin'
option, and implies that suspicious declarations of main should
be warned about.
-ffreestanding
Compile for a freestanding environment; this implies the `-fno-
builtin' option, and implies that main has no special require‐
ments.
-fno-strict-prototype
Treat a function declaration with no arguments, such as `int foo
();', as C would treat it—as saying nothing about the number of
arguments or their types (C++ only). Normally, such a declara‐
tion in C++ means that the function foo takes no arguments.
-trigraphs
Support ANSI C trigraphs. The `-ansi' option implies `-tri‐
graphs'.
-traditional
Attempt to support some aspects of traditional C compilers. For
details, see the GNU C Manual; the duplicate list here has been
deleted so that we won't get complaints when it is out of date.
But one note about C++ programs only (not C). `-traditional'
has one additional effect for C++: assignment to this is permit‐
ted. This is the same as the effect of `-fthis-is-variable'.
-traditional-cpp
Attempt to support some aspects of traditional C preprocessors.
This includes the items that specifically mention the preproces‐
sor above, but none of the other effects of `-traditional'.
-fdollars-in-identifiers
Permit the use of `$' in identifiers (C++ only). You can also
use `-fno-dollars-in-identifiers' to explicitly prohibit use of
`$'. (GNU C++ allows `$' by default on some target systems but
not others.)
-fenum-int-equiv
Permit implicit conversion of int to enumeration types (C++ on‐
ly). Normally GNU C++ allows conversion of enum to int, but not
the other way around.
-fexternal-templates
Produce smaller code for template declarations, by generating
only a single copy of each template function where it is defined
(C++ only). To use this option successfully, you must also mark
all files that use templates with either `#pragma implementa‐
tion' (the definition) or `#pragma interface' (declarations).
When your code is compiled with `-fexternal-templates', all tem‐
plate instantiations are external. You must arrange for all
necessary instantiations to appear in the implementation file;
you can do this with a typedef that references each instantia‐
tion needed. Conversely, when you compile using the default op‐
tion `-fno-external-templates', all template instantiations are
explicitly internal.
-fall-virtual
Treat all possible member functions as virtual, implicitly. All
member functions (except for constructor functions and new or
delete member operators) are treated as virtual functions of the
class where they appear.
This does not mean that all calls to these member functions will
be made through the internal table of virtual functions. Under
some circumstances, the compiler can determine that a call to a
given virtual function can be made directly; in these cases the
calls are direct in any case.
-fcond-mismatch
Allow conditional expressions with mismatched types in the sec‐
ond and third arguments. The value of such an expression is
void.
-fthis-is-variable
Permit assignment to this (C++ only). The incorporation of us‐
er-defined free store management into C++ has made assignment to
`this' an anachronism. Therefore, by default it is invalid to
assign to this within a class member function. However, for
backwards compatibility, you can make it valid with `-fthis-is-
variable'.
-funsigned-char
Let the type char be unsigned, like unsigned char.
Each kind of machine has a default for what char should be. It
is either like unsigned char by default or like signed char by
default.
Ideally, a portable program should always use signed char or un‐
signed char when it depends on the signedness of an object. But
many programs have been written to use plain char and expect it
to be signed, or expect it to be unsigned, depending on the ma‐
chines they were written for. This option, and its inverse, let
you make such a program work with the opposite default.
The type char is always a distinct type from each of signed char
and unsigned char, even though its behavior is always just like
one of those two.
-fsigned-char
Let the type char be signed, like signed char.
Note that this is equivalent to `-fno-unsigned-char', which is
the negative form of `-funsigned-char'. Likewise,
`-fno-signed-char' is equivalent to `-funsigned-char'.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bitfield is signed or unsigned,
when declared with no explicit `signed' or `unsigned' qualifier.
By default, such a bitfield is signed, because this is consis‐
tent: the basic integer types such as int are signed types.
However, when you specify `-traditional', bitfields are all un‐
signed no matter what.
-fwritable-strings
Store string constants in the writable data segment and don't
uniquize them. This is for compatibility with old programs
which assume they can write into string constants. `-tradition‐
al' also has this effect.
Writing into string constants is a very bad idea; “constants”
should be constant.
PREPROCESSOR OPTIONS
These options control the C preprocessor, which is run on each C source
file before actual compilation.
If you use the `-E' option, GCC does nothing except preprocessing.
Some of these options make sense only together with `-E' because they
cause the preprocessor output to be unsuitable for actual compilation.
-include file
Process file as input before processing the regular input file.
In effect, the contents of file are compiled first. Any `-D'
and `-U' options on the command line are always processed before
`-include file', regardless of the order in which they are writ‐
ten. All the `-include' and `-imacros' options are processed in
the order in which they are written.
-imacros file
Process file as input, discarding the resulting output, before
processing the regular input file. Because the output generated
from file is discarded, the only effect of `-imacros file' is to
make the macros defined in file available for use in the main
input. The preprocessor evaluates any `-D' and `-U' options on
the command line before processing `-imacrosfile', regardless of
the order in which they are written. All the `-include' and
`-imacros' options are processed in the order in which they are
written.
-idirafter dir
Add the directory dir to the second include path. The directo‐
ries on the second include path are searched when a header file
is not found in any of the directories in the main include path
(the one that `-I' adds to).
-iprefix prefix
Specify prefix as the prefix for subsequent `-iwithprefix' op‐
tions.
-iwithprefix dir
Add a directory to the second include path. The directory's
name is made by concatenating prefix and dir, where prefix was
specified previously with `-iprefix'.
-nostdinc
Do not search the standard system directories for header files.
Only the directories you have specified with `-I' options (and
the current directory, if appropriate) are searched.
By using both `-nostdinc' and `-I-', you can limit the include-
file search file to only those directories you specify explicit‐
ly.
-nostdinc++
Do not search for header files in the C++-specific standard di‐
rectories, but do still search the other standard directories.
(This option is used when building `libg++'.)
-undef Do not predefine any nonstandard macros. (Including architec‐
ture flags).
-E Run only the C preprocessor. Preprocess all the C source files
specified and output the results to standard output or to the
specified output file.
-C Tell the preprocessor not to discard comments. Used with the
`-E' option.
-P Tell the preprocessor not to generate `#line' commands. Used
with the `-E' option.
-M [ -MG ]
Tell the preprocessor to output a rule suitable for make de‐
scribing the dependencies of each object file. For each source
file, the preprocessor outputs one make-rule whose target is the
object file name for that source file and whose dependencies are
all the files `#include'd in it. This rule may be a single line
or may be continued with `\'-newline if it is long. The list of
rules is printed on standard output instead of the preprocessed
C program.
`-M' implies `-E'.
`-MG' says to treat missing header files as generated files and
assume they live in the same directory as the source file. It
must be specified in addition to `-M'.
-MM [ -MG ]
Like `-M' but the output mentions only the user header files in‐
cluded with `#include file"'. System header files included with
`#include <file>' are omitted.
-MD Like `-M' but the dependency information is written to files
with names made by replacing `.o' with `.d' at the end of the
output file names. This is in addition to compiling the file as
specified—`-MD' does not inhibit ordinary compilation the way
`-M' does.
The Mach utility `md' can be used to merge the `.d' files into a
single dependency file suitable for using with the `make' com‐
mand.
-MMD Like `-MD' except mention only user header files, not system
header files.
-H Print the name of each header file used, in addition to other
normal activities.
-Aquestion(answer)
Assert the answer answer for question, in case it is tested with
a preprocessor conditional such as `#if #question(answer)'.
`-A-' disables the standard assertions that normally describe
the target machine.
-Aquestion
(answer) Assert the answer answer for question, in case it is
tested with a preprocessor conditional such as `#if #ques‐
tion(answer)'. `-A-' disables the standard assertions that nor‐
mally describe the target machine.
-Dmacro
Define macro macro with the string `1' as its definition.
-Dmacro=defn
Define macro macro as defn. All instances of `-D' on the com‐
mand line are processed before any `-U' options.
-Umacro
Undefine macro macro. `-U' options are evaluated after all `-D'
options, but before any `-include' and `-imacros' options.
-dM Tell the preprocessor to output only a list of the macro defini‐
tions that are in effect at the end of preprocessing. Used with
the `-E' option.
-dD Tell the preprocessor to pass all macro definitions into the
output, in their proper sequence in the rest of the output.
-dN Like `-dD' except that the macro arguments and contents are
omitted. Only `#define name' is included in the output.
ASSEMBLER OPTION-Wa,option
Pass option as an option to the assembler. If option contains
commas, it is split into multiple options at the commas.
LINKER OPTIONS
These options come into play when the compiler links object files into
an executable output file. They are meaningless if the compiler is not
doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is
considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If GCC does a link step, these object files are used
as input to the linker.
-llibrary
Use the library named library when linking.
The linker searches a standard list of directories for the li‐
brary, which is actually a file named `liblibrary.a'. The link‐
er then uses this file as if it had been specified precisely by
name.
The directories searched include several standard system direc‐
tories plus any that you specify with `-L'.
Normally the files found this way are library files—archive
files whose members are object files. The linker handles an ar‐
chive file by scanning through it for members which define sym‐
bols that have so far been referenced but not defined. However,
if the linker finds an ordinary object file rather than a li‐
brary, the object file is linked in the usual fashion. The only
difference between using an `-l' option and specifying a file
name is that `-l' surrounds library with `lib' and `.a' and
searches several directories.
-lobjc You need this special case of the -l option in order to link an
Objective C program.
-nostartfiles
Do not use the standard system startup files when linking. The
standard libraries are used normally.
-nostdlib
Don't use the standard system libraries and startup files when
linking. Only the files you specify will be passed to the link‐
er.
-static
On systems that support dynamic linking, this prevents linking
with the shared libraries. On other systems, this option has no
effect.
-shared
Produce a shared object which can then be linked with other ob‐
jects to form an executable. Only a few systems support this
option.
-symbolic
Bind references to global symbols when building a shared object.
Warn about any unresolved references (unless overridden by the
link editor option `-Xlinker -z -Xlinker defs'). Only a few
systems support this option.
-Xlinker option
Pass option as an option to the linker. You can use this to
supply system-specific linker options which GNU CC does not know
how to recognize.
If you want to pass an option that takes an argument, you must
use `-Xlinker' twice, once for the option and once for the argu‐
ment. For example, to pass `-assert definitions', you must
write `-Xlinker -assert -Xlinker definitions'. It does not work
to write `-Xlinker "-assert definitions"', because this passes
the entire string as a single argument, which is not what the
linker expects.
-Wl,option
Pass option as an option to the linker. If option contains com‐
mas, it is split into multiple options at the commas.
-u symbol
Pretend the symbol symbol is undefined, to force linking of li‐
brary modules to define it. You can use `-u' multiple times
with different symbols to force loading of additional library
modules.
DIRECTORY OPTIONS
These options specify directories to search for header files, for li‐
braries and for parts of the compiler:
-Idir Append directory dir to the list of directories searched for in‐
clude files.
-I- Any directories you specify with `-I' options before the `-I-'
option are searched only for the case of `#include "file"'; they
are not searched for `#include <file>'.
If additional directories are specified with `-I' options after
the `-I-', these directories are searched for all `#include' di‐
rectives. (Ordinarily all `-I' directories are used this way.)
In addition, the `-I-' option inhibits the use of the current
directory (where the current input file came from) as the first
search directory for `#include "file"'. There is no way to
override this effect of `-I-'. With `-I.' you can specify
searching the directory which was current when the compiler was
invoked. That is not exactly the same as what the preprocessor
does by default, but it is often satisfactory.
`-I-' does not inhibit the use of the standard system directo‐
ries for header files. Thus, `-I-' and `-nostdinc' are indepen‐
dent.
-Ldir Add directory dir to the list of directories to be searched for
`-l'.
-Bprefix
This option specifies where to find the executables, libraries
and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms
`cpp', `cc1' (or, for C++, `cc1plus'), `as' and `ld'. It tries
prefix as a prefix for each program it tries to run, both with
and without `machine/version/'.
For each subprogram to be run, the compiler driver first tries
the `-B' prefix, if any. If that name is not found, or if `-B'
was not specified, the driver tries two standard prefixes, which
are `/usr/lib/gcc/' and `/usr/local/lib/gcc-lib/'. If neither
of those results in a file name that is found, the compiler
driver searches for the unmodified program name, using the di‐
rectories specified in your `PATH' environment variable.
The run-time support file `libgcc.a' is also searched for using
the `-B' prefix, if needed. If it is not found there, the two
standard prefixes above are tried, and that is all. The file is
left out of the link if it is not found by those means. Most of
the time, on most machines, `libgcc.a' is not actually neces‐
sary.
You can get a similar result from the environment variable
GCC_EXEC_PREFIX; if it is defined, its value is used as a prefix
in the same way. If both the `-B' option and the GCC_EXEC_PRE‐
FIX variable are present, the `-B' option is used first and the
environment variable value second.
WARNING OPTIONS
Warnings are diagnostic messages that report constructions which are
not inherently erroneous but which are risky or suggest there may have
been an error.
These options control the amount and kinds of warnings produced by GNU
CC:
-fsyntax-only
Check the code for syntax errors, but don't emit any output.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-pedantic
Issue all the warnings demanded by strict ANSI standard C; re‐
ject all programs that use forbidden extensions.
Valid ANSI standard C programs should compile properly with or
without this option (though a rare few will require `-ansi').
However, without this option, certain GNU extensions and tradi‐
tional C features are supported as well. With this option, they
are rejected. There is no reason to use this option; it exists
only to satisfy pedants.
`-pedantic' does not cause warning messages for use of the al‐
ternate keywords whose names begin and end with `__'. Pedantic
warnings are also disabled in the expression that follows __ex‐
tension__. However, only system header files should use these
escape routes; application programs should avoid them.
-pedantic-errors
Like `-pedantic', except that errors are produced rather than
warnings.
-W Print extra warning messages for these events:
· A nonvolatile automatic variable might be changed by a call to
longjmp. These warnings are possible only in optimizing compi‐
lation.
The compiler sees only the calls to setjmp. It cannot know
where longjmp will be called; in fact, a signal handler could
call it at any point in the code. As a result, you may get a
warning even when there is in fact no problem because longjmp
cannot in fact be called at the place which would cause a prob‐
lem.
· A function can return either with or without a value. (Falling
off the end of the function body is considered returning without
a value.) For example, this function would evoke such a warn‐
ing:
foo (a)
{
if (a > 0)
return a;
}
Spurious warnings can occur because GNU CC does not realize that
certain functions (including abort and longjmp) will never re‐
turn.
· An expression-statement or the left-hand side of a comma expres‐
sion contains no side effects. To suppress the warning, cast
the unused expression to void. For example, an expression such
as `x[i,j]' will cause a warning, but `x[(void)i,j]' will not.
· An unsigned value is compared against zero with `>' or `<='.
-Wimplicit-int
Warn whenever a declaration does not specify a type.
-Wimplicit-function-declaration
Warn whenever a function is used before being declared.
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration.
-Wmain Warn if the main function is declared or defined with a suspi‐
cious type. Typically, it is a function with external linkage,
returning int, and taking zero or two arguments.
-Wreturn-type
Warn whenever a function is defined with a return-type that de‐
faults to int. Also warn about any return statement with no re‐
turn-value in a function whose return-type is not void.
-Wunused
Warn whenever a local variable is unused aside from its declara‐
tion, whenever a function is declared static but never defined,
and whenever a statement computes a result that is explicitly
not used.
-Wswitch
Warn whenever a switch statement has an index of enumeral type
and lacks a case for one or more of the named codes of that enu‐
meration. (The presence of a default label prevents this warn‐
ing.) case labels outside the enumeration range also provoke
warnings when this option is used.
-Wcomment
Warn whenever a comment-start sequence `/∗' appears in a com‐
ment.
-Wtrigraphs
Warn if any trigraphs are encountered (assuming they are en‐
abled).
-Wformat
Check calls to printf and scanf, etc., to make sure that the ar‐
guments supplied have types appropriate to the format string
specified.
-Wchar-subscripts
Warn if an array subscript has type char. This is a common
cause of error, as programmers often forget that this type is
signed on some machines.
-Wuninitialized
An automatic variable is used without first being initialized.
These warnings are possible only in optimizing compilation, be‐
cause they require data flow information that is computed only
when optimizing. If you don't specify `-O', you simply won't
get these warnings.
These warnings occur only for variables that are candidates for
register allocation. Therefore, they do not occur for a vari‐
able that is declared volatile, or whose address is taken, or
whose size is other than 1, 2, 4 or 8 bytes. Also, they do not
occur for structures, unions or arrays, even when they are in
registers.
Note that there may be no warning about a variable that is used
only to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the
warnings are printed.
These warnings are made optional because GNU CC is not smart
enough to see all the reasons why the code might be correct de‐
spite appearing to have an error. Here is one example of how
this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of y is always 1, 2 or 3, then x is always initial‐
ized, but GNU CC doesn't know this. Here is another common
case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because save_y is used only if it is set.
Some spurious warnings can be avoided if you declare as volatile
all the functions you use that never return.
-Wparentheses
Warn if parentheses are omitted in certain contexts.
-Wtemplate-debugging
When using templates in a C++ program, warn if debugging is not
yet fully available (C++ only).
-Wall All of the above `-W' options combined. These are all the op‐
tions which pertain to usage that we recommend avoiding and that
we believe is easy to avoid, even in conjunction with macros.
The remaining `-W...' options are not implied by `-Wall' because they
warn about constructions that we consider reasonable to use, on occa‐
sion, in clean programs.
-Wtraditional
Warn about certain constructs that behave differently in tradi‐
tional and ANSI C.
· Macro arguments occurring within string constants in the macro
body. These would substitute the argument in traditional C, but
are part of the constant in ANSI C.
· A function declared external in one block and then used after
the end of the block.
· A switch statement has an operand of type long.
-Wshadow
Warn whenever a local variable shadows another local variable.
-Wid-clash-len
Warn whenever two distinct identifiers match in the first len
characters. This may help you prepare a program that will com‐
pile with certain obsolete, brain-damaged compilers.
-Wpointer-arith
Warn about anything that depends on the “size of” a function
type or of void. GNU C assigns these types a size of 1, for
convenience in calculations with void ∗ pointers and pointers to
functions.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if a const char ∗ is
cast to an ordinary char ∗.
-Wcast-align
Warn whenever a pointer is cast such that the required alignment
of the target is increased. For example, warn if a char ∗ is
cast to an int ∗ on machines where integers can only be accessed
at two- or four-byte boundaries.
-Wwrite-strings
Give string constants the type const char[length] so that copy‐
ing the address of one into a non-const char ∗ pointer will get
a warning. These warnings will help you find at compile time
code that can try to write into a string constant, but only if
you have been very careful about using const in declarations and
prototypes. Otherwise, it will just be a nuisance; this is why
we did not make `-Wall' request these warnings.
-Wconversion
Warn if a prototype causes a type conversion that is different
from what would happen to the same argument in the absence of a
prototype. This includes conversions of fixed point to floating
and vice versa, and conversions changing the width or signedness
of a fixed point argument except when the same as the default
promotion.
-Waggregate-return
Warn if any functions that return structures or unions are de‐
fined or called. (In languages where you can return an array,
this also elicits a warning.)
-Wstrict-prototypes
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration which specifies
the argument types.)
-Wmissing-prototypes
Warn if a global function is defined without a previous proto‐
type declaration. This warning is issued even if the definition
itself provides a prototype. The aim is to detect global func‐
tions that fail to be declared in header files.
-Wmissing-declarations
Warn if a global function is defined without a previous declara‐
tion. Do so even if the definition itself provides a prototype.
Use this option to detect global functions that are not declared
in header files.
-Wredundant-decls
Warn if anything is declared more than once in the same scope,
even in cases where multiple declaration is valid and changes
nothing.
-Wnested-externs
Warn if an extern declaration is encountered within an function.
-Wenum-clash
Warn about conversion between different enumeration types (C++
only).
-Wlong-long
Warn if long long type is used. This is default. To inhibit
the warning messages, use flag `-Wno-long-long'. Flags
`-W-long-long' and `-Wno-long-long' are taken into account only
when flag `-pedantic' is used.
-Woverloaded-virtual
(C++ only.) In a derived class, the definitions of virtual
functions must match the type signature of a virtual function
declared in the base class. Use this option to request warnings
when a derived class declares a function that may be an erro‐
neous attempt to define a virtual function: that is, warn when a
function with the same name as a virtual function in the base
class, but with a type signature that doesn't match any virtual
functions from the base class.
-Winline
Warn if a function can not be inlined, and either it was de‐
clared as inline, or else the -finline-functions option was giv‐
en.
-Werror
Treat warnings as errors; abort compilation after any warning.
DEBUGGING OPTIONS
GNU CC has various special options that are used for debugging either
your program or GCC:
-g Produce debugging information in the operating system's native
format (stabs, COFF, XCOFF, or DWARF). GDB can work with this
debugging information.
On most systems that use stabs format, `-g' enables use of extra
debugging information that only GDB can use; this extra informa‐
tion makes debugging work better in GDB but will probably make
other debuggers crash or refuse to read the program. If you
want to control for certain whether to generate the extra infor‐
mation, use `-gstabs+', `-gstabs', `-gxcoff+', `-gxcoff',
`-gdwarf+', or `-gdwarf' (see below).
Unlike most other C compilers, GNU CC allows you to use `-g'
with `-O'. The shortcuts taken by optimized code may occasion‐
ally produce surprising results: some variables you declared may
not exist at all; flow of control may briefly move where you did
not expect it; some statements may not be executed because they
compute constant results or their values were already at hand;
some statements may execute in different places because they
were moved out of loops.
Nevertheless it proves possible to debug optimized output. This
makes it reasonable to use the optimizer for programs that might
have bugs.
The following options are useful when GNU CC is generated with the ca‐
pability for more than one debugging format.
-ggdb Produce debugging information in the native format (if that is
supported), including GDB extensions if at all possible.
-gstabs
Produce debugging information in stabs format (if that is sup‐
ported), without GDB extensions. This is the format used by DBX
on most BSD systems.
-gstabs+
Produce debugging information in stabs format (if that is sup‐
ported), using GNU extensions understood only by the GNU debug‐
ger (GDB). The use of these extensions is likely to make other
debuggers crash or refuse to read the program.
-gcoff Produce debugging information in COFF format (if that is sup‐
ported). This is the format used by SDB on most System V sys‐
tems prior to System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that is sup‐
ported). This is the format used by the DBX debugger on IBM
RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is sup‐
ported), using GNU extensions understood only by the GNU debug‐
ger (GDB). The use of these extensions is likely to make other
debuggers crash or refuse to read the program.
-gdwarf
Produce debugging information in DWARF format (if that is sup‐
ported). This is the format used by SDB on most System V Re‐
lease 4 systems.
-gdwarf+
Produce debugging information in DWARF format (if that is sup‐
ported), using GNU extensions understood only by the GNU debug‐
ger (GDB). The use of these extensions is likely to make other
debuggers crash or refuse to read the program.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel -gxcofflevel
-gdwarflevel
Request debugging information and also use level to specify how
much information. The default level is 2.
Level 1 produces minimal information, enough for making back‐
traces in parts of the program that you don't plan to debug.
This includes descriptions of functions and external variables,
but no information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro defi‐
nitions present in the program. Some debuggers support macro
expansion when you use `-g3'.
-p Generate extra code to write profile information suitable for
the analysis program prof.
-pg Generate extra code to write profile information suitable for
the analysis program gprof.
-a Generate extra code to write profile information for basic
blocks, which will record the number of times each basic block
is executed. This data could be analyzed by a program like
tcov. Note, however, that the format of the data is not what
tcov expects. Eventually GNU gprof should be extended to
process this data.
-ax Generate extra code to read basic block profiling parameters
from file `bb.in' and write profiling results to file `bb.out'.
`bb.in' contains a list of functions. Whenever a function on the
list is entered, profiling is turned on. When the outmost func‐
tion is left, profiling is turned off. If a function name is
prefixed with `-' the function is excluded from profiling. If a
function name is not unique it can be disambiguated by writing
`/path/filename.d:functionname'. `bb.out' will list some avail‐
able filenames. Four function names have a special meaning:
`__bb_jumps__' will cause jump frequencies to be written to
`bb.out'. `__bb_trace__' will cause the sequence of basic
blocks to be piped into `gzip' and written to file `bbtrace.gz'.
`__bb_hidecall__' will cause call instructions to be excluded
from the trace. `__bb_showret__' will cause return instructions
to be included in the trace.
-dletters
Says to make debugging dumps during compilation at times speci‐
fied by letters. This is used for debugging the compiler. The
file names for most of the dumps are made by appending a word to
the source file name (e.g. `foo.c.rtl' or `foo.c.jump').
-dM Dump all macro definitions, at the end of preprocessing, and
write no output.
-dN Dump all macro names, at the end of preprocessing.
-dD Dump all macro definitions, at the end of preprocessing, in ad‐
dition to normal output.
-dy Dump debugging information during parsing, to standard error.
-dr Dump after RTL generation, to `file.rtl'.
-dx Just generate RTL for a function instead of compiling it. Usu‐
ally used with `r'.
-dj Dump after first jump optimization, to `file.jump'.
-ds Dump after CSE (including the jump optimization that sometimes
follows CSE), to `file.cse'.
-dL Dump after loop optimization, to `file.loop'.
-dt Dump after the second CSE pass (including the jump optimization
that sometimes follows CSE), to `file.cse2'.
-df Dump after flow analysis, to `file.flow'.
-dc Dump after instruction combination, to `file.combine'.
-dS Dump after the first instruction scheduling pass, to
`file.sched'.
-dl Dump after local register allocation, to `file.lreg'.
-dg Dump after global register allocation, to `file.greg'.
-dR Dump after the second instruction scheduling pass, to
`file.sched2'.
-dJ Dump after last jump optimization, to `file.jump2'.
-dd Dump after delayed branch scheduling, to `file.dbr'.
-dk Dump after conversion from registers to stack, to `file.stack'.
-da Produce all the dumps listed above.
-dm Print statistics on memory usage, at the end of the run, to
standard error.
-dp Annotate the assembler output with a comment indicating which
pattern and alternative was used.
-fpretend-float
When running a cross-compiler, pretend that the target machine
uses the same floating point format as the host machine. This
causes incorrect output of the actual floating constants, but
the actual instruction sequence will probably be the same as GNU
CC would make when running on the target machine.
-save-temps
Store the usual “temporary” intermediate files permanently;
place them in the current directory and name them based on the
source file. Thus, compiling `foo.c' with `-c -save-temps'
would produce files `foo.cpp' and `foo.s', as well as `foo.o'.
-print-file-name=library
Print the full absolute name of the library file library that
would be used when linking—and do not do anything else. With
this option, GNU CC does not compile or link anything; it just
prints the file name.
-print-libgcc-file-name
Same as `-print-file-name=libgcc.a'.
-print-prog-name=program
Like `-print-file-name', but searches for a program such as
`cpp'.
OPTIMIZATION OPTIONS
These options control various sorts of optimizations:
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and
a lot more memory for a large function.
Without `-O', the compiler's goal is to reduce the cost of com‐
pilation and to make debugging produce the expected results.
Statements are independent: if you stop the program with a
breakpoint between statements, you can then assign a new value
to any variable or change the program counter to any other
statement in the function and get exactly the results you would
expect from the source code.
Without `-O', only variables declared register are allocated in
registers. The resulting compiled code is a little worse than
produced by PCC without `-O'.
With `-O', the compiler tries to reduce code size and execution
time.
When you specify `-O', the two options `-fthread-jumps' and
`-fdefer-pop' are turned on. On machines that have delay slots,
the `-fdelayed-branch' option is turned on. For those machines
that can support debugging even without a frame pointer, the
`-fomit-frame-pointer' option is turned on. On some machines
other flags may also be turned on.
-O2 Optimize even more. Nearly all supported optimizations that do
not involve a space-speed tradeoff are performed. Loop un‐
rolling and function inlining are not done, for example. As
compared to -O, this option increases both compilation time and
the performance of the generated code.
-O3 Optimize yet more. This turns on everything -O2 does, along with
also turning on -finline-functions.
-O0 Do not optimize.
If you use multiple -O options, with or without level numbers,
the last such option is the one that is effective.
Options of the form `-fflag' specify machine-independent flags. Most
flags have both positive and negative forms; the negative form of
`-ffoo' would be `-fno-foo'. The following list shows only one form—
the one which is not the default. You can figure out the other form by
either removing `no-' or adding it.
-ffloat-store
Do not store floating point variables in registers. This pre‐
vents undesirable excess precision on machines such as the 68000
where the floating registers (of the 68881) keep more precision
than a double is supposed to have.
For most programs, the excess precision does only good, but a
few programs rely on the precise definition of IEEE floating
point. Use `-ffloat-store' for such programs.
-fmemoize-lookups
-fsave-memoized
Use heuristics to compile faster (C++ only). These heuristics
are not enabled by default, since they are only effective for
certain input files. Other input files compile more slowly.
The first time the compiler must build a call to a member func‐
tion (or reference to a data member), it must (1) determine
whether the class implements member functions of that name; (2)
resolve which member function to call (which involves figuring
out what sorts of type conversions need to be made); and (3)
check the visibility of the member function to the caller. All
of this adds up to slower compilation. Normally, the second
time a call is made to that member function (or reference to
that data member), it must go through the same lengthy process
again. This means that code like this
cout << "This " << p << " has " << n << " legs.\n";
makes six passes through all three steps. By using a software
cache, a “hit” significantly reduces this cost. Unfortunately,
using the cache introduces another layer of mechanisms which
must be implemented, and so incurs its own overhead. `-fmemo‐
ize-lookups' enables the software cache.
Because access privileges (visibility) to members and member
functions may differ from one function context to the next, g++
may need to flush the cache. With the `-fmemoize-lookups' flag,
the cache is flushed after every function that is compiled. The
`-fsave-memoized' flag enables the same software cache, but when
the compiler determines that the context of the last function
compiled would yield the same access privileges of the next
function to compile, it preserves the cache. This is most help‐
ful when defining many member functions for the same class: with
the exception of member functions which are friends of other
classes, each member function has exactly the same access privi‐
leges as every other, and the cache need not be flushed.
-fno-default-inline
Don't make member functions inline by default merely because
they are defined inside the class scope (C++ only).
-fno-defer-pop
Always pop the arguments to each function call as soon as that
function returns. For machines which must pop arguments after a
function call, the compiler normally lets arguments accumulate
on the stack for several function calls and pops them all at
once.
-fforce-mem
Force memory operands to be copied into registers before doing
arithmetic on them. This may produce better code by making all
memory references potential common subexpressions. When they
are not common subexpressions, instruction combination should
eliminate the separate register-load. I am interested in hear‐
ing about the difference this makes.
-fforce-addr
Force memory address constants to be copied into registers be‐
fore doing arithmetic on them. This may produce better code
just as `-fforce-mem' may. I am interested in hearing about the
difference this makes.
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that
don't need one. This avoids the instructions to save, set up
and restore frame pointers; it also makes an extra register
available in many functions. It also makes debugging impossible
on most machines.
On some machines, such as the Vax, this flag has no effect, be‐
cause the standard calling sequence automatically handles the
frame pointer and nothing is saved by pretending it doesn't ex‐
ist. The machine-description macro FRAME_POINTER_REQUIRED con‐
trols whether a target machine supports this flag.
-finline-functions
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be
worth integrating in this way.
If all calls to a given function are integrated, and the func‐
tion is declared static, then GCC normally does not output the
function as assembler code in its own right.
-fcaller-saves
Enable values to be allocated in registers that will be clob‐
bered by function calls, by emitting extra instructions to save
and restore the registers around such calls. Such allocation is
done only when it seems to result in better code than would oth‐
erwise be produced.
This option is enabled by default on certain machines, usually
those which have no call-preserved registers to use instead.
-fkeep-inline-functions
Even if all calls to a given function are integrated, and the
function is declared static, nevertheless output a separate run-
time callable version of the function.
-fno-function-cse
Do not put function addresses in registers; make each instruc‐
tion that calls a constant function contain the function's ad‐
dress explicitly.
This option results in less efficient code, but some strange
hacks that alter the assembler output may be confused by the op‐
timizations performed when this option is not used.
-fno-peephole
Disable any machine-specific peephole optimizations.
-ffast-math
This option allows GCC to violate some ANSI or IEEE rules/speci‐
fications in the interest of optimizing code for speed. For ex‐
ample, it allows the compiler to assume arguments to the sqrt
function are non-negative numbers.
This option should never be turned on by any `-O' option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ANSI rules/specifications for
math functions.
The following options control specific optimizations. The `-O2' option
turns on all of these optimizations except `-funroll-loops' and `-fun‐
roll-all-loops'.
The `-O' option usually turns on the `-fthread-jumps' and `-fde‐
layed-branch' options, but specific machines may change the default op‐
timizations.
You can use the following flags in the rare cases when “fine-tuning” of
optimizations to be performed is desired.
-fstrength-reduce
Perform the optimizations of loop strength reduction and elimi‐
nation of iteration variables.
-fthread-jumps
Perform optimizations where we check to see if a jump branches
to a location where another comparison subsumed by the first is
found. If so, the first branch is redirected to either the des‐
tination of the second branch or a point immediately following
it, depending on whether the condition is known to be true or
false.
-funroll-loops
Perform the optimization of loop unrolling. This is only done
for loops whose number of iterations can be determined at com‐
pile time or run time.
-funroll-all-loops
Perform the optimization of loop unrolling. This is done for
all loops. This usually makes programs run more slowly.
-fcse-follow-jumps
In common subexpression elimination, scan through jump instruc‐
tions when the target of the jump is not reached by any other
path. For example, when CSE encounters an if statement with an
else clause, CSE will follow the jump when the condition tested
is false.
-fcse-skip-blocks
This is similar to `-fcse-follow-jumps', but causes CSE to fol‐
low jumps which conditionally skip over blocks. When CSE en‐
counters a simple if statement with no else clause,
`-fcse-skip-blocks' causes CSE to follow the jump around the
body of the if.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations
has been performed.
-felide-constructors
Elide constructors when this seems plausible (C++ only). With
this flag, GNU C++ initializes y directly from the call to foo
without going through a temporary in the following code:
A foo (); A y = foo ();
Without this option, GNU C++ first initializes y by calling the
appropriate constructor for type A; then assigns the result of
foo to a temporary; and, finally, replaces the initial value of
`y' with the temporary.
The default behavior (`-fno-elide-constructors') is specified by
the draft ANSI C++ standard. If your program's constructors
have side effects, using `-felide-constructors' can make your
program act differently, since some constructor calls may be
omitted.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively ex‐
pensive.
-fdelayed-branch
If supported for the target machine, attempt to reorder instruc‐
tions to exploit instruction slots available after delayed
branch instructions.
-fschedule-insns
If supported for the target machine, attempt to reorder instruc‐
tions to eliminate execution stalls due to required data being
unavailable. This helps machines that have slow floating point
or memory load instructions by allowing other instructions to be
issued until the result of the load or floating point instruc‐
tion is required.
-fschedule-insns2
Similar to `-fschedule-insns', but requests an additional pass
of instruction scheduling after register allocation has been
done. This is especially useful on machines with a relatively
small number of registers and where memory load instructions
take more than one cycle.
TARGET OPTIONS
By default, GNU CC compiles code for the same type of machine that you
are using. However, it can also be installed as a cross-compiler, to
compile for some other type of machine. In fact, several different
configurations of GNU CC, for different target machines, can be in‐
stalled side by side. Then you specify which one to use with the `-b'
option.
In addition, older and newer versions of GNU CC can be installed side
by side. One of them (probably the newest) will be the default, but
you may sometimes wish to use another.
-b machine
The argument machine specifies the target machine for compila‐
tion. This is useful when you have installed GNU CC as a cross-
compiler.
The value to use for machine is the same as was specified as the
machine type when configuring GNU CC as a cross-compiler. For
example, if a cross-compiler was configured with `configure
i386v', meaning to compile for an 80386 running System V, then
you would specify `-b i386v' to run that cross compiler.
When you do not specify `-b', it normally means to compile for
the same type of machine that you are using.
-V version
The argument version specifies which version of GNU CC to run.
This is useful when multiple versions are installed. For exam‐
ple, version might be `2.0', meaning to run GNU CC version 2.0.
The default version, when you do not specify `-V', is controlled
by the way GNU CC is installed. Normally, it will be a version
that is recommended for general use.
MACHINE DEPENDENT OPTIONS
Each of the target machine types can have its own special options,
starting with `-m', to choose among various hardware models or configu‐
rations—for example, 68010 vs 68020, floating coprocessor or none. A
single installed version of the compiler can compile for any model or
configuration, according to the options specified.
Some configurations of the compiler also support additional special op‐
tions, usually for command-line compatibility with other compilers on
the same platform.
These are the `-m' options defined for the 68000 series:
-m68000
-mc68000
Generate output for a 68000. This is the default when the com‐
piler is configured for 68000-based systems.
-m68020
-mc68020
Generate output for a 68020 (rather than a 68000). This is the
default when the compiler is configured for 68020-based systems.
-m68881
Generate output containing 68881 instructions for floating
point. This is the default for most 68020-based systems unless
-nfp was specified when the compiler was configured.
-m68030
Generate output for a 68030. This is the default when the com‐
piler is configured for 68030-based systems.
-m68040
Generate output for a 68040. This is the default when the com‐
piler is configured for 68040-based systems.
-m68020-40
Generate output for a 68040, without using any of the new in‐
structions. This results in code which can run relatively effi‐
ciently on either a 68020/68881 or a 68030 or a 68040.
-mfpa Generate output containing Sun FPA instructions for floating
point.
-msoft-float
Generate output containing library calls for floating point.
WARNING: the requisite libraries are not part of GNU CC. Nor‐
mally the facilities of the machine's usual C compiler are used,
but this can't be done directly in cross-compilation. You must
make your own arrangements to provide suitable library functions
for cross-compilation.
-mshort
Consider type int to be 16 bits wide, like short int.
-mnobitfield
Do not use the bit-field instructions. `-m68000' implies `-mno‐
bitfield'.
-mbitfield
Do use the bit-field instructions. `-m68020' implies `-mbit‐
field'. This is the default if you use the unmodified sources.
-mrtd Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the rtd in‐
struction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to
pop the arguments there.
This calling convention is incompatible with the one normally
used on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.
Also, you must provide function prototypes for all functions
that take variable numbers of arguments (including printf); oth‐
erwise incorrect code will be generated for calls to those func‐
tions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments
are harmlessly ignored.)
The rtd instruction is supported by the 68010 and 68020 proces‐
sors, but not by the 68000.
These `-m' options are defined for the Vax:
-munix Do not output certain jump instructions (aobleq and so on) that
the Unix assembler for the Vax cannot handle across long ranges.
-mgnu Do output those jump instructions, on the assumption that you
will assemble with the GNU assembler.
-mg Output code for g-format floating point numbers instead of d-
format.
These `-m' switches are supported on the SPARC:
-mfpu
-mhard-float
Generate output containing floating point instructions. This is
the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point.
Warning: there is no GNU floating-point library for SPARC. Nor‐
mally the facilities of the machine's usual C compiler are used,
but this cannot be done directly in cross-compilation. You must
make your own arrangements to provide suitable library functions
for cross-compilation.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program
with this option.
-mno-epilogue
-mepilogue
With -mepilogue (the default), the compiler always emits code
for function exit at the end of each function. Any function ex‐
it in the middle of the function (such as a return statement in
C) will generate a jump to the exit code at the end of the func‐
tion.
With -mno-epilogue, the compiler tries to emit exit code inline
at every function exit.
-mno-v8
-mv8
-msparclite
These three options select variations on the SPARC architecture.
By default (unless specifically configured for the Fujitsu SPAR‐
Clite), GCC generates code for the v7 variant of the SPARC ar‐
chitecture.
-mv8 will give you SPARC v8 code. The only difference from v7
code is that the compiler emits the integer multiply and integer
divide instructions which exist in SPARC v8 but not in SPARC v7.
-msparclite will give you SPARClite code. This adds the integer
multiply, integer divide step and scan (ffs) instructions which
exist in SPARClite but not in SPARC v7.
-mcypress
-msupersparc
These two options select the processor for which the code is op‐
timised.
With -mcypress (the default), the compiler optimises code for
the Cypress CY7C602 chip, as used in the SparcStation/SparcServ‐
er 3xx series. This is also appropriate for the older SparcSta‐
tion 1, 2, IPX etc.
With -msupersparc the compiler optimises code for the SuperSparc
cpu, as used in the SparcStation 10, 1000 and 2000 series. This
flag also enables use of the full SPARC v8 instruction set.
These `-m' options are defined for the Convex:
-mc1 Generate output for a C1. This is the default when the compiler
is configured for a C1.
-mc2 Generate output for a C2. This is the default when the compiler
is configured for a C2.
-margcount
Generate code which puts an argument count in the word preceding
each argument list. Some nonportable Convex and Vax programs
need this word. (Debuggers don't, except for functions with
variable-length argument lists; this info is in the symbol ta‐
ble.)
-mnoargcount
Omit the argument count word. This is the default if you use
the unmodified sources.
These `-m' options are defined for the AMD Am29000:
-mdw Generate code that assumes the DW bit is set, i.e., that byte
and halfword operations are directly supported by the hardware.
This is the default.
-mnodw Generate code that assumes the DW bit is not set.
-mbw Generate code that assumes the system supports byte and halfword
write operations. This is the default.
-mnbw Generate code that assumes the systems does not support byte and
halfword write operations. This implies `-mnodw'.
-msmall
Use a small memory model that assumes that all function address‐
es are either within a single 256 KB segment or at an absolute
address of less than 256K. This allows the call instruction to
be used instead of a const, consth, calli sequence.
-mlarge
Do not assume that the call instruction can be used; this is the
default.
-m29050
Generate code for the Am29050.
-m29000
Generate code for the Am29000. This is the default.
-mkernel-registers
Generate references to registers gr64-gr95 instead of
gr96-gr127. This option can be used when compiling kernel code
that wants a set of global registers disjoint from that used by
user-mode code.
Note that when this option is used, register names in `-f' flags
must use the normal, user-mode, names.
-muser-registers
Use the normal set of global registers, gr96-gr127. This is the
default.
-mstack-check
Insert a call to __msp_check after each stack adjustment. This
is often used for kernel code.
These `-m' options are defined for Motorola 88K architectures:
-m88000
Generate code that works well on both the m88100 and the m88110.
-m88100
Generate code that works best for the m88100, but that also runs
on the m88110.
-m88110
Generate code that works best for the m88110, and may not run on
the m88100.
-midentify-revision
Include an ident directive in the assembler output recording the
source file name, compiler name and version, timestamp, and com‐
pilation flags used.
-mno-underscores
In assembler output, emit symbol names without adding an under‐
score character at the beginning of each name. The default is
to use an underscore as prefix on each name.
-mno-check-zero-division
-mcheck-zero-division
Early models of the 88K architecture had problems with division
by zero; in particular, many of them didn't trap. Use these op‐
tions to avoid including (or to include explicitly) additional
code to detect division by zero and signal an exception. All
GCC configurations for the 88K use `-mcheck-zero-division' by
default.
-mocs-debug-info
-mno-ocs-debug-info
Include (or omit) additional debugging information (about regis‐
ters used in each stack frame) as specified in the 88Open Object
Compatibility Standard, “OCS”. This extra information is not
needed by GDB. The default for DG/UX, SVr4, and Delta 88 SVr3.2
is to include this information; other 88k configurations omit
this information by default.
-mocs-frame-position
-mno-ocs-frame-position
Force (or do not require) register values to be stored in a par‐
ticular place in stack frames, as specified in OCS. The DG/UX,
Delta88 SVr3.2, and BCS configurations use `-mocs-frame-posi‐
tion'; other 88k configurations have the default
`-mno-ocs-frame-position'.
-moptimize-arg-area
-mno-optimize-arg-area
Control how to store function arguments in stack frames. `-mop‐
timize-arg-area' saves space, but may break some debuggers (not
GDB). `-mno-optimize-arg-area' conforms better to standards.
By default GCC does not optimize the argument area.
-mshort-data-num
num Generate smaller data references by making them relative to
r0, which allows loading a value using a single instruction
(rather than the usual two). You control which data references
are affected by specifying num with this option. For example,
if you specify `-mshort-data-512', then the data references af‐
fected are those involving displacements of less than 512 bytes.
`-mshort-data-num' is not effective for num greater than 64K.
-mserialize-volatile
-mno-serialize-volatile
Do, or do not, generate code to guarantee sequential consistency
of volatile memory references.
GNU CC always guarantees consistency by default, for the pre‐
ferred processor submodel. How this is done depends on the sub‐
model.
The m88100 processor does not reorder memory references and so
always provides sequential consistency. If you use `-m88100',
GNU CC does not generate any special instructions for sequential
consistency.
The order of memory references made by the m88110 processor does
not always match the order of the instructions requesting those
references. In particular, a load instruction may execute be‐
fore a preceding store instruction. Such reordering violates
sequential consistency of volatile memory references, when there
are multiple processors. When you use `-m88000' or `-m88110',
GNU CC generates special instructions when appropriate, to force
execution in the proper order.
The extra code generated to guarantee consistency may affect the
performance of your application. If you know that you can safe‐
ly forgo this guarantee, you may use the option `-mno-serialize-
volatile'.
If you use the `-m88100' option but require sequential consis‐
tency when running on the m88110 processor, you should use
`-mserialize-volatile'.
-msvr4
-msvr3 Turn on (`-msvr4') or off (`-msvr3') compiler extensions related
to System V release 4 (SVr4). This controls the following:
· Which variant of the assembler syntax to emit (which you can se‐
lect independently using `-mversion-03.00').
· `-msvr4' makes the C preprocessor recognize `#pragma weak'
· `-msvr4' makes GCC issue additional declaration directives used
in SVr4.
`-msvr3' is the default for all m88K configurations except the SVr4
configuration.
-mtrap-large-shift
-mhandle-large-shift
Include code to detect bit-shifts of more than 31 bits; respec‐
tively, trap such shifts or emit code to handle them properly.
By default GCC makes no special provision for large bit shifts.
-muse-div-instruction
Very early models of the 88K architecture didn't have a divide
instruction, so GCC avoids that instruction by default. Use
this option to specify that it's safe to use the divide instruc‐
tion.
-mversion-03.00
In the DG/UX configuration, there are two flavors of SVr4. This
option modifies -msvr4 to select whether the hybrid-COFF or re‐
al-ELF flavor is used. All other configurations ignore this op‐
tion.
-mwarn-passed-structs
Warn when a function passes a struct as an argument or result.
Structure-passing conventions have changed during the evolution
of the C language, and are often the source of portability prob‐
lems. By default, GCC issues no such warning.
These options are defined for the IBM RS6000:
-mfp-in-toc
-mno-fp-in-toc
Control whether or not floating-point constants go in the Table
of Contents (TOC), a table of all global variable and function
addresses. By default GCC puts floating-point constants there;
if the TOC overflows, `-mno-fp-in-toc' will reduce the size of
the TOC, which may avoid the overflow.
These `-m' options are defined for the IBM RT PC:
-min-line-mul
Use an in-line code sequence for integer multiplies. This is
the default.
-mcall-lib-mul
Call lmul$$ for integer multiples.
-mfull-fp-blocks
Generate full-size floating point data blocks, including the
minimum amount of scratch space recommended by IBM. This is the
default.
-mminimum-fp-blocks
Do not include extra scratch space in floating point data
blocks. This results in smaller code, but slower execution,
since scratch space must be allocated dynamically.
-mfp-arg-in-fpregs
Use a calling sequence incompatible with the IBM calling conven‐
tion in which floating point arguments are passed in floating
point registers. Note that varargs.h and stdargs.h will not
work with floating point operands if this option is specified.
-mfp-arg-in-gregs
Use the normal calling convention for floating point arguments.
This is the default.
-mhc-struct-return
Return structures of more than one word in memory, rather than
in a register. This provides compatibility with the MetaWare
HighC (hc) compiler. Use `-fpcc-struct-return' for compatibili‐
ty with the Portable C Compiler (pcc).
-mnohc-struct-return
Return some structures of more than one word in registers, when
convenient. This is the default. For compatibility with the
IBM-supplied compilers, use either `-fpcc-struct-return' or
`-mhc-struct-return'.
These `-m' options are defined for the MIPS family of computers:
-mcpu=cpu-type
Assume the defaults for the machine type cpu-type when schedul‐
ing instructions. The default cpu-type is default, which picks
the longest cycles times for any of the machines, in order that
the code run at reasonable rates on all MIPS cpu's. Other
choices for cpu-type are r2000, r3000, r4000, and r6000. While
picking a specific cpu-type will schedule things appropriately
for that particular chip, the compiler will not generate any
code that does not meet level 1 of the MIPS ISA (instruction set
architecture) without the -mips2 or -mips3 switches being used.
-mips2 Issue instructions from level 2 of the MIPS ISA (branch likely,
square root instructions). The -mcpu=r4000 or -mcpu=r6000
switch must be used in conjunction with -mips2.
-mips3 Issue instructions from level 3 of the MIPS ISA (64 bit instruc‐
tions). The -mcpu=r4000 switch must be used in conjunction with
-mips2.
-mint64
-mlong64
-mlonglong128
These options don't work at present.
-mmips-as
Generate code for the MIPS assembler, and invoke mips-tfile to
add normal debug information. This is the default for all plat‐
forms except for the OSF/1 reference platform, using the
OSF/rose object format. If any of the -ggdb, -gstabs, or
-gstabs+ switches are used, the mips-tfile program will encapsu‐
late the stabs within MIPS ECOFF.
-mgas Generate code for the GNU assembler. This is the default on the
OSF/1 reference platform, using the OSF/rose object format.
-mrnames
-mno-rnames
The -mrnames switch says to output code using the MIPS software
names for the registers, instead of the hardware names (ie, a0
instead of $4). The GNU assembler does not support the -mrnames
switch, and the MIPS assembler will be instructed to run the
MIPS C preprocessor over the source file. The -mno-rnames
switch is default.
-mgpopt
-mno-gpopt
The -mgpopt switch says to write all of the data declarations
before the instructions in the text section, to all the MIPS as‐
sembler to generate one word memory references instead of using
two words for short global or static data items. This is on by
default if optimization is selected.
-mstats
-mno-stats
For each non-inline function processed, the -mstats switch caus‐
es the compiler to emit one line to the standard error file to
print statistics about the program (number of registers saved,
stack size, etc.).
-mmemcpy
-mno-memcpy
The -mmemcpy switch makes all block moves call the appropriate
string function (memcpy or bcopy) instead of possibly generating
inline code.
-mmips-tfile
-mno-mips-tfile
The -mno-mips-tfile switch causes the compiler not postprocess
the object file with the mips-tfile program, after the MIPS as‐
sembler has generated it to add debug support. If mips-tfile is
not run, then no local variables will be available to the debug‐
ger. In addition, stage2 and stage3 objects will have the tem‐
porary file names passed to the assembler embedded in the object
file, which means the objects will not compare the same.
-msoft-float
Generate output containing library calls for floating point.
WARNING: the requisite libraries are not part of GNU CC. Nor‐
mally the facilities of the machine's usual C compiler are used,
but this can't be done directly in cross-compilation. You must
make your own arrangements to provide suitable library functions
for cross-compilation.
-mhard-float
Generate output containing floating point instructions. This is
the default if you use the unmodified sources.
-mfp64 Assume that the FR bit in the status word is on, and that there
are 32 64-bit floating point registers, instead of 32 32-bit
floating point registers. You must also specify the -mcpu=r4000
and -mips3 switches.
-mfp32 Assume that there are 32 32-bit floating point registers. This
is the default.
-mabicalls
-mno-abicalls
Emit (or do not emit) the .abicalls, .cpload, and .cprestore
pseudo operations that some System V.4 ports use for position
independent code.
-mhalf-pic
-mno-half-pic
The -mhalf-pic switch says to put pointers to extern references
into the data section and load them up, rather than put the ref‐
erences in the text section. This option does not work at
present. -Gnum Put global and static items less than or equal
to num bytes into the small data or bss sections instead of the
normal data or bss section. This allows the assembler to emit
one word memory reference instructions based on the global
pointer (gp or $28), instead of the normal two words used. By
default, num is 8 when the MIPS assembler is used, and 0 when
the GNU assembler is used. The -Gnum switch is also passed to
the assembler and linker. All modules should be compiled with
the same -Gnum value.
-nocpp Tell the MIPS assembler to not run its preprocessor over user
assembler files (with a `.s' suffix) when assembling them.
These `-m' options are defined for the Intel 80386 family of computers:
-m486
-mno-486
Control whether or not code is optimized for a 486 instead of an
386. Code generated for a 486 will run on a 386 and vice versa.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GNU CC. Nor‐
mally the facilities of the machine's usual C compiler are used,
but this can't be done directly in cross-compilation. You must
make your own arrangements to provide suitable library functions
for cross-compilation.
On machines where a function returns floating point results in
the 80387 register stack, some floating point opcodes may be
emitted even if `-msoft-float' is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of
types float and double in an FPU register, even if there is no
FPU. The idea is that the operating system should emulate an
FPU.
The option `-mno-fp-ret-in-387' causes such values to be re‐
turned in ordinary CPU registers instead.
These `-m' options are defined for the HPPA family of computers:
-mpa-risc-1-0
Generate code for a PA 1.0 processor.
-mpa-risc-1-1
Generate code for a PA 1.1 processor.
-mkernel
Generate code which is suitable for use in kernels. Specifical‐
ly, avoid add instructions in which one of the arguments is the
DP register; generate addil instructions instead. This avoids a
rather serious bug in the HP-UX linker.
-mshared-libs
Generate code that can be linked against HP-UX shared libraries.
This option is not fully function yet, and is not on by default
for any PA target. Using this option can cause incorrect code
to be generated by the compiler.
-mno-shared-libs
Don't generate code that will be linked against shared li‐
braries. This is the default for all PA targets.
-mlong-calls
Generate code which allows calls to functions greater than 256K
away from the caller when the caller and callee are in the same
source file. Do not turn this option on unless code refuses to
link with “branch out of range errors from the linker.
-mdisable-fpregs
Prevent floating point registers from being used in any manner.
This is necessary for compiling kernels which perform lazy con‐
text switching of floating point registers. If you use this op‐
tion and attempt to perform floating point operations, the com‐
piler will abort.
-mdisable-indexing
Prevent the compiler from using indexing address modes. This
avoids some rather obscure problems when compiling MIG generated
code under MACH.
-mtrailing-colon
Add a colon to the end of label definitions (for ELF assem‐
blers).
These `-m' options are defined for the Intel 80960 family of computers:
-mcpu-type
Assume the defaults for the machine type cpu-type for instruc‐
tion and addressing-mode availability and alignment. The de‐
fault cpu-type is kb; other choices are ka, mc, ca, cf, sa, and
sb.
-mnumerics
-msoft-float
The -mnumerics option indicates that the processor does support
floating-point instructions. The -msoft-float option indicates
that floating-point support should not be assumed.
-mleaf-procedures
-mno-leaf-procedures
Do (or do not) attempt to alter leaf procedures to be callable
with the bal instruction as well as call. This will result in
more efficient code for explicit calls when the bal instruction
can be substituted by the assembler or linker, but less effi‐
cient code in other cases, such as calls via function pointers,
or using a linker that doesn't support this optimization.
-mtail-call
-mno-tail-call
Do (or do not) make additional attempts (beyond those of the ma‐
chine-independent portions of the compiler) to optimize tail-re‐
cursive calls into branches. You may not want to do this be‐
cause the detection of cases where this is not valid is not to‐
tally complete. The default is -mno-tail-call.
-mcomplex-addr
-mno-complex-addr
Assume (or do not assume) that the use of a complex addressing
mode is a win on this implementation of the i960. Complex ad‐
dressing modes may not be worthwhile on the K-series, but they
definitely are on the C-series. The default is currently -mcom‐
plex-addr for all processors except the CB and CC.
-mcode-align
-mno-code-align
Align code to 8-byte boundaries for faster fetching (or don't
bother). Currently turned on by default for C-series implemen‐
tations only.
-mic-compat
-mic2.0-compat
-mic3.0-compat
Enable compatibility with iC960 v2.0 or v3.0.
-masm-compat
-mintel-asm
Enable compatibility with the iC960 assembler.
-mstrict-align
-mno-strict-align
Do not permit (do permit) unaligned accesses.
-mold-align
Enable structure-alignment compatibility with Intel's gcc re‐
lease version 1.3 (based on gcc 1.37). Currently this is buggy
in that #pragma align 1 is always assumed as well, and cannot be
turned off.
These `-m' options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for
floating-point operations. When -msoft-float is specified,
functions in `libgcc1.c' will be used to perform floating-point
operations. Unless they are replaced by routines that emulate
the floating-point operations, or compiled in such a way as to
call such emulations routines, these routines will issue float‐
ing-point operations. If you are compiling for an Alpha with‐
out floating-point operations, you must ensure that the library
is built so as not to call them.
Note that Alpha implementations without floating-point opera‐
tions are required to have floating-point registers.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point regis‐
ter set. -mno-fp-regs implies -msoft-float. If the floating-
point register set is not used, floating point operands are
passed in integer registers as if they were integers and float‐
ing-point results are passed in $0 instead of $f0. This is a
non-standard calling sequence, so any function with a floating-
point argument or return value called by code compiled with
-mno-fp-regs must also be compiled with that option.
A typical use of this option is building a kernel that does not
use, and hence need not save and restore, any floating-point
registers.
These additional options are available on System V Release 4 for com‐
patibility with other compilers on those systems:
-G On SVr4 systems, gcc accepts the option `-G' (and passes it to
the system linker), for compatibility with other compilers.
However, we suggest you use `-symbolic' or `-shared' as appro‐
priate, instead of supplying linker options on the gcc command
line.
-Qy Identify the versions of each tool used by the compiler, in a
.ident assembler directive in the output.
-Qn Refrain from adding .ident directives to the output file (this
is the default).
-YP,dirs
Search the directories dirs, and no others, for libraries speci‐
fied with `-l'. You can separate directory entries in dirs from
one another with colons.
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The as‐
sembler uses this option.
CODE GENERATION OPTIONS
These machine-independent options control the interface conventions
used in code generation.
Most of them begin with `-f'. These options have both positive and
negative forms; the negative form of `-ffoo' would be `-fno-foo'. In
the table below, only one of the forms is listed—the one which is not
the default. You can figure out the other form by either removing
`no-' or adding it.
-fnonnull-objects
Assume that objects reached through references are not null (C++
only).
Normally, GNU C++ makes conservative assumptions about objects
reached through references. For example, the compiler must
check that a is not null in code like the following:
obj &a = g (); a.f (2);
Checking that references of this sort have non-null values re‐
quires extra code, however, and it is unnecessary for many pro‐
grams. You can use `-fnonnull-objects' to omit the checks for
null, if your program doesn't require checking.
-fpcc-struct-return
Use the same convention for returning struct and union values
that is used by the usual C compiler on your system. This con‐
vention is less efficient for small structures, and on many ma‐
chines it fails to be reentrant; but it has the advantage of al‐
lowing intercallability between GCC-compiled code and PCC-com‐
piled code.
-freg-struct-return
Use the convention that struct and union values are returned in
registers when possible. This is more efficient for small
structures than -fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-re‐
turn, GNU CC defaults to whichever convention is standard for
the target. If there is no standard convention, GNU CC defaults
to -fpcc-struct-return.
-fshort-enums
Allocate to an enum type only as many bytes as it needs for the
declared range of possible values. Specifically, the enum type
will be equivalent to the smallest integer type which has enough
room.
-fshort-double
Use the same size for double as for float .
-fshared-data
Requests that the data and non-const variables of this compila‐
tion be shared data rather than private data. The distinction
makes sense only on certain operating systems, where shared data
is shared between processes running the same program, while pri‐
vate data exists in one copy per process.
-fno-common
Allocate even uninitialized global variables in the bss section
of the object file, rather than generating them as common
blocks. This has the effect that if the same variable is de‐
clared (without extern) in two different compilations, you will
get an error when you link them. The only reason this might be
useful is if you wish to verify that the program will work on
other systems which always work this way.
-fno-ident
Ignore the `#ident' directive.
-fno-gnu-linker
Do not output global initializations (such as C++ constructors
and destructors) in the form used by the GNU linker (on systems
where the GNU linker is the standard method of handling them).
Use this option when you want to use a non-GNU linker, which al‐
so requires using the collect2 program to make sure the system
linker includes constructors and destructors. (collect2 is in‐
cluded in the GNU CC distribution.) For systems which must use
collect2, the compiler driver gcc is configured to do this auto‐
matically.
-finhibit-size-directive
Don't output a .size assembler directive, or anything else that
would cause trouble if the function is split in the middle, and
the two halves are placed at locations far apart in memory.
This option is used when compiling `crtstuff.c'; you should not
need to use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code
to make it more readable. This option is generally only of use
to those who actually need to read the generated assembly code
(perhaps while debugging the compiler itself).
-fvolatile
Consider all memory references through pointers to be volatile.
-fvolatile-global
Consider all memory references to extern and global data items
to be volatile.
-fpic If supported for the target machines, generate position-indepen‐
dent code, suitable for use in a shared library.
-fPIC If supported for the target machine, emit position-independent
code, suitable for dynamic linking, even if branches need large
displacements.
-ffixed-reg
Treat the register named reg as a fixed register; generated code
should never refer to it (except perhaps as a stack pointer,
frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted
are machine-specific and are defined in the REGISTER_NAMES macro
in the machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is
clobbered by function calls. It may be allocated for tempo‐
raries or variables that do not live across a call. Functions
compiled this way will not save and restore the register reg.
Use of this flag for a register that has a fixed pervasive role
in the machine's execution model, such as the stack pointer or
frame pointer, will produce disastrous results.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by
functions. It may be allocated even for temporaries or vari‐
ables that live across a call. Functions compiled this way will
save and restore the register reg if they use it.
Use of this flag for a register that has a fixed pervasive role
in the machine's execution model, such as the stack pointer or
frame pointer, will produce disastrous results.
A different sort of disaster will result from the use of this
flag for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a
three-way choice.
PRAGMAS
Two `#pragma' directives are supported for GNU C++, to permit using the
same header file for two purposes: as a definition of interfaces to a
given object class, and as the full definition of the contents of that
object class.
#pragma interface
(C++ only.) Use this directive in header files that define ob‐
ject classes, to save space in most of the object files that use
those classes. Normally, local copies of certain information
(backup copies of inline member functions, debugging informa‐
tion, and the internal tables that implement virtual functions)
must be kept in each object file that includes class defini‐
tions. You can use this pragma to avoid such duplication. When
a header file containing `#pragma interface' is included in a
compilation, this auxiliary information will not be generated
(unless the main input source file itself uses `#pragma imple‐
mentation'). Instead, the object files will contain references
to be resolved at link time.
#pragma implementation
#pragma implementation "objects.h"
(C++ only.) Use this pragma in a main input file, when you want
full output from included header files to be generated (and made
globally visible). The included header file, in turn, should
use `#pragma interface'. Backup copies of inline member func‐
tions, debugging information, and the internal tables used to
implement virtual functions are all generated in implementation
files.
If you use `#pragma implementation' with no argument, it applies
to an include file with the same basename as your source file;
for example, in `allclass.cc', `#pragma implementation' by it‐
self is equivalent to `#pragma implementation "allclass.h"'.
Use the string argument if you want a single implementation file
to include code from multiple header files.
There is no way to split up the contents of a single header file
into multiple implementation files.
FILES
file.c C source file
file.h C header (preprocessor) file
file.i preprocessed C source file
file.C C++ source file
file.cc C++ source file
file.cxx C++ source file
file.m Objective-C source file
file.s assembly language file
file.o object file
a.out link edited output
TMPDIR/cc∗ temporary files
LIBDIR/cpp preprocessor
LIBDIR/cc1 compiler for C
LIBDIR/cc1plus compiler for C++
LIBDIR/collect linker front end needed on some machines
LIBDIR/libgcc.a GCC subroutine library
/lib/crt[01n].o start-up routine
LIBDIR/ccrt0 additional start-up routine for C++
/lib/libc.a standard C library, see
intro(3)
/usr/include standard directory for #include files
LIBDIR/include standard gcc directory for #include files
LIBDIR/g++-include additional g++ directory for #include
LIBDIR is usually /usr/local/lib/machine/version.
TMPDIR comes from the environment variable TMPDIR (default /usr/tmp if
available, else /tmp).
SEE ALSOcpp(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1).
`gcc', `cpp', `as', `ld', and `gdb' entries in info.
Using and Porting GNU CC (for version 2.0), Richard M. Stallman; The C
Preprocessor, Richard M. Stallman; Debugging with GDB: the GNU Source-
Level Debugger, Richard M. Stallman and Roland H. Pesch; Using as: the
GNU Assembler, Dean Elsner, Jay Fenlason & friends; ld: the GNU linker,
Steve Chamberlain and Roland Pesch.
BUGS
For instructions on reporting bugs, see the GCC manual.
COPYING
Copyright 1991, 1992, 1993 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the en‐
tire resulting derived work is distributed under the terms of a permis‐
sion notice identical to this one.
Permission is granted to copy and distribute translations of this manu‐
al into another language, under the above conditions for modified ver‐
sions, except that this permission notice may be included in transla‐
tions approved by the Free Software Foundation instead of in the origi‐
nal English.
AUTHORS
See the GNU CC Manual for the contributors to GNU CC.
GNU Tools 1998/12/16 GCC(1)
