GCC(1) GNU Tools GCC(1)
NAME
gcc, g++ - GNU project C and C++ Compiler (gcc-2.95)
SYNOPSIS
gcc [ option | filename ]...
g++ [ option | filename ]...
WARNING
The information in this man page is an extract from the
full documentation 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 volun-
teers 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 documentation.
If we find that the things in this man page that are out
of date cause significant confusion or complaints, we will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
obsolete 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: prepro-
cessing, compilation, assembly, and linking. Source file-
name suffixes identify the source language, but which name
you use for the compiler governs default assumptions:
gcc assumes preprocessed (.i) files are C and assumes C
style linking.
g++ assumes preprocessed (.i) files are C++ and assumes
C++ style linking.
Suffixes of source file names indicate the language and
kind of processing 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
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.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. Com-
mon 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
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-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
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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
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-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' op-
tion. Possible values of language are `c',
`objective-c', `c-header', `c++', `cpp-output',
`assembler', and `assembler-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, com-
pile, assemble, link), you can use `-x' (or filename suf-
fixes) 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 compiler output is an object file corre-
sponding 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 require 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 input file specified.
By default, GCC makes the assembler file name for a
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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 com-
pilation.
-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 prepro-
cessing.
-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, unless 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 preprocessed C source on
standard output.
-v Print (on standard error output) the commands exe-
cuted 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 communi-
cation between the various stages of compilation.
This fails to work on some systems where the assem-
bler 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 accepts:
-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
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`$' 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 compila-
tions done with `-ansi'. Alternate predefined
macros such as __unix__ and __vax__ are also avail-
able, with or without `-ansi'.
The `-ansi' option does not cause non-ANSI programs
to be rejected gratuitously. For that, `-pedantic'
is required in addition 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 key-
word. These words may then be used as identifiers.
You can use __asm__, __inline__ and __typeof__ in-
stead. `-ansi' implies `-fno-asm'.
-fno-builtin
Don't recognize built-in functions that do not be-
gin with two leading underscores. Currently, the
functions affected include _exit, abort, abs, allo-
ca, cos, exit, fabs, labs, memcmp, memcpy, 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 im-
plies the `-fno-builtin' option, and implies that
main has no special requirements.
-fno-strict-prototype
Treat a function declaration with no arguments,
such as `int foo ();', as C would treat it--as say-
ing nothing about the number of arguments or their
types (C++ only). Normally, such a declaration in
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C++ means that the function foo takes no arguments.
-trigraphs
Support ANSI C trigraphs. The `-ansi' option im-
plies `-trigraphs'.
-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 permitted. 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 preprocessor 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 oth-
ers.)
-fenum-int-equiv
Permit implicit conversion of int to enumeration
types (C++ only). Normally GNU C++ allows conver-
sion 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
implementation' (the definition) or `#pragma
interface' (declarations).
When your code is compiled with
`-fexternal-templates', all template instantiations
are external. You must arrange for all necessary
instantiations to appear in the implementation
file; you can do this with a typedef that refer-
ences each instantiation needed. Conversely, when
you compile using the default option
`-fno-external-templates', all template instantia-
tions are explicitly internal.
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-fall-virtual
Treat all possible member functions as virtual, im-
plicitly. All member functions (except for con-
structor functions and new or delete member opera-
tors) 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 second and third arguments. The value of
such an expression is void.
-fthis-is-variable
Permit assignment to this (C++ only). The incorpo-
ration of user-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 de-
fault or like signed char by default.
Ideally, a portable program should always use
signed char or unsigned 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 machines 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
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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 consistent:
the basic integer types such as int are signed
types.
However, when you specify `-traditional', bitfields
are all unsigned 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. `-traditional' also has this ef-
fect.
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 pre-
processing. Some of these options make sense only togeth-
er 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
written. All the `-include' and `-imacros' options
are processed in the order in which they are writ-
ten.
-imacros file
Process file as input, discarding the resulting
output, before processing the regular input file.
Because the output generated from file is discard-
ed, the only effect of `-imacros file' is to make
the macros defined in file available for use in the
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main input. The preprocessor evaluates any `-D'
and `-U' options on the command line before pro-
cessing `-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 directories 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' options.
-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 speci-
fied 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 directo-
ries you specify explicitly.
-nostdinc++
Do not search for header files in the C++-specific
standard directories, but do still search the other
standard directories. (This option is used when
building `libg++'.)
-undef Do not predefine any nonstandard macros. (Includ-
ing architecture 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' com-
mands. Used with the `-E' option.
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-M [ -MG ]
Tell the preprocessor to output a rule suitable for
make describing the dependencies of each object
file. For each source file, the preprocessor out-
puts 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 `\'-new-
line 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 gener-
ated files and assume they live in the same direc-
tory 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 included with `#include file"'. Sys-
tem 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 compila-
tion 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' command.
-MMD Like `-MD' except mention only user header files,
not system header files.
-H Print the name of each header file used, in addi-
tion 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 stan-
dard 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 #question(answer)'. `-A-' disables
the standard assertions that normally describe the
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target machine.
-Dmacro
Define macro macro with the string `1' as its defi-
nition.
-Dmacro=defn
Define macro macro as defn. All instances of
`-D' on the command 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 definitions 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 con-
tents are omitted. Only `#define name' is included
in the output.
ASSEMBLER OPTION
-Wa,option
Pass option as an option to the assembler. If op-
tion contains commas, it is split into multiple op-
tions at the commas.
LINKER OPTIONS
These options come into play when the compiler links ob-
ject files into an executable output file. They are mean-
ingless if the compiler is not doing a link step.
object-file-name
A file name that does not end in a special recog-
nized suffix is considered to name an object file
or library. (Object files are distinguished from
libraries by the linker according to the file con-
tents.) 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 library, which is actually a file named
`liblibrary.a'. The linker then uses this file as
if it had been specified precisely by name.
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The directories searched include several standard
system directories 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 archive file by scan-
ning through it for members which define symbols
that have so far been referenced but not defined.
However, if the linker finds an ordinary object
file rather than a library, 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 or-
der 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 linker.
-static
On systems that support dynamic linking, this pre-
vents 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 objects 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 refer-
ences (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 argu-
ment, you must use `-Xlinker' twice, once for the
option and once for the argument. For example, to
pass `-assert definitions', you must write `-Xlink-
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er -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 commas, it is split into multiple options
at the commas.
-u symbol
Pretend the symbol symbol is undefined, to force
linking of library 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 libraries and for parts of the compiler:
-Idir Append directory dir to the list of directories
searched for include files.
-I- Any directories you specify with `-I' options be-
fore 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' directives. (Ordinari-
ly 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 `#in-
clude "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 sys-
tem directories for header files. Thus, `-I-' and
`-nostdinc' are independent.
-Ldir Add directory dir to the list of directories to be
searched for `-l'.
-Bprefix
This option specifies where to find the executa-
bles, libraries and data files of the compiler it-
self.
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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 unmodi-
fied program name, using the directories 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 necessary.
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_PREFIX 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; reject all programs that use forbidden
extensions.
Valid ANSI standard C programs should compile prop-
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erly with or without this option (though a rare few
will require `-ansi'). However, without this op-
tion, certain GNU extensions and traditional C fea-
tures 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 alternate keywords whose names begin and end
with `__'. Pedantic warnings are also disabled in
the expression that follows __extension__. Howev-
er, only system header files should use these es-
cape 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 compilation.
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
problem.
- A function can return either with or without a val-
ue. (Falling off the end of the function body is
considered returning without a value.) For exam-
ple, this function would evoke such a warning:
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 return.
- An expression-statement or the left-hand side of a
comma expression contains no side effects. To sup-
press 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.
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- 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 de-
clared.
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-dec-
laration.
-Wmain Warn if the main function is declared or defined
with a suspicious type. Typically, it is a func-
tion with external linkage, returning int, and tak-
ing zero or two arguments.
-Wreturn-type
Warn whenever a function is defined with a return-
type that defaults to int. Also warn about any re-
turn statement with no return-value in a function
whose return-type is not void.
-Wunused
Warn whenever a local variable is unused aside from
its declaration, 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 enumeration. (The presence
of a default label prevents this warning.) case
labels outside the enumeration range also provoke
warnings when this option is used.
-Wcomment
Warn whenever a comment-start sequence `/*' appears
in a comment.
-Wtrigraphs
Warn if any trigraphs are encountered (assuming
they are enabled).
-Wformat
Check calls to printf and scanf, etc., to make sure
that the arguments supplied have types appropriate
to the format string specified.
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-Wchar-subscripts
Warn if an array subscript has type char. This is
a common cause of error, as programmers often for-
get 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 com-
pilation, because they require data flow informa-
tion 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 variable 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 delet-
ed 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 despite 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 initialized, but GNU CC doesn't know this.
Here is another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
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GCC(1) GNU Tools GCC(1)
...
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 de-
clare as volatile all the functions you use that
never return.
-Wparentheses
Warn if parentheses are omitted in certain con-
texts.
-Wtemplate-debugging
When using templates in a C++ program, warn if de-
bugging is not yet fully available (C++ only).
-Wall All of the above `-W' options combined. These are
all the options which pertain to usage that we rec-
ommend 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 occasion, in clean programs.
-Wtraditional
Warn about certain constructs that behave differ-
ently in traditional and ANSI C.
- Macro arguments occurring within string constants
in the macro body. These would substitute the ar-
gument in traditional C, but are part of the con-
stant 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 lo-
cal variable.
-Wid-clash-len
Warn whenever two distinct identifiers match in the
first len characters. This may help you prepare a
program that will compile with certain obsolete,
brain-damaged compilers.
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-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 re-
quired 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 copying 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 ar-
gument in the absence of a prototype. This in-
cludes 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 defined or called. (In languages where
you can return an array, this also elicits a warn-
ing.)
-Wstrict-prototypes
Warn if a function is declared or defined without
specifying the argument types. (An old-style func-
tion definition is permitted without a warning if
preceded by a declaration which specifies the argu-
ment types.)
-Wmissing-prototypes
Warn if a global function is defined without a pre-
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GCC(1) GNU Tools GCC(1)
vious prototype declaration. This warning is is-
sued even if the definition itself provides a pro-
totype. The aim is to detect global functions that
fail to be declared in header files.
-Wmissing-declarations
Warn if a global function is defined without a pre-
vious declaration. 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 declara-
tion 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 erroneous
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 signa-
ture that doesn't match any virtual functions from
the base class.
-Winline
Warn if a function can not be inlined, and either
it was declared as inline, or else the -fin-
line-functions option was given.
-Werror
Treat warnings as errors; abort compilation after
any warning.
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DEBUGGING OPTIONS
GNU CC has various special options that are used for de-
bugging either your program or GCC:
-g Produce debugging information in the operating sys-
tem'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 information makes debugging
work better in GDB but will probably make other de-
buggers crash or refuse to read the program. If
you want to control for certain whether to generate
the extra information, 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 opti-
mized code may occasionally produce surprising re-
sults: 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 val-
ues were already at hand; some statements may exe-
cute 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 opti-
mizer for programs that might have bugs.
The following options are useful when GNU CC is generated
with the capability 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 supported), without GDB extensions. This
is the format used by DBX on most BSD systems.
-gstabs+
Produce debugging information in stabs format (if
that is supported), using GNU extensions understood
only by the GNU debugger (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 supported). This is the format used by SDB
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GCC(1) GNU Tools GCC(1)
on most System V systems prior to System V Release
4.
-gxcoff
Produce debugging information in XCOFF format (if
that is supported). This is the format used by the
DBX debugger on IBM RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if
that is supported), using GNU extensions understood
only by the GNU debugger (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 supported). This is the format used by SDB
on most System V Release 4 systems.
-gdwarf+
Produce debugging information in DWARF format (if
that is supported), using GNU extensions understood
only by the GNU debugger (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 backtraces in parts of the program that you
don't plan to debug. This includes descriptions of
functions and external variables, but no informa-
tion about local variables and no line numbers.
Level 3 includes extra information, such as all the
macro definitions present in the program. Some de-
buggers 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
GNU Tools 1999/08/23 24
GCC(1) GNU Tools GCC(1)
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 ex-
tended 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
function 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 available 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 specified by letters. This is used for de-
bugging 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 prepro-
cessing, and write no output.
-dN Dump all macro names, at the end of preprocessing.
-dD Dump all macro definitions, at the end of prepro-
cessing, in addition to normal output.
-dy Dump debugging information during parsing, to stan-
dard error.
-dr Dump after RTL generation, to `file.rtl'.
-dx Just generate RTL for a function instead of compil-
ing it. Usually used with `r'.
-dj Dump after first jump optimization, to `file.jump'.
-ds Dump after CSE (including the jump optimization
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GCC(1) GNU Tools GCC(1)
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 indi-
cating 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 ma-
chine.
-save-temps
Store the usual "temporary" intermediate files per-
manently; place them in the current directory and
name them based on the source file. Thus, compil-
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ing `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 func-
tion.
Without `-O', the compiler's goal is to reduce the
cost of compilation 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 re-
sults 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 optimiza-
tions that do not involve a space-speed tradeoff
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are performed. Loop unrolling and function inlin-
ing 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-func-
tions.
-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 prevents undesirable excess precision on ma-
chines such as the 68000 where the floating regis-
ters (of the 68881) keep more precision than a dou-
ble is supposed to have.
For most programs, the excess precision does only
good, but a few programs rely on the precise defi-
nition 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 function (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
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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 intro-
duces another layer of mechanisms which must be im-
plemented, and so incurs its own overhead.
`-fmemoize-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 com-
piled. 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 helpful 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 mere-
ly 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 com-
piler 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 poten-
tial common subexpressions. When they are not com-
mon subexpressions, instruction combination should
eliminate the separate register-load. I am inter-
ested in hearing about the difference this makes.
-fforce-addr
Force memory address constants to be copied into
registers before doing arithmetic on them. This
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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 in-
structions to save, set up and restore frame point-
ers; 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, because the standard calling sequence auto-
matically handles the frame pointer and nothing is
saved by pretending it doesn't exist. The machine-
description macro FRAME_POINTER_REQUIRED controls
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 function is declared static, then GCC nor-
mally does not output the function as assembler
code in its own right.
-fcaller-saves
Enable values to be allocated in registers that
will be clobbered by function calls, by emitting
extra instructions to save and restore the regis-
ters around such calls. Such allocation is done
only when it seems to result in better code than
would otherwise be produced.
This option is enabled by default on certain ma-
chines, usually those which have no call-preserved
registers to use instead.
-fkeep-inline-functions
Even if all calls to a given function are integrat-
ed, and the function is declared static, neverthe-
less output a separate run-time callable version of
the function.
-fno-function-cse
Do not put function addresses in registers; make
each instruction that calls a constant function
contain the function's address explicitly.
This option results in less efficient code, but
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some strange hacks that alter the assembler output
may be confused by the optimizations performed when
this option is not used.
-fno-peephole
Disable any machine-specific peephole optimiza-
tions.
-ffast-math
This option allows GCC to violate some ANSI or IEEE
rules/specifications in the interest of optimizing
code for speed. For example, it allows the compil-
er 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 func-
tions.
The following options control specific optimizations. The
`-O2' option turns on all of these optimizations except
`-funroll-loops' and `-funroll-all-loops'.
The `-O' option usually turns on the `-fthread-jumps' and
`-fdelayed-branch' options, but specific machines may
change the default optimizations.
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 reduc-
tion and elimination of iteration variables.
-fthread-jumps
Perform optimizations where we check to see if a
jump branches to a location where another compari-
son subsumed by the first is found. If so, the
first branch is redirected to either the destina-
tion 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 compile 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.
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-fcse-follow-jumps
In common subexpression elimination, scan through
jump instructions 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 follow jumps which conditionally skip over
blocks. When CSE encounters 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 di-
rectly 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 expensive.
-fdelayed-branch
If supported for the target machine, attempt to re-
order instructions to exploit instruction slots
available after delayed branch instructions.
-fschedule-insns
If supported for the target machine, attempt to re-
order instructions to eliminate execution stalls
due to required data being unavailable. This helps
machines that have slow floating point or memory
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load instructions by allowing other instructions to
be issued until the result of the load or floating
point instruction is required.
-fschedule-insns2
Similar to `-fschedule-insns', but requests an ad-
ditional pass of instruction scheduling after reg-
ister 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 ma-
chine that you are using. However, it can also be in-
stalled as a cross-compiler, to compile for some other
type of machine. In fact, several different configura-
tions 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 in-
stalled side by side. One of them (probably the newest)
will be the default, but you may sometimes wish to use an-
other.
-b machine
The argument machine specifies the target machine
for compilation. This is useful when you have in-
stalled 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 example, 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. Nor-
mally, it will be a version that is recommended for
general use.
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MACHINE DEPENDENT OPTIONS
Each of the target machine types can have its own special
options, starting with `-m', to choose among various hard-
ware models or configurations--for example, 68010 vs
68020, floating coprocessor or none. A single installed
version of the compiler can compile for any model or con-
figuration, according to the options specified.
Some configurations of the compiler also support addition-
al special options, 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 compiler 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 compiler is configured for 68030-based
systems.
-m68040
Generate output for a 68040. This is the default
when the compiler is configured for 68040-based
systems.
-m68020-40
Generate output for a 68040, without using any of
the new instructions. This results in code which
can run relatively efficiently on either a
68020/68881 or a 68030 or a 68040.
-mfpa Generate output containing Sun FPA instructions for
floating point.
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-msoft-float
Generate output containing library calls for float-
ing point. WARNING: the requisite libraries are
not part of GNU CC. Normally 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 li-
brary 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 `-mnobitfield'.
-mbitfield
Do use the bit-field instructions. `-m68020' im-
plies `-mbitfield'. 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 argu-
ments return with the rtd instruction, 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); otherwise incorrect code will
be generated for calls to those functions.
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 processors, 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 assump-
tion that you will assemble with the GNU assembler.
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-mg Output code for g-format floating point numbers in-
stead of d-format.
These `-m' switches are supported on the SPARC:
-mfpu
-mhard-float
Generate output containing floating point instruc-
tions. This is the default.
-mno-fpu
-msoft-float
Generate output containing library calls for float-
ing point. Warning: there is no GNU floating-point
library for SPARC. Normally the facilities of the
machine's usual C compiler are used, but this can-
not 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 exit 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
function.
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 SPARClite), GCC generates code for the v7
variant of the SPARC architecture.
-mv8 will give you SPARC v8 code. The only differ-
ence from v7 code is that the compiler emits the
integer multiply and integer divide instructions
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GCC(1) GNU Tools GCC(1)
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 optimised.
With -mcypress (the default), the compiler optimis-
es code for the Cypress CY7C602 chip, as used in
the SparcStation/SparcServer 3xx series. This is
also appropriate for the older SparcStation 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 non-
portable Convex and Vax programs need this word.
(Debuggers don't, except for functions with vari-
able-length argument lists; this info is in the
symbol table.)
-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 sup-
ported 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 de-
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fault.
-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 addresses 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 de-
fault.
-mkernel-registers
Generate references to registers gr64-gr95 instead
of gr96-gr127. This option can be used when com-
piling kernel code that wants a set of global reg-
isters 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 ad-
justment. This is often used for kernel code.
These `-m' options are defined for Motorola 88K architec-
tures:
-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.
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-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 compilation flags used.
-mno-underscores
In assembler output, emit symbol names without
adding an underscore 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 options to avoid including
(or to include explicitly) additional code to de-
tect 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 registers used in each stack frame) as spec-
ified 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 particular place in stack frames, as
specified in OCS. The DG/UX, Delta88 SVr3.2, and
BCS configurations use `-mocs-frame-position'; oth-
er 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. `-moptimize-arg-area' saves space, but may
break some debuggers (not GDB).
`-mno-optimize-arg-area' conforms better to stan-
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dards. By default GCC does not optimize the argu-
ment 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 ref-
erences affected 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 sequen-
tial consistency of volatile memory references.
GNU CC always guarantees consistency by default,
for the preferred processor submodel. How this is
done depends on the submodel.
The m88100 processor does not reorder memory refer-
ences and so always provides sequential consisten-
cy. If you use `-m88100', GNU CC does not generate
any special instructions for sequential consisten-
cy.
The order of memory references made by the m88110
processor does not always match the order of the
instructions requesting those references. In par-
ticular, a load instruction may execute before a
preceding store instruction. Such reordering vio-
lates sequential consistency of volatile memory
references, when there are multiple processors.
When you use `-m88000' or `-m88110', GNU CC gener-
ates 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 safely forgo this guarantee,
you may use the option `-mno-serialize-volatile'.
If you use the `-m88100' option but require sequen-
tial consistency when running on the m88110 proces-
sor, you should use `-mserialize-volatile'.
-msvr4
-msvr3 Turn on (`-msvr4') or off (`-msvr3') compiler ex-
tensions related to System V release 4 (SVr4).
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This controls the following:
- Which variant of the assembler syntax to emit
(which you can select independently using
`-mversion-03.00').
- `-msvr4' makes the C preprocessor recognize `#prag-
ma weak'
- `-msvr4' makes GCC issue additional declaration di-
rectives 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; respectively, 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 in-
struction by default. Use this option to specify
that it's safe to use the divide instruction.
-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 real-ELF flavor is used.
All other configurations ignore this option.
-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 problems. 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.
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These `-m' options are defined for the IBM RT PC:
-min-line-mul
Use an in-line code sequence for integer multi-
plies. This is the default.
-mcall-lib-mul
Call lmul$$ for integer multiples.
-mfull-fp-blocks
Generate full-size floating point data blocks, in-
cluding the minimum amount of scratch space recom-
mended 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 convention in which floating point argu-
ments are passed in floating point registers. Note
that varargs.h and stdargs.h will not work with
floating point operands if this option is speci-
fied.
-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 compati-
bility with the MetaWare HighC (hc) compiler. Use
`-fpcc-struct-return' for compatibility 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 com-
puters:
-mcpu=cpu-type
Assume the defaults for the machine type cpu-type
when scheduling instructions. The default cpu-type
is default, which picks the longest cycles times
for any of the machines, in order that the code run
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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 instructions). 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 platforms 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 en-
capsulate 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
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GCC(1) GNU Tools GCC(1)
section, to all the MIPS assembler 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 causes 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) in-
stead 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 assembler has generated it
to add debug support. If mips-tfile is not run,
then no local variables will be available to the
debugger. In addition, stage2 and stage3 objects
will have the temporary 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 float-
ing point. WARNING: the requisite libraries are
not part of GNU CC. Normally 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 li-
brary functions for cross-compilation.
-mhard-float
Generate output containing floating point instruc-
tions. This is the default if you use the unmodi-
fied sources.
-mfp64 Assume that the FR bit in the status word is on,
and that there are 32 64-bit floating point regis-
ters, instead of 32 32-bit floating point regis-
ters. You must also specify the -mcpu=r4000 and
-mips3 switches.
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-mfp32 Assume that there are 32 32-bit floating point reg-
isters. 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 ex-
tern references into the data section and load them
up, rather than put the references in the text sec-
tion. 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 in-
stead of the normal data or bss section. This al-
lows the assembler to emit one word memory refer-
ence 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 float-
ing point. Warning: the requisite libraries are
not part of GNU CC. Normally 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 li-
brary 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.
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-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 regis-
ter, 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 returned in ordinary CPU registers instead.
These `-m' options are defined for the HPPA family of com-
puters:
-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.
Specifically, 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 func-
tion yet, and is not on by default for any PA tar-
get. 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 libraries. 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 ker-
nels which perform lazy context switching of float-
ing point registers. If you use this option and
attempt to perform floating point operations, the
compiler will abort.
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-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 assemblers).
These `-m' options are defined for the Intel 80960 family
of computers:
-mcpu-type
Assume the defaults for the machine type cpu-type
for instruction and addressing-mode availability
and alignment. The default 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 sub-
stituted by the assembler or linker, but less effi-
cient code in other cases, such as calls via func-
tion pointers, or using a linker that doesn't sup-
port this optimization.
-mtail-call
-mno-tail-call
Do (or do not) make additional attempts (beyond
those of the machine-independent portions of the
compiler) to optimize tail-recursive calls into
branches. You may not want to do this because the
detection of cases where this is not valid is not
totally 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 addressing modes may not be
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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 implementations 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 In-
tel's gcc release 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 implemen-
tations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point in-
structions 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 floating-point operations. If
you are compiling for an Alpha without floating-
point operations, you must ensure that the library
is built so as not to call them.
Note that Alpha implementations without floating-
point operations are required to have floating-
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point registers.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the float-
ing-point register 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
floating-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 re-
turn 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 re-
store, any floating-point registers.
These additional options are available on System V Release
4 for compatibility 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 appropriate, instead of
supplying linker options on the gcc command line.
-Qy Identify the versions of each tool used by the com-
piler, in a .ident assembler directive in the out-
put.
-Qn Refrain from adding .ident directives to the output
file (this is the default).
-YP,dirs
Search the directories dirs, and no others, for li-
braries specified with `-l'. You can separate di-
rectory entries in dirs from one another with
colons.
-Ym,dir
Look in the directory dir to find the M4 preproces-
sor. The assembler 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
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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 ex-
ample, 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 requires extra code, however, and it is un-
necessary for many programs. 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 convention is less efficient
for small structures, and on many machines it fails
to be reentrant; but it has the advantage of allow-
ing intercallability between GCC-compiled code and
PCC-compiled 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-return, 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 compilation be shared data rather than private
data. The distinction makes sense only on certain
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operating systems, where shared data is shared be-
tween processes running the same program, while
private data exists in one copy per process.
-fno-common
Allocate even uninitialized global variables in the
bss section of the object file, rather than gener-
ating them as common blocks. This has the effect
that if the same variable is declared (without ex-
tern) 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
also requires using the collect2 program to make
sure the system linker includes constructors and
destructors. (collect2 is included in the GNU CC
distribution.) For systems which must use col-
lect2, the compiler driver gcc is configured to do
this automatically.
-finhibit-size-directive
Don't output a .size assembler directive, or any-
thing 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 op-
tion 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 op-
tion 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.
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-fpic If supported for the target machines, generate po-
sition-independent 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 descrip-
tion 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 regis-
ter that is clobbered by function calls. It may be
allocated for temporaries 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 regis-
ter saved by functions. It may be allocated even
for temporaries or variables 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
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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 object classes, to save space in most
of the object files that use those classes. Nor-
mally, local copies of certain information (backup
copies of inline member functions, debugging infor-
mation, and the internal tables that implement vir-
tual functions) must be kept in each object file
that includes class definitions. 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 it-
self uses `#pragma implementation'). 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 mem-
ber functions, debugging information, and the in-
ternal tables used to implement virtual functions
are all generated in implementation files.
If you use `#pragma implementation' with no argu-
ment, it applies to an include file with the same
basename as your source file; for example, in
`allclass.cc', `#pragma implementation' by itself
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 sin-
gle 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
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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 ALSO
cpp(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; Debug-
ging 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 ver-
sions of this manual under the conditions for verbatim
copying, provided that the entire resulting derived work
is distributed under the terms of a permission notice
identical to this one.
Permission is granted to copy and distribute translations
of this manual into another language, under the above con-
ditions for modified versions, except that this permission
notice may be included in translations approved by the
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GCC(1) GNU Tools GCC(1)
Free Software Foundation instead of in the original En-
glish.
AUTHORS
See the GNU CC Manual for the contributors to GNU CC.
GNU Tools 1999/08/23 55
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others. |
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