GCC(1)
NAME
gcc, g++ - GNU project C and C++ Compiler (v2.7)
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If we find that the things in this man page that are out
of date cause significant confusion or complaints, we will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
.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
-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 -Winline -Wmissing-prototypes
-Wmissing-declarations -Wnested-externs -Wno-import
-Wparentheses -Wpointer-arith -Wredundant-decls
-Wreturn-type -Wshadow -Wstrict-prototypes -Wswitch
-Wtemplate-debugging -Wtraditional -Wtrigraphs
-Wuninitialized -Wunused -Wwrite-strings
Debugging Options
-a -dletters -fpretend-float -g -glevel -gcoff
-gxcoff -gxcoff+ -gdwarf -gdwarf+ -gstabs -gstabs+
-ggdb -p -pg -save-temps -print-file-name=library
-print-libgcc-file-name -print-prog-name=program
Optimization Options
-fcaller-saves -fcse-follow-jumps -fcse-skip-blocks
-fdelayed-branch -felide-constructors
-fexpensive-optimizations -ffast-math -ffloat-store
-fforce-addr -fforce-mem -finline-functions
-fkeep-inline-functions -fmemoize-lookups
-fno-default-inline -fno-defer-pop
-fno-function-cse -fno-inline -fno-peephole
-fomit-frame-pointer -frerun-cse-after-loop
-fschedule-insns -fschedule-insns2
-fstrength-reduce -fthread-jumps -funroll-all-loops
-funroll-loops -O -O2
Preprocessor Options
-Aassertion -C -dD -dM -dN -Dmacro[=defn] -E -H
-idirafter dir -include file -imacros file -iprefix
file -iwithprefix dir -M -MD -MM -MMD -nostdinc -P
-Umacro -undef
Assembler Option
-Wa,option
Linker Options
-llibrary -nostartfiles -nostdlib -static -shared
-symbolic -Xlinker option -Wl,option -u symbol
Directory Options
-Bprefix -Idir -I- -Ldir
Target Options
-b machine -V version
Configuration Dependent Options
M680x0 Options
-m68000 -m68020 -m68020-40 -m68030 -m68040 -m68881
-mbitfield -mc68000 -mc68020 -mfpa -mnobitfield
-mrtd -mshort -msoft-float
VAX Options
-mg -mgnu -munix
SPARC Options
-mepilogue -mfpu -mhard-float -mno-fpu
-mno-epilogue -msoft-float -msparclite -mv8
-msupersparc -mcypress
Convex Options
-margcount -mc1 -mc2 -mnoargcount
AMD29K Options
-m29000 -m29050 -mbw -mdw -mkernel-registers
-mlarge -mnbw -mnodw -msmall -mstack-check
-muser-registers
M88K Options
-m88000 -m88100 -m88110 -mbig-pic
-mcheck-zero-division -mhandle-large-shift
-midentify-revision -mno-check-zero-division
-mno-ocs-debug-info -mno-ocs-frame-position
-mno-optimize-arg-area -mno-serialize-volatile
-mno-underscores -mocs-debug-info
-mocs-frame-position -moptimize-arg-area
-mserialize-volatile -mshort-data-num -msvr3 -msvr4
-mtrap-large-shift -muse-div-instruction
-mversion-03.00 -mwarn-passed-structs
RS6000 Options
-mfp-in-toc -mno-fop-in-toc
RT Options
-mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs
-mfull-fp-blocks -mhc-struct-return -min-line-mul
-mminimum-fp-blocks -mnohc-struct-return
MIPS Options
-mcpu=cpu type -mips2 -mips3 -mint64 -mlong64
-mlonglong128 -mmips-as -mgas -mrnames -mno-rnames
-mgpopt -mno-gpopt -mstats -mno-stats -mmemcpy
-mno-memcpy -mno-mips-tfile -mmips-tfile
-msoft-float -mhard-float -mabicalls -mno-abicalls
-mhalf-pic -mno-half-pic -G num -nocpp
i386 Options
-m486 -mno-486 -msoft-float -mno-fp-ret-in-387
HPPA Options
-mpa-risc-1-0 -mpa-risc-1-1 -mkernel -mshared-libs
-mno-shared-libs -mlong-calls -mdisable-fpregs
-mdisable-indexing -mtrailing-colon
i960 Options
-mcpu-type -mnumerics -msoft-float
-mleaf-procedures -mno-leaf-procedures -mtail-call
-mno-tail-call -mcomplex-addr -mno-complex-addr
-mcode-align -mno-code-align -mic-compat
-mic2.0-compat -mic3.0-compat -masm-compat
-mintel-asm -mstrict-align -mno-strict-align
-mold-align -mno-old-align
DEC Alpha Options
-mfp-regs -mno-fp-regs -mno-soft-float -msoft-float
System V Options
-G -Qy -Qn -YP,paths -Ym,dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg
-finhibit-size-directive -fnonnull-objects
-fno-common -fno-ident -fno-gnu-linker
-fpcc-struct-return -fpic -fPIC -freg-struct-return
-fshared-data -fshort-enums -fshort-double
-fvolatile -fvolatile-global -fverbose-asm
OVERALL OPTIONS
-x language
Specify explicitly the language for the following
input files (rather than choosing a default based
on the file name suffix) . This option applies to
all following input files until the next `-x' 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
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
`$' 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.
-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
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.
-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
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
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.
-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
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.
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-
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.
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-
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:
+o 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.
+o 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.
+o 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.
+o An unsigned value is compared against zero with `>>'
or `<<='.
-Wimplicit
Warn whenever a function or parameter is implicitly
declared.
-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.
-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;
...
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.
+o 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.
+o A function declared external in one block and then
used after the end of the block.
+o 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.
-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-
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).
-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.
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
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
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.
-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
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-
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
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
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
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
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.
-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
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.
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.
-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.
-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
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-
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.
-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-
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).
This controls the following:
+o Which variant of the assembler syntax to emit
(which you can select independently using
`-mversion-03.00').
+o `-msvr4' makes the C preprocessor recognize `#prag-
ma weak'
+o `-msvr4' makes GCC issue additional declaration di-
rectives used in SVr4.
`-msvr3' is the default for all m88K configurations except
the SVr4 configuration.