NAME
gcc, g++ - GNU project C and C++ Compiler (gcc2.95)
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 volunteers want to maintain it. If you find a
discrepancy between the man page and the software, please check the Info file, which is the
authoritative 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If we find that the things in this man page that are out of date cause significant confusion or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: preprocessing, compilation, assembly, and linking. Source filename 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 ObjectiveC source; preprocess, compile, assemble
.i preprocessed C; compile, assemble
.ii preprocessed C++; compile, assemble
.s Assembler source; assemble
.S Assembler source; preprocess, assemble
.h Preprocessor file; not usually named on command line
Files with other suffixes are passed to the linker. Common cases include:
.o Object file
.a Archive file
Linking is always the last stage unless you use one of the -c, -S, or -E options to avoid it (or
unless compilation errors stop the whole process). For the link stage, all .o files
corresponding to source files, -l libraries, unrecognized filenames (including named .o object
files and .a archives) are passed to the linker in commandline 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 nondefault forms are shown here.
Here is a summary of all the options, grouped by type. Explanations are in the following
sections.
Overall Options
-c -S -E -o file -pipe -v -x language
Language Options -ansi -fall-virtual -fcond-mismatch -fdollars-in-identifiers
-fenum-int-equiv -fexternal-templates -fno-asm -fno-builtin
-fhosted -fno-hosted -ffreestanding -fno-freestanding
-fno-strict-prototype -fsigned-bitfields -fsigned-char
-fthis-is-variable -funsigned-bitfields -funsigned-char
-fwritable-strings -traditional -traditional-cpp -trigraphs
Warning Options -fsyntax-only -pedantic -pedantic-errors -w -W -Wall
-Waggregate-return -Wcast-align -Wcast-qual -Wchar-subscript
-Wcomment -Wconversion -Wenum-clash -Werror -Wformat
-Wid-clash-len -Wimplicit -Wimplicit-int
-Wimplicit-function-declaration -Winline -Wlong-long -Wmain
-Wmissing-prototypes -Wmissing-declarations -Wnested-externs
-Wno-import -Wparentheses -Wpointer-arith -Wredundant-decls
-Wreturn-type -Wshadow -Wstrict-prototypes -Wswitch
-Wtemplate-debugging -Wtraditional -Wtrigraphs -Wuninitialized
-Wunused -Wwrite-strings
Debugging Options
-a -dletters -fpretend-float -g -glevel -gcoff -gxcoff -gxcoff+
-gdwarf -gdwarf+ -gstabs -gstabs+ -ggdb -p -pg -save-temps
-print-file-name=library -print-libgcc-file-name
-print-prog-name=program
Optimization Options
-fcaller-saves -fcse-follow-jumps -fcse-skip-blocks
-fdelayed-branch -felide-constructors -fexpensive-optimizations
-ffast-math -ffloat-store -fforce-addr -fforce-mem
-finline-functions -fkeep-inline-functions -fmemoize-lookups
-fno-default-inline -fno-defer-pop -fno-function-cse -fno-inline
-fno-peephole -fomit-frame-pointer -frerun-cse-after-loop
-fschedule-insns -fschedule-insns2 -fstrength-reduce
-fthread-jumps -funroll-all-loops -funroll-loops -O -O2 -O3
Preprocessor Options
-Aassertion -C -dD -dM -dN -Dmacro[=defn ] -E -H -idirafter
dir -include file -imacros file -iprefix file -iwithprefix dir -M -MD
-MM -MMD -nostdinc -P -Umacro -undef
Assembler Option -Wa,option
Linker Options -llibrary -nostartfiles -nostdlib -static -shared -symbolic
-Xlinker option -Wl,option -u symbol
Directory Options -Bprefix -Idir -I- -Ldir
Target Options -b machine -V version
Configuration Dependent Options
M680x0 Options
-m68000 -m68020 -m68020-40 -m68030 -m68040 -m68881
-mbitfield -mc68000 -mc68020 -mfpa -mnobitfield -mrtd -mshort
-msoft-float
M68hc1x Options
-m68hc11 -m68hc12 -mshort -msoft-reg-count num -mautoincdec
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
-mcputype -mnumerics -msoft-float -mleaf-procedures
-mno-leaf-procedures -mtail-call -mno-tail-call -mcomplex-addr
-mno-complex-addr -mcode-align -mno-code-align -mic-compat
-mic2.0-compat -mic3.0-compat -masm-compat -mintel-asm
-mstrict-align -mno-strict-align -mold-align -mno-old-align
DEC Alpha Options
-mfp-regs -mno-fp-regs -mno-soft-float -msoft-float
System V Options
-G -Qy -Qn -YP,paths -Ym,dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg
-finhibit-size-directive -fnonnull-objects -fno-common -fno-ident
-fno-gnu-linker -fpcc-struct-return -fpic -fPIC
-freg-struct-return -fshared-data -fshort-enums -fshort-double
-fvolatile -fvolatile-global -fverbose-asm
OVERALL OPTIONS
-x language Specify explicitly the language for the following input files (rather than
choosing a default based on the file name suffix) . This option applies to
all following input files until the next ` -x ' option. Possible values of lan
guage 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, compile, assemble, link), you can use
` -x ' (or filename suffixes) to tell gcc where to start, and one of the options ` -c ', ` -S ', or
` -E ' to say where gcc is to stop. Note that some combinations (for example, ` -x cpp-out
put -E ') instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The compiler out
put is an object file corresponding to each source file.
By default, GCC makes the object file name for a source file by replac
ing 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 nonassembler input file specified.
By default, GCC makes the assembler file name for a source file by re
placing the suffix ` .c ', ` .i ', etc., with ` .s '. Use -o to select another
name.
GCC ignores any input files that don't require compilation.
-E Stop after the preprocessing stage; do not run the compiler proper. The
output is preprocessed source code, which is sent to the standard out
put.
GCC ignores input files which don't require preprocessing.
-o file Place output in file file. This applies regardless to whatever sort of out
put 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 executed to run the
stages of compilation. Also print the version number of the compiler
driver program and of the preprocessor and the compiler proper.
-pipe Use pipes rather than temporary files for communication between the
various stages of compilation. This fails to work on some systems
where the assembler cannot read from a pipe; but the GNU assembler
has no trouble.
LANGUAGE OPTIONS
The following options control the dialect of C that the compiler accepts:
-ansi Support all ANSI standard C programs.
This turns off certain features of GNU C that are incompatible with AN
SI 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 us
ing. It also enables the undesirable and rarely used ANSI trigraph fea
ture, and disallows ` $ ' as part of identifiers.
The alternate keywords __asm__, __extension__, __inline__ and
__typeof__ continue to work despite ` -ansi '. You would not want to
use them in an ANSI C program, of course, but it is useful to put them in
header files that might be included in compilations done with ` -ansi '.
Alternate predefined macros such as __unix__ and __vax__ are also
available, with or without ` -ansi '.
The ` -ansi ' option does not cause nonANSI 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 keyword. These words
may then be used as identifiers. You can use __asm__, __inline__
and __typeof__ instead. ` -ansi ' implies ` -fno-asm '.
-fno-builtin Don't recognize builtin functions that do not begin with two leading un
derscores. Currently, the functions affected include _exit, abort, abs,
alloca, cos, exit, fabs, labs, memcmp, memcpy, sin, sqrt, strcmp, str
cpy, and strlen.
The ` -ansi ' option prevents alloca and _exit from being builtin func
tions.
-fhosted Compile for a hosted environment; this implies the ` -fbuiltin ' option,
and implies that suspicious declarations of main should be warned
about.
-ffreestanding Compile for a freestanding environment; this implies the ` -fnobuiltin '
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 itas saying 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 implies ` -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 others.)
-fenum-int-equiv Permit implicit conversion of int to enumeration types (C++ only). Nor
mally GNU C++ allows conversion of enum to int, but not the other
way around.
-fexternal-templates
Produce smaller code for template declarations, by generating only a
single copy of each template function where it is defined (C++ only). To
use this option successfully, you must also mark all files that use tem
plates 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 instanti
ations to appear in the implementation file; you can do this with a type
def that references each instantiation needed. Conversely, when you
compile using the default option ` -fno-external-templates ', all tem
plate instantiations are explicitly internal.
-fall-virtual Treat all possible member functions as virtual, implicitly. All member
functions (except for constructor functions and new or delete member
operators) are treated as virtual functions of the class where they ap
pear.
This does not mean that all calls to these member functions will be made
through the internal table of virtual functions. Under some circum
stances, the compiler can determine that a call to a given virtual func
tion 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 incorporation of userde
fined free store management into C++ has made assignment to ` this '
an anachronism. Therefore, by default it is invalid to assign to this with
in a class member function. However, for backwards compatibility, you
can make it valid with ` -fthisisvariable '.
-funsigned-char Let the type char be unsigned, like unsigned char.
Each kind of machine has a default for what char should be. It is either
like unsigned char by default or like signed char by default.
Ideally, a portable program should always use signed char or unsigned
char when it depends on the signedness of an object. But many pro
grams 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 neg
ative 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 de
clared 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 effect.
Writing into string constants is a very bad idea; ``constants'' should be
constant.
PREPROCESSOR OPTIONS
These options control the C preprocessor, which is run on each C source file before actual
compilation.
If you use the ` -E ' option, GCC does nothing except preprocessing. Some of these options
make sense only together with ` -E ' because they cause the preprocessor output to be un
suitable 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 ', regard
less 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 processing
` -imacrosfile ', regardless of the order in which they are written. All
the ` -include ' and ` -imacros ' options are processed in the order in
which they are written.
-idirafter dir Add the directory dir to the second include path. The 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 pre
viously with ` -iprefix '.
-nostdinc Do not search the standard system directories for header files. Only the
directories you have specified with ` -I ' options (and the current direc
tory, if appropriate) are searched.
By using both ` -nostdinc ' and ` -I- ', you can limit the includefile
search file to only those directories 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. (Including 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 ' op
tion.
-P Tell the preprocessor not to generate ` #line ' commands. Used with the
` -E ' option.
-M [ -MG ] Tell the preprocessor to output a rule suitable for make describing the
dependencies of each object file. For each source file, the preprocessor
outputs one makerule whose target is the object file name for that
source file and whose dependencies are all the files ` #include 'd in it.
This rule may be a single line or may be continued with ` \ 'newline if it
is long. The list of rules is printed on standard output instead of the pre
processed C program.
` -M ' implies ` -E '.
` -MG ' says to treat missing header files as generated files and assume
they live in the same directory as the source file. It must be specified in
addition to ` -M '.
-MM [ -MG ] Like ` -M ' but the output mentions only the user header files included
with ` #include file"'. System header files included with ` #include
<file>' are omitted.
-MD Like ` -M ' but the dependency information is written to files with
names made by replacing ` .o ' with ` .d ' at the end of the output file
names. This is in addition to compiling the file as specified` -MD '
does not inhibit ordinary compilation the way ` -M ' does.
The Mach utility ` md ' can be used to merge the ` .d ' files into a single
dependency file suitable for using with the ` make ' command.
-MMD Like ` -MD ' except mention only user header files, not system header
files.
-H Print the name of each header file used, in addition to other normal ac
tivities.
-Aquestion(answer)
Assert the answer answer for question, in case it is tested with a pre
processor conditional such as ` #if #question(answer) '. ` -A- ' disables
the standard assertions that normally describe the target machine.
-Aquestion (answer) Assert the answer answer for question, in case it is tested
with a preprocessor conditional such as ` #if #question(answer)'.
` -A ' disables the standard assertions that normally describe the target
machine.
-Dmacro Define macro macro with the string ` 1 ' as its definition.
-Dmacro=defn Define macro macro as defn. All instances of ` -D ' on the command
line are processed before any ` -U ' options.
-Umacro Undefine macro macro. ` -U ' options are evaluated after all ` -D ' op
tions, 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 contents are omitted.
Only ` #define name ' is included in the output.
ASSEMBLER OPTION
-Wa,option Pass option as an option to the assembler. If option contains commas,
it is split into multiple options at the commas.
LINKER OPTIONS
These options come into play when the compiler links object files into an executable output
file. They are meaningless if the compiler is not doing a link step.
objectfilename A file name that does not end in a special recognized suffix is considered
to name an object file or library. (Object files are distinguished from li
braries by the linker according to the file contents.) If GCC does a link
step, these object files are used as input to the linker.
-llibrary Use the library named library when linking.
The linker searches a standard list of directories for the 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 filesarchive files whose
members are object files. The linker handles an archive file by scanning
through it for members which define symbols that have so far been ref
erenced 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 order to link an Objective
C program.
-nostartfiles Do not use the standard system startup files when linking. The standard
libraries are used normally.
-nostdlib Don't use the standard system libraries and startup files when linking.
Only the files you specify will be passed to the linker.
-static On systems that support dynamic linking, this prevents linking with the
shared libraries. On other systems, this option has no effect.
-shared Produce a shared object which can then be linked with other 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 references (unless overridden by the link editor
option ` -Xlinker -z -Xlinker defs '). Only a few systems support this
option.
-Xlinker option Pass option as an option to the linker. You can use this to supply sys
temspecific linker options which GNU CC does not know how to rec
ognize.
If you want to pass an option that takes an argument, you must use
` -Xlinker ' twice, once for the option and once for the argument. For
example, to pass ` -assert definitions ', you must write ` -Xlinker -as
sert -Xlinker definitions '. It does not work to write ` -Xlinker "-as
sert definitions" ', because this passes the entire string as a single argu
ment, 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 mod
ules 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 before the ` -I- ' option
are searched only for the case of ` #include "file" '; they are not
searched for ` #include <file>'.
If additional directories are specified with ` -I ' options after the ` -I- ',
these directories are searched for all ` #include ' directives. (Ordinarily
all ` -I ' directories are used this way.)
In addition, the ` -I- ' option inhibits the use of the current directory
(where the current input file came from) as the first search directory for
` #include "file" '. There is no way to override this effect of ` -I- '.
With ` -I. ' you can specify searching the directory which was current
when the compiler was invoked. That is not exactly the same as what
the preprocessor does by default, but it is often satisfactory.
` -I- ' does not inhibit the use of the standard system 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 executables, libraries and data
files of the compiler itself.
The compiler driver program runs one or more of the subprograms
` cpp ', ` cc1 ' (or, for C++, ` cc1plus '), ` as ' and ` ld '. It tries prefix as a
prefix for each program it tries to run, both with and without
` machine/version/ '.
For each subprogram to be run, the compiler driver first tries the ` -B '
prefix, if any. If that name is not found, or if ` -B ' was not specified, the
driver tries two standard prefixes, which are ` /usr/lib/gcc/ ' and
` /usr/local/lib/gcclib/ '. If neither of those results in a file name that is
found, the compiler driver searches for the unmodified program name,
using the directories specified in your ` PATH ' environment variable.
The runtime 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_EX
EC_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 erro
neous 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 properly with or with
out this option (though a rare few will require ` -ansi '). However, with
out this option, certain GNU extensions and traditional C features are
supported as well. With this option, they are rejected. There is no rea
son 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__. How
ever, only system header files should use these escape routes; applica
tion programs should avoid them.
-pedantic-errors Like ` -pedantic ', except that errors are produced rather than warn
ings.
-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 value. (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 expressionstatement or the lefthand side of a comma expression
contains no side effects. To suppress the warning, cast the unused ex
pression to void. For example, an expression such as ` x[i,j] ' will cause
a warning, but ` x[(void)i,j] ' will not.
· An unsigned value is compared against zero with ` > ' or ` <= '.
-Wimplicitint Warn whenever a declaration does not specify a type.
-Wimplicitfunctiondeclaration
Warn whenever a function is used before being declared.
-Wimplicit Same as Wimplicitint and Wimplicitfunctiondeclaration.
-Wmain Warn if the main function is declared or defined with a suspicious type.
Typically, it is a function with external linkage, returning int, and taking
zero or two arguments.
-Wreturn-type Warn whenever a function is defined with a returntype that defaults to
int. Also warn about any return statement with no returnvalue in a
function whose returntype 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 out
side the enumeration range also provoke warnings when this option is
used.
-Wcomment Warn whenever a commentstart 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 er
ror, as programmers often forget that this type is signed on some ma
chines.
-Wuninitialized An automatic variable is used without first being initialized.
These warnings are possible only in optimizing compilation, because
they require data flow information that is computed only when optimiz
ing. 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 deleted by data flow analysis before the warnings are printed.
These warnings are made optional because GNU CC is not smart
enough to see all the reasons why the code might be correct despite ap
pearing 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 declare as volatile all the
functions you use that never return.
-Wparentheses Warn if parentheses are omitted in certain contexts.
-Wtemplate-debugging
When using templates in a C++ program, warn if debugging is not yet
fully available (C++ only).
-Wall All of the above ` -W ' options combined. These are all the options
which pertain to usage that we recommend avoiding and that we be
lieve is easy to avoid, even in conjunction with macros.
The remaining ` -W... ' options are not implied by ` -Wall ' because they warn about con
structions that we consider reasonable to use, on occasion, in clean programs.
-Wtraditional Warn about certain constructs that behave differently in traditional and
ANSI C.
· Macro arguments occurring within string constants in the macro body.
These would substitute the argument in traditional C, but are part of the
constant in ANSI C.
· A function declared external in one block and then used after the end of
the block.
· A switch statement has an operand of type long.
-Wshadow Warn whenever a local variable shadows another local variable.
-Wid-clash-len Warn whenever two distinct identifiers match in the first len characters.
This may help you prepare a program that will compile with certain ob
solete, braindamaged 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 calcu
lations 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 ordi
nary char *.
-Wcast-align Warn whenever a pointer is cast such that the required alignment of the
target is increased. For example, warn if a char * is cast to an int * on
machines where integers can only be accessed at two or fourbyte
boundaries.
-Wwrite-strings Give string constants the type const char[length] so that copying the
address of one into a nonconst char * pointer will get a warning. These
warnings will help you find at compile time code that can try to write in
to a string constant, but only if you have been very careful about using
const in declarations and prototypes. Otherwise, it will just be a nui
sance; this is why we did not make ` -Wall ' request these warnings.
-Wconversion Warn if a prototype causes a type conversion that is different from what
would happen to the same argument in the absence of a prototype. This
includes conversions of fixed point to floating and vice versa, and con
versions changing the width or signedness of a fixed point argument ex
cept 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
warning.)
-Wstrict-prototypes
Warn if a function is declared or defined without specifying the argu
ment types. (An oldstyle function definition is permitted without a
warning if preceded by a declaration which specifies the argument
types.)
-Wmissing-prototypes
Warn if a global function is defined without a previous prototype decla
ration. This warning is issued 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 previous declaration. Do
so even if the definition itself provides a prototype. Use this option to de
tect global functions that are not declared in header files.
-Wredundantdecls
Warn if anything is declared more than once in the same scope, even in
cases where multiple declaration is valid and changes nothing.
-Wnestedexterns Warn if an extern declaration is encountered within an function.
-Wenum-clash Warn about conversion between different enumeration types (C++ on
ly).
-Wlonglong 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 signature that doesn't match any virtual
functions from the base class.
-Winline Warn if a function can not be inlined, and either it was declared as inline,
or else the -finline-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 debugging either your program or
GCC:
-g Produce debugging information in the operating system's native format
(stabs, COFF, XCOFF, or DWARF). GDB can work with this debug
ging information.
On most systems that use stabs format, ` -g ' enables use of extra de
bugging 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 be
low).
Unlike most other C compilers, GNU CC allows you to use ` -g ' with
` -O '. The shortcuts taken by optimized code may occasionally pro
duce surprising results: some variables you declared may not exist at
all; flow of control may briefly move where you did not expect it; some
statements may not be executed because they compute constant results
or their values were already at hand; some statements may execute in
different places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes
it reasonable to use the optimizer for programs that might have bugs.
The following options are useful when GNU CC is generated with the capability for more
than one debugging format.
-ggdb Produce debugging information in the native format (if that is support
ed), 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), us
ing 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 Sys
tem 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 descrip
tions of functions and external variables, but no information about local
variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions pre
sent in the program. Some debuggers support macro expansion when
you use ` -g3 '.
-p Generate extra code to write profile information suitable for the analysis
program prof.
-pg Generate extra code to write profile information suitable for the analysis
program gprof.
-a Generate extra code to write profile information for basic blocks, which
will record the number of times each basic block is executed. This data
could be analyzed by a program like tcov. Note, however, that the for
mat of the data is not what tcov expects. Eventually GNU gprof should
be extended to process this data.
-ax Generate extra code to read basic block profiling parameters from file
`bb.in' and write profiling results to file `bb.out'. `bb.in' contains a list of
functions. Whenever a function on the list is entered, profiling is turned
on. When the outmost 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 file
names. 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 writ
ten to file `bbtrace.gz'. `__bb_hidecall__' will cause call instructions to
be excluded from the trace. `__bb_showret__' will cause return in
structions to be included in the trace.
-dletters Says to make debugging dumps during compilation at times specified by
letters. This is used for debugging the compiler. The file names for
most of the dumps are made by appending a word to the source file
name (e.g. ` foo.c.rtl ' or ` foo.c.jump ').
-dM Dump all macro definitions, at the end of preprocessing, and write no
output.
-dN Dump all macro names, at the end of preprocessing.
-dD Dump all macro definitions, at the end of preprocessing, in addition to
normal output.
-dy Dump debugging information during parsing, to standard error.
-dr Dump after RTL generation, to ` file.rtl '.
-dx Just generate RTL for a function instead of compiling it. Usually used
with ` r '.
-dj Dump after first jump optimization, to ` file.jump '.
-ds Dump after CSE (including the jump optimization that sometimes fol
lows 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 er
ror.
-dp Annotate the assembler output with a comment indicating which pat
tern and alternative was used.
-fpretend-float When running a crosscompiler, pretend that the target machine uses
the same floating point format as the host machine. This causes incor
rect output of the actual floating constants, but the actual instruction se
quence will probably be the same as GNU CC would make when run
ning on the target machine.
-save-temps Store the usual ``temporary'' intermediate files permanently; place them
in the current directory and name them based on the source file. Thus,
compiling ` foo.c ' with ` -c -save-temps ' would produce files
` foo.cpp ' and ` foo.s ', as well as ` foo.o '.
-print-file-name=library
Print the full absolute name of the library file library that would be used
when linkingand do not do anything else. With this option, GNU CC
does not compile or link anything; it just prints the file name.
-print-libgcc-file-name
Same as ` -print-file-name=libgcc.a '.
-print-prog-name=program
Like ` -print-file-name ', but searches for a program such as `cpp'.
OPTIMIZATION OPTIONS
These options control various sorts of optimizations:
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot
more memory for a large function.
Without ` -O ', the compiler's goal is to reduce the cost of compilation
and to make debugging produce the expected results. Statements are
independent: if you stop the program with a breakpoint between state
ments, you can then assign a new value to any variable or change the
program counter to any other statement in the function and get exactly
the results you would expect from the source code.
Without ` -O ', only variables declared register are allocated in regis
ters. 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 oth
er flags may also be turned on.
-O2 Optimize even more. Nearly all supported optimizations that do not in
volve a spacespeed tradeoff are performed. Loop unrolling and func
tion inlining are not done, for example. As compared to -O, this option
increases both compilation time and the performance of the generated
code.
-O3 Optimize yet more. This turns on everything -O2 does, along with also
turning on -finline-functions.
-O0 Do not optimize.
If you use multiple -O options, with or without level numbers, the last
such option is the one that is effective.
Options of the form ` -fflag ' specify machineindependent flags. Most flags have both posi
tive and negative forms; the negative form of ` -ffoo ' would be ` -fno-foo '. The following
list shows only one formthe 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 undesir
able excess precision on machines such as the 68000 where the floating
registers (of the 68881) keep more precision than a double is supposed
to have.
For most programs, the excess precision does only good, but a few pro
grams rely on the precise definition of IEEE floating point. Use
` -ffloat-store ' for such programs.
-fmemoize-lookups
-fsave-memoized Use heuristics to compile faster (C++ only). These heuristics are not
enabled by default, since they are only effective for certain input files.
Other input files compile more slowly.
The first time the compiler must build a call to a member 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 conver
sions need to be made); and (3) check the visibility of the member func
tion 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 in
troduces another layer of mechanisms which must be implemented, and
so incurs its own overhead. ` -fmemoize-lookups ' enables the soft
ware cache.
Because access privileges (visibility) to members and member func
tions may differ from one function context to the next, g++ may need to
flush the cache. With the ` -fmemoize-lookups ' flag, the cache is
flushed after every function that is compiled. The `-fsave-memoized'
flag enables the same software cache, but when the compiler deter
mines 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 ac
cess privileges as every other, and the cache need not be flushed.
-fno-default-inline
Don't make member functions inline by default merely because they are
defined inside the class scope (C++ only).
-fno-defer-pop Always pop the arguments to each function call as soon as that function
returns. For machines which must pop arguments after a function call,
the compiler normally lets arguments accumulate on the stack for sev
eral function calls and pops them all at once.
-fforce-mem Force memory operands to be copied into registers before doing arith
metic on them. This may produce better code by making all memory
references potential common subexpressions. When they are not com
mon subexpressions, instruction combination should eliminate the sepa
rate registerload. I am interested in hearing about the difference this
makes.
-fforce-addr Force memory address constants to be copied into registers before do
ing arithmetic on them. This may produce better code just as
` -fforce-mem ' may. I am interested in hearing about the difference
this makes.
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that don't need
one. This avoids the instructions to save, set up and restore frame
pointers; it also makes an extra register available in many functions. It
also makes debugging impossible on most machines.
On some machines, such as the Vax, this flag has no effect, because the
standard calling sequence automatically handles the frame pointer and
nothing is saved by pretending it doesn't exist. The machinedescription
macro FRAME_POINTER_REQUIRED controls whether a target
machine supports this flag.
-finline-functions Integrate all simple functions into their callers. The compiler heuristical
ly 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 normally does not output the function as assembler
code in its own right.
-fcaller-saves Enable values to be allocated in registers that will be clobbered by func
tion calls, by emitting extra instructions to save and restore the registers
around such calls. Such allocation is done only when it seems to result
in better code than would otherwise be produced.
This option is enabled by default on certain machines, usually those
which have no callpreserved registers to use instead.
-fkeep-inline-functions
Even if all calls to a given function are integrated, and the function is de
clared static, nevertheless output a separate runtime 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 al
ter the assembler output may be confused by the optimizations per
formed when this option is not used.
-fno-peephole Disable any machinespecific peephole optimizations.
-ffastmath This option allows GCC to violate some ANSI or IEEE rules/specifica
tions in the interest of optimizing code for speed. For example, it allows
the compiler to assume arguments to the sqrt function are nonnegative
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 imple
mentation of IEEE or ANSI rules/specifications for math functions.
The following options control specific optimizations. The ` -O2 ' option turns on all of these
optimizations except ` -funroll-loops ' and ` -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 ``finetuning'' of optimizations to be
performed is desired.
-fstrength-reduce Perform the optimizations of loop strength reduction and elimination of
iteration variables.
-fthread-jumps Perform optimizations where we check to see if a jump branches to a lo
cation where another comparison subsumed by the first is found. If so,
the first branch is redirected to either the destination 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 ex
ample, 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
Rerun common subexpression elimination after loop optimizations has
been performed.
-felide-constructors
Elide constructors when this seems plausible (C++ only). With this flag,
GNU C++ initializes y directly from the call to foo without going through
a temporary in the following code:
A foo (); A y = foo ();
Without this option, GNU C++ first initializes y by calling the appropri
ate 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 ef
fects, using ` -felideconstructors ' can make your program act differ
ently, 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 reorder instructions to
exploit instruction slots available after delayed branch instructions.
-fschedule-insns If supported for the target machine, attempt to reorder instructions to
eliminate execution stalls due to required data being unavailable. This
helps machines that have slow floating point or memory load instruc
tions 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 additional pass of in
struction scheduling after register allocation has been done. This is es
pecially useful on machines with a relatively small number of registers
and where memory load instructions take more than one cycle.
TARGET OPTIONS
By default, GNU CC compiles code for the same type of machine that you are using. How
ever, it can also be installed as a crosscompiler, to compile for some other type of machine.
In fact, several different configurations of GNU CC, for different target machines, can be in
stalled side by side. Then you specify which one to use with the ` -b ' option.
In addition, older and newer versions of GNU CC can be installed side by side. One of them
(probably the newest) will be the default, but you may sometimes wish to use another.
-b machine The argument machine specifies the target machine for compilation.
This is useful when you have installed GNU CC as a crosscompiler.
The value to use for machine is the same as was specified as the ma
chine type when configuring GNU CC as a crosscompiler. For exam
ple, if a crosscompiler 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. Normally, it will be a version that is recom
mended for general use.
MACHINE DEPENDENT OPTIONS
Each of the target machine types can have its own special options, starting with ` -m ', to
choose among various hardware models or configurationsfor example, 68010 vs 68020,
floating coprocessor or none. A single installed version of the compiler can compile for any
model or configuration, according to the options specified.
Some configurations of the compiler also support additional special options, usually for com
mandline 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 68000based systems.
-m68020
-mc68020 Generate output for a 68020 (rather than a 68000). This is the default
when the compiler is configured for 68020based systems.
-m68881 Generate output containing 68881 instructions for floating point. This is
the default for most 68020based 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 68030based systems.
-m68040 Generate output for a 68040. This is the default when the compiler is
configured for 68040based 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 floating 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 crosscompilation. You must make your own arrangements to pro
vide suitable library functions for crosscompilation.
-mshort Consider type int to be 16 bits wide, like short int.
-mnobitfield Do not use the bitfield instructions. ` -m68000 ' implies
` -mnobitfield '.
-mbitfield Do use the bitfield instructions. ` -m68020 ' implies ` -mbitfield '. This
is the default if you use the unmodified sources.
-mrtd Use a different functioncalling convention, in which functions that take
a fixed number of arguments 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 ig
nored.)
The rtd instruction is supported by the 68010 and 68020 processors, but
not by the 68000.
These ` -m ' options are defined for the Motorola 68HC11 and 68HC12:
-m68hc11 Generate output for a 68HC11. This is the default when the compiler is
configured for a 68HC11based target.
-m68hc12 Generate output for a 68HC12. This is the default when the compiler is
configured for a 68HC12based target.
-mshort Consider type int to be 16 bits wide, like short int.
-msoftregcount=num
Specify the number of pseudosoft registers which are used for the code
generation. The maximum number is 32. Using more pseudosoft regis
ter may or may not result in better code depending on the program. The
default is 4 for 68HC11 and 2 for 68HC12.
-mautoincdec Enable the use of 68HC12 pre and post autoincrement and autodecre
ment addressing modes.
These ` -m ' options are defined for the Vax:
-munix Do not output certain jump instructions (aobleq and so on) that the Unix
assembler for the Vax cannot handle across long ranges.
-mgnu Do output those jump instructions, on the assumption that you will as
semble with the GNU assembler.
-mg Output code for gformat floating point numbers instead of dformat.
These ` -m ' switches are supported on the SPARC:
-mfpu
-mhard-float Generate output containing floating point instructions. This is the de
fault.
-mno-fpu
-msoft-float Generate output containing library calls for floating point. Warning:
there is no GNU floatingpoint library for SPARC. Normally the facili
ties of the machine's usual C compiler are used, but this cannot be done
directly in crosscompilation. You must make your own arrangements
to provide suitable library functions for crosscompilation.
-msoft-float changes the calling convention in the output file; there
fore, 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 difference from v7 code
is that the compiler emits the integer multiply and integer divide instruc
tions which exist in SPARC v8 but not in SPARC v7.
-msparclite will give you SPARClite code. This adds the integer multi
ply, integer divide step and scan (ffs) instructions which exist in SPAR
Clite 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 optimises code for the Cy
press CY7C602 chip, as used in the SparcStation/SparcServer 3xx se
ries. 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 con
figured for a C1.
-mc2 Generate output for a C2. This is the default when the compiler is con
figured for a C2.
-margcount Generate code which puts an argument count in the word preceding
each argument list. Some nonportable Convex and Vax programs need
this word. (Debuggers don't, except for functions with variablelength
argument lists; this info is in the symbol table.)
-mnoargcount Omit the argument count word. This is the default if you use the unmod
ified 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 half
word operations are directly supported by the hardware. This is the de
fault.
-mnodw Generate code that assumes the DW bit is not set.
-mbw Generate code that assumes the system supports byte and halfword
write operations. This is the default.
-mnbw Generate code that assumes the systems does not support byte and
halfword write operations. This implies ` -mnodw '.
-msmall Use a small memory model that assumes that all function 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 default.
-mkernel-registers
Generate references to registers gr64gr95 instead of gr96gr127. This
option can be used when compiling kernel code that wants a set of glob
al registers disjoint from that used by usermode code.
Note that when this option is used, register names in ` -f ' flags must use
the normal, usermode, names.
-muser-registers Use the normal set of global registers, gr96gr127. This is the default.
-mstack-check Insert a call to __msp_check after each stack adjustment. This is often
used for kernel code.
These ` -m ' options are defined for Motorola 88K architectures:
-m88000 Generate code that works well on both the m88100 and the m88110.
-m88100 Generate code that works best for the m88100, but that also runs on the
m88110.
-m88110 Generate code that works best for the m88110, and may not run on the
m88100.
-midentify-revision
Include an ident directive in the assembler output recording the source
file name, compiler name and version, timestamp, and 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 under
score as prefix on each name.
-mno-check-zero-division
-mcheck-zero-division
Early models of the 88K architecture had problems with division by ze
ro; in particular, many of them didn't trap. Use these options to avoid in
cluding (or to include explicitly) additional code to detect division by ze
ro 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 specified in the 88Open Object Compatibility
Standard, ``OCS''. This extra information is not needed by GDB. The
default for DG/UX, SVr4, and Delta 88 SVr3.2 is to include this informa
tion; 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 '; other
88k configurations have the default ` -mno-ocs-frame-position '.
-moptimize-arg-area
-mno-optimize-arg-area
Control how to store function arguments in stack frames.
` -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 argument area.
-mshort-data-num
num Generate smaller data references by making them relative to r0,
which allows loading a value using a single instruction (rather than the
usual two). You control which data references are affected by specify
ing num with this option. For example, if you specify
` -mshort-data-512 ', then the data references affected are those in
volving displacements of less than 512 bytes. ` -mshort-data-num ' is
not effective for num greater than 64K.
-mserializevolatile
-mnoserializevolatile
Do, or do not, generate code to guarantee sequential 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 references and so al
ways provides sequential consistency. 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 refer
ences. In particular, a load instruction may execute before a preceding
store instruction. Such reordering violates sequential consistency of
volatile memory references, when there are multiple processors. When
you use ` -m88000 ' or ` -m88110 ', GNU CC generates special in
structions when appropriate, to force execution in the proper order.
The extra code generated to guarantee consistency may affect the per
formance of your application. If you know that you can safely forgo this
guarantee, you may use the option ` -mnoserializevolatile '.
If you use the ` -m88100 ' option but require sequential consistency
when running on the m88110 processor, you should use
` -mserializevolatile '.
-msvr4
-msvr3 Turn on (` -msvr4 ') or off (` -msvr3 ') compiler extensions related to
System V release 4 (SVr4). This controls the following:
· Which variant of the assembler syntax to emit (which you can select in
dependently using ` -mversion-03.00 ').
· ` -msvr4 ' makes the C preprocessor recognize ` #pragma weak '
· ` -msvr4 ' makes GCC issue additional declaration directives used in
SVr4.
` -msvr3 ' is the default for all m88K configurations except the SVr4 configuration.
-mtrap-large-shift
-mhandle-large-shift
Include code to detect bitshifts 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 instruc
tion, so GCC avoids that instruction by default. Use this option to speci
fy 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 hybridCOFF or realELF flavor
is used. All other configurations ignore this option.
-mwarn-passed-structs
Warn when a function passes a struct as an argument or result. Struc
turepassing 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 floatingpoint constants go in the Table of Con
tents (TOC), a table of all global variable and function addresses. By
default GCC puts floatingpoint constants there; if the TOC overflows,
` -mno-fp-in-toc ' will reduce the size of the TOC, which may avoid
the overflow.
These ` -m ' options are defined for the IBM RT PC:
-min-line-mul Use an inline code sequence for integer multiplies. This is the default.
-mcall-lib-mul Call lmul$$ for integer multiples.
-mfull-fp-blocks Generate fullsize floating point data blocks, including the minimum
amount of scratch space recommended by IBM. This is the default.
-mminimum-fp-blocks
Do not include extra scratch space in floating point data blocks. This re
sults 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 arguments are passed in floating point registers.
Note that varargs.h and stdargs.h will not work with floating point
operands if this option is specified.
-mfp-arg-in-gregs
Use the normal calling convention for floating point arguments. This is
the default.
-mhc-struct-return
Return structures of more than one word in memory, rather than in a
register. This provides compatibility with the MetaWare HighC (hc)
compiler. Use ` -fpcc-struct-return ' for compatibility with the
Portable C Compiler (pcc).
-mnohc-struct-return
Return some structures of more than one word in registers, when con
venient. This is the default. For compatibility with the IBMsupplied
compilers, use either ` -fpcc-struct-return ' or
` -mhc-struct-return '.
These ` -m ' options are defined for the MIPS family of computers:
-mcpu=cputype Assume the defaults for the machine type cputype when scheduling in
structions. The default cputype is default, which picks the longest cy
cles times for any of the machines, in order that the code run at reason
able rates on all MIPS cpu's. Other choices for cputype are r2000,
r3000, r4000, and r6000. While picking a specific cputype will sched
ule things appropriately for that particular chip, the compiler will not
generate any code that does not meet level 1 of the MIPS ISA (instruc
tion 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 encapsulate 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 prepro
cessor over the source file. The -mno-rnames switch is default.
-mgpopt
-mno-gpopt The -mgpopt switch says to write all of the data declarations before
the instructions in the text section, to all the MIPS assembler to gener
ate one word memory references instead of using two words for short
global or static data items. This is on by default if optimization is select
ed.
-mstats
-mno-stats For each noninline 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) instead of possibly generating inline
code.
-mmips-tfile
-mno-mips-tfile The -mno-mips-tfile switch causes the compiler not postprocess the
object file with the mips-tfile program, after the MIPS 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 assem
bler embedded in the object file, which means the objects will not com
pare the same.
-msoft-float Generate output containing library calls for floating 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 crosscompilation. You must make your own arrangements to pro
vide suitable library functions for crosscompilation.
-mhard-float Generate output containing floating point instructions. This is the de
fault if you use the unmodified sources.
-mfp64 Assume that the FR bit in the status word is on, and that there are 32
64bit floating point registers, instead of 32 32bit floating point registers.
You must also specify the -mcpu=r4000 and -mips3 switches.
-mfp32 Assume that there are 32 32bit floating point registers. This is the de
fault.
-mabicalls
-mno-abicalls Emit (or do not emit) the .abicalls, .cpload, and .cprestore pseudo oper
ations that some System V.4 ports use for position independent code.
-mhalf-pic
-mno-half-pic The -mhalf-pic switch says to put pointers to extern references into
the data section and load them up, rather than put the references in the
text section. This option does not work at present. -Gnum Put global
and static items less than or equal to num bytes into the small data or bss
sections instead of the normal data or bss section. This allows the as
sembler to emit one word memory reference instructions based on the
global pointer (gp or $28), instead of the normal two words used. By
default, num is 8 when the MIPS assembler is used, and 0 when the
GNU assembler is used. The -Gnum switch is also passed to the as
sembler and linker. All modules should be compiled with the same
-Gnum value.
-nocpp Tell the MIPS assembler to not run its preprocessor over user assem
bler files (with a ` .s ' suffix) when assembling them.
These ` -m ' options are defined for the Intel 80386 family of computers: -m486
-mno-486 Control whether or not code is optimized for a 486 instead of an 386.
Code generated for a 486 will run on a 386 and vice versa.
-msoft-float Generate output containing library calls for floating point. Warning: the
requisite libraries are not part of GNU CC. Normally the facilities of the
machine's usual C compiler are used, but this can't be done directly in
crosscompilation. You must make your own arrangements to provide
suitable library functions for crosscompilation.
On machines where a function returns floating point results in the 80387
register stack, some floating point opcodes may be emitted even if
` -msoftfloat ' is used.
-mnofpretin387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types float
and double in an FPU register, even if there is no FPU. The idea is that
the operating system should emulate an FPU.
The option ` -mnofpretin387 ' causes such values to be returned in
ordinary CPU registers instead.
These ` -m ' options are defined for the HPPA family of computers:
-mparisc10 Generate code for a PA 1.0 processor.
-mparisc11 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; gener
ate addil instructions instead. This avoids a rather serious bug in the
HPUX linker.
-msharedlibs Generate code that can be linked against HPUX shared libraries. This
option is not fully function yet, and is not on by default for any PA target.
Using this option can cause incorrect code to be generated by the com
piler.
-mnosharedlibs Don't generate code that will be linked against shared libraries. This is
the default for all PA targets.
-mlongcalls 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.
-mdisablefpregs Prevent floating point registers from being used in any manner. This is
necessary for compiling kernels which perform lazy context switching
of floating point registers. If you use this option and attempt to perform
floating point operations, the compiler will abort.
-mdisableindexing
Prevent the compiler from using indexing address modes. This avoids
some rather obscure problems when compiling MIG generated code
under MACH.
-mtrailingcolon Add a colon to the end of label definitions (for ELF assemblers).
These ` -m ' options are defined for the Intel 80960 family of computers:
-mcputype Assume the defaults for the machine type cputype for instruction and
addressingmode availability and alignment. The default cputype is kb;
other choices are ka, mc, ca, cf, sa, and sb.
-mnumerics
-msoft-float The -mnumerics option indicates that the processor does support float
ingpoint instructions. The -msoft-float option indicates that float
ingpoint 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 ex
plicit calls when the bal instruction can be substituted by the assembler
or linker, but less efficient code in other cases, such as calls via function
pointers, or using a linker that doesn't support this optimization.
-mtail-call
-mno-tail-call Do (or do not) make additional attempts (beyond those of the ma
chineindependent portions of the compiler) to optimize tailrecursive
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 worthwhile on the Kseries, but they definitely are on the
Cseries. The default is currently -mcomplex-addr for all processors
except the CB and CC.
-mcode-align
-mno-code-align Align code to 8byte boundaries for faster fetching (or don't bother).
Currently turned on by default for Cseries 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 structurealignment compatibility with Intel's gcc release ver
sion 1.3 (based on gcc 1.37). Currently this is buggy in that #pragma
align 1 is always assumed as well, and cannot be turned off.
These ` -m ' options are defined for the DEC Alpha implementations:
-mnosoftfloat
-msoftfloat Use (do not use) the hardware floatingpoint instructions for float
ingpoint operations. When -msoftfloat is specified, functions in
` libgcc1.c ' will be used to perform floatingpoint operations. Unless
they are replaced by routines that emulate the floatingpoint operations,
or compiled in such a way as to call such emulations routines, these rou
tines will issue floatingpoint operations. If you are compiling for an Al
pha without floatingpoint operations, you must ensure that the library is
built so as not to call them.
Note that Alpha implementations without floatingpoint operations are
required to have floatingpoint registers.
-mfpreg
-mnofpregs Generate code that uses (does not use) the floatingpoint register set.
-mnofpregs implies -msoftfloat. If the floatingpoint register set is
not used, floating point operands are passed in integer registers as if
they were integers and floatingpoint results are passed in $0 instead of
$f0. This is a nonstandard calling sequence, so any function with a
floatingpoint argument or return value called by code compiled with
-mnofpregs must also be compiled with that option.
A typical use of this option is building a kernel that does not use, and
hence need not save and restore, any floatingpoint 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 sys
tem linker), for compatibility with other compilers. However, we sug
gest you use ` -symbolic ' or ` -shared ' as appropriate, instead of sup
plying linker options on the gcc command line.
-Qy Identify the versions of each tool used by the compiler, in a .ident as
sembler directive in the output.
-Qn Refrain from adding .ident directives to the output file (this is the de
fault).
-YP,dirs Search the directories dirs, and no others, for libraries specified with
` -l '. You can separate directory entries in dirs from one another with
colons.
-Ym,dir Look in the directory dir to find the M4 preprocessor. The assembler
uses this option.
CODE GENERATION OPTIONS
These machineindependent options control the interface conventions used in code genera
tion.
Most of them begin with `-f'. These options have both positive and negative forms; the neg
ative form of ` -ffoo ' would be ` -fno-foo '. In the table below, only one of the forms is list
edthe one which is not the default. You can figure out the other form by either removing
` no- ' or adding it.
-fnonnull-objects Assume that objects reached through references are not null (C++ on
ly).
Normally, GNU C++ makes conservative assumptions about objects
reached through references. For example, the compiler must check
that a is not null in code like the following:
obj &a = g (); a.f (2);
Checking that references of this sort have nonnull values requires extra
code, however, and it is unnecessary for many programs. You can use
` -fnonnullobjects ' 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 ef
ficient for small structures, and on many machines it fails to be reen
trant; but it has the advantage of allowing intercallability between
GCCcompiled code and PCCcompiled code.
-freg-struct-return
Use the convention that struct and union values are returned in regis
ters 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-re
turn.
-fshort-enums Allocate to an enum type only as many bytes as it needs for the de
clared 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 nonconst variables of this compilation be
shared data rather than private data. The distinction makes sense only
on certain operating systems, where shared data is shared between
processes running the same program, while private data exists in one
copy per process.
-fno-common Allocate even uninitialized global variables in the bss section of the ob
ject file, rather than generating them as common blocks. This has the
effect that if the same variable is declared (without extern) in two differ
ent 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 de
structors) 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 nonGNU 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 collect2, the compiler driver gcc is configured
to do this automatically.
-finhibitsizedirective
Don't output a .size assembler directive, or anything else that would
cause trouble if the function is split in the middle, and the two halves are
placed at locations far apart in memory. This option is used when com
piling ` crtstuff.c '; you should not need to use it for anything else.
-fverboseasm Put extra commentary information in the generated assembly code to
make it more readable. This option is generally only of use to those who
actually need to read the generated assembly code (perhaps while de
bugging the compiler itself).
-fvolatile Consider all memory references through pointers to be volatile.
-fvolatile-global Consider all memory references to extern and global data items to be
volatile.
-fpic If supported for the target machines, generate positionindependent
code, suitable for use in a shared library.
-fPIC If supported for the target machine, emit positionindependent 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
machinespecific and are defined in the REGISTER_NAMES macro in
the machine description macro file.
This flag does not have a negative form, because it specifies a
threeway choice.
-fcall-used-reg Treat the register named reg as an allocable register 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 ma
chine'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
threeway choice.
-fcall-saved-reg Treat the register named reg as an allocable register 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 ma
chine'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 regis
ter in which function values may be returned.
This flag does not have a negative form, because it specifies a
threeway 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. Normal
ly, local copies of certain information (backup copies of inline member
functions, debugging information, and the internal tables that implement
virtual functions) must be kept in each object file that includes class defi
nitions. You can use this pragma to avoid such duplication. When a
header file containing ` #pragma interface ' is included in a compilation,
this auxiliary information will not be generated (unless the main input
source file itself uses ` #pragma 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 member functions, debugging infor
mation, and the internal tables used to implement virtual functions are all
generated in implementation files.
If you use ` #pragma implementation ' with no argument, it applies to
an include file with the same basename as your source file; for example,
in ` allclass.cc ', ` #pragma implementation ' by 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 single header file into multi
ple implementation files.
FILES
file.c C source file
file.h C header (preprocessor) file
file.i preprocessed C source file
file.C C++ source file
file.cc C++ source file
file.cxx C++ source file
file.m ObjectiveC 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 startup routine
LIBDIR/ccrt0 additional startup 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; Debugging with GDB: the GNU SourceLevel Debugger, Richard M.
Stallman and Roland H. Pesch; Using as: the GNU Assembler, Dean Elsner, Jay Fenlason &
friends; ld: the GNU linker, Steve Chamberlain and Roland Pesch.
BUGS
For instructions on reporting bugs, see the GCC manual.
COPYING
Copyright © 1991, 1992, 1993 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this manual provided the
copyright notice and this permission notice are preserved on all copies.
Permission is granted to copy and distribute modified versions of this manual under the condi
tions for verbatim copying, provided that the entire resulting derived work is distributed under
the terms of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this manual into another language,
under the above conditions for modified versions, except that this permission notice may be
included in translations approved by the Free Software Foundation instead of in the original
English.
AUTHORS
See the GNU CC Manual for the contributors to GNU CC.