GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below for the remain-
der. g++ accepts mostly the same options as gcc.
Note: In Apple's version of GCC, both cc and gcc are actually symbolic
links to gcc-3.3, while c++ and g++ are links to g++-3.3, unless the
ggcccc_sseelleecctt(1) command has been used to point them at another compiler
version.
Note that Apple's GCC includes a number of extensions to standard GCC
(flagged below with ``APPLE ONLY''), and that not all generic GCC
options are available or supported on Darwin / Mac OS X. In particu-
lar, Apple does not currently support the compilation of Fortran, Ada,
or Java, although there are third parties who have made these work.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The ``overall options'' allow you to stop this
process at an intermediate stage. For example, the -c option says not
to run the linker. Then the output consists of object files output by
the assembler.
Other options are passed on to one stage of processing. Some options
control the preprocessor and others the compiler itself. Yet other
options control the assembler and linker; most of these are not docu-
mented here, since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful
for C programs; when an option is only useful with another language
(usually C++), the explanation says so explicitly. If the description
for a particular option does not mention a source language, you can use
that option with all supported languages.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dr is very different from -d -r.
You can mix options and other arguments. For the most part, the order
you use doesn't matter. Order does matter when you use several options
of the same kind; for example, if you specify -L more than once, the
directories are searched in the order specified.
Many options have long names starting with -f or with -W---for example,
-fforce-mem, -fstrength-reduce, -Wformat and so on. Most of these have
both positive and negative forms; the negative form of -ffoo would be
-fno-foo. This manual documents only one of these two forms, whichever
one is not the default.
OPTIONS
Option Summary
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 -pass-exit-codes -x language -ObjC
(APPLE ONLY) -ObjC++ (APPLE ONLY) -arch arch (APPLE ONLY) -v -###
--help --target-help --version
C Language Options
-ansi -std=standard -aux-info filename -faltivec (APPLE ONLY)
-fasm-blocks (APPLE ONLY) -fno-asm -fno-builtin -fno-builtin-func-
tion -fhosted -ffreestanding -fms-extensions -trigraphs
-no-integrated-cpp -traditional -traditional-cpp -fallow-sin-
gle-precision -fcond-mismatch -fconstant-cfstrings (APPLE ONLY)
-fpch-preprocess (APPLE ONLY) -fsigned-bitfields -fsigned-char
-funsigned-bitfields -funsigned-char -fpascal-strings (APPLE ONLY)
-fcoalesce (APPLE ONLY) -fweak-coalesced (APPLE ONLY) -Wno-#warn-
ings (APPLE ONLY) -Wextra-tokens (APPLE ONLY) -Wpragma-once (APPLE
ONLY) -Wnewline-eof (APPLE ONLY) -Wno-altivec-long-deprecated
(APPLE ONLY) -fwritable-strings
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space
-fno-const-strings -fdollars-in-identifiers -fno-elide-construc-
tors -fno-enforce-eh-specs -fexternal-templates -falt-exter-
nal-templates -ffor-scope -fno-for-scope -fno-gnu-keywords
-fno-implicit-templates -fno-implicit-inline-templates -fno-imple-
ment-inlines -findirect-virtual-calls (APPLE ONLY) -fapple-kext
(APPLE ONLY) -fcoalesce-templates (APPLE ONLY) -fms-extensions
-fno-nonansi-builtins -fno-operator-names -fno-optional-diags
-fpermissive -frepo -fno-rtti -fstats -ftemplate-depth-n
-fuse-cxa-atexit -fvtable-gc -fno-weak -nostdinc++
-fno-default-inline -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor
-Wreorder -Weffc++ -Wno-deprecated -Wno-non-template-friend
-Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions
-Wsign-promo -Wsynth
Objective-C Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers (APPLE ONLY) -fobjc-direct-dispatch (APPLE ONLY)
-fobjc-exceptions (APPLE ONLY) -freplace-objc-classes (APPLE ONLY)
-fzero-link (APPLE ONLY) -gen-decls -Wno-protocol -Wselector -Wun-
declared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -W -Wall -Waggre-
gate-return -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment
-Wconversion -Wno-deprecated-declarations -Wdisabled-optimization
-Wno-div-by-zero -Werror -Wfloat-equal -Wformat -Wformat=2
-Wformat-nonliteral -Wformat-security -Wimplicit -Wimplicit-int
-Wimplicit-function-declaration -Werror-implicit-function-declara-
tion -Wimport -Winline -Winvalid-pch -Wno-endif-labels
-Wno-invalid-offsetof (APPLE ONLY) -Wlarger-than-len -Wlong-long
-Wno-long-double (APPLE ONLY) -Wmain -Wmissing-braces -Wmiss-
ing-format-attribute -Wmissing-noreturn -Wmost (APPLE ONLY)
-Wno-multichar -Wno-format-extra-args -Wno-format-y2k -Wno-import
-Wnonnull -Wpacked -Wpadded -Wparentheses -Wpointer-arith -Wre-
dundant-decls -Wreturn-type -Wsequence-point -Wshadow -Wsign-com-
pare -Wstrict-aliasing -Wswitch -Wswitch-default -Wswitch-enum
-Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized -Wun-
known-pragmas -Wunreachable-code -Wunused -Wunused-function
-Wunused-label -Wunused-parameter -Wunused-value -Wunused-vari-
able -Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes
-Wnested-externs -Wstrict-prototypes -Wtraditional
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered
-fdump-translation-unit[-n] -fdump-class-hierarchy[-n]
-fdump-tree-original[-n] -fdump-tree-optimized[-n]
-fdump-tree-inlined[-n] -feliminate-dwarf2-dups -fmem-report
-fprofile-arcs -fsched-verbose=n -ftest-coverage -ftime-report -g
-glevel -gcoff -gdwarf -gdwarf-1 -gdwarf-1+ -gdwarf-2 -ggdb
-gstabs -gstabs+ -gvms -gxcoff -gxcoff+ -p -pg
-print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib -print-prog-name=program
-print-search-dirs -Q -save-temps -time -fsave-repository=loca-
tion -grepository
Optimization Options
-falign-functions=n -falign-jumps=n -falign-labels=n
-falign-loops=n -falign-loops-max-skip=n -falign-jumps-max-skip=n
-fbranch-probabilities -fcaller-saves -fcprop-registers -fcse-fol-
low-jumps -fcse-skip-blocks -fdata-sections -fdelayed-branch
-fdelete-null-pointer-checks -fexpensive-optimizations -ffast-math
-ffloat-store -fforce-addr -fforce-mem -ffunction-sections -fgcse
-fgcse-lm -fgcse-sm -floop-optimize -fcrossjumping -fif-conversion
-fif-conversion2 -finline-functions -finline-limit=n
-fkeep-inline-functions -fkeep-static-consts -fmerge-constants
-fmerge-all-constants -fmove-all-movables -fnew-ra
-fno-branch-count-reg -fno-default-inline -fno-defer-pop
-fno-function-cse -fno-guess-branch-probability -fno-inline
-fno-math-errno -fno-peephole -fno-peephole2 -funsafe-math-opti-
mizations -ffinite-math-only -fno-trapping-math -fno-zero-initial-
ized-in-bss -fomit-frame-pointer -foptimize-register-move -fopti-
mize-sibling-calls -fprefetch-loop-arrays -freduce-all-givs -freg-
move -frename-registers -freorder-blocks -freorder-functions -fre-
run-cse-after-loop -frerun-loop-opt -fschedule-insns -fsched-
ule-insns2 -fno-sched-interblock -fno-sched-spec
-fsched-spec-load -fsched-spec-load-dangerous -fsignaling-nans
-fsingle-precision-constant -fssa -fssa-ccp -fssa-dce
-fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps -fun-
roll-all-loops -funroll-loops --param name=value -O -O0 -O1 -O2
-O3 -Os -fast (APPLE ONLY)
Preprocessor Options
-$ -Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN
-Dmacro[=defn] -E -H -idirafter dir -include file -imacros file
-iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem
dir -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -remap -depen-
dency-file (APPLE ONLY) -trigraphs -undef -Umacro -Wp,option
Assembler Option
-Wa,option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs -nost-
dlib -no-c++filt (APPLE ONLY) -s -static -static-libgcc -shared
-shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -I- -Fdir (APPLE ONLY) -Ldir -specs=file
Target Options
-V version -b machine
Machine Dependent Options
RS/6000 and PowerPC Options
-mcpu=cpu-type -mtune=cpu-type -mpower -mno-power -mpower2
-mno-power2 -mpowerpc -mpowerpc64 -mno-powerpc -maltivec
-mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt
-mno-powerpc-gfxopt -mnew-mnemonics -mold-mnemonics -mfull-toc
-mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32
-mxl-call -mno-xl-call -mpe -malign-mac68k (APPLE ONLY)
-malign-power (APPLE ONLY) -malign-natural (APPLE ONLY)
-msoft-float -mhard-float -mmultiple -mno-multiple -mstring
-mno-string -mupdate -mno-update -mfused-madd -mno-fused-madd
-mbit-align -mno-bit-align -mstrict-align -mno-strict-align
-mrelocatable -mno-relocatable -mrelocatable-lib -mno-relocat-
able-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig
-mbig-endian -mdynamic-no-pic (APPLE ONLY) -mlong-branch (APPLE
ONLY) -mcall-aix -mcall-sysv -mcall-netbsd -maix-struct-return
-msvr4-struct-return -mabi=altivec -mabi=no-altivec -mabi=spe
-mabi=no-spe -misel=yes -misel=no -mprototype -mno-prototype -msim
-mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks
-mwindiss -G num -pthread
Darwin Options
-all_load -allowable_client -arch -arch_errors_fatal -arch_only
-bind_at_load -bundle -bundle_loader -client_name -compatibil-
ity_version -current_version -dead_strip
-no_dead_strip_inits_and_terms -dependency-file -dylib_file
-dylinker_install_name -dynamic -dynamiclib -exported_symbols_list
-filelist -flat_namespace -force_cpusubtype_ALL -force_flat_names-
pace -headerpad_max_install_names -image_base -init -install_name
-keep_private_externs -multi_module -multiply_defined -multi-
ply_defined_unused -noall_load -nomultidefs -noprebind
-noseglinkedit -pagezero_size -prebind -prebind_all_twolevel_mod-
ules -private_bundle -read_only_relocs -sectalign -sectobjectsym-
bols -whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder
-seg_addr_table -seg_addr_table_filename -seglinkedit -segprot
-segs_read_only_addr -segs_read_write_addr -single_module -static
-sub_library -sub_umbrella -twolevel_namespace -umbrella -undefined
-unexported_symbols_list -weak_reference_mismatches -whatsloaded
-mone-byte-bool
i386 and x86-64 Options
-mcpu=cpu-type -march=cpu-type -mfpmath=unit -masm=dialect
-mno-fancy-math-387 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib
-mno-wide-multiply -mrtd -malign-double -mpreferred-stack-bound-
ary=num -mmmx -msse -msse2 -m3dnow -mthreads -mno-align-stringops
-minline-all-stringops -mpush-args -maccumulate-outgoing-args
-m128bit-long-double -m96bit-long-double -mregparm=num
-momit-leaf-frame-pointer -mno-red-zone -mcmodel=code-model -m32
-m64
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -funwind-tables -fasynchronous-unwind-tables
-finhibit-size-directive -finstrument-functions -fno-common
-fno-ident -fno-gnu-linker -fpcc-struct-return -fpic -fPIC
-freg-struct-return -fshared-data -fshort-enums -fshort-double
-fshort-wchar -fvolatile -fvolatile-global -fvolatile-static
-fverbose-asm -fpack-struct -fstack-check -fstack-limit-regis-
ter=reg -fstack-limit-symbol=sym -fargument-alias -fargu-
ment-noalias -fargument-noalias-global -fleading-underscore
-ftls-model=model -ftrapv -fbounds-check
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order. The first three
stages apply to an individual source file, and end by producing an
object file; linking combines all the object files (those newly com-
piled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of
compilation is done:
file.c
C source code which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the library
libobjc.a to make an Objective-C program work.
file.mi
Objective-C source code which should not be preprocessed.
file.h
C or C++ header file to be turned into a precompiled header.
file.cc
file.cp
file.cxx
file.cpp
file.c++
file.C
C++ source code which must be preprocessed. Note that in .cxx, the
last two letters must both be literally x. Likewise, .C refers to
a literal capital C.
file.mm
file.M
Objective-C++ source code which must be preprocessed. (APPLE ONLY)
file.mii
Objective-C++ source code which should not be preprocessed. (APPLE
ONLY)
file.hh
file.H
C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR
Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fortran source code which must be preprocessed (with the tradi-
tional preprocessor).
file.r
Fortran source code which must be preprocessed with a RATFOR pre-
processor (not included with GCC).
file.ads
Ada source code file which contains a library unit declaration (a
declaration of a package, subprogram, or generic, or a generic
instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration). Such files are also
called specs.
file.adb
Ada source code file containing a library unit body (a subprogram
or package body). Such files are also called bodies.
file.s
Assembler code. Apple's version of GCC runs the preprocessor on
these files as well as those ending in .S.
file.S
Assembler code which must be preprocessed.
other
An object file to be fed straight into linking. Any file name with
no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on the
file name suffix). This option applies to all following input
files until the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objc-cpp-output
objective-c++ (APPLE ONLY) objective-c++-header (APPLE ONLY)
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
java
treelang
-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).
-ObjC
-ObjC++
These are similar in effect to -x objective-c and -x objective-c++,
but also cause the -ObjC flag to be passed to lldd(1), causing static
archive libraries that define an Objective-C class or category to
be linked in in their entirety. (APPLE ONLY)
-arch arch
Compile for the specified target architecture arch. The allowable
values are i386 and ppc. Multiple options work, and direct the
compiler to produce ``fat'' binaries including object code for each
architecture specified with -arch. This option only works if
assembler and libraries are available for each architecture speci-
fied. (APPLE ONLY)
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase
of the compiler returns a non-success return code. If you specify
-pass-exit-codes, the gcc program will instead return with numeri-
cally highest error produced by any phase that returned an error
indication.
If you only want some of the stages of compilation, you can use -x (or
filename suffixes) to tell gcc where to start, and one of the options
-c, -S, or -E to say where gcc is to stop. Note that some combinations
(for example, -x cpp-output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking
stage simply is not done. The ultimate output is in the form of an
object file for each source file.
By default, the object file name for a source file is made by
replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly,
are ignored.
-S Stop after the stage of compilation proper; do not assemble. The
output is in the form of an assembler code file for each non-assem-
bler input file specified.
By default, the assembler file name for a source file is made by
replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper.
The output is in the form of preprocessed source code, which is
sent to the standard output.
Input files which don't require preprocessing are ignored.
-o file
Place output in file file. This applies regardless to whatever
sort of output is being produced, 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 -o is not specified, 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 com-
piler driver program and of the preprocessor and the compiler
proper.
-###
Like -v except the commands are not executed and all command argu-
ments are quoted. This is useful for shell scripts to capture the
driver-generated command lines.
-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 is unable to read from a pipe; but the GNU
assembler has no trouble.
--help
Print (on the standard output) a description of the command line
options understood by gcc. If the -v option is also specified then
--help will also be passed on to the various processes invoked by
gcc, so that they can display the command line options they accept.
If the -W option is also specified then command line options which
have no documentation associated with them will also be displayed.
--target-help
Print (on the standard output) a description of target specific
command line options for each tool.
--version
Display the version number and copyrights of the invoked GCC.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp,
.CPP, .c++, .cp, or .cxx; C++ header files often use .hh or .H; prepro-
cessed C++ files use the suffix .ii. GCC recognizes files with these
names and compiles them as C++ programs even if you call the compiler
the same way as for compiling C programs (usually with the name gcc).
However, C++ programs often require class libraries as well as a com-
piler that understands the C++ language---and under some circumstances,
you might want to compile programs or header files from standard input,
or otherwise without a suffix that flags them as C++ programs. You
might also like to precompile a C header file with a .h extension to be
used in C++ compilations. g++ is a program that calls GCC with the
default language set to C++, and automatically specifies linking
against the C++ library. On many systems, g++ is also installed with
the name c++.
When you compile C++ programs, you may specify many of the same com-
mand-line options that you use for compiling programs in any language;
or command-line options meaningful for C and related languages; or
options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived
from C, such as C++ and Objective-C) that the compiler accepts:
-ansi
In C mode, support all ISO C90 programs. In C++ mode, remove GNU
extensions that conflict with ISO C++.
This turns off certain features of GCC that are incompatible with
ISO C90 (when compiling C code), or of standard C++ (when compiling
C++ code), such as the "asm" and "typeof" keywords, and predefined
macros such as "unix" and "vax" that identify the type of system
you are using. It also enables the undesirable and rarely used ISO
trigraph feature. For the C compiler, it disables recognition of
C++ style // comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and
"__typeof__" continue to work despite -ansi. You would not want to
use them in an ISO C program, of course, but it is useful to put
them in header files that might be included in compilations done
with -ansi. Alternate predefined macros such as "__unix__" and
"__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected
gratuitously. For that, -pedantic is required in addition to
-ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is
used. Some header files may notice this macro and refrain from
declaring certain functions or defining certain macros that the ISO
standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions which would normally be built in but do not have seman-
tics defined by ISO C (such as "alloca" and "ffs") are not built-in
functions with -ansi is used.
-std=
Determine the language standard. This option is currently only
supported when compiling C or C++. A value for this option must be
provided; possible values are
c89
iso9899:1990
ISO C90 (same as -ansi).
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported;
see <http://gcc.gnu.org/gcc-3.3/c99status/> for more infor-
mation. The names c9x and iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including some C99 fea-
tures).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented
in GCC, this will become the default. The name gnu9x is depre-
cated.
c++98
The 1998 ISO C++ standard plus amendments.
gnu++98
The same as -std=c++98 plus GNU extensions. This is the
default for C++ code.
Even when this option is not specified, you can still use some of
the features of newer standards in so far as they do not conflict
with previous C standards. For example, you may use "__restrict__"
even when -std=c99 is not specified.
The -std options specifying some version of ISO C have the same
effects as -ansi, except that features that were not in ISO C90 but
are in the specified version (for example, // comments and the
"inline" keyword in ISO C99) are not disabled.
-aux-info filename
Output to the given filename prototyped declarations for all func-
tions declared and/or defined in a translation unit, including
those in header files. This option is silently ignored in any lan-
guage other than C.
Besides declarations, the file indicates, in comments, the origin
of each declaration (source file and line), whether the declaration
was implicit, prototyped or unprototyped (I, N for new or O for
old, respectively, in the first character after the line number and
the colon), and whether it came from a declaration or a definition
(C or F, respectively, in the following character). In the case of
function definitions, a K&R-style list of arguments followed by
their declarations is also provided, inside comments, after the
declaration.
-faltivec
Enable the AltiVec language extensions, as defined in Motorola's
AltiVec PIM. This includes the recognition of "vector" and "pixel"
as (context-dependent) keywords, the definition of built-in func-
tions such as "vec_add", and other extensions. Note that unlike
the option -maltivec, the extensions do not require the inclusion
of any special header files. (APPLE ONLY)
-fasm-blocks
Enable the use of blocks and entire functions of assembly code
within a C or C++ file. The syntax follows that used in CodeWar-
rior. (APPLE ONLY)
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that
code can use these words as identifiers. You can use the keywords
"__asm__", "__inline__" and "__typeof__" instead. -ansi implies
-fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm"
and "inline" are standard keywords. You may want to use the
-fno-gnu-keywords flag instead, which has the same effect. In C99
mode (-std=c99 or -std=gnu99), this switch only affects the "asm"
and "typeof" keywords, since "inline" is a standard keyword in ISO
C99.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with
__builtin_ as prefix.
GCC normally generates special code to handle certain built-in
functions more efficiently; for instance, calls to "alloca" may
become single instructions that adjust the stack directly, and
calls to "memcpy" may become inline copy loops. The resulting code
is often both smaller and faster, but since the function calls no
longer appear as such, you cannot set a breakpoint on those calls,
nor can you change the behavior of the functions by linking with a
different library.
With the -fno-builtin-function option only the built-in function
function is disabled. function must not begin with __builtin_. If
a function is named this is not built-in in this version of GCC,
this option is ignored. There is no corresponding -fbuiltin-func-
tion option; if you wish to enable built-in functions selectively
when using -fno-builtin or -ffreestanding, you may define macros
such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation takes place in a hosted environment. This
implies -fbuiltin. A hosted environment is one in which the entire
standard library is available, and in which "main" has a return
type of "int". Examples are nearly everything except a kernel.
This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding environment.
This implies -fno-builtin. A freestanding environment is one in
which the standard library may not exist, and program startup may
not necessarily be at "main". The most obvious example is an OS
kernel. This is equivalent to -fno-hosted.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for
strict ISO C conformance) implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling.
This option allows a user supplied "cc1", "cc1plus", or "cc1obj"
via the -B option. The user supplied compilation step can then add
in an additional preprocessing step after normal preprocessing but
before compiling. The default is to use the integrated cpp (inter-
nal cpp)
The semantics of this option will change if "cc1", "cc1plus", and
"cc1obj" are merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-
standard C compiler. They are now only supported with the -E
switch. The preprocessor continues to support a pre-standard mode.
See the GNU CPP manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second
and third arguments. The value of such an expression is void.
This option is not supported for C++.
-fpch-preprocess
Enable PCH processing even when -E or -save-temps is used.
-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 programs have been written to use plain "char" and expect
it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let
you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed
char" or "unsigned char", even though its behavior is always just
like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the
negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned,
when the declaration does not use either "signed" or "unsigned".
By default, such a bit-field is signed, because this is consistent:
the basic integer types such as "int" are signed types.
-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.
Writing into string constants is a very bad idea; ``constants''
should be constant.
-fconstant-cfstrings
Enable the automatic creation of a CoreFoundation-type constant
string whenever a special builtin "__builtin__CFStringMakeCon-
stantString" is called on a literal string, and for "@"..."" liter-
als in Objective-C (thus overriding the -fconstant-string-class
setting). This option will also be enabled if the environment
variable "MACOSX_DEPLOYMENT_TARGET" exists and is set to "10.2" or
greater. (APPLE ONLY)
-fpascal-strings
Allow Pascal-style string literals to be constructed. (APPLE ONLY)
-fcoalesce
Coalesce duplicated functions and data. The linker will discard all
but one, saving space. Enabled by default. (APPLE ONLY)
-fweak-coalesced
Use the new OS X "weak_definitions" section attribute for coalesced
items. A single "normal" definition will be chosen by the linker
over any number of weakly-coalesced ones. (APPLE ONLY)
Options Controlling C++ Dialect
This section describes the command-line options that are only meaning-
ful for C++ programs; but you can also use most of the GNU compiler
options regardless of what language your program is in. For example,
you might compile a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs;
you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 1 is the version of the C++
ABI that first appeared in G++ 3.2. Version 0 will always be the
version that conforms most closely to the C++ ABI specification.
Therefore, the ABI obtained using version 0 will change as ABI bugs
are fixed.
The default is version 1.
-fno-access-control
Turn off all access checking. This switch is mainly useful for
working around bugs in the access control code.
-fcheck-new
Check that the pointer returned by "operator new" is non-null
before attempting to modify the storage allocated. The current
Working Paper requires that "operator new" never return a null
pointer, so this check is normally unnecessary.
An alternative to using this option is to specify that your "opera-
tor new" does not throw any exceptions; if you declare it tthhrrooww(()),
G++ will check the return value. See also new (nothrow).
-fconserve-space
Put uninitialized or runtime-initialized global variables into the
common segment, as C does. This saves space in the executable at
the cost of not diagnosing duplicate definitions. If you compile
with this flag and your program mysteriously crashes after "main()"
has completed, you may have an object that is being destroyed twice
because two definitions were merged.
This option is no longer useful on most targets, now that support
has been added for putting variables into BSS without making them
common.
-fno-const-strings
Give string constants type "char *" instead of type "const char *".
By default, G++ uses type "const char *" as required by the stan-
dard. Even if you use -fno-const-strings, you cannot actually mod-
ify the value of a string constant, unless you also use
-fwritable-strings.
This option might be removed in a future release of G++. For maxi-
mum portability, you should structure your code so that it works
with string constants that have type "const char *".
-fdollars-in-identifiers
Accept $ in identifiers. You can also explicitly prohibit use of $
with the option -fno-dollars-in-identifiers. (GNU C allows $ by
default on most target systems, but there are a few exceptions.)
Traditional C allowed the character $ to form part of identifiers.
However, ISO C and C++ forbid $ in identifiers.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a tempo-
rary which is only used to initialize another object of the same
type. Specifying this option disables that optimization, and
forces G++ to call the copy constructor in all cases.
-fno-enforce-eh-specs
Don't check for violation of exception specifications at runtime.
This option violates the C++ standard, but may be useful for reduc-
ing code size in production builds, much like defining NDEBUG. The
compiler will still optimize based on the exception specifications.
-fexternal-templates
instantiation; template instances are emitted or not according to
the location of the template definition.
This option is deprecated.
-falt-external-templates
Similar to -fexternal-templates, but template instances are emitted
or not according to the place where they are first instantiated.
This option is deprecated.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a
for-init-statement is limited to the for loop itself, as specified
by the C++ standard. If -fno-for-scope is specified, the scope of
variables declared in a for-init-statement extends to the end of
the enclosing scope, as was the case in old versions of G++, and
other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to
allow and give a warning for old-style code that would otherwise be
invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this
word as an identifier. You can use the keyword "__typeof__"
instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated
implicitly (i.e. by use); only emit code for explicit instantia-
tions.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates,
either. The default is to handle inlines differently so that com-
piles with and without optimization will need the same set of
explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
errors if these functions are not inlined everywhere they are
called.
-findirect-virtual-calls
Do not make direct calls to virtual functions; instead, always go
through the vtable. (APPLE ONLY)
-fapple-kext
Alter vtables, destructors, and other implementation details to
more closely resemble the GCC 2.95 ABI. This is to make kernel
extensions loadable by Darwin kernels, and is required to build any
Darwin kernel extension. -fno-exceptions and -static must also be
used with this flag. (APPLE ONLY)
-fcoalesce-templates
Mark instantiated templates as "coalesced": the linker will discard
all but one, thus saving space. (APPLE ONLY)
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as
implicit int and getting a pointer to member function via non-stan-
dard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C. These include "ffs", "alloca", "_exit", "index",
"bzero", "conjf", and other related functions.
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor",
"compl", "not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need
to issue. Currently, the only such diagnostic issued by G++ is the
one for a name having multiple meanings within a class.
-fpermissive
Downgrade messages about nonconformant code from errors to warn-
ings. By default, G++ effectively sets -pedantic-errors without
-pedantic; this option reverses that. This behavior and this
option are superseded by -pedantic, which works as it does for GNU
C.
-frepo
Enable automatic template instantiation at link time. This option
also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual
functions for use by the C++ runtime type identification features
(dynamic_cast and typeid). If you don't use those parts of the
language, you can save some space by using this flag. Note that
exception handling uses the same information, but it will generate
it as needed.
-fskip-unused-source
Skip declarations in the input files that aren't actually needed.
This flag doesn't change the meaning of correct source code, but it
can suppress diagnostics about incorrect source code and suppress
output of debugging information for the parts of the input that are
skipped.
-fstats
Emit statistics about front-end processing at the end of the compi-
lation. This information is generally only useful to the G++
development team.
-ftemplate-depth-n
Set the maximum instantiation depth for template classes to n. A
limit on the template instantiation depth is needed to detect end-
less recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than
17.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with
the "__cxa_atexit" function rather than the "atexit" function.
This option is required for fully standards-compliant handling of
static destructors, but will only work if your C library supports
"__cxa_atexit". This option is not supported on Mac OS X.
-fvtable-gc
Emit special relocations for vtables and virtual function refer-
ences so that the linker can identify unused virtual functions and
zero out vtable slots that refer to them. This is most useful with
-ffunction-sections and -Wl,--gc-sections, in order to also discard
the functions themselves.
This optimization requires GNU as and GNU ld. Not all systems sup-
port this option. -Wl,--gc-sections is ignored without -static.
-fno-weak
Do not use weak symbol support, even if it is provided by the
linker. By default, G++ will use weak symbols if they are avail-
able. This option exists only for testing, and should not be used
by end-users; it will result in inferior code and has no benefits.
This option may be removed in a future release of G++.
-nostdinc++
Do not search for header files in the standard directories specific
to C++, but do still search the other standard directories. (This
option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline func-
tions; they just won't be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with
the vendor-neutral C++ ABI. Although an effort has been made to
warn about all such cases, there are probably some cases that are
not warned about, even though G++ is generating incompatible code.
There may also be cases where warnings are emitted even though the
code that is generated will be compatible.
You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be
binary compatible with code generated by other compilers.
The known incompatibilities at this point include:
* Incorrect handling of tail-padding for bit-fields. G++ may
attempt to pack data into the same byte as a base class. For
example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte
as"A::f1"; other compilers will not. You can avoid this prob-
lem by explicitly padding "A" so that its size is a multiple of
the byte size on your platform; that will cause G++ and other
compilers to layout "B" identically.
* Incorrect handling of tail-padding for virtual bases. G++ does
not use tail padding when laying out virtual bases. For exam-
ple:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for
"A"; other compilers will. You can avoid this problem by
explicitly padding "A" so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++
and other compilers to layout "C" identically.
* Incorrect handling of bit-fields with declared widths greater
than that of their underlying types, when the bit-fields appear
in a union. For example:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make
the union too small by the number of bits in an "int".
* Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes
that the "A" data member of "B" is already at offset zero.
* Names of template functions whose types involve "typename" or
template template parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable, because all the constructors or
destructors in a class are private and the class has no friends or
public static member functions. This warning is enabled by
default.
-Wnon-virtual-dtor (C++ only)
Warn when a class declares a non-virtual destructor that should
probably be virtual, because it looks like the class will be used
polymorphically. This warning is enabled by -Wall.
-Wreorder (C++ only)
Warn when the order of member initializers given in the code does
not match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
Here the compiler will warn that the member initializers for i and
j will be rearranged to match the declaration order of the members.
This warning is enabled by -Wall.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)
Warn about violations of the following style guidelines from Scott
Meyers' Effective C++ book:
* Item 11: Define a copy constructor and an assignment operator
for classes with dynamically allocated memory.
* Item 12: Prefer initialization to assignment in constructors.
* Item 14: Make destructors virtual in base classes.
* Item 15: Have "operator=" return a reference to *this.
* Item 23: Don't try to return a reference when you must return
an object.
and about violations of the following style guidelines from Scott
Meyers' More Effective C++ book:
* Item 6: Distinguish between prefix and postfix forms of incre-
ment and decrement operators.
* Item 7: Never overload "&&", "||", or ",".
If you use this option, you should be aware that the standard
library headers do not obey all of these guidelines; you can use
grep -v to filter out those warnings.
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared
within a template. With the advent of explicit template specifica-
tion support in G++, if the name of the friend is an unqualified-id
(i.e., friend foo(int)), the C++ language specification demands
that the friend declare or define an ordinary, nontemplate func-
tion. (Section 14.5.3). Before G++ implemented explicit specifi-
cation, unqualified-ids could be interpreted as a particular spe-
cialization of a templatized function. Because this non-conforming
behavior is no longer the default behavior for G++, -Wnon-tem-
plate-friend allows the compiler to check existing code for poten-
tial trouble spots, and is on by default. This new compiler behav-
ior can be turned off with -Wno-non-template-friend which keeps the
conformant compiler code but disables the helpful warning.
-Wold-style-cast (C++ only)
Warn if an old-style (C-style) cast to a non-void type is used
within a C++ program. The new-style casts (static_cast, reinter-
pret_cast, and const_cast) are less vulnerable to unintended
effects, and much easier to grep for.
-Woverloaded-virtual (C++ only)
Warn when a function declaration hides virtual functions from a
base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like this:
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
-Wsign-promo (C++ only)
Warn when overload resolution chooses a promotion from unsigned or
enumeral type to a signed type over a conversion to an unsigned
type of the same size. Previous versions of G++ would try to pre-
serve unsignedness, but the standard mandates the current behavior.
-Wsynth (C++ only)
Warn when G++'s synthesis behavior does not match that of cfront.
For instance:
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator = (const
A&);, while cfront will use the user-defined operator =.
Options Controlling Objective-C Dialect
This section describes the command-line options that are only meaning-
ful for Objective-C programs; but you can also use most of the GNU com-
piler options regardless of what language your program is in. For
example, you might compile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, only -fgnu-runtime is an option meant only for Objec-
tive-C programs; you can use the other options with any language sup-
ported by GCC.
Here is a list of options that are only for compiling Objective-C pro-
grams:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each
literal string specified with the syntax "@"..."". The default
class name is "NXConstantString" if the GNU runtime is being used,
and "NSConstantString" if the NeXT runtime is being used (see
below). The -fconstant-cfstrings option, if also present, will
override the -fconstant-string-class setting and cause "@"...""
literals to be laid out as constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C
runtime. This is the default for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the
default for NeXT-based systems, including Darwin and Mac OS X. The
macro "__NEXT_RUNTIME__" is predefined if (and only if) this option
is used.
-fno-nil-receivers (APPLE ONLY)
Assume that all Objective-C message dispatches (e.g., "[receiver
message:arg]") in this translation unit ensure that the receiver is
not "nil". This allows for more efficient entry points in the run-
time to be used. Currently, this option is only available in con-
junction with the NeXT runtime.
-fobjc-direct-dispatch (APPLE ONLY)
For some functions (such as "objc_msgSend") called very frequently
by Objective-C programs, special entry points exist in high memory
that may be jumped to directly (e.g., via the "bla" instruction on
the PowerPC) for improved performance. The -fobjc-direct-dispatch
option will cause such jumps to be generated. This option is only
available in conjunction with the NeXT runtime; furthermore, pro-
grams built with the -fobjc-direct-dispatch option will only run on
Mac OS X 10.4 (Tiger) or later systems.
-fobjc-exceptions (APPLE ONLY)
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++ and Java. This
option will also be enabled if the environment variable
"MACOSX_DEPLOYMENT_TARGET" exists and is set to "10.3" or greater.
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
The @throw statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a @catch block, the
@throw may appear without an argument (as shown above), in which
case the object caught by the @catch will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and
caught using this scheme. When an object is thrown, it will be
caught by the nearest @catch clause capable of handling objects of
that type, analogously to how "catch" blocks work in C++ and Java.
A "@catch(id ...)" clause (as shown above) may also be provided to
catch any and all Objective-C exceptions not caught by previous
@catch clauses (if any).
The @finally clause, if present, will be executed upon exit from
the immediately preceding "@try ... @catch" section. This will
happen regardless of whether any exceptions are thrown, caught or
rethrown inside the "@try ... @catch" section, analogously to the
behavior of the "finally" clause in Java.
There are several caveats to using the new exception mechanism:
* Although currently designed to be binary compatible with
"NS_HANDLER"-style idioms provided by the "NSException" class,
the new exceptions can only be used on Mac OS X 10.3 (Panther)
and later systems, due to additional functionality needed in
the (NeXT) Objective-C runtime.
* As mentioned above, the new exceptions do not support handling
types other than Objective-C objects. Furthermore, when used
from Objective-C++, the Objective-C exception model does not
interoperate with C++ exceptions at this time. This means you
cannot @throw an exception from Objective-C and "catch" it in
C++, or vice versa (i.e., "throw ... @catch").
The -fobjc-exceptions switch also enables the use of synchroniza-
tion blocks for thread-safe execution:
ObjCClass *lockObject = ...;
...
@synchronized (lockObject) {
...
@throw expr;
...
}
Unlike Java, Objective-C does not allow for entire methods to be
marked @synchronized. Note that throwing exceptions out of @syn-
chronized blocks is allowed, and will cause the guarding object to
be unlocked properly.
-freplace-objc-classes (APPLE ONLY)
When compiling for the NeXT runtime, the compiler ordinarily
replaces calls to "objc_getClass("...")" (when the name of the
class is known at compile time) with static class references that
get initialized at load time, which improves run-time performance.
Specifying the -freplace-objc-classes flag suppresses this behavior
and causes calls to "objc_getClass("...")" to be retained. This is
useful in Fix-and-Continue debugging mode, since it allows for
individual class implementations to be modified during program exe-
cution.
-fzero-link (APPLE ONLY)
Emit a special marker instructing lldd(1) not to statically link in
the resulting object file, and allow ddyylldd(1) to load it in at run
time instead. This is used in conjunction with the Zero Link
debugging mode.
-gen-decls
Dump interface declarations for all classes seen in the source file
to a file named sourcename.decl.
-Wno-protocol
If a class is declared to implement a protocol, a warning is issued
for every method in the protocol that is not implemented by the
class. The default behavior is to issue a warning for every method
not explicitly implemented in the class, even if a method implemen-
tation is inherited from the superclass. If you use the "-Wno-pro-
tocol" option, then methods inherited from the superclass are con-
sidered to be implemented, and no warning is issued for them.
-Wselector
Warn if multiple methods of different types for the same selector
are found during compilation. The check is performed on the list
of methods in the final stage of compilation. Additionally, a
check is performed that for each selector appearing in a "@selec-
tor(...)" expression, a corresponding method with that selector
has been found during compilation. Because these checks scan the
method table only at the end of compilation, these warnings are not
produced if the final stage of compilation is not reached, for
example because an error is found during compilation, or because
the "-fsyntax-only" option is being used.
-Wundeclared-selector
Warn if a "@selector(...)" expression referring to an undeclared
selector is found. A selector is considered undeclared if no
method with that name has been declared (explicitly, in an @inter-
face or @protocol declaration, or implicitly, in an @implementation
section) before the "@selector(...)" expression. This option
always performs its checks as soon as a "@selector(...)" expression
is found (while "-Wselector" only performs its checks in the final
stage of compilation), and so additionally enforces the coding
style convention that methods and selectors must be declared before
being used.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of
the output device's aspect (e.g. its width, ...). The options
described below can be used to control the diagnostic messages format-
ting algorithm, e.g. how many characters per line, how often source
location information should be reported. Right now, only the C++ front
end can honor these options. However it is expected, in the near
future, that the remaining front ends would be able to digest them cor-
rectly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n
characters. The default is 72 characters for g++ and 0 for the
rest of the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a
single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit once source location information; that
is, in case the message is too long to fit on a single physical
line and has to be wrapped, the source location won't be emitted
(as prefix) again, over and over, in subsequent continuation lines.
This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location information (as
prefix) for physical lines that result from the process of breaking
a message which is too long to fit on a single line.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are
not inherently erroneous but which are risky or suggest there may have
been an error.
You can request many specific warnings with options beginning -W, for
example -Wimplicit to request warnings on implicit declarations. Each
of these specific warning options also has a negative form beginning
-Wno- to turn off warnings; for example, -Wno-implicit. This manual
lists only one of the two forms, whichever is not the default.
The following options control the amount and kinds of warnings produced
by GCC; for further, language-specific options also refer to @ref{C++
Dialect Options} and @ref{Objective-C Dialect Options}.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond
that.
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject
all programs that use forbidden extensions, and some other programs
that do not follow ISO C and ISO C++. For ISO C, follows the ver-
sion of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or
without this option (though a rare few will require -ansi or a -std
option specifying the required version of ISO C). However, without
this option, certain GNU extensions and traditional C and C++ fea-
tures are supported as well. With this option, they are rejected.
-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;
application programs should avoid them.
Some users try to use -pedantic to check programs for strict ISO C
conformance. They soon find that it does not do quite what they
want: it finds some non-ISO practices, but not all---only those for
which ISO C requires a diagnostic, and some others for which diag-
nostics have been added.
A feature to report any failure to conform to ISO C might be useful
in some instances, but would require considerable additional work
and would be quite different from -pedantic. We don't have plans
to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended
dialect of C, such as gnu89 or gnu99, there is a corresponding base
standard, the version of ISO C on which the GNU extended dialect is
based. Warnings from -pedantic are given where they are required
by the base standard. (It would not make sense for such warnings
to be given only for features not in the specified GNU C dialect,
since by definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be nothing
to warn about.)
-pedantic-errors
Like -pedantic, except that errors are produced rather than warn-
ings.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-Wno-#warnings
Inhibit warning messages issued by #warning.
-Wpragma-once
-Wextra-tokens
Warn about extra tokens at the end of prepreprocessor directives.
(APPLE ONLY)
-Wnewline-eof
Warn about files missing a newline at the end of the file. (APPLE
ONLY)
-Wno-altivec-long-deprecated
Do not warn about the use of the deprecated 'long' keyword in
AltiVec data types. (APPLE ONLY)
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause
of error, as programmers often forget that this type is signed on
some machines.
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment,
or whenever a Backslash-Newline appears in a // comment.
-Wformat
Check calls to "printf" and "scanf", etc., to make sure that the
arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string
make sense. This includes standard functions, and others specified
by format attributes, in the "printf", "scanf", "strftime" and
"strfmon" (an X/Open extension, not in the C standard) families.
The formats are checked against the format features supported by
GNU libc version 2.2. These include all ISO C90 and C99 features,
as well as features from the Single Unix Specification and some BSD
and GNU extensions. Other library implementations may not support
all these features; GCC does not support warning about features
that go beyond a particular library's limitations. However, if
-pedantic is used with -Wformat, warnings will be given about for-
mat features not in the selected standard version (but not for
"strfmon" formats, since those are not in any version of the C
standard).
Since -Wformat also checks for null format arguments for several
functions, -Wformat also implies -Wnonnull.
-Wformat is included in -Wall. For more control over some aspects
of format checking, the options -Wno-format-y2k, -Wno-for-
mat-extra-args, -Wno-format-zero-length, -Wformat-nonliteral,
-Wformat-security, and -Wformat=2 are available, but are not
included in -Wall.
-Wno-format-y2k
If -Wformat is specified, do not warn about "strftime" formats
which may yield only a two-digit year.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a
"printf" or "scanf" format function. The C standard specifies that
such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with $ operand number specifications, normally warnings
are still given, since the implementation could not know what type
to pass to "va_arg" to skip the unused arguments. However, in the
case of "scanf" formats, this option will suppress the warning if
the unused arguments are all pointers, since the Single Unix Speci-
fication says that such unused arguments are allowed.
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats.
The C standard specifies that zero-length formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a
string literal and so cannot be checked, unless the format function
takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions
that represent possible security problems. At present, this warns
about calls to "printf" and "scanf" functions where the format
string is not a string literal and there are no format arguments,
as in "printf (foo);". This may be a security hole if the format
string came from untrusted input and contains %n. (This is cur-
rently a subset of what -Wformat-nonliteral warns about, but in
future warnings may be added to -Wformat-security that are not
included in -Wformat-nonliteral.)
-Wformat=2
Enable -Wformat plus format checks not included in -Wformat. Cur-
rently equivalent to -Wformat -Wformat-nonliteral -Wformat-secu-
rity.
-Wnonnull
Enable warning about passing a null pointer for arguments marked as
requiring a non-null value by the "nonnull" function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled
with the -Wno-nonnull option.
-Wimplicit-int
Warn when a declaration does not specify a type.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being
declared.
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration.
-Wmain
Warn if the type of main is suspicious. main should be a function
with external linkage, returning int, taking either zero arguments,
two, or three arguments of appropriate types.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed.
In the following example, the initializer for a is not fully brack-
eted, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is
expected, or when operators are nested whose precedence people
often get confused about.
Also warn about constructions where there may be confusion to which
"if" statement an "else" branch belongs. Here is an example of
such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every "else" branch belongs to the innermost possible "if"
statement, which in this example is "if (b)". This is often not
what the programmer expected, as illustrated in the above example
by indentation the programmer chose. When there is the potential
for this confusion, GCC will issue a warning when this flag is
specified. To eliminate the warning, add explicit braces around
the innermost "if" statement so there is no way the "else" could
belong to the enclosing "if". The resulting code would look like
this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
-Wsequence-point
Warn about code that may have undefined semantics because of viola-
tions of sequence point rules in the C standard.
The C standard defines the order in which expressions in a C pro-
gram are evaluated in terms of sequence points, which represent a
partial ordering between the execution of parts of the program:
those executed before the sequence point, and those executed after
it. These occur after the evaluation of a full expression (one
which is not part of a larger expression), after the evaluation of
the first operand of a "&&", "||", "? :" or "," (comma) operator,
before a function is called (but after the evaluation of its argu-
ments and the expression denoting the called function), and in cer-
tain other places. Other than as expressed by the sequence point
rules, the order of evaluation of subexpressions of an expression
is not specified. All these rules describe only a partial order
rather than a total order, since, for example, if two functions are
called within one expression with no sequence point between them,
the order in which the functions are called is not specified. How-
ever, the standards committee have ruled that function calls do not
overlap.
It is not specified when between sequence points modifications to
the values of objects take effect. Programs whose behavior depends
on this have undefined behavior; the C standard specifies that
``Between the previous and next sequence point an object shall have
its stored value modified at most once by the evaluation of an
expression. Furthermore, the prior value shall be read only to
determine the value to be stored.''. If a program breaks these
rules, the results on any particular implementation are entirely
unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] =
b[n++]" and "a[i++] = i;". Some more complicated cases are not
diagnosed by this option, and it may give an occasional false posi-
tive result, but in general it has been found fairly effective at
detecting this sort of problem in programs.
The present implementation of this option only works for C pro-
grams. A future implementation may also work for C++ programs.
The C standard is worded confusingly, therefore there is some
debate over the precise meaning of the sequence point rules in sub-
tle cases. Links to discussions of the problem, including proposed
formal definitions, may be found on our readings page, at
<http://gcc.gnu.org/readings/>.
-Wreturn-type
Warn whenever a function is defined with a return-type that
defaults to "int". Also warn about any "return" statement with no
return-value in a function whose return-type is not "void".
For C++, a function without return type always produces a diagnos-
tic message, even when -Wno-return-type is specified. The only
exceptions are main and functions defined in system headers.
-Wswitch
Warn whenever a "switch" statement has an index of enumeral type
and lacks a "case" for one or more of the named codes of that enu-
meration. (The presence of a "default" label prevents this warn-
ing.) "case" labels outside the enumeration range also provoke
warnings when this option is used.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumeral type
and lacks a "case" for one or more of the named codes of that enu-
meration. "case" labels outside the enumeration range also provoke
warnings when this option is used.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning
of the program (trigraphs within comments are not warned about).
-Wunused-function
Warn whenever a static function is declared but not defined or a
non\-inline static function is unused.
-Wunused-label
Warn whenever a label is declared but not used.
To suppress this warning use the unused attribute.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its decla-
ration.
To suppress this warning use the unused attribute.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is
unused aside from its declaration
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not
used.
To suppress this warning cast the expression to void.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you
must either specify -W -Wunused or separately specify
-Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being initial-
ized or if a variable may be clobbered by a "setjmp" call.
These warnings are possible only in optimizing compilation, because
they require data flow information that is computed only when opti-
mizing. If you don't specify -O, you simply won't get these warn-
ings.
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 warn-
ings are printed.
These warnings are made optional because GCC is not smart enough to
see all the reasons why the code might be correct despite appearing
to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always ini-
tialized, but GCC 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.
This option also warns when a non-volatile automatic variable might
be changed by a call to "longjmp". These warnings as well are pos-
sible 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.
Some spurious warnings can be avoided if you declare all the func-
tions you use that never return as "noreturn".
-Wunknown-pragmas
stood by GCC. If this command line option is used, warnings will
even be issued for unknown pragmas in system header files. This is
not the case if the warnings were only enabled by the -Wall command
line option.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It
warns about code which might break the strict aliasing rules that
the compiler is using for optimization. The warning does not catch
all cases, but does attempt to catch the more common pitfalls. It
is included in -Wall.
-Wall
All of the above -W options combined. This enables all the warn-
ings about constructions that some users consider questionable, and
that are easy to avoid (or modify to prevent the warning), even in
conjunction with macros. This also enables some language-specific
warnings described in @ref{C++ Dialect Options} and @ref{Objec-
tive-C Dialect Options}.
-Wmost
This is equivalent to -Wall -Wno-parentheses. (APPLE ONLY)
The following -W... options are not implied by -Wall. Some of them
warn about constructions that users generally do not consider question-
able, but which occasionally you might wish to check for; others warn
about constructions that are necessary or hard to avoid in some cases,
and there is no simple way to modify the code to suppress the warning.
-W Print extra warning messages for these events:
* A function can return either with or without a value. (Falling
off the end of the function body is considered returning with-
out a value.) For example, this function would evoke such a
warning:
foo (a)
{
if (a > 0)
return a;
}
* An expression-statement or the left-hand side of a comma
expression contains no side effects. To suppress the warning,
cast the unused expression to void. For example, an expression
such as x[i,j] will cause a warning, but x[(void)i,j] will not.
* An unsigned value is compared against zero with < or <=.
* A comparison like x<=y<=z appears; this is equivalent to (x<=y
? 1 : 0) <= z, which is a different interpretation from that of
ordinary mathematical notation.
* Storage-class specifiers like "static" are not the first things
in a declaration. According to the C Standard, this usage is
obsolescent.
* The return type of a function has a type qualifier such as
"const". Such a type qualifier has no effect, since the value
returned by a function is not an lvalue. (But don't warn about
the GNU extension of "volatile void" return types. That exten-
sion will be warned about if -pedantic is specified.)
* If -Wall or -Wunused is also specified, warn about unused argu-
ments.
* A comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to
unsigned. (But don't warn if -Wno-sign-compare is also speci-
fied.)
* An aggregate has a partly bracketed initializer. For example,
the following code would evoke such a warning, because braces
are missing around the initializer for "x.h":
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
* An aggregate has an initializer which does not initialize all
members. For example, the following code would cause such a
warning, because "x.h" would be implicitly initialized to zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating
point division by zero is not warned about, as it can be a legiti-
mate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the
assumption that they usually do not indicate real problems and
would only make the compiler output harder to read. Using this
command line option tells GCC to emit warnings from system headers
as if they occurred in user code. However, note that using -Wall
in conjunction with this option will not warn about unknown pragmas
in system headers---for that, -Wunknown-pragmas must also be used.
-Wfloat-equal
Warn if floating point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers. If you are doing this, then you
need to compute (by analyzing the code, or in some other way) the
maximum or likely maximum error that the computation introduces,
and allow for it when performing comparisons (and when producing
output, but that's a different problem). In particular, instead of
testing for equality, you would check to see whether the two values
have ranges that overlap; and this is done with the relational
operators, so equality comparisons are probably mistaken.
-Wtraditional (C only)
Warn about certain constructs that behave differently in tradi-
tional and ISO C. Also warn about ISO C constructs that have no
traditional C equivalent, and/or problematic constructs which
should be avoided.
* Macro parameters that appear within string literals in the
macro body. In traditional C macro replacement takes place
within string literals, but does not in ISO C.
* In traditional C, some preprocessor directives did not exist.
Traditional preprocessors would only consider a line to be a
directive if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C under-
stands but would ignore because the # does not appear as the
first character on the line. It also suggests you hide direc-
them. Some traditional implementations would not recognize
#elif, so it suggests avoiding it altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating point
constant suffixes. (Traditional C does support the L suffix on
integer constants.) Note, these suffixes appear in macros
defined in the system headers of most modern systems, e.g. the
_MIN/_MAX macros in "<limits.h>". Use of these macros in user
code might normally lead to spurious warnings, however gcc's
integrated preprocessor has enough context to avoid warning in
these cases.
* 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".
* A non-"static" function declaration follows a "static" one.
This construct is not accepted by some traditional C compilers.
* The ISO type of an integer constant has a different width or
signedness from its traditional type. This warning is only
issued if the base of the constant is ten. I.e. hexadecimal or
octal values, which typically represent bit patterns, are not
warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a sepa-
rate namespace for labels.
* Initialization of unions. If the initializer is zero, the
warning is omitted. This is done under the assumption that the
zero initializer in user code appears conditioned on e.g.
"__STDC__" to avoid missing initializer warnings and relies on
default initialization to zero in the traditional C case.
* Conversions by prototypes between fixed/floating point values
and vice versa. The absence of these prototypes when compiling
with traditional C would cause serious problems. This is a
subset of the possible conversion warnings, for the full set
use -Wconversion.
* Use of ISO C style function definitions. This warning inten-
tionally is not issued for prototype declarations or variadic
functions because these ISO C features will appear in your code
when using libiberty's traditional C compatibility macros,
"PARAMS" and "VPARAMS". This warning is also bypassed for
nested functions because that feature is already a gcc exten-
sion and thus not relevant to traditional C compatibility.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wendif-labels
Warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable shadows another local variable,
parameter or global variable or whenever a built-in function is
shadowed.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
-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 conve-
nience in calculations with "void *" pointers and pointers to func-
tions.
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type. For
example, warn if "int malloc()" is cast to "anything *".
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if a "const char *" is
cast to an ordinary "char *".
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of
the target is increased. For example, warn if a "char *" is cast
to an "int *" on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const
char[length]" so that copying the address of one into a non-"const"
"char *" pointer will get a warning; when compiling C++, warn about
the deprecated conversion from string constants to "char *". These
warnings will help you find at compile time code that can try to
write into a string constant, but only if you have been very care-
ful about using "const" in declarations and prototypes. Otherwise,
it will just be a nuisance; this is why we did not make -Wall
request these warnings.
-Wconversion
Warn if a prototype causes a type conversion that is different from
what would happen to the same argument in the absence of a proto-
type. This includes conversions of fixed point to floating and
vice versa, and conversions changing the width or signedness of a
fixed point argument except when the same as the default promotion.
Also, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the assign-
ment "x = -1" if "x" is unsigned. But do not warn about explicit
casts like "(unsigned) -1".
-Wsign-compare
Warn when a comparison between signed and unsigned values could
produce an incorrect result when the signed value is converted to
unsigned. This warning is enabled by -W, and by -Wall in C++ only.
-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 (C only)
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration which specifies the
argument types.)
-Wmissing-prototypes (C only)
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. 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 declara-
tion. Do so even if the definition itself provides a prototype.
Use this option to detect global functions that are not declared in
header files.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute "nore-
turn". Note these are only possible candidates, not absolute ones.
Care should be taken to manually verify functions actually do not
ever return before adding the "noreturn" attribute, otherwise sub-
tle code generation bugs could be introduced. You will not get a
warning for "main" in hosted C environments.
-Wmissing-format-attribute
If -Wformat is enabled, also warn about functions which might be
candidates for "format" attributes. Note these are only possible
candidates, not absolute ones. GCC will guess that "format"
attributes might be appropriate for any function that calls a func-
tion like "vprintf" or "vscanf", but this might not always be the
case, and some functions for which "format" attributes are appro-
priate may not be detected. This option has no effect unless
-Wformat is enabled (possibly by -Wall).
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually
they indicate a typo in the user's code, as they have implementa-
tion-defined values, and should not be used in portable code.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as
deprecated by using the "deprecated" attribute. (@pxref{Function
Attributes}, @pxref{Variable Attributes}, @pxref{Type Attributes}.)
-Wpacked
Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable "f.x" in "struct bar" will be
misaligned even though "struct bar" does not itself have the packed
attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpadded
Warn if padding is included in a structure, either to align an ele-
ment of the structure or to align the whole structure. Sometimes
when this happens it is possible to rearrange the fields of the
structure to reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even
in cases where multiple declaration is valid and changes nothing.
-Wnested-externs (C only)
Warn if an "extern" declaration is encountered within a function.
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at
least a whole line of source code will never be executed, because
some condition is never satisfied or because it is after a proce-
dure that never returns.
It is possible for this option to produce a warning even though
there are circumstances under which part of the affected line can
be executed, so care should be taken when removing apparently-
unreachable code.
For instance, when a function is inlined, a warning may mean that
the line is unreachable in only one inlined copy of the function.
This option is not made part of -Wall because in a debugging ver-
sion of a program there is often substantial code which checks cor-
rect functioning of the program and is, hopefully, unreachable
because the program does work. Another common use of unreachable
code is to provide behavior which is selectable at compile-time.
-Winline
Warn if a function can not be inlined and it was declared as
inline.
-Wno-invalid-offsetof (C++ only)
Suppress warnings from applying the offsetof macro to a non-POD
type.
-Wno-long-double
Inhibit warning if the long double type is used. (APPLE ONLY)
-Winvalid-pch
Warn if a precompiled header is found in the search path but can't
be used.
-Wlong-long
Warn if long long type is used. This is default. To inhibit the
warning messages, use -Wno-long-long. Flags -Wlong-long and
-Wno-long-long are taken into account only when -pedantic flag is
used.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning
does not generally indicate that there is anything wrong with your
code; it merely indicates that GCC's optimizers were unable to han-
dle the code effectively. Often, the problem is that your code is
too big or too complex; GCC will refuse to optimize programs when
the optimization itself is likely to take inordinate amounts of
time.
-Werror
Make all warnings into errors.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your
program or GCC:
-g Produce debugging information in the operating system's native for-
mat (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
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but will probably make other
debuggers crash or refuse to read the program. If you want to con-
trol for certain whether to generate the extra information, use
-gstabs+ or -gstabs (see below).
Unlike most other C compilers, GCC allows you to use -g with -O.
The shortcuts taken by optimized code may occasionally produce sur-
prising 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 GCC is generated with the
capability for more than one debugging format.
-ggdb
Produce debugging information for use by GDB. This means to use
the most expressive format available (DWARF 2, stabs, or the native
format if neither of those are supported), including GDB extensions
if at all possible.
-gstabs
Produce debugging information in stabs format (if that is sup-
ported), without GDB extensions. This is the format used by DBX on
most BSD systems. On MIPS, Alpha and System V Release 4 systems
this option produces stabs debugging output which is not understood
by DBX or SDB. On System V Release 4 systems this option requires
the GNU assembler.
-gstabs+
Produce debugging information in stabs format (if that is sup-
ported), using GNU extensions understood only by the GNU debugger
(GDB). The use of these extensions is likely to make other debug-
gers crash or refuse to read the program.
(Other debug formats, such as -gcoff, are not supported in Darwin
or Mac OS X.)
-glevel
-ggdblevel
-gstabslevel
Request debugging information and also use level to specify how
much information. The default level is 2.
Level 1 produces minimal information, enough for making backtraces
in parts of the program that you don't plan to debug. This
includes descriptions of functions and external variables, but no
information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro defini-
tions present in the program. Some debuggers support macro expan-
sion when you use -g3.
Note that in order to avoid confusion between DWARF1 debug level 2,
and DWARF2, neither -gdwarf nor -gdwarf-2 accept a concatenated
debug level. Instead use an additional -glevel option to change
the debug level for DWARF1 or DWARF2.
-feliminate-dwarf2-dups
Compress DWARF2 debugging information by eliminating duplicated
information about each symbol. This option only makes sense when
generating DWARF2 debugging information with -gdwarf-2.
-p Generate extra code to write profile information suitable for the
analysis program prof. You must use this option when compiling the
source files you want data about, and you must also use it when
linking.
-pg Generate extra code to write profile information suitable for the
analysis program gprof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-Q Makes the compiler print out each function name as it is compiled,
and print some statistics about each pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by
each pass when it finishes.
-fmem-report
Makes the compiler print some statistics about permanent memory
allocation when it finishes.
-fprofile-arcs
Instrument arcs during compilation to generate coverage data or for
profile-directed block ordering. During execution the program
records how many times each branch is executed and how many times
it is taken. When the compiled program exits it saves this data to
a file called auxname.da for each source file. auxname is gener-
ated from the name of the output file, if explicitly specified and
it is not the final executable, otherwise it is the basename of the
source file. In both cases any suffix is removed (e.g. foo.da for
input file dir/foo.c, or dir/foo.da for output file specified as -o
dir/foo.o).
For profile-directed block ordering, compile the program with
-fprofile-arcs plus optimization and code generation options, gen-
erate the arc profile information by running the program on a
selected workload, and then compile the program again with the same
optimization and code generation options plus -fbranch-probabili-
ties.
The other use of -fprofile-arcs is for use with gcov, when it is
used with the -ftest-coverage option.
With -fprofile-arcs, for each function of your program GCC creates
a program flow graph, then finds a spanning tree for the graph.
Only arcs that are not on the spanning tree have to be instru-
mented: the compiler adds code to count the number of times that
these arcs are executed. When an arc is the only exit or only
entrance to a block, the instrumentation code can be added to the
block; otherwise, a new basic block must be created to hold the
instrumentation code.
-ftest-coverage
Create data files for the gcov code-coverage utility. See -fpro-
file-arcs option above for a description of auxname.
auxname.bb
A mapping from basic blocks to line numbers, which gcov uses to
associate basic block execution counts with line numbers.
auxname.bbg
A list of all arcs in the program flow graph. This allows gcov
to reconstruct the program flow graph, so that it can compute
all basic block and arc execution counts from the information
in the auxname.da file.
Use -ftest-coverage with -fprofile-arcs; the latter option adds
instrumentation to the program, which then writes execution counts
to another data file:
auxname.da
Runtime arc execution counts, used in conjunction with the arc
information in the file auxname.bbg.
Coverage data will map better to the source files if -ftest-cover-
age is used without optimization.
-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 pass number and
a word to the dumpname. dumpname is generated from the name of the
output file, if explicitly specified and it is not an executable,
otherwise it is the basename of the source file. In both cases any
suffix is removed (e.g. foo.00.rtl or foo.01.sibling). Here are
the possible letters for use in letters, and their meanings:
A Annotate the assembler output with miscellaneous debugging
information.
b Dump after computing branch probabilities, to file.14.bp.
B Dump after block reordering, to file.32.bbro.
c Dump after instruction combination, to the file file.19.com-
bine.
C Dump after the first if conversion, to the file file.15.ce1.
d Dump after delayed branch scheduling, to file.34.dbr.
D Dump all macro definitions, at the end of preprocessing, in
addition to normal output.
e Dump after SSA optimizations, to file.04.ssa and file.07.ussa.
E Dump after the second if conversion, to file.29.ce3.
f Dump after life analysis, to file.18.life.
F Dump after purging "ADDRESSOF" codes, to file.10.addressof.
g Dump after global register allocation, to file.24.greg.
h Dump after finalization of EH handling code, to file.02.eh.
k Dump after reg-to-stack conversion, to file.31.stack.
o Dump after post-reload optimizations, to file.25.postreload.
G Dump after GCSE, to file.11.gcse.
i Dump after sibling call optimizations, to file.01.sibling.
j Dump after the first jump optimization, to file.03.jump.
k Dump after conversion from registers to stack, to
file.31.stack.
l Dump after local register allocation, to file.23.lreg.
L Dump after loop optimization, to file.12.loop.
M Dump after performing the machine dependent reorganization
pass, to file.33.mach.
n Dump after register renumbering, to file.28.rnreg.
N Dump after the register move pass, to file.21.regmove.
r Dump after RTL generation, to file.00.rtl.
R Dump after the second scheduling pass, to file.30.sched2.
s Dump after CSE (including the jump optimization that sometimes
follows CSE), to file.09.cse.
S Dump after the first scheduling pass, to file.22.sched.
t Dump after the second CSE pass (including the jump optimization
that sometimes follows CSE), to file.17.cse2.
u Dump after null pointer elimination pass to file.08.null.
w Dump after the second flow pass, to file.26.flow2.
X Dump after SSA dead code elimination, to file.06.ssadce.
z Dump after the peephole pass, to file.27.peephole2.
a Produce all the dumps listed above.
m Print statistics on memory usage, at the end of the run, to
standard error.
p Annotate the assembler output with a comment indicating which
pattern and alternative was used. The length of each instruc-
tion is also printed.
P Dump the RTL in the assembler output as a comment before each
instruction. Also turns on -dp annotation.
v For each of the other indicated dump files (except for
file.00.rtl), dump a representation of the control flow graph
suitable for viewing with VCG to file.pass.vcg.
x Just generate RTL for a function instead of compiling it. Usu-
ally used with r.
y Dump debugging information during parsing, to standard error.
-fdump-unnumbered
When doing debugging dumps (see -d option above), suppress instruc-
tion numbers and line number note output. This makes it more fea-
sible to use diff on debugging dumps for compiler invocations with
different options, in particular with and without -g.
-fdump-translation-unit (C and C++ only)
-fdump-translation-unit-options (C and C++ only)
Dump a representation of the tree structure for the entire transla-
tion unit to a file. The file name is made by appending .tu to the
source file name. If the -options form is used, options controls
the details of the dump as described for the -fdump-tree options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class's hierarchy and virtual func-
tion table layout to a file. The file name is made by appending
.class to the source file name. If the -options form is used,
options controls the details of the dump as described for the
-fdump-tree options.
-fdump-tree-switch (C++ only)
-fdump-tree-switch-options (C++ only)
Control the dumping at various stages of processing the intermedi-
ate language tree to a file. The file name is generated by append-
ing a switch specific suffix to the source file name. If the
-options form is used, options is a list of - separated options
that control the details of the dump. Not all options are applica-
ble to all dumps, those which are not meaningful will be ignored.
The following options are available
address
Print the address of each node. Usually this is not meaningful
as it changes according to the environment and source file. Its
primary use is for tying up a dump file with a debug environ-
ment.
slim
Inhibit dumping of members of a scope or body of a function
merely because that scope has been reached. Only dump such
items when they are directly reachable by some other path.
all Turn on all options.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
inlined
Dump after function inlining, to file.inlined.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls
the amount of debugging output the scheduler prints. This informa-
tion is written to standard error, unless -dS or -dR is specified,
in which case it is output to the usual dump listing file, .sched
or .sched2 respectively. However for n greater than nine, the out-
put is always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same informa-
tion as -dRS. For n greater than one, it also output basic block
probabilities, detailed ready list information and unit/insn info.
For n greater than two, it includes RTL at abort point, control-
flow and regions info. And for n over four, -fsched-verbose also
includes dependence info.
-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.i and foo.s, as well as foo.o. This creates a prepro-
cessed foo.i output file even though the compiler now normally uses
an integrated preprocessor.
-time
Report the CPU time taken by each subprocess in the compilation
sequence. For C source files, this is the compiler proper and
assembler (plus the linker if linking is done). The output looks
like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the ``user time,'' that is time
spent executing the program itself. The second number is ``system
time,'' time spent executing operating system routines on behalf of
the program. Both numbers are in seconds.
-fsave-repository=location
Creates separate symbol repository at location for given input
header file. Separate repository contains only debugging symbols in
stabs format.
-grepository
Instructs compiler to use separate symbol repository with debugging
symbols. Compiler searches for such repositories in include paths.
-print-file-name=library
Print the full absolute name of the library file library that would
be used when linking---and don't do anything else. With this
option, GCC does not compile or link anything; it just prints the
file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by
any other switches present in the command line. This directory is
supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler
switches that enable them. The directory name is separated from
the switches by ;, and each switch starts with an @} instead of the
@samp{-, without spaces between multiple switches. This is sup-
posed to ease shell-processing.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do
want to link with libgcc.a. You can do
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a list
of program and library directories gcc will search---and don't do
anything else.
This is useful when gcc prints the error message installation prob-
lem, cannot exec cpp0: No such file or directory. To resolve this
you either need to put cpp0 and the other compiler components where
gcc expects to find them, or you can set the environment variable
GCC_EXEC_PREFIX to the directory where you installed them. Don't
forget the trailing '/'.
-dumpmachine
Print the compiler's target machine (for example,
i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0)---and don't do any-
thing else.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else.
(This is used when GCC itself is being built.)
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the
cost of compilation and to make debugging produce the expected results.
Statements are independent: if you stop the program with a breakpoint
between statements, you can then assign a new value to any variable or
change the program counter to any other statement in the function and
get exactly the results you would expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the
performance and/or code size at the expense of compilation time and
possibly the ability to debug the program.
Not all optimizations are controlled directly by a flag. Only opti-
mizations that have a flag are listed.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time,
without performing any optimizations that take a great deal of com-
pilation time.
In Apple's version of GCC, -fstrict-aliasing, -freorder-blocks, and
-fsched-interblock are disabled by default when optimizing.
-O2 Optimize even more. GCC performs nearly all supported optimiza-
tions that do not involve a space-speed tradeoff. The compiler
does not perform loop unrolling or function inlining when you spec-
ify -O2. As compared to -O, this option increases both compilation
time and the performance of the generated code.
-O2 turns on all optimization flags specified by -O. It also turns
on the following optimization flags: -fforce-mem -foptimize-sib-
ling-calls -fstrength-reduce -fcse-follow-jumps -fcse-skip-blocks
-frerun-cse-after-loop -frerun-loop-opt -fgcse -fgcse-lm
-fgcse-sm -fdelete-null-pointer-checks -fexpensive-optimizations
-fregmove -fschedule-insns -fschedule-insns2 -fsched-interblock
-fsched-spec -fcaller-saves -fpeephole2 -freorder-blocks -fre-
order-functions -fstrict-aliasing -falign-functions -falign-jumps
-falign-loops -falign-labels
Please note the warning under -fgcse about invoking -O2 on programs
that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2
and also turns on the -finline-functions and -frename-registers
options.
-O0 Do not optimize. This is the default.
-fast
Optimize for maximum performance. -fast changes the overall opti-
mization strategy of GCC in order to produce the fastest possible
running code for PPC7450 and G5 architectures. By default, -fast
optimizes for G5. Programs optimized for G5 will not run on
PPC7450. To optimize for PPC7450, add -mcpu=7450 on command line.
-fast currently enables the following optimization flags (for G5
and PPC7450). These flags may change in the future. You cannot
override any of these options if you use -fast except by setting
-mcpu=7450. Note that -ffast-math, -fstrict-aliasing and
-malign-natural are unsafe in some situations. To build shared
libraries with -fast, specify -fPIC on command line.
-O3 -funroll-loops -fstrict-aliasing -fsched-interblock
-falign-loops=16 -falign-jumps=16 -falign-functions=16
-falign-jumps-max-skip=15 -falign-loops-max-skip=15 -malign-natural
-ffast-math -mdynamic-no-pic -mpowerpc-gpopt -force_cpusubtype_ALL
-fstrict-aliasing -mtune=G5 -mcpu=G5 -mpowerpc64
-Os Optimize for size. -Os enables all -O2 optimizations that do not
typically increase code size. It also performs further optimiza-
tions designed to reduce code size.
-Os disables the following optimization flags: -falign-functions
-falign-jumps -falign-loops -falign-labels -freorder-blocks
-fprefetch-loop-arrays
If you use multiple -O options, with or without level numbers, the
last such option is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most
flags have both positive and negative forms; the negative form of -ffoo
would be -fno-foo. In the table below, only one of the forms is
listed---the one you typically will use. You can figure out the other
form by either removing no- or adding it.
The following options control specific optimizations. They are either
activated by -O options or are related to ones that are. You can use
the following flags in the rare cases when ``fine-tuning'' of optimiza-
tions to be performed is desired.
-fno-default-inline
Do not make member functions inline by default merely because they
are defined inside the class scope (C++ only). Otherwise, when you
specify -O, member functions defined inside class scope are com-
piled inline by default; i.e., you don't need to add inline in
front of the member function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that
function returns. For machines which must pop arguments after a
function call, the compiler normally lets arguments accumulate on
the stack for several function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
-fforce-mem
Force memory operands to be copied into registers before doing
arithmetic on them. This produces better code by making all memory
references potential common subexpressions. When they are not com-
mon subexpressions, instruction combination should eliminate the
separate register-load.
Enabled at levels -O2, -O3, -Os.
-fforce-addr
Force memory address constants to be copied into registers before
doing arithmetic on them. This may produce better code just as
-fforce-mem may.
-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 some 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
machine-description macro "FRAME_POINTER_REQUIRED" controls whether
a target machine supports this flag.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-fno-inline
Don't pay attention to the "inline" keyword. Normally this option
is used to keep the compiler from expanding any functions inline.
Note that if you are not optimizing, no functions can be expanded
inline.
-finline-functions
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be worth
integrating in this way.
If all calls to a given function are integrated, and the function
is declared "static", then the function is normally not output as
assembler code in its own right.
Enabled at level -O3.
-finline-limit=n
By default, gcc limits the size of functions that can be inlined.
This flag allows the control of this limit for functions that are
explicitly marked as inline (i.e., marked with the inline keyword
or defined within the class definition in c++). n is the size of
functions that can be inlined in number of pseudo instructions (not
counting parameter handling). The default value of n is 600.
Increasing this value can result in more inlined code at the cost
of compilation time and memory consumption. Decreasing usually
makes the compilation faster and less code will be inlined (which
presumably means slower programs). This option is particularly
useful for programs that use inlining heavily such as those based
on recursive templates with C++.
Inlining is actually controlled by a number of parameters, which
may be specified individually by using --param name=value. The
-finline-limit=n option sets some of these parameters as follows:
@item max-inline-insns
is set to I<n>.
@item max-inline-insns-single
is set to I<n>/2.
@item max-inline-insns-single-auto
is set to I<n>/2.
@item min-inline-insns
is set to 130 or I<n>/4, whichever is smaller.
@item max-inline-insns-rtl
is set to I<n>.
Using -finline-limit=600 thus results in the default settings for
these parameters. See below for a documentation of the individual
parameters controlling inlining.
Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way, it represents
a count of assembly instructions and as such its exact meaning
might change from one release to an another.
-fkeep-inline-functions
Even if all calls to a given function are integrated, and the func-
tion is declared "static", nevertheless output a separate run-time
callable version of the function. This switch does not affect
"extern inline" functions.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't
turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the com-
piler to check if the variable was referenced, regardless of
whether or not optimization is turned on, use the
-fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating
point constants) across compilation units.
This option is the default for optimized compilation if the assem-
bler and linker support it. Use -fno-merge-constants to inhibit
this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-con-
stants this considers e.g. even constant initialized arrays or ini-
tialized constant variables with integral or floating point types.
Languages like C or C++ require each non-automatic variable to have
distinct location, so using this option will result in non-conform-
ing behavior.
-fno-branch-count-reg
Do not use ``decrement and branch'' instructions on a count regis-
ter, but instead generate a sequence of instructions that decrement
a register, compare it against zero, then branch based upon the
result. This option is only meaningful on architectures that sup-
port such instructions, which include x86, PowerPC, IA-64 and
S/390.
The default is -fbranch-count-reg, enabled when -fstrength-reduce
is enabled.
-fno-function-cse
Do not put function addresses in registers; make each instruction
that calls a constant function contain the function's address
explicitly.
This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the optimiza-
tions performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables
that are initialized to zero into BSS. This can save space in the
resulting code.
This option turns off this behavior because some programs explic-
itly rely on variables going to the data section. E.g., so that
the resulting executable can find the beginning of that section
and/or make assumptions based on that.
The default is -fzero-initialized-in-bss.
-fstrength-reduce
Perform the optimizations of loop strength reduction and elimina-
tion of iteration variables.
Enabled at levels -O2, -O3, -Os.
-fthread-jumps
Perform optimizations where we check to see if a jump branches to a
location where another comparison subsumed by the first is found.
If so, the first branch is redirected to either the destination of
the second branch or a point immediately following it, depending on
whether the condition is known to be true or false.
Enabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination, scan through jump instructions
when the target of the jump is not reached by any other path. For
example, when CSE encounters an "if" statement with an "else"
clause, CSE will follow the jump when the condition tested is
false.
Enabled at levels -O2, -O3, -Os.
-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".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations
has been performed.
Enabled at levels -O2, -O3, -Os.
-frerun-loop-opt
Run the loop optimizer twice.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass
also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC exten-
sion, you may get better runtime performance if you disable the
global common subexpression elimination pass by adding -fno-gcse to
the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination
will attempt to move loads which are only killed by stores into
themselves. This allows a loop containing a load/store sequence to
be changed to a load outside the loop, and a copy/store within the
loop.
Enabled by default when gcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, A store motion pass is run after global
common subexpression elimination. This pass will attempt to move
stores out of loops. When used in conjunction with -fgcse-lm,
loops containing a load/store sequence can be changed to a load
before the loop and a store after the loop.
Enabled by default when gcse is enabled.
-floop-optimize
Perform loop optimizations: move constant expressions out of loops,
simplify exit test conditions and optionally do strength-reduction
and loop unrolling as well.
Enabled at levels -O, -O2, -O3, -Os.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies
equivalent code and save code size. The resulting code may or may
not perform better than without cross-jumping.
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion
Attempt to transform conditional jumps into branch-less equiva-
lents. This include use of conditional moves, min, max, set flags
and abs instructions, and some tricks doable by standard arith-
metics. The use of conditional execution on chips where it is
available is controlled by "if-conversion2".
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Use conditional execution (where available) to transform condi-
tional jumps into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os.
-fdelete-null-pointer-checks
Use global dataflow analysis to identify and eliminate useless
checks for null pointers. The compiler assumes that dereferencing
a null pointer would have halted the program. If a pointer is
checked after it has already been dereferenced, it cannot be null.
In some environments, this assumption is not true, and programs can
safely dereference null pointers. Use
-fno-delete-null-pointer-checks to disable this optimization for
programs which depend on that behavior.
Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expen-
sive.
Enabled at levels -O2, -O3, -Os.
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instructions and as
operands of other simple instructions in order to maximize the
amount of register tying. This is especially helpful on machines
with two-operand instructions.
Note -fregmove and -foptimize-register-move are the same optimiza-
tion.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder instruc-
tions to exploit instruction slots available after delayed branch
instructions.
Enabled at levels -O, -O2, -O3, -Os.
-fschedule-insns
If supported for the target machine, attempt to reorder instruc-
tions to eliminate execution stalls due to required data being
unavailable. This helps machines that have slow floating point or
memory load instructions by allowing other instructions to be
issued until the result of the load or floating point instruction
is required.
Enabled at levels -O2, -O3, -Os.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of
instruction scheduling after register allocation has been done.
This is especially useful on machines with a relatively small num-
ber of registers and where memory load instructions take more than
one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Don't schedule instructions across basic blocks. This is normally
enabled by default when scheduling before register allocation, i.e.
with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Don't allow speculative motion of non-load instructions. This is
normally enabled by default when scheduling before register alloca-
tion, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load
Allow speculative motion of some load instructions. This only
makes sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only
makes sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-fcaller-saves
Enable values to be allocated in registers that will be clobbered
by function calls, by emitting extra instructions to save and
restore the 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 always enabled by default on certain machines, usu-
ally those which have no call-preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
-fmove-all-movables
Forces all invariant computations in loops to be moved outside the
loop.
-freduce-all-givs
Forces all general-induction variables in loops to be
strength-reduced.
Note: When compiling programs written in Fortran, -fmove-all-mov-
ables and -freduce-all-givs are enabled by default when you use the
optimizer.
These options may generate better or worse code; results are highly
dependent on the structure of loops within the source code.
These two options are intended to be removed someday, once they
have helped determine the efficacy of various approaches to improv-
ing loop optimizations.
Please let us (<gcc@gcc.gnu.org> and <fortran@gnu.org>) know how
use of these options affects the performance of your production
code. We're very interested in code that runs slower when these
options are enabled.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The differ-
ence between -fno-peephole and -fno-peephole2 is in how they are
implemented in the compiler; some targets use one, some use the
other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels
-O2, -O3, -Os.
-fbranch-probabilities
-fno-guess-branch-probability
Do not guess branch probabilities using a randomized model.
Sometimes gcc will opt to use a randomized model to guess branch
probabilities, when none are available from either profiling feed-
back (-fprofile-arcs) or __builtin_expect. This means that differ-
ent runs of the compiler on the same program may produce different
object code.
In a hard real-time system, people don't want different runs of the
compiler to produce code that has different behavior; minimizing
non-determinism is of paramount import. This switch allows users
to reduce non-determinism, possibly at the expense of inferior
optimization.
The default is -fguess-branch-probability at levels -O, -O2, -O3,
-Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce
number of taken branches and improve code locality.
Enabled at levels -O2, -O3, -Os.
-freorder-functions
Reorder basic blocks in the compiled function in order to reduce
number of taken branches and improve code locality. This is imple-
mented by using special subsections "text.hot" for most frequently
executed functions and "text.unlikely" for unlikely executed func-
tions. Reordering is done by the linker so object file format must
support named sections and linker must place them in a reasonable
way.
Also profile feedback must be available in to make this option
effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allows the compiler to assume the strictest aliasing rules applica-
ble to the language being compiled. For C (and C++), this acti-
vates optimizations based on the type of expressions. In particu-
lar, an object of one type is assumed never to reside at the same
address as an object of a different type, unless the types are
almost the same. For example, an "unsigned int" can alias an
"int", but not a "void*" or a "double". A character type may alias
any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one
most recently written to (called ``type-punning'') is common. Even
with -fstrict-aliasing, type-punning is allowed, provided the mem-
ory is accessed through the union type. So, the code above will
work as expected. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Every language that wishes to perform language-specific alias anal-
ysis should define a function that computes, given an "tree" node,
an alias set for the node. Nodes in different alias sets are not
allowed to alias. For an example, see the C front-end function
"c_get_alias_set".
Enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two greater than
n, skipping up to n bytes. For instance, -falign-functions=32
aligns functions to the next 32-byte boundary, but -falign-func-
tions=24 would align to the next 32-byte boundary only if this can
be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and
mean that functions will not be aligned.
Some assemblers only support this flag when n is a power of two; in
that case, it is rounded up.
If n is not specified, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up to
n bytes like -falign-functions. This option can easily make code
slower, because it must insert dummy operations for when the branch
target is reached in the usual flow of the code.
If -falign-loops or -falign-jumps are applicable and are greater
than this value, then their values are used instead.
If n is not specified, use a machine-dependent default which is
very likely to be 1, meaning no alignment.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes like
-falign-functions. The hope is that the loop will be executed many
times, which will make up for any execution of the dummy opera-
tions.
-falign-loops-max-skip
-falign-loops-max-skip=n
When aligning loops to a power-of-two boundary, only do so if can
skip by up to n bytes.
If n is not specified, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets
where the targets can only be reached by jumping, skipping up to n
bytes like -falign-functions. In this case, no dummy operations
need be executed.
-falign-jumps-max-skip
-falign-jumps-max-skip=n
When aligning branch targets to a power-of-two boundary, only do so
if can skip by up to n bytes.
If n is not specified, use a machine-dependent default.
Enabled at levels -O2, -O3.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use
of registers left over after register allocation. This optimiza-
tion will most benefit processors with lots of registers. It can,
however, make debugging impossible, since variables will no longer
stay in a ``home register''.
Enabled at levels -O3.
-fno-cprop-registers
After register allocation and post-register allocation instruction
splitting, we perform a copy-propagation pass to try to reduce
scheduling dependencies and occasionally eliminate the copy.
Disabled at levels -O, -O2, -O3, -Os.
The following options control compiler behavior regarding floating
point arithmetic. These options trade off between speed and correct-
ness. All must be specifically enabled.
-ffloat-store
Do not store floating point variables in registers, and inhibit
other options that might change whether a floating point value is
taken from a register or memory.
This option prevents undesirable excess precision on machines such
as the 68000 where the floating registers (of the 68881) keep more
precision than a "double" is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does
only good, but a few programs rely on the precise definition of
IEEE floating point. Use -ffloat-store for such programs, after
modifying them to store all pertinent intermediate computations
into variables.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trap-
ping-math, -ffinite-math-only and -fno-signaling-nans.
This option causes the preprocessor macro "__FAST_MATH__" to be
defined.
This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math func-
tions.
-fno-math-errno
Do not set ERRNO after calling math functions that are executed
with a single instruction, e.g., sqrt. A program that relies on
IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math func-
tions.
The default is -fmath-errno.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may violate IEEE or
ANSI standards. When used at link-time, it may include libraries
or startup files that change the default FPU control word or other
similar optimizations.
This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math func-
tions.
The default is -fno-unsafe-math-optimizations.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that
arguments and results are not NaNs or +-Infs.
This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications.
The default is -fno-finite-math-only.
-fno-trapping-math
Compile code assuming that floating-point operations cannot gener-
ate user-visible traps. These traps include division by zero,
overflow, underflow, inexact result and invalid operation. This
option implies -fno-signaling-nans. Setting this option may allow
faster code if one relies on ``non-stop'' IEEE arithmetic, for
example.
This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math func-
tions.
The default is -ftrapping-math.
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-
visible traps during floating-point operations. Setting this
option disables optimizations that may change the number of excep-
tions visible with signaling NaNs. This option implies -ftrap-
ping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be
defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that affect signaling NaN behavior.
-fsingle-precision-constant
Treat floating point constant as single precision constant instead
of implicitly converting it to double precision constant.
The following options control optimizations that may improve perfor-
mance, but are not enabled by any -O options. This section includes
experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can com-
pile it a second time using -fbranch-probabilities, to improve
optimizations based on the number of times each branch was taken.
When the program compiled with -fprofile-arcs exits it saves arc
execution counts to a file called sourcename.da for each source
file The information in this data file is very dependent on the
structure of the generated code, so you must use the same source
code and the same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each
JUMP_INSN and CALL_INSN. These can be used to improve optimiza-
tion. Currently, they are only used in one place: in reorg.c,
instead of guessing which path a branch is mostly to take, the
REG_BR_PROB values are used to exactly determine which path is
taken more often.
-fnew-ra
Use a graph coloring register allocator. Currently this option is
meant for testing, so we are interested to hear about miscompila-
tions with -fnew-ra.
-ftracer
Perform tail duplication to enlarge superblock size. This transfor-
mation simplifies the control flow of the function allowing other
optimizations to do better job.
-funroll-loops
Unroll loops whose number of iterations can be determined at com-
pile time or upon entry to the loop. -funroll-loops implies both
-fstrength-reduce and -frerun-cse-after-loop. This option makes
code larger, and may or may not make it run faster.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain
when the loop is entered. This usually makes programs run more
slowly. -funroll-all-loops implies the same options as -fun-
roll-loops,
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to
prefetch memory to improve the performance of loops that access
large arrays.
Disabled at level -Os.
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output
file if the target supports arbitrary sections. The name of the
function or the name of the data item determines the section's name
in the output file.
Use these options on systems where the linker can perform optimiza-
tions to improve locality of reference in the instruction space.
HPPA processors running HP-UX and SPARC processors running Solaris
2 have linkers with such optimizations. Other systems using the
ELF object format as well as AIX may have these optimizations in
the future.
Only use these options when there are significant benefits from
doing so. When you specify these options, the assembler and linker
will create larger object and executable files and will also be
slower. You will not be able to use "gprof" on all systems if you
specify this option and you may have problems with debugging if you
specify both this option and -g.
-fssa
Perform optimizations in static single assignment form. Each func-
tion's flow graph is translated into SSA form, optimizations are
performed, and the flow graph is translated back from SSA form.
Users should not specify this option, since it is not yet ready for
production use.
-fssa-ccp
Perform Sparse Conditional Constant Propagation in SSA form.
Requires -fssa. Like -fssa, this is an experimental feature.
-fssa-dce
Perform aggressive dead-code elimination in SSA form. Requires
-fssa. Like -fssa, this is an experimental feature.
--param name=value
In some places, GCC uses various constants to control the amount of
optimization that is done. For example, GCC will not inline func-
tions that contain more that a certain number of instructions. You
can control some of these constants on the command-line using the
--param option.
In each case, the value is an integer. The allowable choices for
name are given in the following table:
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for
an instruction to fill a delay slot. If more than this arbi-
trary number of instructions is searched, the time savings from
filling the delay slot will be minimal so stop searching.
Increasing values mean more aggressive optimization, making the
compile time increase with probably small improvement in exe-
cutable run time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instruc-
tions to consider when searching for a block with valid live
register information. Increasing this arbitrarily chosen value
means more aggressive optimization, increasing the compile
time. This parameter should be removed when the delay slot
code is rewritten to maintain the control-flow graph.
max-gcse-memory
The approximate maximum amount of memory that will be allocated
in order to perform the global common subexpression elimination
optimization. If more memory than specified is required, the
optimization will not be done.
max-gcse-passes
The maximum number of passes of GCSE to run.
max-pending-list-length
The maximum number of pending dependencies scheduling will
allow before flushing the current state and starting over.
Large functions with few branches or calls can create exces-
sively large lists which needlessly consume memory and
resources.
max-inline-insns-single
Several parameters control the tree inliner used in gcc. This
number sets the maximum number of instructions (counted in
gcc's internal representation) in a single function that the
tree inliner will consider for inlining. This only affects
functions declared inline and methods implemented in a class
declaration (C++). The default value is 300.
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of
functions that would otherwise not be considered for inlining
by the compiler will be investigated. To those functions, a
different (more restrictive) limit compared to functions
declared inline can be applied. The default value is 300.
max-inline-insns
The tree inliner does decrease the allowable size for single
functions to be inlined after we already inlined the number of
instructions given here by repeated inlining. This number
should be a factor of two or more larger than the single func-
tion limit. Higher numbers result in better runtime perfor-
mance, but incur higher compile-time resource (CPU time, mem-
ory) requirements and result in larger binaries. Very high
values are not advisable, as too large binaries may adversely
affect runtime performance. The default value is 600.
max-inline-slope
After exceeding the maximum number of inlined instructions by
repeated inlining, a linear function is used to decrease the
allowable size for single functions. The slope of that func-
tion is the negative reciprocal of the number specified here.
The default value is 32.
min-inline-insns
The repeated inlining is throttled more and more by the linear
function after exceeding the limit. To avoid too much throt-
tling, a minimum for this function is specified here to allow
repeated inlining for very small functions even when a lot of
repeated inlining already has been done. The default value is
130.
max-inline-insns-rtl
For languages that use the RTL inliner (this happens at a later
stage than tree inlining), you can set the maximum allowable
size (counted in RTL instructions) for the RTL inliner with
this parameter. The default value is 600.
max-unrolled-insns
The maximum number of instructions that a loop should have if
that loop is unrolled, and if the loop is unrolled, it deter-
mines how many times the loop code is unrolled.
hot-bb-count-fraction
Select fraction of the maximal count of repetitions of basic
block in program given basic block needs to have to be consid-
ered hot.
hot-bb-frequency-fraction
Select fraction of the maximal frequency of executions of basic
block in function given basic block needs to have to be consid-
ered hot
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given
percentage of executed instructions is covered. This limits
unnecessary code size expansion.
The tracer-dynamic-coverage-feedback is used only when profile
feedback is available. The real profiles (as opposed to stati-
cally estimated ones) are much less balanced allowing the
threshold to be larger value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given per-
centage. This is rather hokey argument, as most of the dupli-
cates will be eliminated later in cross jumping, so it may be
set to much higher values than is the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge
is less than this threshold (in percent).
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge do have probability lower
than this threshold.
Similarly to tracer-dynamic-coverage two values are present,
one for compilation for profile feedback and one for compila-
tion without. The value for compilation with profile feedback
needs to be more conservative (higher) in order to make tracer
effective.
ggc-min-expand
GCC uses a garbage collector to manage its own memory alloca-
tion. This parameter specifies the minimum percentage by which
the garbage collector's heap should be allowed to expand
between collections. Tuning this may improve compilation
speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of
100% when RAM >= 1GB. If "getrlimit" is available, the notion
of "RAM" is the smallest of actual RAM, RLIMIT_RSS, RLIMIT_DATA
and RLIMIT_AS. If GCC is not able to calculate RAM on a par-
ticular platform, the lower bound of 30% is used. Setting this
parameter and ggc-min-heapsize to zero causes a full collection
to occur at every opportunity. This is extremely slow, but can
be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins
bothering to collect garbage. The first collection occurs
after the heap expands by ggc-min-expand% beyond ggc-min-heap-
size. Again, tuning this may improve compilation speed, and
has no effect on code generation.
The default is RAM/8, with a lower bound of 4096 (four
megabytes) and an upper bound of 131072 (128 megabytes). If
"getrlimit" is available, the notion of "RAM" is the smallest
of actual RAM, RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If GCC
is not able to calculate RAM on a particular platform, the
lower bound is used. Setting this parameter very large effec-
tively disables garbage collection. Setting this parameter and
ggc-min-expand to zero causes a full collection to occur at
every opportunity.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source
file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some
of these options make sense only together with -E because they cause
the preprocessor output to be unsuitable for actual compilation.
You can use -Wp,option to bypass the compiler driver and pass option
directly through to the preprocessor. If option contains commas, it is
split into multiple options at the commas. However, many options are
modified, translated or interpreted by the compiler driver before being
passed to the preprocessor, and -Wp forcibly bypasses this phase. The
preprocessor's direct interface is undocumented and subject to change,
so whenever possible you should avoid using -Wp and let the driver han-
dle the options instead.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
Predefine name as a macro, with definition definition. There are
no restrictions on the contents of definition, but if you are
invoking the preprocessor from a shell or shell-like program you
may need to use the shell's quoting syntax to protect characters
such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line,
write its argument list with surrounding parentheses before the
equals sign (if any). Parentheses are meaningful to most shells,
so you will need to quote the option. With sh and csh,
-D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the
command line. All -imacros file and -include file options are pro-
cessed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or provided
with a -D option.
-undef
Do not predefine any system-specific macros. The common predefined
macros remain defined.
-I dir
Add the directory dir to the list of directories to be searched for
header files. Directories named by -I are searched before the
standard system include directories. If the directory dir is a
standard system include directory, the option is ignored to ensure
that the default search order for system directories and the spe-
cial treatment of system headers are not defeated .
-o file
Write output to file. This is the same as specifying file as the
second non-option argument to cpp. gcc has a different interpreta-
tion of a second non-option argument, so you must use -o to specify
the output file.
-Wall
Turns on all optional warnings which are desirable for normal code.
At present this is -Wcomment and -Wtrigraphs. Note that many of
the preprocessor's warnings are on by default and have no options
to control them.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment,
or whenever a backslash-newline appears in a // comment. (Both
forms have the same effect.)
-Wtrigraphs
Warn if any trigraphs are encountered. This option used to take
effect only if -trigraphs was also specified, but now works inde-
pendently. Warnings are not given for trigraphs within comments,
as they do not affect the meaning of the program.
-Wtraditional
Warn about certain constructs that behave differently in tradi-
tional and ISO C. Also warn about ISO C constructs that have no
traditional C equivalent, and problematic constructs which should
be avoided.
-Wimport
Warn the first time #import is used.
-Wundef
Warn whenever an identifier which is not a macro is encountered in
an #if directive, outside of defined. Such identifiers are
replaced with zero.
-Wunused-macros
Warn about macros defined in the main file that are unused. A
macro is used if it is expanded or tested for existence at least
once. The preprocessor will also warn if the macro has not been
used at the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros
defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped condi-
tional blocks, then CPP will report it as unused. To avoid the
warning in such a case, you might improve the scope of the macro's
definition by, for example, moving it into the first skipped block.
Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This
usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are not
in older programs. This warning is on by default.
-Werror
Make all warnings into hard errors. Source code which triggers
warnings will be rejected.
-Wsystem-headers
Issue warnings for code in system headers. These are normally
unhelpful in finding bugs in your own code, therefore suppressed.
If you are responsible for the system library, you may want to see
them.
-w Suppress all warnings, including those which GNU CPP issues by
default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some
of them are left out by default, since they trigger frequently on
harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory diag-
nostics into errors. This includes mandatory diagnostics that GCC
issues without -pedantic but treats as warnings.
-M Instead of outputting the result of preprocessing, output a rule
suitable for make describing the dependencies of the main source
file. The preprocessor outputs one make rule containing the object
file name for that source file, a colon, and the names of all the
included files, including those coming from -include or -imacros
command line options.
Unless specified explicitly (with -MT or -MQ), the object file name
consists of the basename of the source file with any suffix
replaced with object file suffix. If there are many included files
then the rule is split into several lines using \-newline. The
rule has no commands.
This option does not suppress the preprocessor's debug output, such
as -dM. To avoid mixing such debug output with the dependency
rules you should explicitly specify the dependency output file with
-MF, or use an environment variable like DEPENDENCIES_OUTPUT.
Debug output will still be sent to the regular output stream as
normal.
Passing -M to the driver implies -E, and suppresses warnings with
an implicit -w.
-MM Like -M but do not mention header files that are found in system
header directories, nor header files that are included, directly or
indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in
an #include directive does not in itself determine whether that
header will appear in -MM dependency output. This is a slight
change in semantics from GCC versions 3.0 and earlier.
-MF file
@anchor{-MF} When used with -M or -MM, specifies a file to write
the dependencies to. If no -MF switch is given the preprocessor
sends the rules to the same place it would have sent preprocessed
output.
When used with the driver options -MD or -MMD, -MF overrides the
default dependency output file.
-dependency-file
Like -MF. (APPLE ONLY)
-MG In conjunction with an option such as -M requesting dependency gen-
eration, -MG assumes missing header files are generated files and
adds them to the dependency list without raising an error. The
dependency filename is taken directly from the "#include" directive
without prepending any path. -MG also suppresses preprocessed out-
put, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each dependency
other than the main file, causing each to depend on nothing. These
dummy rules work around errors make gives if you remove header
files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By
default CPP takes the name of the main input file, including any
path, deletes any file suffix such as .c, and appends the plat-
form's usual object suffix. The result is the target.
An -MT option will set the target to be exactly the string you
specify. If you want multiple targets, you can specify them as a
single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given
with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied.
The driver determines file based on whether an -o option is given.
If it is, the driver uses its argument but with a suffix of .d,
otherwise it take the basename of the input file and applies a .d
suffix.
If -MD is used in conjunction with -E, any -o switch is understood
to specify the dependency output file (but @pxref{-MF}), but if
used without -E, each -o is understood to specify a target object
file.
Since -E is not implied, -MD can be used to generate a dependency
output file as a side-effect of the compilation process.
-MMD
Like -MD except mention only user header files, not system -header
files.
-fpch-deps
When using precompiled headers, this flag will cause the depen-
dency-output flags to also list the files from the precompiled
header's dependencies. If not specified only the precompiled
header would be listed and not the files that were used to create
it because those files are not consulted when a precompiled header
is used.
-x c
-x c++
-x objective-c
-x objective-c++
-x assembler-with-cpp
Specify the source language: C, C++, Objective-C, Objective-C++, or
assembly. This has nothing to do with standards conformance or
extensions; it merely selects which base syntax to expect. If you
give none of these options, cpp will deduce the language from the
extension of the source file: .c, .cc, .m, .mm, or .S. Some other
common extensions for C++ and assembly are also recognized. If cpp
does not recognize the extension, it will treat the file as C; this
is the most generic mode.
Note: Previous versions of cpp accepted a -lang option which
selected both the language and the standards conformance level.
This option has been removed, because it conflicts with the -l
option.
-std=standard
-ansi
Specify the standard to which the code should conform. Currently
CPP knows about C and C++ standards; others may be added in the
future.
standard may be one of:
"iso9899:1990"
"c89"
The ISO C standard from 1990. c89 is the customary shorthand
for this version of the standard.
The -ansi option is equivalent to -std=c89.
"iso9899:199409"
The 1990 C standard, as amended in 1994.
"iso9899:1999"
"c99"
"iso9899:199x"
"c9x"
The revised ISO C standard, published in December 1999. Before
publication, this was known as C9X.
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"c++98"
The 1998 ISO C++ standard plus amendments.
"gnu++98"
The same as -std=c++98 plus GNU extensions. This is the
default for C++ code.
-I- Split the include path. Any directories specified with -I options
before -I- are searched only for headers requested with
"#include "file""; they are not searched for "#include <file>". If
additional directories are specified with -I options after the -I-,
those directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current
file directory as the first search directory for "#include "file"".
-nostdinc
Do not search the standard system directories for header files.
Only the directories you have specified with -I options (and the
directory of the current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard direc-
tories, but do still search the other standard directories. (This
option is used when building the C++ library.)
-include file
Process file as if "#include "file"" appeared as the first line of
the primary source file. However, the first directory searched for
file is the preprocessor's working directory instead of the direc-
tory containing the main source file. If not found there, it is
searched for in the remainder of the "#include "..."" search chain
as normal.
If multiple -include options are given, the files are included in
the order they appear on the command line.
-imacros file
Exactly like -include, except that any output produced by scanning
file is thrown away. Macros it defines remain defined. This
allows you to acquire all the macros from a header without also
processing its declarations.
All files specified by -imacros are processed before all files
specified by -include.
-idirafter dir
Search dir for header files, but do it after all directories speci-
fied with -I and the standard system directories have been
exhausted. dir is treated as a system include directory.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options.
If the prefix represents a directory, you should include the final
/.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and
add the resulting directory to the include search path. -iwithpre-
fixbefore puts it in the same place -I would; -iwithprefix puts it
where -idirafter would.
Use of these options is discouraged.
-isystem dir
Search dir for header files, after all directories specified by -I
but before the standard system directories. Mark it as a system
directory, so that it gets the same special treatment as is applied
to the standard system directories.
-fpreprocessed
Indicate to the preprocessor that the input file has already been
preprocessed. This suppresses things like macro expansion, tri-
graph conversion, escaped newline splicing, and processing of most
directives. The preprocessor still recognizes and removes com-
ments, so that you can pass a file preprocessed with -C to the com-
piler without problems. In this mode the integrated preprocessor
is little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the exten-
sions .i, .ii or .mi. These are the extensions that GCC uses for
preprocessed files created by -save-temps.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor
report correct column numbers in warnings or errors, even if tabs
appear on the line. If the value is less than 1 or greater than
100, the option is ignored. The default is 8.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary
if diagnostics are being scanned by a program that does not under-
stand the column numbers, such as dejagnu.
-A predicate=answer
Make an assertion with the predicate predicate and answer answer.
This form is preferred to the older form -A predicate(answer),
which is still supported, because it does not use shell special
characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-A- Cancel all predefined assertions and all assertions preceding it on
the command line. Also, undefine all predefined macros and all
macros preceding it on the command line. (This is a historical
wart and may change in the future.)
-dCHARS
CHARS is a sequence of one or more of the following characters, and
must not be preceded by a space. Other characters are interpreted
by the compiler proper, or reserved for future versions of GCC, and
so are silently ignored. If you specify characters whose behavior
conflicts, the result is undefined.
M Instead of the normal output, generate a list of #define direc-
tives for all the macros defined during the execution of the
preprocessor, including predefined macros. This gives you a
way of finding out what is predefined in your version of the
preprocessor. Assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
will show all the predefined macros.
D Like M except in two respects: it does not include the prede-
fined macros, and it outputs both the #define directives and
the result of preprocessing. Both kinds of output go to the
standard output file.
N Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of prepro-
cessing.
-P Inhibit generation of linemarkers in the output from the preproces-
sor. This might be useful when running the preprocessor on some-
thing that is not C code, and will be sent to a program which might
be confused by the linemarkers.
-C Do not discard comments. All comments are passed through to the
output file, except for comments in processed directives, which are
deleted along with the directive.
You should be prepared for side effects when using -C; it causes
the preprocessor to treat comments as tokens in their own right.
For example, comments appearing at the start of what would be a
directive line have the effect of turning that line into an ordi-
nary source line, since the first token on the line is no longer a
#.
-CC Do not discard comments, including during macro expansion. This is
like -C, except that comments contained within macros are also
passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the -CC option
causes all C++-style comments inside a macro to be converted to
C-style comments. This is to prevent later use of that macro from
inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
-gcc
Define the macros __GNUC__, __GNUC_MINOR__ and __GNUC_PATCHLEVEL__.
These are defined automatically when you use gcc -E; you can turn
them off in that case with -no-gcc.
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as
opposed to ISO C preprocessors.
-trigraphs
Process trigraph sequences. These are three-character sequences,
all starting with ??, that are defined by ISO C to stand for single
characters. For example, ??/ stands for \, so '??/n' is a charac-
ter constant for a newline. By default, GCC ignores trigraphs, but
in standard-conforming modes it converts them. See the -std and
-ansi options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
-remap
Enable special code to work around file systems which only permit
very short file names, such as MS-DOS.
--help
--target-help
Print text describing all the command line options instead of pre-
processing anything.
-v Verbose mode. Print out GNU CPP's version number at the beginning
of execution, and report the final form of the include path.
-H Print the name of each header file used, in addition to other nor-
mal activities. Each name is indented to show how deep in the
#include stack it is. Precompiled header files are also printed,
even if they are found to be invalid; an invalid precompiled header
file is printed with ...x and a valid one with ...! .
-version
--version
Print out GNU CPP's version number. With one dash, proceed to pre-
process as normal. With two dashes, exit immediately.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains com-
mas, it is split into multiple options at the commas.
Options for Linking
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.
In addition to the options listed below, Apple's GCC also accepts and
passes nearly all of the options defined by the linker ld and by the
library tool libtool. Common options include -framework, -dynamic,
-bundle, -flat_namespace, and so forth. See the ld and libtool man
pages for further details.
object-file-name
A file name that does not end in a special recognized suffix is
considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as
input to the linker.
-c
-S
-E If any of these options is used, then the linker is not run, and
object file names should not be used as arguments.
-llibrary
-l library
Search the library named library when linking. (The second alter-
native with the library as a separate argument is only for POSIX
compliance and is not recommended.)
It makes a difference where in the command you write this option;
the linker searches and processes libraries and object files in the
order they are specified. Thus, foo.o -lz bar.o searches library z
after file foo.o but before bar.o. If bar.o refers to functions in
z, those functions may not be loaded.
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 directo-
ries plus any that you specify with -L.
Normally the files found this way are library files---archive files
whose members are object files. The linker handles an archive file
by scanning through it for members which define symbols that have
so far been referenced but not defined. But if the file that is
found is an ordinary object file, it is linked in the usual fash-
ion. 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 system libraries are used normally, unless -nostdlib or
-nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the
libraries you specify will be passed to the linker. The standard
startup files are used normally, unless -nostartfiles is used. The
compiler may generate calls to memcmp, memset, and memcpy for Sys-
tem V (and ISO C) environments or to bcopy and bzero for BSD envi-
ronments. These entries are usually resolved by entries in libc.
These entry points should be supplied through some other mechanism
when this option is specified.
-nostdlib
Do not use the standard system startup files or libraries when
linking. No startup files and only the libraries you specify will
be passed to the linker. The compiler may generate calls to mem-
cmp, memset, and memcpy for System V (and ISO C) environments or to
bcopy and bzero for BSD environments. These entries are usually
resolved by entries in libc. These entry points should be supplied
through some other mechanism when this option is specified.
-no-c++filt
By default all linker diagnostic output is piped through c++filt.
This option suppresses that behavior. (APPLE ONLY)
One of the standard libraries bypassed by -nostdlib and -nodefault-
libs is libgcc.a, a library of internal subroutines that GCC uses
to overcome shortcomings of particular machines, or special needs
for some languages.
In most cases, you need libgcc.a even when you want to avoid other
standard libraries. In other words, when you specify -nostdlib or
-nodefaultlibs you should usually specify -lgcc as well. This
ensures that you have no unresolved references to internal GCC
library subroutines. (For example, __main, used to ensure C++ con-
structors will be called.)
-s Remove all symbol table and relocation information from the exe-
cutable.
-static
On systems that support dynamic linking, this prevents linking with
the shared libraries. On other systems, this option has no effect.
This option will not work on Mac OS X unless all of your libraries
(including libgcc.a) have also been compiled with -static.
-shared
Produce a shared object which can then be linked with other objects
to form an executable. Not all systems support this option. For
predictable results, you must also specify the same set of options
that were used to generate code (-fpic, -fPIC, or model suboptions)
when you specify this option.[1]
This option is not supported on Mac OS X.
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options
force the use of either the shared or static version respectively.
If no shared version of libgcc was built when the compiler was con-
figured, these options have no effect.
There are several situations in which an application should use the
shared libgcc instead of the static version. The most common of
these is when the application wishes to throw and catch exceptions
across different shared libraries. In that case, each of the
libraries as well as the application itself should use the shared
libgcc.
Therefore, the G++ and GCJ drivers automatically add -shared-libgcc
whenever you build a shared library or a main executable, because
C++ and Java programs typically use exceptions, so this is the
right thing to do.
If, instead, you use the GCC driver to create shared libraries, you
may find that they will not always be linked with the shared
libgcc. If GCC finds, at its configuration time, that you have a
GNU linker that does not support option --eh-frame-hdr, it will
link the shared version of libgcc into shared libraries by default.
Otherwise, it will take advantage of the linker and optimize away
the linking with the shared version of libgcc, linking with the
static version of libgcc by default. This allows exceptions to
propagate through such shared libraries, without incurring reloca-
tion costs at library load time.
However, if a library or main executable is supposed to throw or
catch exceptions, you must link it using the G++ or GCJ driver, as
appropriate for the languages used in the program, or using the
option -shared-libgcc, such that it is linked with the shared
libgcc.
-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 sup-
port this option.
-Xlinker option
Pass option as an option to the linker. You can use this to supply
system-specific linker options which GCC 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
-assert -Xlinker definitions. It does not work to write -Xlinker
"-assert definitions", because this passes the entire string as a
single argument, which is not what the linker expects.
-Wl,option
Pass option as an option to the linker. If option contains commas,
it is split into multiple options at the commas.
-u symbol
Pretend the symbol symbol is undefined, to force linking of library
modules to define it. You can use -u multiple times with different
symbols to force loading of additional library modules.
Options for Directory Search
These options specify directories to search for header files, for
libraries and for parts of the compiler:
-Idir
Add the directory dir to the head of the list of directories to be
searched for header files. This can be used to override a system
header file, substituting your own version, since these directories
are searched before the system header file directories. However,
you should not use this option to add directories that contain ven-
dor-supplied system header files (use -isystem for that). If you
use more than one -I option, the directories are scanned in left-
to-right order; the standard system directories come after.
If a standard system include directory, or a directory specified
with -isystem, is also specified with -I, the -I option will be
ignored. The directory will still be searched but as a system
directory at its normal position in the system include chain. This
is to ensure that GCC's procedure to fix buggy system headers and
the ordering for the include_next directive are not inadvertently
changed. If you really need to change the search order for system
directories, use the -nostdinc and/or -isystem options.
-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 direc-
tory (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.
-Fdir
In Apple's version of GCC only, add the directory dir to the head
of the list of directories to be searched for frameworks.
The framework search algorithm is, for an inclusion of
<Fmwk/Header.h>, to look for files named path/Fmwk.framework/Head-
ers/Header.h or path/Fmwk.framework/PrivateHeaders/Header.h where
path includes /System/Library/Frameworks/ /Library/Frameworks/, and
/Local/Library/Frameworks/, plus any additional paths specified by
-F.
All the -F options are also passed to the linker.
-Bprefix
This option specifies where to find the executables, libraries,
include files, and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms
cpp, cc1, as and ld. It tries prefix as a prefix for each program
it tries to run, both with and without machine/version/.
For each subprogram to be run, the compiler driver first tries the
-B prefix, if any. If that name is not found, or if -B was not
specified, the driver tries two standard prefixes, which are
/usr/lib/gcc/ and /usr/local/lib/gcc-lib/. If neither of those
results in a file name that is found, the unmodified program name
is searched for using the directories specified in your PATH envi-
ronment variable.
The compiler will check to see if the path provided by the -B
refers to a directory, and if necessary it will add a directory
separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to
libraries in the linker, because the compiler translates these
options into -L options for the linker. They also apply to
includes files in the preprocessor, because the compiler translates
these options into -isystem options for the preprocessor. In this
case, the compiler appends include to the prefix.
The run-time support file libgcc.a can also be searched for using
the -B prefix, if needed. If it is not found there, the two stan-
dard prefixes above are tried, and that is all. The file is left
out of the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use
the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/,
where N is a number in the range 0 to 9, then it will be replaced
by [dir/]include. This is to help with boot-strapping the com-
piler.
-specs=file
Process file after the compiler reads in the standard specs file,
in order to override the defaults that the gcc driver program uses
when determining what switches to pass to cc1, cc1plus, as, ld,
etc. More than one -specs=file can be specified on the command
line, and they are processed in order, from left to right.
Specifying Target Machine and Compiler Version
The usual way to run GCC is to run the executable called gcc, or
<machine>-gcc when cross-compiling, or <machine>-gcc-<version> to run a
version other than the one that was installed last. Sometimes this is
inconvenient, so GCC provides options that will switch to another
cross-compiler or version.
-b machine
The argument machine specifies the target machine for compilation.
The value to use for machine is the same as was specified as the
machine type when configuring GCC as a cross-compiler. For exam-
ple, if a cross-compiler was configured with configure i386v, mean-
ing to compile for an 80386 running System V, then you would spec-
ify -b i386v to run that cross compiler.
-V version
The argument version specifies which version of GCC to run. This
is useful when multiple versions are installed. For example, ver-
sion might be 2.0, meaning to run GCC version 2.0.
The -V and -b options work by running the <machine>-gcc-<version> exe-
cutable, so there's no real reason to use them if you can just run that
directly.
Hardware Models and Configurations
Earlier we discussed the standard option -b which chooses among differ-
ent installed compilers for completely different target machines, such
as VAX vs. 68000 vs. 80386.
In addition, each of these target machine types can have its own spe-
cial options, starting with -m, to choose among various hardware models
or configurations---for example, 68010 vs 68020, floating coprocessor
or none. A single installed version of the compiler can compile for
any model or configuration, according to the options specified.
Some configurations of the compiler also support additional special
options, usually for compatibility with other compilers on the same
platform.
These options are defined by the macro "TARGET_SWITCHES" in the machine
description. The default for the options is also defined by that
macro, which enables you to change the defaults.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
GCC supports two related instruction set architectures for the
RS/6000 and PowerPC. The POWER instruction set are those instruc-
tions supported by the rios chip set used in the original RS/6000
systems and the PowerPC instruction set is the architecture of the
Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx
microprocessors.
Neither architecture is a subset of the other. However there is a
large common subset of instructions supported by both. An MQ reg-
ister is included in processors supporting the POWER architecture.
You use these options to specify which instructions are available
on the processor you are using. The default value of these options
is determined when configuring GCC. Specifying the -mcpu=cpu_type
overrides the specification of these options. We recommend you use
the -mcpu=cpu_type option rather than the options listed above.
The -mpower option allows GCC to generate instructions that are
found only in the POWER architecture and to use the MQ register.
Specifying -mpower2 implies -power and also allows GCC to generate
instructions that are present in the POWER2 architecture but not
the original POWER architecture.
The -mpowerpc option allows GCC to generate instructions that are
found only in the 32-bit subset of the PowerPC architecture. Spec-
ifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to use
the optional PowerPC architecture instructions in the General Pur-
pose group, including floating-point square root. Specifying
-mpowerpc-gfxopt implies -mpowerpc and also allows GCC to use the
optional PowerPC architecture instructions in the Graphics group,
including floating-point select.
The -mpowerpc64 option allows GCC to generate the additional 64-bit
instructions that are found in the full PowerPC64 architecture and
to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
-mno-powerpc64.
If you specify both -mno-power and -mno-powerpc, GCC will use only
the instructions in the common subset of both architectures plus
some special AIX common-mode calls, and will not use the MQ regis-
ter. Specifying both -mpower and -mpowerpc permits GCC to use any
instruction from either architecture and to allow use of the MQ
register; specify this for the Motorola MPC601.
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code.
With -mnew-mnemonics, GCC uses the assembler mnemonics defined for
the PowerPC architecture. With -mold-mnemonics it uses the assem-
bler mnemonics defined for the POWER architecture. Instructions
defined in only one architecture have only one mnemonic; GCC uses
that mnemonic irrespective of which of these options is specified.
GCC defaults to the mnemonics appropriate for the architecture in
use. Specifying -mcpu=cpu_type sometimes overrides the value of
these option. Unless you are building a cross-compiler, you should
normally not specify either -mnew-mnemonics or -mold-mnemonics, but
should instead accept the default.
-mcpu=cpu_type
Set architecture type, register usage, choice of mnemonics, and
instruction scheduling parameters for machine type cpu_type. Sup-
ported values for cpu_type are rios, rios1, rsc, rios2, rs64a, 601,
602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, power,
power2, powerpc, 403, 505, 801, 821, 823, and 860 and common.
-mcpu=common selects a completely generic processor. Code gener-
ated under this option will run on any POWER or PowerPC processor.
GCC will use only the instructions in the common subset of both
architectures, and will not use the MQ register. GCC assumes a
generic processor model for scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 spec-
ify generic POWER, POWER2, pure 32-bit PowerPC (i.e., not MPC601),
and 64-bit PowerPC architecture machine types, with an appropriate,
generic processor model assumed for scheduling purposes.
The other options specify a specific processor. Code generated
under those options will run best on that processor, and may not
run at all on others.
The -mcpu options automatically enable or disable other -m options
as follows:
common
-mno-power, -mno-powerpc
power
power2
rios1
rios2
rsc -mpower, -mno-powerpc, -mno-new-mnemonics
powerpc
rs64a
602
603
603e
604
620
630
740
7400
7450
750
505 -mno-power, -mpowerpc, -mnew-mnemonics
601 -mpower, -mpowerpc, -mnew-mnemonics
403
821
860 -mno-power, -mpowerpc, -mnew-mnemonics, -msoft-float
-mtune=cpu_type
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the architecture type, register usage, or
choice of mnemonics, as -mcpu=cpu_type would. The same values for
cpu_type are used for -mtune as for -mcpu. If both are specified,
the code generated will use the architecture, registers, and
mnemonics set by -mcpu, but the scheduling parameters set by
-mtune.
-maltivec
-mno-altivec
These switches enable or disable the use of built-in functions that
allow access to the AltiVec instruction set. You may also need to
set -mabi=altivec to adjust the current ABI with AltiVec ABI
enhancements.
This option is not supported on Mac OS X; use -faltivec instead.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not
change the default ABI, instead it adds the SPE ABI extensions to
the current ABI.
-mabi=no-spe
Disable Booke SPE ABI extensions for the current ABI.
-misel=yes/no
-misel
This switch enables or disables the generation of ISEL instruc-
tions.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created
for every executable file. The -mfull-toc option is selected by
default. In that case, GCC will allocate at least one TOC entry
for each unique non-automatic variable reference in your program.
GCC will also place floating-point constants in the TOC. However,
only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have over-
flowed the available TOC space, you can reduce the amount of TOC
space used with the -mno-fp-in-toc and -mno-sum-in-toc options.
-mno-fp-in-toc prevents GCC from putting floating-point constants
in the TOC and -mno-sum-in-toc forces GCC to generate code to cal-
culate the sum of an address and a constant at run-time instead of
putting that sum into the TOC. You may specify one or both of
these options. Each causes GCC to produce very slightly slower and
larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both
of these options, specify -mminimal-toc instead. This option
causes GCC to make only one TOC entry for every file. When you
specify this option, GCC will produce code that is slower and
larger but which uses extremely little TOC space. You may wish to
use this option only on files that contain less frequently executed
code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers,
64-bit "long" type, and the infrastructure needed to support them.
Specifying -maix64 implies -mpowerpc64 and -mpowerpc, while -maix32
disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults
to -maix32.
-mxl-call
-mno-xl-call
On AIX, pass floating-point arguments to prototyped functions
beyond the register save area (RSA) on the stack in addition to
argument FPRs. The AIX calling convention was extended but not
initially documented to handle an obscure K&R C case of calling a
function that takes the address of its arguments with fewer argu-
ments than declared. AIX XL compilers access floating point argu-
ments which do not fit in the RSA from the stack when a subroutine
is compiled without optimization. Because always storing floating-
point arguments on the stack is inefficient and rarely needed, this
option is not enabled by default and only is necessary when calling
subroutines compiled by AIX XL compilers without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an applica-
tion written to use message passing with special startup code to
enable the application to run. The system must have PE installed
in the standard location (/usr/lpp/ppe.poe/), or the specs file
must be overridden with the -specs= option to specify the appropri-
ate directory location. The Parallel Environment does not support
threads, so the -mpe option and the -pthread option are incompati-
ble.
-malign-mac68k
-malign-power
-malign-natural
The option -malign-mac68k causes structure fields to be aligned on
2-byte boundaries, in order to be compatible with m68k compiler
output. The option -malign-power is the standard alignment mode
for the PowerPC. The option -malign-natural is an extension of
PowerPC alignment that aligns larger data types such as doubles on
their natural boundaries. (APPLE ONLY)
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register
set. Software floating point emulation is provided if you use the
-msoft-float option, and pass the option to GCC when linking.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word
instructions and the store multiple word instructions. These
instructions are generated by default on POWER systems, and not
generated on PowerPC systems. Do not use -mmultiple on little
endian PowerPC systems, since those instructions do not work when
the processor is in little endian mode. The exceptions are PPC740
and PPC750 which permit the instructions usage in little endian
mode.
-mstring
-mno-string
Generate code that uses (does not use) the load string instructions
and the store string word instructions to save multiple registers
and do small block moves. These instructions are generated by
default on POWER systems, and not generated on PowerPC systems. Do
not use -mstring on little endian PowerPC systems, since those
instructions do not work when the processor is in little endian
mode. The exceptions are PPC740 and PPC750 which permit the
instructions usage in little endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instruc-
tions that update the base register to the address of the calcu-
lated memory location. These instructions are generated by
default. If you use -mno-update, there is a small window between
the time that the stack pointer is updated and the address of the
previous frame is stored, which means code that walks the stack
frame across interrupts or signals may get corrupted data.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply
and accumulate instructions. These instructions are generated by
default if hardware floating is used.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force struc-
tures and unions that contain bit-fields to be aligned to the base
type of the bit-field.
For example, by default a structure containing nothing but 8
"unsigned" bit-fields of length 1 would be aligned to a 4 byte
boundary and have a size of 4 bytes. By using -mno-bit-align, the
structure would be aligned to a 1 byte boundary and be one byte in
size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that
unaligned memory references will be handled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows (does not
allow) the program to be relocated to a different address at run-
time. If you use -mrelocatable on any module, all objects linked
together must be compiled with -mrelocatable or -mrelocatable-lib.
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows (does not
allow) the program to be relocated to a different address at run-
time. Modules compiled with -mrelocatable-lib can be linked with
either modules compiled without -mrelocatable and -mrelocatable-lib
or with modules compiled with the -mrelocatable options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that
register 2 contains a pointer to a global area pointing to the
addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in little endian mode. The -mlittle-endian option is the
same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in big endian mode. The -mbig-endian option is the same
as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not
relocatable, but that its external references are relocatable. The
resulting code is suitable for applications, but not shared
libraries. (APPLE ONLY)
-mlong-branch
On Darwin and Mac OS X systems, compile calls to use a 32-bit des-
tination address. This is to support kernel extensions, which may
load anywhere within the kernel address space. (APPLE ONLY)
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using call-
ing conventions that adheres to the March 1995 draft of the System
V Application Binary Interface, PowerPC processor supplement. This
is the default unless you configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aix
On System V.4 and embedded PowerPC systems compile code using call-
ing conventions that are similar to those used on AIX. This is the
default if you configured GCC using powerpc-*-eabiaix.
-mcall-solaris
On System V.4 and embedded PowerPC systems compile code for the
Solaris operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the
Linux-based GNU system.
-mcall-gnu
On System V.4 and embedded PowerPC systems compile code for the
Hurd-based GNU system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the
NetBSD operating system.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified
by the SVR4 ABI).
-mabi=altivec
Extend the current ABI with AltiVec ABI extensions. This does not
change the default ABI, instead it adds the AltiVec ABI extensions
to the current ABI.
This option is effectively permanently enabled on Mac OS X.
-mabi=no-altivec
Disable AltiVec ABI extensions for the current ABI.
This option will not work on Mac OS X.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to
variable argument functions are properly prototyped. Otherwise,
the compiler must insert an instruction before every non prototyped
call to set or clear bit 6 of the condition code register (CR) to
indicate whether floating point values were passed in the floating
point registers in case the function takes a variable arguments.
With -mprototype, only calls to prototyped variable argument func-
tions will set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is
called sim-crt0.o and that the standard C libraries are libsim.a
and libc.a. This is the default for powerpc-*-eabisim. configura-
tions.
-mmvme
On embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are libmvme.a and
libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are
compiling for a VxWorks system.
-mwindiss
Specify that you are compiling for the WindISS simulation environ-
ment.
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags
header to indicate that eabi extended relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to
the Embedded Applications Binary Interface (eabi) which is a set of
modifications to the System V.4 specifications. Selecting -meabi
means that the stack is aligned to an 8 byte boundary, a function
"__eabi" is called to from "main" to set up the eabi environment,
and the -msdata option can use both "r2" and "r13" to point to two
separate small data areas. Selecting -mno-eabi means that the
stack is aligned to a 16 byte boundary, do not call an initializa-
tion function from "main", and the -msdata option will only use
"r13" to point to a single small data area. The -meabi option is
on by default if you configured GCC using one of the pow-
erpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized
"const" global and static data in the .sdata2 section, which is
pointed to by register "r2". Put small initialized non-"const"
global and static data in the .sdata section, which is pointed to
by register "r13". Put small uninitialized global and static data
in the .sbss section, which is adjacent to the .sdata section. The
-msdata=eabi option is incompatible with the -mrelocatable option.
The -msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and
static data in the .sdata section, which is pointed to by register
"r13". Put small uninitialized global and static data in the .sbss
section, which is adjacent to the .sdata section. The -msdata=sysv
option is incompatible with the -mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used, com-
pile code the same as -msdata=eabi, otherwise compile code the same
as -msdata=sysv.
-msdata-data
On System V.4 and embedded PowerPC systems, put small global and
static data in the .sdata section. Put small uninitialized global
and static data in the .sbss section. Do not use register "r13" to
address small data however. This is the default behavior unless
other -msdata options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static
data in the .data section, and all uninitialized data in the .bss
section.
-G num
On embedded PowerPC systems, 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. By default, num is 8. The -G
num switch is also passed to the linker. All modules should be
compiled with the same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit regis-
ter names in the assembly language output using symbolic forms.
-mlongcall
-mno-longcall
Default to making all function calls via pointers, so that func-
tions which reside further than 64 megabytes (67,108,864 bytes)
from the current location can be called. This setting can be over-
call(0)".
Some linkers are capable of detecting out-of-range calls and gener-
ating glue code on the fly. On these systems, long calls are
unnecessary and generate slower code. As of this writing, the AIX
linker can do this, as can the GNU linker for PowerPC/64. It is
planned to add this feature to the GNU linker for 32-bit PowerPC
systems as well.
In the future, we may cause GCC to ignore all longcall specifica-
tions when the linker is known to generate glue.
-pthread
Adds support for multithreading with the pthreads library. This
option sets flags for both the preprocessor and linker.
Darwin Options
-all_load
Loads all members of static archive libraries. See man ld(1) for
more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong archi-
tecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker
will bind all undefined references when the file is loaded or
launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more infor-
mation.
-bundle_loader executable
This specifies the executable that will be loading the build output
file being linked. See man ld(1) for more information.
-mone-byte-bool
Override the defaults for bool so that sizeof(bool)==1. By default
sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when com-
piling for Darwin/x86, so this option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate code
that is not binary compatible with code generated without that
switch. Using this switch may require recompiling all other mod-
ules in a program, including system libraries. Use this switch to
conform to a non-default data model.
-allowable_client client_name
-arch_only
-client_name
-compatibility_version
-current_version
-dead_strip
-no_dead_strip_inits_and_terms
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-dynamiclib
-exported_symbols_list
-filelist
-flat_namespace
-force_cpusubtype_ALL
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
This options are available for Darwin linker. Darwin linker man
page describes them in detail.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of comput-
ers:
-mcpu=cpu-type
Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions. The
choices for cpu-type are i386, i486, i586, i686, pentium, pentium-
mmx, pentiumpro, pentium2, pentium3, pentium4, k6, k6-2, k6-3,
athlon, athlon-tbird, athlon-4, athlon-xp, athlon-mp, winchip-c6,
winchip2 and c3.
While picking a specific cpu-type will schedule things appropri-
ately for that particular chip, the compiler will not generate any
code that does not run on the i386 without the -march=cpu-type
option being used. i586 is equivalent to pentium and i686 is
equivalent to pentiumpro. k6 and athlon are the AMD chips as
opposed to the Intel ones.
-march=cpu-type
Generate instructions for the machine type cpu-type. The choices
for cpu-type are the same as for -mcpu. Moreover, specifying
-march=cpu-type implies -mcpu=cpu-type.
-m386
-m486
-mpentium
-mpentiumpro
These options are synonyms for -mcpu=i386, -mcpu=i486, -mcpu=pen-
tium, and -mcpu=pentiumpro respectively. These synonyms are depre-
cated.
-mfpmath=unit
generate floating point arithmetics for selected unit unit. the
choices for unit are:
387 Use the standard 387 floating point coprocessor present major-
ity of chips and emulated otherwise. Code compiled with this
option will run almost everywhere. The temporary results are
computed in 80bit precision instead of precision specified by
the type resulting in slightly different results compared to
most of other chips. See -ffloat-store for more detailed
description.
This is the default choice for i386 compiler.
sse Use scalar floating point instructions present in the SSE
instruction set. This instruction set is supported by Pentium3
and newer chips, in the AMD line by Athlon-4, Athlon-xp and
Athlon-mp chips. The earlier version of SSE instruction set
supports only single precision arithmetics, thus the double and
extended precision arithmetics is still done using 387. Later
version, present only in Pentium4 and the future AMD x86-64
chips supports double precision arithmetics too.
For i387 you need to use -march=cpu-type, -msse or -msse2
switches to enable SSE extensions and make this option effec-
tive. For x86-64 compiler, these extensions are enabled by
default.
The resulting code should be considerably faster in majority of
cases and avoid the numerical instability problems of 387 code,
but may break some existing code that expects temporaries to be
80bit.
This is the default choice for x86-64 compiler.
sse,387
Attempt to utilize both instruction sets at once. This effec-
tively double the amount of available registers and on chips
with separate execution units for 387 and SSE the execution
resources too. Use this option with care, as it is still
experimental, because gcc register allocator does not model
separate functional units well resulting in instable perfor-
mance.
-masm=dialect
Output asm instructions using selected dialect. Supported choices
are intel or att (the default one).
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating point com-
parisons. These handle correctly the case where the result of a
comparison is unordered.
-msoft-float
Generate output containing library calls for floating point. Warn-
ing: the requisite libraries are not part of GCC. Normally the
facilities of the machine's usual C compiler are used, but this
can't be done directly in cross-compilation. You must make your
own arrangements to provide suitable library functions for
cross-compilation.
On machines where a function returns floating point results in the
80387 register stack, some floating point opcodes may be emitted
even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types
"float" and "double" in an FPU register, even if there is no FPU.
The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in
ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt"
instructions for the 387. Specify this option to avoid generating
those instructions. This option is the default on FreeBSD, OpenBSD
and NetBSD. This option is overridden when -march indicates that
the target cpu will always have an FPU and so the instruction will
not need emulation. As of revision 2.6.1, these instructions are
not generated unless you also use the -funsafe-math-optimizations
switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long"
variables on a two word boundary or a one word boundary. Aligning
"double" variables on a two word boundary will produce code that
runs somewhat faster on a Pentium at the expense of more memory.
Warning: if you use the -malign-double switch, structures contain-
ing the above types will be aligned differently than the published
application binary interface specifications for the 386 and will
not be binary compatible with structures in code compiled without
that switch.
-m128bit-long-double
Control the size of "long double" type. i386 application binary
interface specify the size to be 12 bytes, while modern architec-
tures (Pentium and newer) prefer "long double" aligned to 8 or 16
byte boundary. This is impossible to reach with 12 byte long dou-
bles in the array accesses.
Warning: if you use the -m128bit-long-double switch, the structures
and arrays containing "long double" will change their size as well
as function calling convention for function taking "long double"
will be modified.
-m96bit-long-double
Set the size of "long double" to 96 bits as required by the i386
application binary interface. This is the default.
-msvr3-shlib
-mno-svr3-shlib
Control whether GCC places uninitialized local variables into the
"bss" or "data" segments. -msvr3-shlib places them into "bss".
These options are meaningful only on System V Release 3.
-mrtd
Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the "ret" num
instruction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to pop
the arguments there.
You can specify that an individual function is called with this
calling sequence with the function attribute stdcall. You can also
override the -mrtd option by using the function attribute cdecl.
Warning: this calling convention is incompatible with the one nor-
mally used on Unix, so you cannot use it if you need to call
libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including "printf"); otherwise
incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments. By
default, no registers are used to pass arguments, and at most 3
registers can be used. You can control this behavior for a spe-
cific function by using the function attribute regparm.
Warning: if you use this switch, and num is nonzero, then you must
build all modules with the same value, including any libraries.
This includes the system libraries and startup modules.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num
byte boundary. If -mpreferred-stack-boundary is not specified, the
default is 4 (16 bytes or 128 bits), except when optimizing for
code size (-Os), in which case the default is the minimum correct
alignment (4 bytes for x86, and 8 bytes for x86-64).
On Pentium and PentiumPro, "double" and "long double" values should
be aligned to an 8 byte boundary (see -malign-double) or suffer
significant run time performance penalties. On Pentium III, the
Streaming SIMD Extension (SSE) data type "__m128" suffers similar
penalties if it is not 16 byte aligned.
To ensure proper alignment of this values on the stack, the stack
boundary must be as aligned as that required by any value stored on
the stack. Further, every function must be generated such that it
keeps the stack aligned. Thus calling a function compiled with a
higher preferred stack boundary from a function compiled with a
lower preferred stack boundary will most likely misalign the stack.
It is recommended that libraries that use callbacks always use the
default setting.
This extra alignment does consume extra stack space, and generally
increases code size. Code that is sensitive to stack space usage,
such as embedded systems and operating system kernels, may want to
reduce the preferred alignment to -mpreferred-stack-boundary=2.
-mmmx
-mno-mmx
-msse
-mno-sse
-msse2
-mno-sse2
-m3dnow
-mno-3dnow
These switches enable or disable the use of built-in functions that
allow direct access to the MMX, SSE and 3Dnow extensions of the
instruction set.
To have SSE/SSE2 instructions generated automatically from float-
ing-point code, see -mfpmath=sse.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is
shorter and usually equally fast as method using SUB/MOV operations
and is enabled by default. In some cases disabling it may improve
performance because of improved scheduling and reduced dependen-
cies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing argu-
ments will be computed in the function prologue. This is faster on
most modern CPUs because of reduced dependencies, improved schedul-
ing and reduced stack usage when preferred stack boundary is not
equal to 2. The drawback is a notable increase in code size. This
switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on Mingw32. Code that
relies on thread-safe exception handling must compile and link all
code with the -mthreads option. When compiling, -mthreads defines
-D_MT; when linking, it links in a special thread helper library
-lmingwthrd which cleans up per thread exception handling data.
-mno-align-stringops
Do not align destination of inlined string operations. This switch
reduces code size and improves performance in case the destination
is already aligned, but gcc don't know about it.
-minline-all-stringops
By default GCC inlines string operations only when destination is
known to be aligned at least to 4 byte boundary. This enables more
inlining, increase code size, but may improve performance of code
that depends on fast memcpy, strlen and memset for short lengths.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions.
This avoids the instructions to save, set up and restore frame
pointers and makes an extra register available in leaf functions.
The option -fomit-frame-pointer removes the frame pointer for all
functions which might make debugging harder.
These -m switches are supported in addition to the above on AMD x86-64
processors in 64-bit environments.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit envi-
ronment sets int, long and pointer to 32 bits and generates code
that runs on any i386 system. The 64-bit environment sets int to
32 bits and long and pointer to 64 bits and generates code for
AMD's x86-64 architecture.
-mno-red-zone
Do not use a so called red zone for x86-64 code. The red zone is
mandated by the x86-64 ABI, it is a 128-byte area beyond the loca-
tion of the stack pointer that will not be modified by signal or
interrupt handlers and therefore can be used for temporary data
without adjusting the stack pointer. The flag -mno-red-zone dis-
ables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols
must be linked in the lower 2 GB of the address space. Pointers
are 64 bits. Programs can be statically or dynamically linked.
This is the default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the
negative 2 GB of the address space. This model has to be used for
Linux kernel code.
-mcmodel=medium
Generate code for the medium model: The program is linked in the
lower 2 GB of the address space but symbols can be located anywhere
in the address space. Programs can be statically or dynamically
linked, but building of shared libraries are not supported with the
medium model.
-mcmodel=large
Generate code for the large model: This model makes no assumptions
about addresses and sizes of sections. Currently GCC does not
implement this model.
Options for Code Generation Conventions
These machine-independent options control the interface conventions
used in code generation.
Most of them have both positive and negative forms; the negative form
of -ffoo would be -fno-foo. In the table below, only one of the forms
is listed---the one which is not the default. You can figure out the
other form by either removing no- or adding it.
-fbounds-check
For front-ends that support it, generate additional code to check
that indices used to access arrays are within the declared range.
This is currently only supported by the Java and Fortran 77
front-ends, where this option defaults to true and false respec-
tively.
-ftrapv
This option generates traps for signed overflow on addition, sub-
traction, multiplication operations.
-fexceptions
Enable exception handling. Generates extra code needed to propa-
gate exceptions. For some targets, this implies GCC will generate
frame unwind information for all functions, which can produce sig-
nificant data size overhead, although it does not affect execution.
If you do not specify this option, GCC will enable it by default
for languages like C++ which normally require exception handling,
and disable it for languages like C that do not normally require
it. However, you may need to enable this option when compiling C
code that needs to interoperate properly with exception handlers
written in C++. You may also wish to disable this option if you
are compiling older C++ programs that don't use exception handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw excep-
tions. Note that this requires platform-specific runtime support
that does not exist everywhere. Moreover, it only allows trapping
instructions to throw exceptions, i.e. memory references or float-
ing point instructions. It does not allow exceptions to be thrown
from arbitrary signal handlers such as "SIGALRM".
-funwind-tables
Similar to -fexceptions, except that it will just generate any
needed static data, but will not affect the generated code in any
other way. You will normally not enable this option; instead, a
language processor that needs this handling would enable it on your
behalf.
-fasynchronous-unwind-tables
Generate unwind table in dwarf2 format, if supported by target
machine. The table is exact at each instruction boundary, so it
can be used for stack unwinding from asynchronous events (such as
debugger or garbage collector).
-fpcc-struct-return
Return ``short'' "struct" and "union" values in memory like longer
ones, rather than in registers. This convention is less efficient,
but it has the advantage of allowing intercallability between GCC-
compiled files and files compiled with other compilers, particu-
larly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends
on the target configuration macros.
Short structures and unions are those whose size and alignment
match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not
binary compatible with code compiled with the -freg-struct-return
switch. Use it to conform to a non-default application binary
interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible.
This is more efficient for small structures than
-fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return,
GCC defaults to whichever convention is standard for the target.
If there is no standard convention, GCC defaults to
-fpcc-struct-return, except on targets where GCC is the principal
compiler. In those cases, we can choose the standard, and we chose
the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not
binary compatible with code compiled with the -fpcc-struct-return
switch. Use it to conform to a non-default application binary
interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the
declared range of possible values. Specifically, the "enum" type
will be equivalent to the smallest integer type which has enough
room.
Warning: the -fshort-enums switch causes GCC to generate code that
is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fshort-double
Use the same size for "double" as for "float".
Warning: the -fshort-double switch causes GCC to generate code that
is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fshort-wchar
Override the underlying type for wchar_t to be short unsigned int
instead of the default for the target. This option is useful for
building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that
is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fshared-data
Requests that the data and non-"const" variables of this compila-
tion be shared data rather than private data. The distinction
makes sense only on certain operating systems, where shared data is
shared between processes running the same program, while private
data exists in one copy per process.
-fno-common
In C, allocate even uninitialized global variables in the data sec-
tion of the object file, rather than generating them as common
blocks. This has the effect that if the same variable is declared
(without "extern") in two different compilations, you will get an
error when you link them. The only reason this might be useful is
if you wish to verify that the program will work on other systems
which always work this way.
-fno-ident
Ignore the #ident directive.
-fno-gnu-linker
Do not output global initializations (such as C++ constructors and
destructors) in the form used by the GNU linker (on systems where
the GNU linker is the standard method of handling them). Use this
option when you want to use a non-GNU linker, which also requires
using the collect2 program to make sure the system linker includes
constructors and destructors. (collect2 is included in the GCC
distribution.) For systems which must use collect2, the compiler
driver gcc is configured to do this automatically.
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the
two halves are placed at locations far apart in memory. This
option is used when compiling crtstuff.c; you should not need to
use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to
make it more readable. This option is generally only of use to
those who actually need to read the generated assembly code (per-
haps while debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be
omitted and is useful when comparing two assembler files.
-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. GCC does not consider static data items to be
volatile because of this switch.
-fvolatile-static
Consider all memory references to static data to be volatile.
-fpic
Generate position-independent code (PIC) suitable for use in a
shared library, if supported for the target machine. Such code
accesses all constant addresses through a global offset table
(GOT). The dynamic loader resolves the GOT entries when the pro-
gram starts (the dynamic loader is not part of GCC; it is part of
the operating system). If the GOT size for the linked executable
exceeds a machine-specific maximum size, you get an error message
from the linker indicating that -fpic does not work; in that case,
recompile with -fPIC instead. (These maximums are 16k on the m88k,
8k on the SPARC, and 32k on the m68k and RS/6000. The 386 has no
such limit.)
Position-independent code requires special support, and therefore
works only on certain machines. For the 386, GCC supports PIC for
System V but not for the Sun 386i. Code generated for the IBM
RS/6000 is always position-independent.
-fpic is not supported on Mac OS X.
-fPIC
If supported for the target machine, emit position-independent
code, suitable for dynamic linking and avoiding any limit on the
size of the global offset table. This option makes a difference on
the m68k, m88k, and the SPARC.
Position-independent code requires special support, and therefore
works only on certain machines.
-fPIC is the default on Darwin and Mac OS X.
-ffixed-reg
Treat the register named reg as a fixed register; generated code
should never refer to it (except perhaps as a stack pointer, frame
pointer or in some other fixed role).
reg must be the name of a register. The register names accepted
are machine-specific and are defined in the "REGISTER_NAMES" macro
in the machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is clob-
bered 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.
It is an error to used this flag with the frame pointer or stack
pointer. Use of this flag for other registers that have fixed per-
vasive roles in the machine's execution model will produce disas-
trous results.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by
functions. It may be allocated even for temporaries or variables
that live across a call. Functions compiled this way will save and
restore the register reg if they use it.
It is an error to used this flag with the frame pointer or stack
pointer. Use of this flag for other registers that have fixed per-
vasive roles in the machine's execution model will produce disas-
trous results.
A different sort of disaster will result from the use of this flag
for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a
three-way choice.
-fpack-struct
Pack all structure members together without holes.
Warning: the -fpack-struct switch causes GCC to generate code that
is not binary compatible with code generated without that switch.
Additionally, it makes the code suboptimal. Use it to conform to a
non-default application binary interface.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions.
Just after function entry and just before function exit, the fol-
lowing profiling functions will be called with the address of the
current function and its call site. (On some platforms,
"__builtin_return_address" does not work beyond the current func-
tion, so the call site information may not be available to the pro-
filing functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current func-
tion, which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in
other functions. The profiling calls will indicate where, concep-
tually, the inline function is entered and exited. This means that
addressable versions of such functions must be available. If all
your uses of a function are expanded inline, this may mean an addi-
tional expansion of code size. If you use extern inline in your C
code, an addressable version of such functions must be provided.
(This is normally the case anyways, but if you get lucky and the
optimizer always expands the functions inline, you might have got-
ten away without providing static copies.)
A function may be given the attribute "no_instrument_function", in
which case this instrumentation will not be done. This can be
used, for example, for the profiling functions listed above, high-
priority interrupt routines, and any functions from which the pro-
filing functions cannot safely be called (perhaps signal handlers,
if the profiling routines generate output or allocate memory).
-fstack-check
Generate code to verify that you do not go beyond the boundary of
the stack. You should specify this flag if you are running in an
environment with multiple threads, but only rarely need to specify
it in a single-threaded environment since stack overflow is auto-
matically detected on nearly all systems if there is only one
stack.
Note that this switch does not actually cause checking to be done;
the operating system must do that. The switch causes generation of
code to ensure that the operating system sees the stack being
extended.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a cer-
tain value, either the value of a register or the address of a sym-
bol. If the stack would grow beyond the value, a signal is raised.
For most targets, the signal is raised before the stack overruns
the boundary, so it is possible to catch the signal without taking
special precautions.
For instance, if the stack starts at absolute address 0x80000000
and grows downwards, you can use the flags -fstack-limit-sym-
bol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to
enforce a stack limit of 128KB. Note that this may only work with
the GNU linker.
-fargument-alias
-fargument-noalias
-fargument-noalias-global
Specify the possible relationships among parameters and between
parameters and global data.
-fargument-alias specifies that arguments (parameters) may alias
each other and may alias global storage.-fargument-noalias speci-
fies that arguments do not alias each other, but may alias global
storage.-fargument-noalias-global specifies that arguments do not
alias each other and do not alias global storage.
Each language will automatically use whatever option is required by
the language standard. You should not need to use these options
yourself.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly
change the way C symbols are represented in the object file. One
use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate
code that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary
interface. Not all targets provide complete support for this
switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model argu-
ment should be one of "global-dynamic", "local-dynamic", "ini-
tial-exec" or "local-exec".
The default without -fpic is "initial-exec"; with -fpic the default
is "global-dynamic".
ENVIRONMENT
This section describes several environment variables that affect how
GCC operates. Some of them work by specifying directories or prefixes
to use when searching for various kinds of files. Some are used to
specify other aspects of the compilation environment.
Note that you can also specify places to search using options such as
-B, -I and -L. These take precedence over places specified using envi-
ronment variables, which in turn take precedence over those specified
by the configuration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses localiza-
tion information that allow GCC to work with different national
conventions. GCC inspects the locale categories LC_CTYPE and
LC_MESSAGES if it has been configured to do so. These locale cate-
gories can be set to any value supported by your installation. A
typical value is en_UK for English in the United Kingdom.
The LC_CTYPE environment variable specifies character classifica-
tion. GCC uses it to determine the character boundaries in a
string; this is needed for some multibyte encodings that contain
quote and escape characters that would otherwise be interpreted as
a string end or escape.
The LC_MESSAGES environment variable specifies the language to use
in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value
of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES
default to the value of the LANG environment variable. If none of
these variables are set, GCC defaults to traditional C English
behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary
files. GCC uses temporary files to hold the output of one stage of
compilation which is to be used as input to the next stage: for
example, the output of the preprocessor, which is the input to the
compiler proper.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler. No slash is
added when this prefix is combined with the name of a subprogram,
but you can specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an
appropriate prefix to use based on the pathname it was invoked
with.
If GCC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc-lib/ where
prefix is the value of "prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are
used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with /usr/local/lib/gcc-lib
(more precisely, with the value of GCC_INCLUDE_DIR), GCC tries
replacing that beginning with the specified prefix to produce an
alternate directory name. Thus, with -Bfoo/, GCC will search
foo/bar where it would normally search /usr/local/lib/bar. These
alternate directories are searched first; the standard directories
come next.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directo-
ries, much like PATH. GCC tries the directories thus specified
when searching for subprograms, if it can't find the subprograms
using GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories,
much like PATH. When configured as a native compiler, GCC tries
the directories thus specified when searching for special linker
files, if it can't find them using GCC_EXEC_PREFIX. Linking using
GCC also uses these directories when searching for ordinary
libraries for the -l option (but directories specified with -L come
first).
LANG
This variable is used to pass locale information to the compiler.
One way in which this information is used is to determine the char-
acter set to be used when character literals, string literals and
comments are parsed in C and C++. When the compiler is configured
to allow multibyte characters, the following values for LANG are
recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the
compiler will use mblen and mbtowc as defined by the default locale
to recognize and translate multibyte characters.
Some additional environments variables affect the behavior of the pre-
processor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a spe-
cial character, much like PATH, in which to look for header files.
The special character, "PATH_SEPARATOR", is target-dependent and
determined at GCC build time. For Windows-based targets it is a
semicolon, and for almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if speci-
fied with -I, but after any paths given with -I options on the com-
mand line. This environment variable is used regardless of which
language is being preprocessed.
The remaining environment variables apply only when preprocessing
the particular language indicated. Each specifies a list of direc-
tories to be searched as if specified with -isystem, but after any
paths given with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to
search its current working directory. Empty elements can appear at
the beginning or end of a path. For instance, if the value of
CPATH is ":/special/include", that has the same effect as
-I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output depen-
dencies for Make based on the non-system header files processed by
the compiler. System header files are ignored in the dependency
output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which
case the Make rules are written to that file, guessing the target
name from the source file name. Or the value can have the form
file target, in which case the rules are written to file file using
target as the target name.
In other words, this environment variable is equivalent to combin-
ing the options -MM and -MF, with an optional -MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above),
except that system header files are not ignored, so it implies -M
rather than -MM. However, the dependence on the main input file is
omitted.
BUGS
To report bugs to Apple, see <http://developer.apple.com/bugreporter/>.
FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code
for constructors to work. On multi-libbed systems, gcc -shared
must select the correct support libraries to link against. Failing
to supply the correct flags may lead to subtle defects. Supplying
them in cases where they are not necessary is innocuous.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), g77(1), as(1), ld(1),
gdb(1), adb(1), dbx(1), sdb(1), gcc_select(1) and the Info entries for
gcc, cpp, g77, as, ld, binutils and gdb.
AUTHOR
See the Info entry for gcc, or <http://gcc.gnu.org/onlinedocs/gcc/Con-tributors/>, for
contributors to GCC.
COPYRIGHT
Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
1999, 2000, 2001, 2002, 2003 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'' and ``Funding
Free Software'', the Front-Cover texts being (a) (see below), and with
the Back-Cover Texts being (b) (see below). A copy of the license is
included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
gcc-3.3 2005-03-20
Mac OS X 10.4 - Generated Fri Apr 29 06:57:20 CDT 2005
