Red Hat gcc options

gcc.gnu.org

gcc - GNU project C and C++ compiler 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 remainder. g++ accepts mostly the same options as gcc.

In Apple's version of GCC, both cc and gcc are actually symbolic links to a compiler named like gcc-version; which compiler is linked to may be changed using the command gcc_select. Similarly, c++ and g++ are links to a compiler named like g++-version.

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 particular, Apple does not currently support the compilation of Fortran, Ada, or Java, although there are third parties who have made these work. Environment variables

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 documented 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. C dialect
C ++ dialect
Machine Dependent
OPTIONS Overall Options -c -S -E -o file -combine -pipe -pass-exit-codes -ObjC (APPLE ONLY) -ObjC++ (APPLE ONLY) -arch arch (APPLE ONLY) -fsave-repository=file -x language -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-function -fhosted -ffreestanding -fms-extensions -trigraphs -no-integrated-cpp -traditional -traditional-cpp -fallow-single-precision -fcond-mismatch -fconstant-cfstrings (APPLE ONLY) -fnon-lvalue-assign (APPLE ONLY) -fno-nested-functions -fpch-preprocess (APPLE ONLY) -fsigned-bitfields -fsigned-char -fpascal-strings (APPLE ONLY) -Wno-#warnings (APPLE ONLY) -Wextra-tokens (APPLE ONLY) -Wnewline-eof (APPLE ONLY) -Wno-altivec-long-deprecated (APPLE ONLY) -funsigned-bitfields -funsigned-char -fwritable-strings C++ Language Options -fabi-version=n -fno-access-control -fcheck-new -fconserve-space -fno-const-strings -fno-elide-constructors -fno-enforce-eh-specs -ffor-scope -fno-for-scope -fno-gnu-keywords -fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines -fms-extensions -fno-nonansi-builtins -fno-operator-names -fno-optional-diags -fpermissive -frepo -fno-rtti -fstats -ftemplate-depth-n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline -fvisibility-inlines-hidden -fvisibility-ms-compat -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Weffc++ -Wno-deprecated -Wstrict-null-sentinel -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo

Objective-C and Objective-C++ Language Options -fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers -fobjc-call-cxx-cdtors (APPLE ONLY) -fobjc-sjlj-exceptions -fobjc-gc -freplace-objc-classes -fzero-link -gen-decls -Wno-protocol -Wselector -Wstrict-selector-match -Wundeclared-selector Language Independent Options -fmessage-length=n -fdiagnostics-show-location=[once|every-line]

Warning Options -fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall -Waggregate-return -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Wno-deprecated-declarations -Wdisabled-optimization -Wno-div-by-zero -Wno-endif-labels -Werror -Werror-implicit-function-declaration -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k -Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Wimport -Wno-import -Winit-self -Winline -Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than-len -Wlong-long -Wmain -Wmissing-braces -Wmissing-field-initializers -Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-noreturn -Wmost (APPLE ONLY) -Wno-multichar -Wnonnull -Wpacked -Wpadded -Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast -Wredundant-decls -Wreturn-type -Wsequence-point -Wshadow -Wstack-protector -Wsign-compare -Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch -Wswitch-default -Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized -Wunknown-pragmas -Wunreachable-code -Wunused -Wunused-function -Wunused-label -Wunused-parameter -Wunused-value -Wunused-variable -Wwrite-strings -Wvariadic-macros

C-only Warning Options -Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wold-style-definition -Wstrict-prototypes -Wtraditional -Wdeclaration-after-statement -Wno-discard-qual -Wno-pointer-sign Debugging Options -dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered-fdump-translation-unit[-n] -fdump-class-hierarchy[-n] -fdump-ipa-all -fdump-ipa-cgraph -fdump-tree-all -fdump-tree-original[-n] -fdump-tree-optimized[-n] -fdump-tree-inlined[-n] -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias -fdump-tree-ch -fdump-tree-ssa[-n] -fdump-tree-pre[-n] -fdump-tree-ccp[-n] -fdump-tree-dce[-n] -fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n] -fdump-tree-scev [-n] -fdump-tree-ddall [-n] -fdump-tree-elck [-n] -fdump-tree-dom[-n] -fdump-tree-dse[-n] -fdump-tree-phiopt[-n] -fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n] -fdump-tree-nrv -fdump-tree-vect -fdump-tree-sra[-n] -fdump-tree-fre[-n] -fdump-tree-loop[-n] -fdump-tree-vect[-n] -ftree-vectorizer-verbose=n -flimit-debug-info -feliminate-dwarf2-dups -feliminate-unused-debug-types -feliminate-unused-debug-symbols -fmem-report -fopt-diary -fprofile-arcs -ftree-based-profiling -frandom-seed=string -fsched-verbose=n -ftest-coverage -ftime-report -fvar-tracking -g -glevel -gcoff -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 Optimization Options -falign-functions=n -falign-jumps=n -falign-labels=n -falign-loops=n -falign-loops-max-skip=n -falign-jumps-max-skip=n -fbounds-check -fmudflap -fmudflapth -fmudflapir -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcprop-registers -fcreate-profile -fcse-follow-jumps -fcse-skip-blocks -fcx-limited-range -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 -fgcse-las -fgcse-after-reload -floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 -finline-functions -finline-limit=n -fkeep-inline-functions -fkeep-static-consts -fmerge-constants -fmerge-all-constants -fmodulo-sched -fno-branch-count-reg -fno-default-inline -fno-defer-pop -floop-optimize2 -fmove-loop-invariants -fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -funsafe-math-optimizations -ffinite-math-only -fno-trapping-math -fno-zero-initialized-in-bss -mstackrealign -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls -fprefetch-loop-arrays -fprofile-generate -fprofile-use -fregmove -frename-registers -freorder-blocks -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -frerun-loop-opt -frounding-math -fschedule-insns -fschedule-insns2 -fno-sched-interblock -fno-sched-spec -fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns=n -sched-stalled-insns-dep=n -fsched2-use-superblocks -fsched2-use-traces -freschedule-modulo-scheduled-loops -fsignaling-nans -fsingle-precision-constant -fspeculative-prefetching -fstack-protector -fstack-protector-all -fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps -funroll-all-loops -funroll-loops -fpeel-loops -fsplit-ivs-in-unroller -funswitch-loops -fvariable-expansion-in-unroller -ftree-pre -ftree-ccp -ftree-dce -ftree-loop-optimize -ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon -fivopts -ftree-dominator-opts -ftree-dse -ftree-copyrename -ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize -fuse-profile -fweb -fscalar-evolutions -fall-data-deps --param name=value -O -O0 -O1 -O2 -O3 -Os -Oz (APPLE ONLY) -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-fworking-directory -remap -trigraphs -undef -Umacro -Wp,option -Xpreprocessor option Assembler Option -Wa,option -Xassembler option Linker Options object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -s -static -static-libgcc -shared -shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol Directory Options -Bprefix -Idir -iquotedir -Ldir -specs=file -I- Target Options -V version -b machine

Machine Dependent Options ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check -mapcs-float -mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant -msched-prolog -mno-sched-prolog -mlittle-endian -mbig-endian -mwords-little-endian -mfloat-abi=name -msoft-float -mhard-float -mfpe -mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name -mfpu=name -mstructure-size-boundary=n -mabort-on-noreturn -mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-function-name -mthumb -marm -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking Darwin Options -all_load -allowable_client -arch -arch_errors_fatal -arch_only -bind_at_load -bundle -bundle_loader -client_name -compatibility_version -current_version -dead_strip -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 -iframework -image_base -init -install_name -keep_private_externs -multi_module -multiply_defined -multiply_defined_unused -noall_load -no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs -noprebind -noseglinkedit -pagezero_size -prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs -sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder -segaddr -segs_read_only_addr -segs_read_write_addr -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 -F -gused -gfull -mmacosx-version-min=version -mkernel -mone-byte-bool i386 and x86-64 Options -mtune=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-boundary=num -mmmx -msse -msse2 -msse3 -mssse3 -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 -mno-tls-direct-seg-refs -mcmodel=code-model -m32 -m64 PowerPC Options See RS/6000 and PowerPC 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 -mpim-altivec -mno-pim-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-compat -mno-xl-compat -mpe -malign-power -malign-natural -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-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -mdynamic-no-pic -mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type -minsert-sched-nops=scheme -mcall-sysv -mcall-netbsd -maix-struct-return -msvr4-struct-return -mabi=altivec -mabi=no-altivec -mabi=spe -mabi=no-spe -misel=yes -misel=no -mspe=yes -mspe=no -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double -mprototype -mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -mwindiss -G num -pthread 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 -fpcc-struct-return -fpic -fPIC -fpie -fPIE -freg-struct-return -fshared-data -fshort-enums -fshort-double -fshort-wchar -fverbose-asm -fpack-struct[=n] -fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym -fargument-alias -fargument-noalias -fargument-noalias-global -fleading-underscore -ftls-model=model -ftrapv -fwrapv -fbounds-check -fvisibility Options Controlling the Kind of Output Compilation involves stages: preprocessing, compilation proper, assembly and linking, always in that order. GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file.

suffix determines what kind of compilation is done:

file.c C must be preprocessed.
file.i C not be preprocessed.
file.ii C++ should not be preprocessed.
file.m Objective-C . link with the libobjc library to make an Objective-C program work.
file.mi Objective-C should not be preprocessed.
file.mm, file.M Objective-C++ source code. link with the libobjc library to make an Objective-C++ program work. Note that .M refers to a literal capital M.
file.mii Objective-C++ should not be preprocessed.
file.h C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header.
file.cc, file.cp, file.cxx, file.cpp, 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++ must be preprocessed. (APPLE ONLY)
file.mii Objective-C++ 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 should not be preprocessed.
file.F
file.fpp
file.FPP Fortran must be preprocessed (with the traditional preprocessor).
file.r Fortran must be preprocessed with a RATFOR preprocessor (not included with GCC).
file.f90 file.f95 Fortran 90/95 should not be preprocessed.
file.ads Ada 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 library unit body (a subprogram or package body). Such files are also called bodies.
file.s Assembler Apple's version of GCC runs the preprocessor on these files as well as those ending in .S.

file.S Assembler 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.


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. c c-header c-cpp-output c++ c++-header c++-cpp-output objective-c objective-c-header objective-c-cpp-output objective-c++ objective-c++-header objective-c++-cpp-output assembler assembler-with-cpp ada f77 f77-cpp-input ratfor f95 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 affect only the choice of compiler for files already identified as source files. (APPLE ONLY)
-arch arch Compile for the specified target architecture arch. The allowable values are i386, ppc and ppc64. Multiple options work, and direct the compiler to produce ``universal'' binaries including object code for each architecture specified with -arch. This option only works if assembler and libraries are available for each architecture specified. (APPLE ONLY) -fsave-repository=file Save debug info in separate object file. This is available only while building PCH in -gfull mode. file.for file.FOR Fortran should not be preprocessed. file.F, file.fpp, file.FPP Fortran must be preprocessed (with the traditional preprocessor). file.r Fortran must be preprocessed with a RATFOR preprocessor (not included with GCC). file.f90 file.f95 Fortran 90/95 source code which should not be preprocessed. 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 c-cpp-output c++ c++-header c++-cpp-output objective-c objective-c-header objective-c-cpp-output objective-c++ objective-c++-header objective-c++-cpp-output assembler assembler-with-cpp ada f77 f77-cpp-input ratfor f95 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 affect only the choice of compiler for files already identified as source files. (APPLE ONLY)
-arch arch Compile for the specified target architecture arch. The allowable values are i386, ppc and ppc64. Multiple options work, and direct the compiler to produce ``universal'' binaries including object code for each architecture specified with -arch. This option only works if assembler and libraries are available for each architecture specified. (APPLE ONLY)
-fsave-repository=file Save debug info in separate object file. This is available only while building PCH in -gfull mode.
-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 numerically 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-assembler 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. 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, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.
-v Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.
-### Like -v except the commands are not executed and all command arguments 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.
-combine If you are compiling multiple source files, this option tells the driver to pass all the source files to the compiler at once (for those languages for which the compiler can handle this). This will allow intermodule analysis (IMA) to be performed by the compiler. Currently the only language for which this is supported is C. If you pass source files for multiple languages to the driver, using this option, the driver will invoke the compiler(s) that support IMA once each, passing each compiler all the source files appropriate for it. For those languages that do not support IMA this option will be ignored, and the compiler will be invoked once for each source file in that language. If you use this option in conjunction with -save-temps, the compiler will generate multiple pre-processed files (one for each source file), but only one (combined) .o or .s file.
--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 -Wextra 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; and preprocessed 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, the use of gcc does not add the C++ library. g++ is a program that calls GCC and treats .c, .h and .i files as C++ source files instead of C source files unless -x is used, and automatically specifies linking against the C++ library. This is also useful when precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed with the name c++.

When you compile C++ programs, you may specify many of the same command-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++, Objective-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 semantics 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 for more information. The names c9x and iso9899:199x are deprecated. gnu89 Default, ISO C90 plus GNU extensions (including some C99 features). gnu99 gnu9x ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will become the default. The name gnu9x is deprecated. 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 functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language 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
  • 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 This flag is provided for compatibility with Metrowerks CodeWarrior and MrC compilers as well as previous Apple versions of GCC. It causes the -mpim-altivec option to be turned on.
    -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 CodeWarrior. (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. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to "printf", when "printf" is built in, and "strlen" is known not to modify global memory. 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-function 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. Some cases of unnamed fields in structures and unions are only accepted with this option.
    -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 (internal 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++.
    -fno-nested-functions Disable nested functions. This option is not supported for C++ or Objective-C++. On Darwin, nested functions are disabled by default.
    -fpch-preprocess Enable PCH processing even when -E or -save-temps is used.
    -fnon-lvalue-assign C and C++ forbid the use of casts and conditional expressions as lvalues, e.g.: float *p, q, r; ((int *)p)++; (cond ? q : r) = 3.0; As a transitional measure, the Apple version of GCC 4.0 allows casts and conditional expressions to be used as lvalues in certain situations. This is accomplished via the -fnon-lvalue-assign switch, which is on by default. Whenever an lvalue cast or an lvalue conditional expression is encountered, the compiler will issue a deprecation warning and then rewrite the expression as follows: (type)expr ---becomes---> *(type *)&expr cond ? expr1 : expr2 ---becomes---> *(cond ? &expr1 : &expr2) To disallow lvalue casts and lvalue conditional expressions altogether, specify
    -fno-non-lvalue-assign; lvalue casts and lvalue conditional expressions will be disallowed in future versions of Apple's GCC.
    -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.
    -fconstant-cfstrings Enable the automatic creation of a CoreFoundation-type constant string whenever a special builtin "__builtin__CFStringMakeConstantString" is called on a literal string. (APPLE ONLY)
    -fpascal-strings Allow Pascal-style string literals to be constructed. (APPLE ONLY)
    -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. This option is deprecated.

    Options Controlling C++ Dialect

    This section describes the command-line options that are only meaningful 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 2 is the version of the C++ ABI that first appeared in G++ 3.4. 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 2.
    -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. This check is normally unnecessary because the C++ standard specifies that "operator new" will only return 0 if it is declared throw(), in which case the compiler will always check the return value even without this option. In all other cases, when "operator new" has a non-empty exception specification, memory exhaustion is signalled by throwing "std::bad_alloc". 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 standard. Even if you use -fno-const-strings, you cannot actually modify the value of a string constant, unless you also use -fwritable-strings. This option might be removed in a future release of G++. For maximum portability, you should structure your code so that it works with string constants that have type "const char *".
    -fno-elide-constructors The C++ standard allows an implementation to omit creating a temporary 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 reducing code size in production builds, much like defining NDEBUG. The compiler will still optimize based on the exception specifications.
    -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 instantiations.
    -fno-implicit-inline-templates Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles 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 controlled by #pragma implementation. This will cause linker errors if these functions are not inlined everywhere they are called.
    -fms-extensions Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard 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 some diagnostics about nonconformant code from errors to warnings. Thus, using
    -fpermissive will allow some nonconforming code to compile.
    -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.
    -fstats Emit statistics about front-end processing at the end of the compilation. 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 endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17.
    -fno-threadsafe-statics Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.
    -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".
    -fno-use-cxa-get-exception-ptr Don't use the "__cxa_get_exception_ptr" runtime routine. This will cause "std::uncaught_exception" to be incorrect, but is necessary if the runtime routine is not available.
    -fvisibility-inlines-hidden This switch declares that the user does not attempt to compare pointers to inline methods where the addresses of the two functions were taken in different shared objects. The effect of this is that GCC may, effectively, mark inline methods with "__attribute__ ((visibility ("hidden")))" so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. The behaviour of this switch is not quite the same as marking the methods as hidden directly. Normally if there is a class with default visibility which has a hidden method, the effect of this is that the method must be defined in only one shared object. This switch does not have this restriction. You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility.
    -fvisibility-ms-compat This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio. The flag makes these changes to GCC's linkage model: 1. It sets the default visibility to 'hidden', like -fvisibility=hidden. 2. Types, but not their members, are not hidden by default. 3. The One Definition Rule is relaxed for types without explicit visibility specifications which are defined in more than one different shared object: those declarations are permitted if they would have been permitted when this option was not used. This option is discouraged, rather, it is preferable for types to be explicitly exported as desired on a per-class basis. Unfortunately because Visual Studio can't compare two different hidden types as unequal for the purposes of type_info and exception handling, users are able to write code that relies upon this behavior. Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects will be different, so changing one will not change the other; and that pointers to function members defined in different shared objects will not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.
    -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 available. 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 functions; 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 problem 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 example: 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 void f(typename Q::X) {} template