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.
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.
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 -v -### --help --target-help --version C Language Options -ansi -std=standard -aux-info filename -fno-asm -fno-builtin -fno-builtin-function -fhosted -ffreestanding -fms-extensions -trigraphs -no-integrated-cpp -traditional -traditional-cpp -fallow-single-precision -fcond-mismatch -fsigned-bitfields -fsigned-char -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 -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 Objective-C Language Options -fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers -fobjc-exceptions -freplace-objc-classes -fzero-link -gen-decls -Wno-protocol -Wselector -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 -Wendif-labels -Werror -Werror-implicit-function-declaration -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-invalid-offsetof -Winvalid-pch -Wlarger-than-len -Wlong-long -Wmain -Wmissing-braces -Wmissing-format-attribute -Wmissing-noreturn -Wno-multichar -Wnonnull -Wpacked -Wpadded -Wparentheses -Wpointer-arith -Wredundant-decls -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare -Wstrict-aliasing -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 C-only Warning Options -Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wold-style-definition -Wstrict-prototypes -Wtraditional -Wdeclaration-after-statement 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 -feliminate-unused-debug-types -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs -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 -fbranch-probabilities -fprofile-values -fvpt -fbranch-target-load-optimize -fbranch-target-load-optimize2 -fcaller-saves -fcprop-registers -fcse-follow-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 -fgcse-las -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-optimizations -ffinite-math-only -fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls -fprefetch-loop-arrays -fprofile-generate -fprofile-use -freduce-all-givs -fregmove -frename-registers -freorder-blocks -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 -fsignaling-nans -fsingle-precision-constant -fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps -funroll-all-loops -funroll-loops -fpeel-loops -funswitch-loops -fold-unroll-loops -fold-unroll-all-loops --param name=value -O -O0 -O1 -O2 -O3 -Os 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 -I- -Ldir -specs=file Target Options -V version -b machine Machine Dependent Options M680x0 Options -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 -mnobitfield -mrtd -mshort -msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data -mshared-library-id=n -mid-shared-library -mno-id-shared-library M68hc1x Options -m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 -mauto-incdec -minmax -mlong-calls -mshort -msoft-reg-count=count
VAX Options -mg -mgnu -munix
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -m32 -m64 -mapp-regs -mno-app-regs -mfaster-structs -mno-faster-structs -mflat -mno-flat -mfpu -mno-fpu -mhard-float -msoft-float -mhard-quad-float -msoft-quad-float -mimpure-text -mno-impure-text -mlittle-endian -mstack-bias -mno-stack-bias -munaligned-doubles -mno-unaligned-doubles -mv8plus -mno-v8plus -mvis -mno-vis -mcypress -mf930 -mf934 -msparclite -msupersparc -mv8 -threads -pthreads
ARM Options -mapcs-frame -mno-apcs-frame -mapcs-26 -mapcs-32 -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 -malignment-traps -mno-alignment-traps -msoft-float -mhard-float -mfpe -mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name -mfpe=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
MN10300 Options -mmult-bug -mno-mult-bug -mam33 -mno-am33 -mam33-2 -mno-am33-2 -mno-crt0 -mrelax
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -missue-rate=number -mbranch-cost=number -mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func -mflush-func=name -mno-flush-trap -mflush-trap=number -G num
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-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 -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 -compatibility_version -current_version -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 -nofixprebinding -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
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 -mips16 -mno-mips16 -mabi=abi -mabicalls -mno-abicalls -mxgot -mno-xgot -membedded-pic -mno-embedded-pic -mgp32 -mgp64 -mfp32 -mfp64 -mhard-float -msoft-float -msingle-float -mdouble-float -mint64 -mlong64 -mlong32 -Gnum -membedded-data -mno-embedded-data -muninit-const-in-rodata -mno-uninit-const-in-rodata -msplit-addresses -mno-split-addresses -mexplicit-relocs -mno-explicit-relocs -mrnames -mno-rnames -mcheck-zero-division -mno-check-zero-division -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad -mfused-madd -mno-fused-madd -nocpp -mfix-sb1 -mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-likely -mno-branch-likely
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 -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
HPPA Options -march=architecture-type -mbig-switch -mdisable-fpregs -mdisable-indexing -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld -mjump-in-delay -mlinker-opt -mlong-calls -mlong-load-store -mno-big-switch -mno-disable-fpregs -mno-disable-indexing -mno-fast-indirect-calls -mno-gas -mno-jump-in-delay -mno-long-load-store -mno-portable-runtime -mno-soft-float -mno-space-regs -msoft-float -mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -mschedule=cpu-type -mspace-regs -msio -mwsio -nolibdld -static -threads
Intel 960 Options -mcpu-type -masm-compat -mclean-linkage -mcode-align -mcomplex-addr -mleaf-procedures -mic-compat -mic2.0-compat -mic3.0-compat -mintel-asm -mno-clean-linkage -mno-code-align -mno-complex-addr -mno-leaf-procedures -mno-old-align -mno-strict-align -mno-tail-call -mnumerics -mold-align -msoft-float -mstrict-align -mtail-call
DEC Alpha Options -mno-fp-regs -msoft-float -malpha-as -mgas -mieee -mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data -mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
DEC Alpha/VMS Options -mvms-return-codes
H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300
SH Options -m1 -m2 -m2e -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4 -m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu -m5-compact -m5-compact-nofpu -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mnomacsave -mieee -misize -mpadstruct -mspace -mprefergot -musermode
System V Options -Qy -Qn -YP,paths -Ym,dir
ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section -mdata=data-section -mrodata=readonly-data-section
TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm -mmemparm -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload -mrpts=count -mrptb -mdb -mloop-unsigned -mparallel-insns -mparallel-mpy -mpreserve-float
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-prolog-function -mspace -mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-disable-callt -mv850e1 -mv850e -mv850 -mbig-switch
NS32K Options -m32032 -m32332 -m32532 -m32081 -m32381 -mmult-add -mnomult-add -msoft-float -mrtd -mnortd -mregparam -mnoregparam -msb -mnosb -mbitfield -mnobitfield -mhimem -mnohimem
AVR Options -mmcu=mcu -msize -minit-stack=n -mno-interrupts -mcall-prologues -mno-tablejump -mtiny-stack
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields -m4byte-functions -mno-4byte-functions -mcallgraph-data -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict -mbase-addresses -mno-base-addresses -msingle-exit -mno-single-exit
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mb-step -mregister-names -mno-sdata -mconstant-gp -mauto-pic -minline-float-divide-min-latency -minline-float-divide-max-throughput -minline-int-divide-min-latency -minline-int-divide-max-throughput -minline-sqrt-min-latency -minline-sqrt-max-throughput -mno-dwarf2-asm -mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type -mt -pthread -milp32 -mlp64
D30V Options -mextmem -mextmemory -monchip -mno-asm-optimize -masm-optimize -mbranch-cost=n -mcond-exec=n
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float -mbackchain -mno-backchain -mpacked-stack -mno-packed-stack -msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch -mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -melinux-stacksize=n -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt -melf -maout -melinux -mlinux -sim -sim2 -mmul-bug-workaround -mno-mul-bug-workaround
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy -mbcopy-builtin -mint32 -mno-int16 -mint16 -mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32 -mabshi -mno-abshi -mbranch-expensive -mbranch-cheap -msplit -mno-split -munix-asm -mdec-asm
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mtext-section-literals -mno-text-section-literals -mtarget-align -mno-target-align -mlongcalls -mno-longcalls
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -malloc-cc -mfixed-cc -mdword -mno-dword -mdouble -mno-double -mmedia -mno-media -mmuladd -mno-muladd -mlibrary-pic -macc-4 -macc-8 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move -mscc -mno-scc -mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec -mtomcat-stats -mcpu=cpu
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 -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
Compilation can involve up to four 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.For any given input file, the file name suffix determines what kind of compilation is done:
You can specify the input language explicitly with the -x option:
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.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.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 traditional preprocessor). file.r Fortran source code which must be preprocessed with a RATFOR preprocessor (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. 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. 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.
-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 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). -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.
-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 you specify -o when compiling more than one input file, or you are producing an executable file as output, all the source files on the command line will be compiled at once.
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 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. --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.
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, C++ programs often require class libraries as well as a compiler 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 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.
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 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 <http://gcc.gnu.org/gcc-3.4/c99status.html> 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 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.
-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-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. -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++. -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.
This option is deprecated.
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.CIn 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:
In addition, these optimization, warning, and code generation options have meanings only for 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. -fvisibility-inlines-hidden Causes all inlined methods to be marked 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. While it can cause bloating through duplication of code within each DSO where it is used, often the wastage is less than the considerable space occupied by a long symbol name in the export table which is typical when using templates and namespaces. For even more savings, combine with the -fvisibility=hidden switch. -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.) The following -W... options are not affected by -Wall.
-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 asA::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 <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 that class are private, and it has neither friends nor public static member functions. -Wnon-virtual-dtor (C++ only) Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it declares a non-virtual one. 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) { } };The compiler will rearrange the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect. This warning is enabled 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. Also warn about violations of the following style guidelines from Scott Meyers’ More Effective C++ book:
* Item 6: Distinguish between prefix and postfix forms of increment and decrement operators. * Item 7: Never overload &&, ||, or ,. When selecting this option, be aware that the standard library headers do not obey all of these guidelines; 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. Since the advent of explicit template specification 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 function. (Section 14.5.3). Before G++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a templatized function. Because this non-conforming behavior is no longer the default behavior for G++, -Wnon-template-friend allows the compiler to check existing code for potential trouble spots and is on by default. This new compiler behavior 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, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search 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:
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 enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ would try to preserve unsignedness, but the standard mandates the current behavior.
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 =.
(NOTE: This manual does not describe the Objective-C language itself. See <http://gcc.gnu.org/readings.html> for references.)This section describes the command-line options that are only meaningful for Objective-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 some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.mIn this example, -fgnu-runtime is an option meant only for Objective-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 Objective-C programs:
-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 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 runtime to be used. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later. -fobjc-exceptions Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++ and Java. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
@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 synchronization blocks for thread-safe execution:
@synchronized (ObjCClass *guard) { ... }Upon entering the @synchronized block, a thread of execution shall first check whether a lock has been placed on the corresponding guard object by another thread. If it has, the current thread shall wait until the other thread relinquishes its lock. Once guard becomes available, the current thread will place its own lock on it, execute the code contained in the @synchronized block, and finally relinquish the lock (thereby making guard available to other threads).
Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized. Note that throwing exceptions out of @synchronized blocks is allowed, and will cause the guarding object to be unlocked properly.
-freplace-objc-classes Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later. -fzero-link 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 -fzero-link flag suppresses this behavior and causes calls to objc_getClass("...") to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. -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 implementation is inherited from the superclass. If you use the -Wno-protocol option, then methods inherited from the superclass are considered 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 for each selector appearing in a @selector(...) expression, and a corresponding method for 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 before the @selector(...) expression, either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. 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. This also enforces the coding style convention that methods and selectors must be declared before being used. -print-objc-runtime-info Generate C header describing the largest structure that is passed by value, if any.
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 formatting 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 correctly.
-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.
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 C++ Dialect Options and 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 version 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++ features 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__. However, 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 diagnostics 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