GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below for the remain-
der. g++ accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The "overall options" allow you to stop this
process at an intermediate stage. For example, the -c option says not
to run the linker. Then the output consists of object files output by
the assembler.
Other options are passed on to one stage of processing. Some options
control the preprocessor and others the compiler itself. Yet other
options control the assembler and linker; most of these are not docu-
mented here, since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful
for C programs; when an option is only useful with another language
(usually C++), the explanation says so explicitly. If the description
for a particular option does not mention a source language, you can use
that option with all supported languages.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dr is very different from -d -r.
You can mix options and other arguments. For the most part, the order
you use doesn't matter. Order does matter when you use several options
of the same kind; for example, if you specify -L more than once, the
directories are searched in the order specified.
Many options have long names starting with -f or with -W---for example,
-fstrength-reduce, -Wformat and so on. Most of these have both posi-
tive and negative forms; the negative form of -ffoo would be -fno-foo.
This manual documents only one of these two forms, whichever one is not
the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations
are in the following sections.
Overall Options
-c -S -E -o file -combine -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
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space
-ffriend-injection -fno-const-strings -fno-elide-constructors
-fno-enforce-eh-specs -ffor-scope -fno-for-scope -fno-gnu-key-
words -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-vir-
tual-dtor -Wreorder -Weffc++ -Wno-deprecated -Wstrict-null-sen-
tinel -Wno-non-template-friend -Wold-style-cast -Woverloaded-vir-
tual -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 -fobjc-direct-dispatch
-fobjc-exceptions -fobjc-gc -freplace-objc-classes -fzero-link
-gen-decls -Wassign-intercept -Wno-protocol -Wselector
-Wstrict-selector-match -Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
-fdiagnostics-show-options
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall
-Waggregate-return -Wno-attributes -Wc++-compat -Wcast-align
-Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Wno-depre-
cated-declarations -Wdisabled-optimization -Wno-div-by-zero
-Wno-endif-labels -Werror -Werror-implicit-function-declaration
-Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-for-
mat-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k
-Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Wim-
port -Wno-import -Winit-self -Winline -Wno-int-to-pointer-cast
-Wno-invalid-offsetof -Winvalid-pch -Wlarger-than-len -Wun-
safe-loop-optimizations -Wlong-long -Wmain -Wmissing-braces
-Wmissing-field-initializers -Wmissing-format-attribute -Wmiss-
ing-include-dirs -Wmissing-noreturn -Wno-multichar -Wnonnull
-Wpacked -Wpadded -Wparentheses -Wpointer-arith
-Wno-pointer-to-int-cast -Wredundant-decls -Wreturn-type -Wse-
quence-point -Wshadow -Wsign-compare -Wstack-protector
-Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch -Wswitch-default
-Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef -Wuninitial-
ized -Wunknown-pragmas -Wno-pragmas -Wunreachable-code -Wunused
-Wunused-function -Wunused-label -Wunused-parameter
-Wunused-value -Wunused-variable -Wvariadic-macros
-Wvolatile-register-var -Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes
-Wnested-externs -Wold-style-definition -Wstrict-prototypes
-Wtraditional -Wdeclaration-after-statement -Wpointer-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-origi-
nal[-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-mud-
flap[-n] -fdump-tree-dom[-n] -fdump-tree-dse[-n]
-fdump-tree-phiopt[-n] -fdump-tree-forwprop[-n] -fdump-tree-copyre-
name[-n] -fdump-tree-nrv -fdump-tree-vect -fdump-tree-sink
-fdump-tree-sra[-n] -fdump-tree-salias -fdump-tree-fre[-n]
-fdump-tree-vrp[-n] -ftree-vectorizer-verbose=n
-fdump-tree-storeccp[-n] -feliminate-dwarf2-dups -felimi-
nate-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 -fbounds-check -fmudflap -fmudflapth -fmudflapir
-fbranch-probabilities -fprofile-values -fvpt -fbranch-tar-
get-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive
-fcaller-saves -fcprop-registers -fcse-follow-jumps
-fcse-skip-blocks -fcx-limited-range -fdata-sections -fde-
layed-branch -fdelete-null-pointer-checks -fearly-inlining -fex-
pensive-optimizations -ffast-math -ffloat-store -fforce-addr
-ffunction-sections -fgcse -fgcse-lm -fgcse-sm -fgcse-las
-fgcse-after-reload -floop-optimize -fcrossjumping -fif-conversion
-fif-conversion2 -finline-functions -finline-functions-called-once
-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 -fun-
safe-loop-optimizations -ffinite-math-only -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-reg-
ister-move -foptimize-sibling-calls -fprefetch-loop-arrays -fpro-
file-generate -fprofile-use -fregmove -frename-registers -fre-
order-blocks -freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -frerun-loop-opt -frounding-math -fsched-
ule-insns -fschedule-insns2 -fno-sched-interblock -fno-sched-spec
-fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns=n -fsched-stalled-insns-dep=n
-fsched2-use-superblocks -fsched2-use-traces -freschedule-mod-
ulo-scheduled-loops -fsignaling-nans -fsingle-precision-constant
-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-sink -ftree-ch -ftree-sra
-ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize
-ftree-vect-loop-version -ftree-salias -fweb -ftree-copy-prop
-ftree-store-ccp -ftree-store-copy-prop -fwhole-program --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 -isysroot 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 -nost-
dlib -pie -rdynamic -s -static -static-libgcc -shared
-shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -iquotedir -Ldir -specs=file -I- --sysroot=dir
Target Options
-V version -b machine
Machine Dependent Options
ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section
-mdata=data-section -mrodata=readonly-data-section
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-pro-
log -mno-sched-prolog -mlittle-endian -mbig-endian -mwords-lit-
tle-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-sin-
gle-pic-base -mpic-register=reg -mnop-fun-dllimport -mcir-
rus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-func-
tion-name -mthumb -marm -mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking -mtp=name
AVR Options -mmcu=mcu -msize -minit-stack=n -mno-interrupts
-mcall-prologues -mno-tablejump -mtiny-stack -mint8
Blackfin Options -momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer -mspecld-anomaly -mno-specld-anomaly
-mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k
-mid-shared-library -mno-id-shared-library -mshared-library-id=n
-mlong-calls -mno-long-calls
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
CRX Options -mmac -mpush-args
Darwin Options -all_load -allowable_client -arch
-arch_errors_fatal -arch_only -bind_at_load -bundle -bun-
dle_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 -image_base -init -install_name
-keep_private_externs -multi_module -multiply_defined -multi-
ply_defined_unused -noall_load -no_dead_strip_inits_and_terms
-nofixprebinding -nomultidefs -noprebind -noseglinkedit
-pagezero_size -prebind -prebind_all_twolevel_modules -pri-
vate_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 -unde-
fined -unexported_symbols_list -weak_reference_mismatches -what-
sloaded -F -gused -gfull -mmacosx-version-min=version
-mone-byte-bool
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
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 -mfdpic
-minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp
-mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8 -mpack
-mno-pack -mno-eflags -mcond-move -mno-cond-move -moptimize-mem-
bar -mno-optimize-membar -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 -mTLS -mtls -mcpu=cpu
H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300
HPPA Options -march=architecture-type -mbig-switch -mdis-
able-fpregs -mdisable-indexing -mfast-indirect-calls -mgas
-mgnu-ld -mhp-ld -mfixed-range=register-range -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-porta-
ble-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 -munix=unix-std
-nolibdld -static -threads
i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfp-
math=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387
-msoft-float -msvr3-shlib -mno-wide-multiply -mrtd -malign-dou-
ble -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 -msseregparm
-momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs
-mcmodel=code-model -m32 -m64 -mlarge-data-threshold=num
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld
-mno-pic -mvolatile-asm-stop -mregister-names -mno-sdata -mcon-
stant-gp -mauto-pic -minline-float-divide-min-latency -min-
line-float-divide-max-throughput -minline-int-divide-min-latency
-minline-int-divide-max-throughput -minline-sqrt-min-latency -min-
line-sqrt-max-throughput -mno-dwarf2-asm -mearly-stop-bits
-mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type
-mt -pthread -milp32 -mlp64
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
M32C Options -mcpu=cpu -msim -memregs=number
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
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mre-
lax-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
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 -mgp32
-mgp64 -mfp32 -mfp64 -mhard-float -msoft-float -msingle-float
-mdouble-float -mdsp -mpaired-single -mips3d -mlong64 -mlong32
-msym32 -mno-sym32 -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 -mcheck-zero-division -mno-check-zero-divi-
sion -mdivide-traps -mdivide-breaks -mmemcpy -mno-memcpy
-mlong-calls -mno-long-calls -mmad -mno-mad -mfused-madd
-mno-fused-madd -nocpp -mfix-r4000 -mno-fix-r4000 -mfix-r4400
-mno-fix-r4400 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130
-mfix-sb1 -mno-fix-sb1 -mflush-func=func -mno-flush-func
-mbranch-likely -mno-branch-likely -mfp-exceptions -mno-fp-excep-
tions -mvr4130-align -mno-vr4130-align
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon
-mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv -mto-
plevel-symbols -melf -mbranch-predict -mno-branch-predict
-mbase-addresses -mno-base-addresses -msingle-exit -mno-sin-
gle-exit
MN10300 Options -mmult-bug -mno-mult-bug -mam33 -mno-am33
-mam33-2 -mno-am33-2 -mreturn-pointer-on-d0 -mno-crt0 -mrelax
MT Options -mno-crt0 -mbacc -msim -march=cpu-type
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 -mab-
shi -mno-abshi -mbranch-expensive -mbranch-cheap -msplit
-mno-split -munix-asm -mdec-asm
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-pow-
erpc -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt
-mpowerpc-gfxopt -mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopc-
ntb -mno-popcntb -mfprnd -mno-fprnd -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 -mmulti-
ple -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 -maltivec
-mswdiv -mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme
-mcall-sysv -mcall-netbsd -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt -misel
-mno-isel -misel=yes -misel=no -mspe -mno-spe -mspe=yes -mspe=no
-mvrsave -mno-vrsave -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
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type
-mhard-float -msoft-float -mlong-double-64 -mlong-double-128
-mbackchain -mno-backchain -mpacked-stack -mno-packed-stack
-msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mde-
bug -mno-debug -mesa -mzarch -mtpf-trace -mno-tpf-trace
-mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack
-mstack-size -mstack-guard
SH Options -m1 -m2 -m2e -m3 -m3e -m4-nofpu -m4-single-only
-m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al
-m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu
-m5-compact -m5-compact-nofpu -mb -ml -mdalign -mrelax
-mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave
-mieee -misize -mpadstruct -mspace -mprefergot -musermode
-multcost=number -mdiv=strategy -mdivsi3_libfunc=name -mad-
just-unroll -mindexed-addressing -mgettrcost=number -mpt-fixed
-minvalid-symbols
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
-m32 -m64 -mapp-regs -mno-app-regs -mfaster-structs
-mno-faster-structs -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 -threads -pthreads -pthread
System V Options -Qy -Qn -YP,paths -Ym,dir
TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm -mmem-
parm -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 -mpro-
log-function -mno-prolog-function -mspace -mtda=n -msda=n
-mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-dis-
able-callt -mv850e1 -mv850e -mv850 -mbig-switch
VAX Options -mg -mgnu -munix
x86-64 Options See i386 and x86-64 Options.
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
zSeries Options See S/390 and zSeries Options.
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
-fno-jump-tables -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 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 exe-
cutable file.
For any given input file, the file name suffix determines what kind of
compilation is done:
file.c
C source code which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc
library to make an Objective-C program work.
file.mi
Objective-C source code which should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must 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++ source code which 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++ source code which must be preprocessed.
file.mii
Objective-C++ source code which should not be preprocessed.
file.hh
file.H
C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR
Fixed form Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fixed form Fortran source code which must be preprocessed (with the
traditional preprocessor).
file.f90
file.f95
Free form Fortran source code which should not be preprocessed.
file.F90
file.F95
Free form Fortran source code which must be preprocessed (with the
traditional preprocessor).
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.
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
f95 f95-cpp-input
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 numeri-
cally highest error produced by any phase that returned an error
indication.
If you only want some of the stages of compilation, you can use -x (or
filename suffixes) to tell gcc where to start, and one of the options
-c, -S, or -E to say where gcc is to stop. Note that some combinations
(for example, -x cpp-output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking
stage simply is not done. The ultimate output is in the form of an
object file for each source file.
By default, the object file name for a source file is made by
replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly,
are ignored.
-S Stop after the stage of compilation proper; do not assemble. The
output is in the form of an assembler code file for each non-assem-
bler input file specified.
By default, the assembler file name for a source file is made by
replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper.
The output is in the form of preprocessed source code, which is
sent to the standard output.
Input files which don't require preprocessing are ignored.
-o file
Place output in file file. This applies regardless to whatever
sort of output is being produced, whether it be an executable file,
an object file, an assembler file or preprocessed C code.
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 com-
piler driver program and of the preprocessor and the compiler
proper.
-###
Like -v except the commands are not executed and all command argu-
ments are quoted. This is useful for shell scripts to capture the
driver-generated command lines.
-pipe
Use pipes rather than temporary files for communication between the
various stages of compilation. This fails to work on some systems
where the assembler is unable to read from a pipe; but the GNU
assembler has no trouble.
-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 appropri-
ate 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 con-
junction 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 dis-
played.
--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 com-
piler the same way as for compiling C programs (usually with the name
gcc).
However, C++ programs often require class libraries as well as a com-
piler that understands the C++ language---and under some circumstances,
you might want to compile programs or header files from standard input,
or otherwise without a suffix that flags them as C++ programs. You
might also like to precompile a C header file with a .h extension to be
used in C++ compilations. g++ is a program that calls GCC with the
default language set to C++, and automatically specifies linking
against the C++ library. On many systems, g++ is also installed with
the name c++.
When you compile C++ programs, you may specify many of the same com-
mand-line options that you use for compiling programs in any language;
or command-line options meaningful for C and related languages; or
options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived
from C, such as C++, 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 seman-
tics defined by ISO C (such as "alloca" and "ffs") are not built-in
functions with -ansi is used.
-std=
Determine the language standard. This option is currently only
supported when compiling C or C++. A value for this option must be
provided; possible values are
c89
iso9899:1990
ISO C90 (same as -ansi).
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported;
see <http://gcc.gnu.org/gcc-4.1/c99status.html> for more infor-
mation. The names c9x and iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including some C99 fea-
tures).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented
in GCC, this will become the default. The name gnu9x is depre-
cated.
c++98
The 1998 ISO C++ standard plus amendments.
gnu++98
The same as -std=c++98 plus GNU extensions. This is the
default for C++ code.
Even when this option is not specified, you can still use some of
the features of newer standards in so far as they do not conflict
with previous C standards. For example, you may use "__restrict__"
even when -std=c99 is not specified.
The -std options specifying some version of ISO C have the same
effects as -ansi, except that features that were not in ISO C90 but
are in the specified version (for example, // comments and the
"inline" keyword in ISO C99) are not disabled.
-aux-info filename
Output to the given filename prototyped declarations for all func-
tions declared and/or defined in a translation unit, including
those in header files. This option is silently ignored in any lan-
guage other than C.
Besides declarations, the file indicates, in comments, the origin
of each declaration (source file and line), whether the declaration
was implicit, prototyped or unprototyped (I, N for new or O for
old, respectively, in the first character after the line number and
the colon), and whether it came from a declaration or a definition
(C or F, respectively, in the following character). In the case of
function definitions, a K&R-style list of arguments followed by
their declarations is also provided, inside comments, after the
declaration.
-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-func-
tion option; if you wish to enable built-in functions selectively
when using -fno-builtin or -ffreestanding, you may define macros
such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation takes place in a hosted environment. This
implies -fbuiltin. A hosted environment is one in which the entire
standard library is available, and in which "main" has a return
type of "int". Examples are nearly everything except a kernel.
This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding environment.
This implies -fno-builtin. A freestanding environment is one in
which the standard library may not exist, and program startup may
not necessarily be at "main". The most obvious example is an OS
kernel. This is equivalent to -fno-hosted.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
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 (inter-
nal cpp)
The semantics of this option will change if "cc1", "cc1plus", and
"cc1obj" are merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-
standard C compiler. They are now only supported with the -E
switch. The preprocessor continues to support a pre-standard mode.
See the GNU CPP manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second
and third arguments. The value of such an expression is void.
This option is not supported for C++.
-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.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaning-
ful for C++ programs; but you can also use most of the GNU compiler
options regardless of what language your program is in. For example,
you might compile a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs;
you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 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 specifica-
tion. 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 "oper-
ator new" will only return 0 if it is declared tthhrrooww(()), 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.
-ffriend-injection
Inject friend functions into the enclosing namespace, so that they
are visible outside the scope of the class in which they are
declared. Friend functions were documented to work this way in the
old Annotated C++ Reference Manual, and versions of G++ before 4.1
always worked that way. However, in ISO C++ a friend function
which is not declared in an enclosing scope can only be found using
argument dependent lookup. This option causes friends to be
injected as they were in earlier releases.
This option is for compatibility, and may be removed in a future
release of G++.
-fno-const-strings
Give string constants type "char *" instead of type "const char *".
By default, G++ uses type "const char *" as required by the stan-
dard. Even if you use -fno-const-strings, you cannot actually mod-
ify the value of a string constant.
This option might be removed in a future release of G++. For maxi-
mum portability, you should structure your code so that it works
with string constants that have type "const char *".
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a tempo-
rary which is only used to initialize another object of the same
type. Specifying this option disables that optimization, and
forces G++ to call the copy constructor in all cases.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception specifica-
tions at runtime. This option violates the C++ standard, but may
be useful for reducing code size in production builds, much like
defining NDEBUG. This does not give user code permission to throw
exceptions in violation of the exception specifications; the com-
piler will still optimize based on the specifications, so throwing
an unexpected exception will result in undefined behavior.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a
for-init-statement is limited to the for loop itself, as specified
by the C++ standard. If -fno-for-scope is specified, the scope of
variables declared in a for-init-statement extends to the end of
the enclosing scope, as was the case in old versions of G++, and
other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to
allow and give a warning for old-style code that would otherwise be
invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this
word as an identifier. You can use the keyword "__typeof__"
instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated
implicitly (i.e. by use); only emit code for explicit instantia-
tions.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates,
either. The default is to handle inlines differently so that com-
piles with and without optimization will need the same set of
explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
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-stan-
dard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C. These include "ffs", "alloca", "_exit", "index",
"bzero", "conjf", and other related functions.
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor",
"compl", "not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need
to issue. Currently, the only such diagnostic issued by G++ is the
one for a name having multiple meanings within a class.
-fpermissive
Downgrade 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 compi-
lation. This information is generally only useful to the G++
development team.
-ftemplate-depth-n
Set the maximum instantiation depth for template classes to n. A
limit on the template instantiation depth is needed to detect end-
less recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than
17.
-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__ ((visi-
bility ("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 sav-
ings, 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 avail-
able. This option exists only for testing, and should not be used
by end-users; it will result in inferior code and has no benefits.
This option may be removed in a future release of G++.
-nostdinc++
Do not search for header files in the standard directories specific
to C++, but do still search the other standard directories. (This
option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline func-
tions; they just won't be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with
the vendor-neutral C++ ABI. Although an effort has been made to
warn about all such cases, there are probably some cases that are
not warned about, even though G++ is generating incompatible code.
There may also be cases where warnings are emitted even though the
code that is generated will be compatible.
You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be
binary compatible with code generated by other compilers.
The known incompatibilities at this point include:
* Incorrect handling of tail-padding for bit-fields. G++ may
attempt to pack data into the same byte as a base class. For
example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte
as"A::f1"; other compilers will not. You can avoid this prob-
lem by explicitly padding "A" so that its size is a multiple of
the byte size on your platform; that will cause G++ and other
compilers to layout "B" identically.
* Incorrect handling of tail-padding for virtual bases. G++ does
not use tail padding when laying out virtual bases. For exam-
ple:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for
"A"; other compilers will. You can avoid this problem by
explicitly padding "A" so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++
and other compilers to layout "C" identically.
* Incorrect handling of bit-fields with declared widths greater
than that of their underlying types, when the bit-fields appear
in a union. For example:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make
the union too small by the number of bits in an "int".
* Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes
that the "A" data member of "B" is already at offset zero.
* Names of template functions whose types involve "typename" or
template template parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable because all the constructors or
destructors in 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 warn-
ing 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.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)
Warn about violations of the following style guidelines from Scott
Meyers' Effective C++ book:
* Item 11: Define a copy constructor and an assignment operator
for classes with dynamically allocated memory.
* Item 12: Prefer initialization to assignment in constructors.
* Item 14: Make destructors virtual in base classes.
* Item 15: Have "operator=" return a reference to *this.
* Item 23: Don't try to return a reference when you must return
an object.
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 incre-
ment 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.
-Wstrict-null-sentinel (C++ only)
Warn also about the use of an uncasted "NULL" as sentinel. When
compiling only with GCC this is a valid sentinel, as "NULL" is
defined to "__null". Although it is a null pointer constant not a
null pointer, it is guaranteed to of the same size as a pointer.
But this use is not portable across different compilers.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared
within a template. Since the advent of explicit template specifi-
cation support in G++, if the name of the friend is an unqualified-
id (i.e., friend foo(int)), the C++ language specification demands
that the friend declare or define an ordinary, nontemplate func-
tion. (Section 14.5.3). Before G++ implemented explicit specifi-
cation, unqualified-ids could be interpreted as a particular spe-
cialization of a templatized function. Because this non-conforming
behavior is no longer the default behavior for G++, -Wnon-tem-
plate-friend allows the compiler to check existing code for poten-
tial trouble spots and is on by default. This new compiler behav-
ior can be turned off with -Wno-non-template-friend which keeps the
conformant compiler code but disables the helpful warning.
-Wold-style-cast (C++ only)
Warn if an old-style (C-style) cast to a non-void type is used
within a C++ program. The new-style casts (dynamic_cast,
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 pre-
serve 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 =.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++
languages themselves. See
This section describes the command-line options that are only meaning-
ful for Objective-C and Objective-C++ programs, but you can also use
most of the language-independent GNU compiler options. For example,
you might compile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C
and Objective-C++ programs; you can use the other options with any lan-
guage supported by GCC.
Note that since Objective-C is an extension of the C language, Objec-
tive-C compilations may also use options specific to the C front-end
(e.g., -Wtraditional). Similarly, Objective-C++ compilations may use
C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and
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 run-
time to be used. Currently, this option is only available in con-
junction with the NeXT runtime on Mac OS X 10.3 and later.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables
is a C++ object with a non-trivial default constructor. If so,
synthesize a special "- (id) .cxx_construct" instance method that
will run non-trivial default constructors on any such instance
variables, in order, and then return "self". Similarly, check if
any instance variable is a C++ object with a non-trivial destruc-
tor, and if so, synthesize a special "- (void) .cxx_destruct"
method that will run all such default destructors, in reverse
order.
The "- (id) .cxx_construct" and/or "- (void) .cxx_destruct" methods
thusly generated will only operate on instance variables declared
in the current Objective-C class, and not those inherited from
superclasses. It is the responsibility of the Objective-C runtime
to invoke all such methods in an object's inheritance hierarchy.
The "- (id) .cxx_construct" methods will be invoked by the runtime
immediately after a new object instance is allocated; the "- (void)
.cxx_destruct" methods will be invoked immediately before the run-
time deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and
later has support for invoking the "- (id) .cxx_construct" and "-
(void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is
accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++ and Java. Cur-
rently, 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 synchroniza-
tion 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 @syn-
chronized blocks is allowed, and will cause the guarding object to
be unlocked properly.
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++
programs.
-freplace-objc-classes
Emit a special marker instructing lldd(1) not to statically link in
the resulting object file, and allow ddyylldd(1) to load it in at run
time instead. This is used in conjunction with the Fix-and-Con-
tinue debugging mode, where the object file in question may be
recompiled and dynamically reloaded in the course of program execu-
tion, 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.
-Wassign-intercept
Warn whenever an Objective-C assignment is being intercepted by the
garbage collector.
-Wno-protocol
If a class is declared to implement a protocol, a warning is issued
for every method in the protocol that is not implemented by the
class. The default behavior is to issue a warning for every method
not explicitly implemented in the class, even if a method implemen-
tation is inherited from the superclass. If you use the -Wno-pro-
tocol option, then methods inherited from the superclass are con-
sidered to be implemented, and no warning is issued for them.
-Wselector
Warn if multiple methods of different types for the same selector
are found during compilation. The check is performed on the list
of methods in the final stage of compilation. Additionally, a
check is performed for each selector appearing in a "@selec-
tor(...)" 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.
-Wstrict-selector-match
Warn if multiple methods with differing argument and/or return
types are found for a given selector when attempting to send a mes-
sage using this selector to a receiver of type "id" or "Class".
When this flag is off (which is the default behavior), the compiler
will omit such warnings if any differences found are confined to
types which share the same size and alignment.
-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 decla-
ration, 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 conven-
tion 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.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of
the output device's aspect (e.g. its width, ...). The options
described below can be used to control the diagnostic messages format-
ting algorithm, e.g. how many characters per line, how often source
location information should be reported. Right now, only the C++ front
end can honor these options. However it is expected, in the near
future, that the remaining front ends would be able to digest them cor-
rectly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n
characters. The default is 72 characters for g++ and 0 for the
rest of the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a
single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit once source location information; that
is, in case the message is too long to fit on a single physical
line and has to be wrapped, the source location won't be emitted
(as prefix) again, over and over, in subsequent continuation lines.
This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location information (as
prefix) for physical lines that result from the process of breaking
a message which is too long to fit on a single line.
-fdiagnostics-show-options
This option instructs the diagnostic machinery to add text to each
diagnostic emitted, which indicates which command line option
directly controls that diagnostic, when such an option is known to
the diagnostic machinery.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are
not inherently erroneous but which are risky or suggest there may have
been an error.
You can request many specific warnings with options beginning -W, for
example -Wimplicit to request warnings on implicit declarations. Each
of these specific warning options also has a negative form beginning
-Wno- to turn off warnings; for example, -Wno-implicit. This manual
lists only one of the two forms, whichever is not the default.
The following options control the amount and kinds of warnings produced
by GCC; for further, language-specific options also refer to C++
Dialect Options and Objective-C 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 ver-
sion of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or
without this option (though a rare few will require -ansi or a -std
option specifying the required version of ISO C). However, without
this option, certain GNU extensions and traditional C and C++ fea-
tures are supported as well. With this option, they are rejected.
-pedantic does not cause warning messages for use of the alternate
keywords whose names begin and end with __. Pedantic warnings are
also disabled in the expression that follows "__extension__". How-
ever, only system header files should use these escape routes;
application programs should avoid them.
Some users try to use -pedantic to check programs for strict ISO C
conformance. They soon find that it does not do quite what they
want: it finds some non-ISO practices, but not all---only those for
which ISO C requires a diagnostic, and some others for which diag-
nostics have been added.
A feature to report any failure to conform to ISO C might be useful
in some instances, but would require considerable additional work
and would be quite different from -pedantic. We don't have plans
to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended
dialect of C, such as gnu89 or gnu99, there is a corresponding base
standard, the version of ISO C on which the GNU extended dialect is
based. Warnings from -pedantic are given where they are required
by the base standard. (It would not make sense for such warnings
to be given only for features not in the specified GNU C dialect,
since by definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be nothing
to warn about.)
-pedantic-errors
Like -pedantic, except that errors are produced rather than warn-
ings.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause
of error, as programmers often forget that this type is signed on
some machines. This warning is enabled by -Wall.
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment,
or whenever a Backslash-Newline appears in a // comment. This
warning is enabled by -Wall.
-Wfatal-errors
This option causes the compiler to abort compilation on the first
error occurred rather than trying to keep going and printing fur-
ther error messages.
-Wformat
Check calls to "printf" and "scanf", etc., to make sure that the
arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string
make sense. This includes standard functions, and others specified
by format attributes, in the "printf", "scanf", "strftime" and
"strfmon" (an X/Open extension, not in the C standard) families (or
other target-specific families). Which functions are checked with-
out format attributes having been specified depends on the standard
version selected, and such checks of functions without the
attribute specified are disabled by -ffreestanding o