From Linux Man Pages
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 remainder. 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 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 use-
ful 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 mul-
tiple 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 positive and negative forms; the negative form of -ffoo would be -fno-foo. This manual
documents only one of these two forms, whichever one is not the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
Overall Options
-c -S -E -o file -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 -ffreestand-
ing -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-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-depre-
cated -Wstrict-null-sentinel -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-con-
versions -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 -Was-
sign-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++-com-
pat -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Wno-deprecated-declarations -Wdis-
abled-optimization -Wno-div-by-zero -Wno-endif-labels -Werror -Werror-implicit-function-declaration -Wfa-
tal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-format-extra-args -Wformat-nonliteral -Wformat-security
-Wformat-y2k -Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Wimport -Wno-import -Winit-self
-Winline -Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than-len -Wunsafe-loop-opti-
mizations -Wlong-long -Wmain -Wmissing-braces -Wmissing-field-initializers -Wmissing-format-attribute
-Wmissing-include-dirs -Wmissing-noreturn -Wno-multichar -Wnonnull -Wpacked -Wpadded -Wparentheses
-Wpointer-arith -Wno-pointer-to-int-cast -Wredundant-decls -Wreturn-type -Wsequence-point -Wshadow
-Wsign-compare -Wstack-protector -Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch -Wswitch-default
-Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized -Wunknown-pragmas -Wno-pragmas -Wun-
reachable-code -Wunused -Wunused-function -Wunused-label -Wunused-parameter -Wunused-value -Wunused-vari-
able -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-original[-n]
-fdump-tree-optimized[-n] -fdump-tree-inlined[-n] -fdump-tree-cfg -fdump-tree-vcg -fdump-tree-alias
-fdump-tree-ch -fdump-tree-ssa[-n] -fdump-tree-pre[-n] -fdump-tree-ccp[-n] -fdump-tree-dce[-n]
-fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n] -fdump-tree-dom[-n] -fdump-tree-dse[-n]
-fdump-tree-phiopt[-n] -fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n] -fdump-tree-nrv -fdump-tree-vect
-fdump-tree-sink -fdump-tree-sra[-n] -fdump-tree-salias -fdump-tree-fre[-n] -fdump-tree-vrp[-n] -ftree-vec-
torizer-verbose=n -fdump-tree-storeccp[-n] -feliminate-dwarf2-dups -feliminate-unused-debug-types -felimi-
nate-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-target-load-optimize -fbranch-tar-
get-load-optimize2 -fbtr-bb-exclusive -fcaller-saves -fcprop-registers -fcse-follow-jumps -fcse-skip-blocks
-fcx-limited-range -fdata-sections -fdelayed-branch -fdelete-null-pointer-checks -fearly-inlining -fexpen-
sive-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 -fin-
line-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 -funsafe-loop-opti-
mizations -ffinite-math-only -fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer -fopti-
mize-register-move -foptimize-sibling-calls -fprefetch-loop-arrays -fprofile-generate -fprofile-use -freg-
move -frename-registers -freorder-blocks -freorder-blocks-and-partition -freorder-functions -fre-
run-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
-fsched-stalled-insns-dep=n -fsched2-use-superblocks -fsched2-use-traces -freschedule-modulo-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-vector-
ize -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 -nostdlib -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-prolog -mno-sched-prolog -mlittle-endian
-mbig-endian -mwords-little-endian -mfloat-abi=name -msoft-float -mhard-float -mfpe -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name -mfpu=name -mstructure-size-boundary=n -mabort-on-noreturn
-mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport
-mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-function-name -mthumb -marm -mtpcs-frame
-mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking -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
-bundle_loader -client_name -compatibility_version -current_version -dead_strip -dependency-file
-dylib_file -dylinker_install_name -dynamic -dynamiclib -exported_symbols_list -filelist -flat_namespace
-force_cpusubtype_ALL -force_flat_namespace -headerpad_max_install_names -image_base -init -install_name
-keep_private_externs -multi_module -multiply_defined -multiply_defined_unused -noall_load
-no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs -noprebind -noseglinkedit -pagezero_size
-prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs -sectalign -sectobjectsymbols
-whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder -segaddr -segs_read_only_addr
-segs_read_write_addr -seg_addr_table -seg_addr_table_filename -seglinkedit -segprot -segs_read_only_addr
-segs_read_write_addr -single_module -static -sub_library -sub_umbrella -twolevel_namespace -umbrella
-undefined -unexported_symbols_list -weak_reference_mismatches -whatsloaded -F -gused -gfull -mmacosx-ver-
sion-min=version -mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -malpha-as -mgas -mieee -mieee-with-inexact -mieee-confor-
mant -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-membar -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 -mdisable-fpregs -mdisable-indexing -mfast-indi-
rect-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-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 -munix=unix-std -nolibdld -static -threads
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 -mpre-
ferred-stack-boundary=num -mmmx -msse -msse2 -msse3 -mssse3 -m3dnow -mthreads -mno-align-stringops -min-
line-all-stringops -mpush-args -maccumulate-outgoing-args -m128bit-long-double -m96bit-long-double -mreg-
parm=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 -mregis-
ter-names -mno-sdata -mconstant-gp -mauto-pic -minline-float-divide-min-latency -min-
line-float-divide-max-throughput -minline-int-divide-min-latency -minline-int-divide-max-throughput -min-
line-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
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 -mrelax-immediates -mno-relax-immediates -mwide-bit-
fields -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-division -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-exceptions -mvr4130-align -mno-vr4130-align
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
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 -mabshi -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 -mpow-
erpc -mpowerpc64 -mno-powerpc -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt
-mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -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 -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 -maltivec -mswdiv -mprior-
itize-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 -mdebug -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-com-
pact-nofpu -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave
-mieee -misize -mpadstruct -mspace -mprefergot -musermode -multcost=number -mdiv=strategy -mdivsi3_lib-
func=name -madjust-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 -mmemparm -mfast-fix -mmpyi -mbk -mti
-mdp-isr-reload -mrpts=count -mrptb -mdb -mloop-unsigned -mparallel-insns -mparallel-mpy -mpre-
serve-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
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-sec-
tion-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 -fasyn-
chronous-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 -fargu-
ment-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 com-
bines 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:
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 subpro-
gram 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
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 numerically highest
error produced by any phase that returned an error indication.
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to
start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for exam-
ple, -x cpp-output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate
output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code
file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of prepro-
cessed 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 prepro-
cessed C source on standard output.
-v Print (on standard error output) the commands executed to run the stages of compilation. Also print the ver-
sion number of the compiler driver program and of the preprocessor and the compiler proper.
-###
Like -v except the commands are not executed and all command arguments are quoted. This is useful for shell
scripts to capture the driver-generated command lines.
-pipe
Use pipes rather than temporary files for communication between the various stages of compilation. This
fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no
trouble.
-combine
If you are compiling multiple source files, this option tells the driver to pass all the source files to the
compiler at once (for those languages for which the compiler can handle this). This will allow intermodule
analysis (IMA) to be performed by the compiler. Currently the only language for which this is supported is
C. If you pass source files for multiple languages to the driver, using this option, the driver will invoke
the compiler(s) that support IMA once each, passing each compiler all the source files appropriate for it.
For those languages that do not support IMA this option will be ignored, and the compiler will be invoked
once for each source file in that language. If you use this option in conjunction with -save-temps, the com-
piler will generate multiple pre-processed files (one for each source file), but only one (combined) .o or .s
file.
--help
Print (on the standard output) a description of the command line options understood by gcc. If the -v option
is also specified then --help will also be passed on to the various processes invoked by gcc, so that they
can display the command line options they accept. If the -Wextra option is also specified then command line
options which have no documentation associated with them will also be displayed.
--target-help
Print (on the standard output) a description of target specific command line options for each tool.
--version
Display the version number and copyrights of the invoked GCC.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files
often use .hh or .H; and preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and
compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually
with the name gcc).
However, 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.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objec-
tive-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-4.1/c99sta-
tus.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 transla-
tion 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 defini-
tion (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 exam-
ple, warnings are given with -Wformat for bad calls to "printf", when "printf" is built in, and "strlen" is
known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must not
begin with __builtin_. If a function is named this is not built-in in this version of GCC, this option is
ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions
selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation takes place in a hosted environment. This implies -fbuiltin. A hosted environment
is one in which the entire standard library is available, and in which "main" has a return type of "int".
Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding environment. This implies -fno-builtin. A freestand-
ing 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 -tri-
graphs.
-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 sup-
ported with the -E switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP man-
ual 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 signed-
ness 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 inte-
ger types such as "int" are signed types.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++ programs; but you can also use
most of the GNU compiler options regardless of what language your program is in. For example, you might compile
a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use the other options with any
language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4. Ver-
sion 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 allo-
cated. 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 with-
out 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++ Ref-
erence 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 standard. Even if you use -fno-const-strings, you cannot actually modify the value of a
string constant.
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 generate code to 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. 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 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 diag-
nostic 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 gen-
erate 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.)
In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline functions; they just won't be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Although an
effort has been made to warn about all such cases, there are probably some cases that are not warned about,
even though G++ is generating incompatible code. There may also be cases where warnings are emitted even
though the code that is generated will be compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated
by G++ may not be binary compatible with code generated by other compilers.
The known incompatibilities at this point include:
* Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same byte as a
base class. For example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte as"A::f1"; other compilers will not. You can
avoid this problem by explicitly padding "A" so that its size is a multiple of the byte size on your
platform; that will cause G++ and other compilers to layout "B" identically.
* Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out vir-
tual 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 mem-
bers, 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 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.
-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 sen-
tinel, 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 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
(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 preserve unsigned-
ness, 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 meaningful for Objective-C and Objective-C++ pro-
grams, but you can also use most of the language-independent GNU compiler options. For example, you might com-
pile 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 language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options spe-
cific 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 "NSCon-
stantString" 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 con-
stant 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 Dar-
win 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-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 destructor, 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 hier-
archy. 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 runtime
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. 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, analo-
gously 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 "NSEx-
ception" 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 prop-
erly.
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++ programs.
-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.
-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 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.
-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 message 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 con-
fined 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 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.
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 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 loca-
tion 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 machin-
ery.
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 exten-
sions, 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, cer-
tain 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 sys-
tem 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 con-
siderable 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 warnings.
-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 further 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 without format attributes having been specified depends on the standard version
selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or
-fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO
C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions.
Other library implementations may not support all these features; GCC does not support warning about features
that go beyond a particular library's limitations. However, if -pedantic is used with -Wformat, warnings
will be given about format features not in the selected standard version (but not for "strfmon" formats,
since those are not in any version of the C standard).
Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.
-Wformat is included in -Wall. For more control over some aspects of format checking, the options -Wfor-
mat-y2k, -Wno-format-extra-args, -Wno-format-zero-length, -Wformat-nonliteral, -Wformat-security, and -Wfor-
mat=2 are available, but are not included in -Wall.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats which may yield only a two-digit year.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a "printf" or "scanf" format function. The C
standard specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are specified with $ operand number specifica-
tions, normally warnings are still given, since the implementation could not know what type to pass to
"va_arg" to skip the unused arguments. However, in the case of "scanf" formats, this option will suppress
the warning if the unused arguments are all pointers, since the Single Unix Specification says that such
unused arguments are allowed.
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length
formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked,
unless the format function takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions that represent possible security problems.
At present, this warns about calls to "printf" and "scanf" functions where the format string is not a string
literal and there are no format arguments, as in "printf (foo);". This may be a security hole if the format
string came from untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral
warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-non-
literal.)
-Wformat=2
Enable -Wformat plus format checks not included in -Wformat. Currently equivalent to -Wformat -Wformat-non-
literal -Wformat-security -Wformat-y2k.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a non-null value by the "nonnull" func-
tion attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables which are initialized with themselves. Note this option can only be used
with the -Wuninitialized option, which in turn only works with -O1 and above.
For example, GCC will warn about "i" being uninitialized in the following snippet only when -Winit-self has
been specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int
Warn when a declaration does not specify a type. This warning is enabled by -Wall.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being declared. The form
-Wno-error-implicit-function-declaration is not supported. This warning is enabled by -Wall (as a warning,
not an error).
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
-Wmain
Warn if the type of main is suspicious. main should be a function with external linkage, returning int, tak-
ing either zero arguments, two, or three arguments of appropriate types. This warning is enabled by -Wall.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer
for a is not fully bracketed, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a
truth value is expected, or when operators are nested whose precedence people often get confused about. Only
the warning for an assignment used as a truth value is supported when compiling C++; the other warnings are
only supported when compiling C.
Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a differ-
ent interpretation from that of ordinary mathematical notation.
Also warn about constructions where there may be confusion to which "if" statement an "else" branch belongs.
Here is an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every "else" branch belongs to the innermost possible "if" statement, which in this example is "if
(b)". This is often not what the programmer expected, as illustrated in the above example by indentation the
programmer chose. When there is the potential for this confusion, GCC will issue a warning when this flag is
specified. To eliminate the warning, add explicit braces around the innermost "if" statement so there is no
way the "else" could belong to the enclosing "if". The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of violations of sequence point rules in the C
standard.
The C standard defines the order in which expressions in a C program are evaluated in terms of sequence
points, which represent a partial ordering between the execution of parts of the program: those executed
before the sequence point, and those executed after it. These occur after the evaluation of a full expres-
sion (one which is not part of a larger expression), after the evaluation of the first operand of a "&&",
"||", "? :" or "," (comma) operator, before a function is called (but after the evaluation of its arguments
and the expression denoting the called function), and in certain other places. Other than as expressed by
the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All
these rules describe only a partial order rather than a total order, since, for example, if two functions are
called within one expression with no sequence point between them, the order in which the functions are called
is not specified. However, the standards committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect. Pro-
grams whose behavior depends on this have undefined behavior; the C standard specifies that "Between the pre-
vious and next sequence point an object shall have its stored value modified at most once by the evaluation
of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.". If
a program breaks these rules, the results on any particular implementation are entirely unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and "a[i++] = i;". Some more com-
plicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in
general it has been found fairly effective at detecting this sort of problem in programs.
The present implementation of this option only works for C programs. A future implementation may also work
for C++ programs.
The C standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence
point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may
be found on the GCC readings page, at <http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall.
-Wreturn-type
Warn whenever a function is defined with a return-type that defaults to "int". Also warn about any "return"
statement with no return-value in a function whose return-type is not "void".
For C, also warn if the return type of a function has a type qualifier such as "const". Such a type quali-
fier has no effect, since the value returned by a function is not an lvalue. ISO C prohibits qualified
"void" return types on function definitions, so such return types always receive a warning even without this
option.
For C++, a function without return type always produces a diagnostic message, even when -Wno-return-type is
specified. The only exceptions are main and functions defined in system headers.
This warning is enabled by -Wall.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the
named codes of that enumeration. (The presence of a "default" label prevents this warning.) "case" labels
outside the enumeration range also provoke warnings when this option is used. This warning is enabled by
-Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the
named codes of that enumeration. "case" labels outside the enumeration range also provoke warnings when this
option is used.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within comments
are not warned about). This warning is enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a non-inline static function is unused. This
warning is enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused aside from its declaration. This
warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used. This warning is enabled by -Wall.
To suppress this warning cast the expression to void.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note
that -Wall implies -Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a
"setjmp" call.
These warnings are possible only in optimizing compilation, because they require data flow information that
is computed only when optimizing. If you don't specify -O, you simply won't get these warnings.
If you want to warn about code which uses the uninitialized value of the variable in its own initializer, use
the -Winit-self option.
These warnings occur for individual uninitialized or clobbered elements of structure, union or array vari-
ables as well as for variables which are uninitialized or clobbered as a whole. They do not occur for vari-
ables or elements declared "volatile". Because these warnings depend on optimization, the exact variables or
elements for which there are warnings will depend on the precise optimization options and version of GCC
used.
Note that there may be no warning about a variable that is used only to compute a value that itself is never
used, because such computations may be deleted by data flow analysis before the warnings are printed.
These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be
correct despite appearing to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn't know this. Here is
another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because "save_y" is used only if it is set.
This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp". These
warnings as well are possible only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be called; in fact, a sig-
nal handler could call it at any point in the code. As a result, you may get a warning even when there is in
fact no problem because "longjmp" cannot in fact be called at the place which would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return as "nore-
turn".
This warning is enabled by -Wall.
-Wunknown-pragmas
Warn when a #pragma directive is encountered which is not understood by GCC. If this command line option is
used, warnings will even be issued for unknown pragmas in system header files. This is not the case if the
warnings were only enabled by the -Wall command line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between
pragmas. See also -Wunknown-pragmas.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It warns about code which might break the
strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but
does attempt to catch the more common pitfalls. It is included in -Wall.
-Wstrict-aliasing=2
This option is only active when -fstrict-aliasing is active. It warns about code which might break the
strict aliasing rules that the compiler is using for optimization. This warning catches more cases than
-Wstrict-aliasing, but it will also give a warning for some ambiguous cases that are safe.
-Wall
All of the above -W options combined. This enables all the warnings about constructions that some users con-
sider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with
macros. This also enables some language-specific warnings described in C++ Dialect Options and Objective-C
and Objective-C++ Dialect Options.
The following -W... options are not implied by -Wall. Some of them warn about constructions that users generally
do not consider questionable, but which occasionally you might wish to check for; others warn about constructions
that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the
warning.
-Wextra
(This option used to be called -W. The older name is still supported, but the newer name is more descrip-
tive.) Print extra warning messages for these events:
* A function can return either with or without a value. (Falling off the end of the function body is con-
sidered returning without a value.) For example, this function would evoke such a warning:
foo (a)
{
if (a > 0)
return a;
}
* An expression-statement or the left-hand side of a comma expression contains no side effects. To sup-
press the warning, cast the unused expression to void. For example, an expression such as x[i,j] will
cause a warning, but x[(void)i,j] will not.
* An unsigned value is compared against zero with < or >=.
* Storage-class specifiers like "static" are not the first things in a declaration. According to the C
Standard, this usage is obsolescent.
* If -Wall or -Wunused is also specified, warn about unused arguments.
* A comparison between signed and unsigned values could produce an incorrect result when the signed value
is converted to unsigned. (But don't warn if -Wno-sign-compare is also specified.)
* An aggregate has an initializer which does not initialize all members. This warning can be independently
controlled by -Wmissing-field-initializers.
* A function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }
* An empty body occurs in an if or else statement.
* A pointer is compared against integer zero with <, <=, >, or >=.
* A variable might be changed by longjmp or vfork.
* Any of several floating-point events that often indicate errors, such as overflow, underflow, loss of
precision, etc.
*<(C++ only)>
An enumerator and a non-enumerator both appear in a conditional expression.
*<(C++ only)>
A non-static reference or non-static const member appears in a class without constructors.
*<(C++ only)>
Ambiguous virtual bases.
*<(C++ only)>
Subscripting an array which has been declared register.
*<(C++ only)>
Taking the address of a variable which has been declared register.
*<(C++ only)>
A base class is not initialized in a derived class' copy constructor.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating point division by zero is not warned
about, as it can be a legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from system headers are nor-
mally suppressed, on the assumption that they usually do not indicate real problems and would only make the
compiler output harder to read. Using this command line option tells GCC to emit warnings from system head-
ers as if they occurred in user code. However, note that using -Wall in conjunction with this option will
not warn about unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.
-Wfloat-equal
Warn if floating point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point val-
ues as approximations to infinitely precise real numbers. If you are doing this, then you need to compute
(by analyzing the code, or in some other way) the maximum or likely maximum error that the computation intro-
duces, and allow for it when performing comparisons (and when producing output, but that's a different prob-
lem). In particular, instead of testing for equality, you would check to see whether the two values have
ranges that overlap; and this is done with the relational operators, so equality comparisons are probably
mistaken.
-Wtraditional (C only)
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C con-
structs that have no traditional C equivalent, and/or problematic constructs which should be avoided.
* Macro parameters that appear within string literals in the macro body. In traditional C macro replace-
ment takes place within string literals, but does not in ISO C.
* In traditional C, some preprocessor directives did not exist. Traditional preprocessors would only con-
sider a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns
about directives that traditional C understands but would ignore because the # does not appear as the
first character on the line. It also suggests you hide directives like #pragma not understood by tradi-
tional C by indenting them. Some traditional implementations would not recognize #elif, so it suggests
avoiding it altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating point constant suffixes. (Traditional C does sup-
port the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system
headers of most modern systems, e.g. the _MIN/_MAX macros in "<limits.h>". Use of these macros in user
code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context
to avoid warning in these cases.
* A function declared external in one block and then used after the end of the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static" one. This construct is not accepted by some tra-
ditional C compilers.
* The ISO type of an integer constant has a different width or signedness from its traditional type. This
warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typi-
cally represent bit patterns, are not warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
* Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the
assumption that the zero initializer in user code appears conditioned on e.g. "__STDC__" to avoid missing
initializer warnings and relies on default initialization to zero in the traditional C case.
* Conversions by prototypes between fixed/floating point values and vice versa. The absence of these pro-
totypes when compiling with traditional C would cause serious problems. This is a subset of the possible
conversion warnings, for the full set use -Wconversion.
* Use of ISO C style function definitions. This warning intentionally is not issued for prototype declara-
tions or variadic functions because these ISO C features will appear in your code when using libiberty's
traditional C compatibility macros, "PARAMS" and "VPARAMS". This warning is also bypassed for nested
functions because that feature is already a GCC extension and thus not relevant to traditional C compati-
bility.
-Wdeclaration-after-statement (C only)
Warn when a declaration is found after a statement in a block. This construct, known from C++, was intro-
duced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90 and was not supported by
GCC versions before GCC 3.0.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable shadows another local variable, parameter or global variable or whenever a
built-in function is shadowed.
-Wlarger-than-len
Warn whenever a