GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c│-S│-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below for the 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 assem-
bler 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 pro-
grams; when an option is only useful with another language (usually C++), the
explanation says so explicitly. If the description for a particular option does
not mention a source language, you can use that option with all supported lan-
guages.
The gcc program accepts options and file names as operands. Many options have
multi-letter names; therefore multiple single-letter options may not be grouped:
-dr is very different from -d -r.
You can mix options and other arguments. For the most part, the order you use
doesn’t matter. Order does matter when you use several options of the same kind;
for example, if you specify -L more than once, the directories are searched in the
order specified.
Many options have long names starting with -f or with -W---for example,
-fforce-mem, -fstrength-reduce, -Wformat and so on. Most of these have both posi-
tive and negative forms; the negative form of -ffoo would be -fno-foo. This manual
documents only one of these two forms, whichever one is not the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the
following sections.
Overall Options
-c -S -E -o file -combine -pipe -pass-exit-codes -x language -v -###
--help --target-help --version
C Language Options
-ansi -std=standard -aux-info filename -fno-asm -fno-builtin
-fno-builtin-function -fhosted -ffreestanding -fms-extensions -trigraphs
-no-integrated-cpp -traditional -traditional-cpp -fallow-single-precision
-fcond-mismatch -fsigned-bitfields -fsigned-char -funsigned-bitfields -fun-
signed-char
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space
-fno-const-strings -fno-elide-constructors -fno-enforce-eh-specs -ffor-scope
-fno-for-scope -fno-gnu-keywords -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines -fms-extensions
-fno-nonansi-builtins -fno-operator-names -fno-optional-diags -fpermissive
-frepo -fno-rtti -fstats -ftemplate-depth-n -fno-threadsafe-statics
-fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline -fvisibil-
ity-inlines-hidden -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder
-Weffc++ -Wno-deprecated -Wstrict-null-sentinel -Wno-non-template-friend
-Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers -fobjc-exceptions -freplace-objc-classes -fzero-link
-gen-decls -Wno-protocol -Wselector -Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once│every-line]
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall -Waggre-
gate-return -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wconver-
sion -Wno-deprecated-declarations -Wdisabled-optimization -Wno-div-by-zero
-Wno-endif-labels -Werror -Werror-implicit-function-declaration -Wfatal-errors
-Wfloat-equal -Wformat -Wformat=2 -Wno-format-extra-args -Wformat-nonliteral
-Wformat-security -Wformat-y2k -Wimplicit -Wimplicit-function-declaration
-Wimplicit-int -Wimport -Wno-import -Winit-self -Winline -Wno-invalid-off-
setof -Winvalid-pch -Wlarger-than-len -Wlong-long -Wmain -Wmissing-braces
-Wmissing-field-initializers -Wmissing-format-attribute -Wmissing-include-dirs
-Wmissing-noreturn -Wno-multichar -Wnonnull -Wpacked -Wpadded -Wparentheses
-Wpointer-arith -Wredundant-decls -Wreturn-type -Wsequence-point -Wshadow
-Wsign-compare -Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch
-Wswitch-default -Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef
-Wuninitialized -Wunknown-pragmas -Wunreachable-code -Wunused -Wunused-func-
tion -Wunused-label -Wunused-parameter -Wunused-value -Wunused-variable
-Wwrite-strings -Wvariadic-macros
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes
-Wnested-externs -Wold-style-definition -Wstrict-prototypes -Wtraditional
-Wdeclaration-after-statement -Wno-pointer-sign
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered
-fdump-translation-unit[-n] -fdump-class-hierarchy[-n] -fdump-ipa-all
-fdump-ipa-cgraph -fdump-tree-all -fdump-tree-original[-n] -fdump-tree-opti-
mized[-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-sra[-n] -fdump-tree-fre[-n]
-ftree-vectorizer-verbose=n -feliminate-dwarf2-dups -felimi-
nate-unused-debug-types -feliminate-unused-debug-symbols -fmem-report -fpro-
file-arcs -ftree-based-profiling -frandom-seed=string -fsched-verbose=n
-ftest-coverage -ftime-report -fvar-tracking -g -glevel -gcoff -gdwarf-2
-ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ -p -pg
-print-file-name=library -print-libgcc-file-name -print-multi-directory
-print-multi-lib -print-prog-name=program -print-search-dirs -Q -save-temps
-time
Optimization Options
-falign-functions=n -falign-jumps=n -falign-labels=n -falign-loops=n
-fbounds-check -fmudflap -fmudflapth -fmudflapir -fbranch-probabilities
-fprofile-values -fvpt -fbranch-target-load-optimize -fbranch-target-load-opti-
mize2 -fbtr-bb-exclusive -fcaller-saves -fcprop-registers -fcse-follow-jumps
-fcse-skip-blocks -fcx-limited-range -fdata-sections -fdelayed-branch
-fdelete-null-pointer-checks -fexpensive-optimizations -ffast-math
-ffloat-store -fforce-addr -fforce-mem -ffunction-sections -fgcse -fgcse-lm
-fgcse-sm -fgcse-las -fgcse-after-reload -floop-optimize -fcrossjumping
-fif-conversion -fif-conversion2 -finline-functions -finline-func-
tions-called-once -finline-limit=n -fkeep-inline-functions -fkeep-static-con-
sts -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-opti-
mizations -ffinite-math-only -fno-trapping-math -fno-zero-initialized-in-bss
-fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls
-fprefetch-loop-arrays -fprofile-generate -fprofile-use -fregmove -fre-
name-registers -freorder-blocks -freorder-blocks-and-partition -freorder-func-
tions -frerun-cse-after-loop -frerun-loop-opt -frounding-math -fschedule-insns
-fschedule-insns2 -fno-sched-interblock -fno-sched-spec -fsched-spec-load
-fsched-spec-load-dangerous -fsched-stalled-insns=n -sched-stalled-insns-dep=n
-fsched2-use-superblocks -fsched2-use-traces -freschedule-modulo-sched-
uled-loops -fsignaling-nans -fsingle-precision-constant -fspecula-
tive-prefetching -fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps
-funroll-all-loops -funroll-loops -fpeel-loops -fsplit-ivs-in-unroller
-funswitch-loops -fvariable-expansion-in-unroller -ftree-pre -ftree-ccp
-ftree-dce -ftree-loop-optimize -ftree-loop-linear -ftree-loop-im
-ftree-loop-ivcanon -fivopts -ftree-dominator-opts -ftree-dse -ftree-copyrename
-ftree-ch -ftree-sra -ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize -fweb
--param name=value -O -O0 -O1 -O2 -O3 -Os
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn]
-E -H -idirafter dir -include file -imacros file -iprefix file -iwithprefix
dir -iwithprefixbefore dir -isystem dir -M -MM -MF -MG -MP -MQ -MT
-nostdinc -P -fworking-directory -remap -trigraphs -undef -Umacro
-Wp,option -Xpreprocessor option
Assembler Option
-Wa,option -Xassembler option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -s
-static -static-libgcc -shared -shared-libgcc -symbolic -Wl,option
-Xlinker option -u symbol
Directory Options
-Bprefix -Idir -iquotedir -Ldir -specs=file -I-
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-cir-
rus-fix-invalid-insns -mpoke-function-name -mthumb -marm -mtpcs-frame
-mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking
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
Darwin Options -all_load -allowable_client -arch -arch_errors_fatal
-arch_only -bind_at_load -bundle -bundle_loader -client_name -compatibil-
ity_version -current_version -dead_strip -dependency-file -dylib_file
-dylinker_install_name -dynamic -dynamiclib -exported_symbols_list -filelist
-flat_namespace -force_cpusubtype_ALL -force_flat_namespace -header-
pad_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 -pri-
vate_bundle -read_only_relocs -sectalign -sectobjectsymbols -whyload
-seg1addr -sectcreate -sectobjectsymbols -sectorder -segaddr
-segs_read_only_addr -segs_read_write_addr -seg_addr_table -seg_addr_ta-
ble_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_ref-
erence_mismatches -whatsloaded -F -gused -gfull -mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -malpha-as -mgas -mieee
-mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode -mfp-round-
ing-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu-type
-mtune=cpu-type -mbwx -mmax -mfix -mcix -mfloat-vax -mfloat-ieee -mex-
plicit-relocs -msmall-data -mlarge-data -msmall-text -mlarge-text -mmem-
ory-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 -multi-
lib-library-pic -mlinked-fp -mlong-calls -malign-labels -mlibrary-pic
-macc-4 -macc-8 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move
-mscc -mno-scc -mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch
-mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec
-mno-nested-cond-exec -mtomcat-stats -mTLS -mtls -mcpu=cpu
H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300
HPPA Options -march=architecture-type -mbig-switch -mdisable-fpregs -mdis-
able-indexing -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld
-mfixed-range=register-range -mjump-in-delay -mlinker-opt -mlong-calls
-mlong-load-store -mno-big-switch -mno-disable-fpregs -mno-disable-indexing
-mno-fast-indirect-calls -mno-gas -mno-jump-in-delay -mno-long-load-store
-mno-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 -mpreferred-stack-bound-
ary=num -mmmx -msse -msse2 -msse3 -m3dnow -mthreads -mno-align-stringops
-minline-all-stringops -mpush-args -maccumulate-outgoing-args
-m128bit-long-double -m96bit-long-double -mregparm=num
-momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-
model -m32 -m64
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic
-mvolatile-asm-stop -mregister-names -mno-sdata -mconstant-gp -mauto-pic
-minline-float-divide-min-latency -minline-float-divide-max-throughput -min-
line-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 -mis-
sue-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
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-bitfields -mno-wide-bitfields -m4byte-functions
-mno-4byte-functions -mcallgraph-data -mno-callgraph-data -mslow-bytes
-mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340
-mstack-increment
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4
-mips32 -mips32r2 -mips64 -mips16 -mno-mips16 -mabi=abi -mabicalls
-mno-abicalls -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 -mhard-float
-msoft-float -msingle-float -mdouble-float -mpaired-single -mips3d -mint64
-mlong64 -mlong32 -msym32 -mno-sym32 -Gnum -membedded-data -mno-embed-
ded-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 -mno-crt0 -mrelax
NS32K Options -m32032 -m32332 -m32532 -m32081 -m32381 -mmult-add -mno-
mult-add -msoft-float -mrtd -mnortd -mregparam -mnoregparam -msb -mnosb
-mbitfield -mnobitfield -mhimem -mnohimem
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 -mpowerpc -mpowerpc64 -mno-powerpc -maltivec
-mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-pow-
erpc-gfxopt -mnew-mnemonics -mold-mnemonics -mfull-toc -mminimal-toc
-mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe
-malign-power -malign-natural -msoft-float -mhard-float -mmultiple
-mno-multiple -mstring -mno-string -mupdate -mno-update -mfused-madd
-mno-fused-madd -mbit-align -mno-bit-align -mstrict-align -mno-strict-align
-mrelocatable -mno-relocatable -mrelocatable-lib -mno-relocatable-lib -mtoc
-mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -mdynamic-no-pic
-mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type -min-
sert-sched-nops=scheme -mcall-sysv -mcall-netbsd -maix-struct-return
-msvr4-struct-return -mabi=altivec -mabi=no-altivec -mabi=spe -mabi=no-spe
-misel=yes -misel=no -mspe=yes -mspe=no -mfloat-gprs=yes -mfloat-gprs=no
-mfloat-gprs=single -mfloat-gprs=double -mprototype -mno-prototype -msim
-mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -mwindiss
-G num -pthread
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float
-msoft-float -mbackchain -mno-backchain -mpacked-stack -mno-packed-stack
-msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug
-mno-debug -mesa -mzarch -mtpf-trace -mno-tpf-trace -mfused-madd
-mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size
-mstack-guard
SH Options -m1 -m2 -m2e -m3 -m3e -m4-nofpu -m4-single-only -m4-single
-m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -m5-64media
-m5-64media-nofpu -m5-32media -m5-32media-nofpu -m5-compact -m5-compact-nofpu
-mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mrenesas -mno-rene-
sas -mnomacsave -mieee -misize -mpadstruct -mspace -mprefergot -musermode
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
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 -mpreserve-float
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function
-mno-prolog-function -mspace -mtda=n -msda=n -mzda=n -mapp-regs
-mno-app-regs -mdisable-callt -mno-disable-callt -mv850e1 -mv850e -mv850
-mbig-switch
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-sec-
tion-literals -mno-text-section-literals -mtarget-align -mno-target-align
-mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-excep-
tions -funwind-tables -fasynchronous-unwind-tables -finhibit-size-directive
-finstrument-functions -fno-common -fno-ident -fpcc-struct-return -fpic
-fPIC -fpie -fPIE -freg-struct-return -fshared-data -fshort-enums
-fshort-double -fshort-wchar -fverbose-asm -fpack-struct[=n] -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym -fargument-alias
-fargument-noalias -fargument-noalias-global -fleading-underscore
-ftls-model=model -ftrapv -fwrapv -fbounds-check -fvisibility
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper,
assembly and linking, always in that order. GCC is capable of preprocessing and
compiling several files either into several assembler input files, or into one
assembler input file; then each assembler input file produces an object file, and
linking combines all the object files (those newly compiled, and those specified as
input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation
is done:
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 precom-
piled header.
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code which must be preprocessed. Note that in .cxx, the last two
letters must both be literally x. Likewise, .C refers to a literal capital C.
file.hh
file.H
C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR
Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fortran source code which must be preprocessed (with the traditional preproces-
sor).
file.r
Fortran source code which must be preprocessed with a RATFOR preprocessor (not
included with GCC).
file.f90
file.f95
Fortran 90/95 source code which should not be preprocessed.
file.ads
Ada source code file which contains a library unit declaration (a declaration
of a package, subprogram, or generic, or a generic instantiation), or a library
unit renaming declaration (a package, generic, or subprogram renaming declara-
tion). 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 recog-
nized 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 let-
ting the compiler choose a default based on the file name suffix). This option
applies to all following input files until the next -x option. Possible values
for language are:
c c-header c-cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
f95
java
treelang
-x none
Turn off any specification of a language, so that subsequent files are handled
according to their file name suffixes (as they are if -x has not been used at
all).
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase of the com-
piler returns a non-success return code. If you specify -pass-exit-codes, the
gcc program will instead return with numerically highest error produced by any
phase that returned an error indication.
If you only want some of the stages of compilation, you can use -x (or filename
suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say
where gcc is to stop. Note that some combinations (for example, -x cpp-output -E)
instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking stage sim-
ply is not done. The ultimate output is in the form of an object file for each
source file.
By default, the object file name for a source file is made by replacing the
suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S Stop after the stage of compilation proper; do not assemble. The output is in
the form of an assembler code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the
suffix .c, .i, etc., with .s.
Input files that don’t require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper. The output
is in the form of preprocessed source code, which is sent to the standard out-
put.
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 assem-
bler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the
object file for source.suffix in source.o, its assembler file in source.s, a
precompiled header file in source.suffix.gch, and all preprocessed C source on
standard output.
-v Print (on standard error output) the commands executed to run the stages of
compilation. Also print the version number of the compiler driver program and
of the preprocessor and the compiler proper.
-###
Like -v except the commands are not executed and all command arguments are
quoted. This is useful for shell scripts to capture the driver-generated com-
mand lines.
-pipe
Use pipes rather than temporary files for communication between the various
stages of compilation. This fails to work on some systems where the assembler
is unable to read from a pipe; but the GNU assembler has no trouble.
-combine
If you are compiling multiple source files, this option tells the driver to
pass all the source files to the compiler at once (for those languages for
which the compiler can handle this). This will allow intermodule analysis
(IMA) to be performed by the compiler. Currently the only language for which
this is supported is C. If you pass source files for multiple languages to the
driver, using this option, the driver will invoke the compiler(s) that support
IMA once each, passing each compiler all the source files appropriate for it.
For those languages that do not support IMA this option will be ignored, and
the compiler will be invoked once for each source file in that language. If
you use this option in conjunction with -save-temps, the compiler will generate
multiple pre-processed files (one for each source file), but only one (com-
bined) .o or .s file.
--help
Print (on the standard output) a description of the command line options under-
stood 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 com-
pile 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 Objective-C++) that the compiler accepts:
-ansi
In C mode, support all ISO C90 programs. In C++ mode, remove GNU extensions
that conflict with ISO C++.
This turns off certain features of GCC that are incompatible with ISO C90 (when
compiling C code), or of standard C++ (when compiling C++ code), such as the
"asm" and "typeof" keywords, and predefined macros such as "unix" and "vax"
that identify the type of system you are using. It also enables the undesir-
able 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-4.0/c99status.html> for more information. The
names c9x and iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including some C99 features).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
this will become the default. The name gnu9x is deprecated.
c++98
The 1998 ISO C++ standard plus amendments.
gnu++98
The same as -std=c++98 plus GNU extensions. This is the default for C++
code.
Even when this option is not specified, you can still use some of the features
of newer standards in so far as they do not conflict with previous C standards.
For example, you may use "__restrict__" even when -std=c99 is not specified.
The -std options specifying some version of ISO C have the same effects as
-ansi, except that features that were not in ISO C90 but are in the specified
version (for example, // comments and the "inline" keyword in ISO C99) are not
disabled.
-aux-info filename
Output to the given filename prototyped declarations for all functions declared
and/or defined in a translation unit, including those in header files. This
option is silently ignored in any language other than C.
Besides declarations, the file indicates, in comments, the origin of each dec-
laration (source file and line), whether the declaration was implicit, proto-
typed 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 dec-
laration or a definition (C or F, respectively, in the following character).
In the case of function definitions, a K&R-style list of arguments followed by
their declarations is also provided, inside comments, after the declaration.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use
these words as identifiers. You can use the keywords "__asm__", "__inline__"
and "__typeof__" instead. -ansi implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm" and "inline"
are standard keywords. You may want to use the -fno-gnu-keywords flag instead,
which has the same effect. In C99 mode (-std=c99 or -std=gnu99), this switch
only affects the "asm" and "typeof" keywords, since "inline" is a standard key-
word in ISO C99.
-fno-builtin
-fno-builtin-function
Don’t recognize built-in functions that do not begin with __builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more
efficiently; for instance, calls to "alloca" may become single instructions
that adjust the stack directly, and calls to "memcpy" may become inline copy
loops. The resulting code is often both smaller and faster, but since the
function calls no longer appear as such, you cannot set a breakpoint on those
calls, nor can you change the behavior of the functions by linking with a
different library. In addition, when a function is recognized as a built-in
function, GCC may use information about that function to warn about problems
with calls to that function, or to generate more efficient code, even if the
resulting code still contains calls to that function. For example, warnings
are given with -Wformat for bad calls to "printf", when "printf" is built in,
and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is
disabled. function must not begin with __builtin_. If a function is named
this is not built-in in this version of GCC, this option is ignored. There is
no corresponding -fbuiltin-function option; if you wish to enable built-in
functions selectively when using -fno-builtin or -ffreestanding, you may define
macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation takes place in a hosted environment. This implies
-fbuiltin. A hosted environment is one in which the entire standard library is
available, and in which "main" has a return type of "int". Examples are nearly
everything except a kernel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding environment. This
implies -fno-builtin. A freestanding environment is one in which the standard
library may not exist, and program startup may not necessarily be at "main".
The most obvious example is an OS kernel. This is equivalent to -fno-hosted.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
Some cases of unnamed fields in structures and unions are only accepted with
this option.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C
conformance) implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling. This option
allows a user supplied "cc1", "cc1plus", or "cc1obj" via the -B option. The
user supplied compilation step can then add in an additional preprocessing step
after normal preprocessing but before compiling. The default is to use the
integrated cpp (internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are
merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-standard C com-
piler. They are now only supported with the -E switch. The preprocessor con-
tinues to support a pre-standard mode. See the GNU CPP manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third
arguments. The value of such an expression is void. This option is not sup-
ported for C++.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It is either
like "unsigned char" by default or like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char"
when it depends on the signedness of an object. But many programs have been
written to use plain "char" and expect it to be signed, or expect it to be
unsigned, depending on the machines they were written for. This option, and
its inverse, let you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed char" or
"unsigned char", even though its behavior is always just like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the negative form
of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to
-funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the dec-
laration does not use either "signed" or "unsigned". By default, such a bit-
field is signed, because this is consistent: the basic integer types such as
"int" are signed types.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++
programs; but you can also use most of the GNU compiler options regardless of what
language your program is in. For example, you might compile a file "firstClass.C"
like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use
the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that
first appeared in G++ 3.4. Version 1 is the version of the C++ ABI that first
appeared in G++ 3.2. Version 0 will always be the version that conforms most
closely to the C++ ABI specification. Therefore, the ABI obtained using ver-
sion 0 will change as ABI bugs are fixed.
The default is version 2.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around
bugs in the access control code.
-fcheck-new
Check that the pointer returned by "operator new" is non-null before attempting
to modify the storage allocated. This check is normally unnecessary because
the C++ standard specifies that "operator new" will only return 0 if it is
declared throw(), in which case the compiler will always check the return value
even without this option. In all other cases, when "operator new" has a non-
empty exception specification, memory exhaustion is signalled by throwing
"std::bad_alloc". See also new (nothrow).
-fconserve-space
Put uninitialized or runtime-initialized global variables into the common seg-
ment, as C does. This saves space in the executable at the cost of not diag-
nosing duplicate definitions. If you compile with this flag and your program
mysteriously crashes after "main()" has completed, you may have an object that
is being destroyed twice because two definitions were merged.
This option is no longer useful on most targets, now that support has been
added for putting variables into BSS without making them common.
-fno-const-strings
Give string constants type "char *" instead of type "const char *". By
default, G++ uses type "const char *" as required by the standard. Even if you
use -fno-const-strings, you cannot actually modify the value of a string con-
stant.
This option might be removed in a future release of G++. For maximum portabil-
ity, you should structure your code so that it works with string constants that
have type "const char *".
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary which is
only used to initialize another object of the same type. Specifying this
option disables that optimization, and forces G++ to call the copy constructor
in all cases.
-fno-enforce-eh-specs
Don’t check for violation of exception specifications at runtime. This option
violates the C++ standard, but may be useful for reducing code size in produc-
tion builds, much like defining NDEBUG. The compiler will still optimize based
on the exception specifications.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a for-init-
statement is limited to the for loop itself, as specified by the C++ standard.
If -fno-for-scope is specified, the scope of variables declared in a for-init-
statement extends to the end of the enclosing scope, as was the case in old
versions of G++, and other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to allow and
give a warning for old-style code that would otherwise be invalid, or have dif-
ferent behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this word as an
identifier. You can use the keyword "__typeof__" instead. -ansi implies
-fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated implicitly
(i.e. by use); only emit code for explicit instantiations.
-fno-implicit-inline-templates
Don’t emit code for implicit instantiations of inline templates, either. The
default is to handle inlines differently so that compiles with and without
optimization will need the same set of explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions controlled by
#pragma implementation. This will cause linker errors if these functions are
not inlined everywhere they are called.
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as implicit int
and getting a pointer to member function via non-standard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by ANSI/ISO C.
These include "ffs", "alloca", "_exit", "index", "bzero", "conjf", and other
related functions.
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor", "compl",
"not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to issue.
Currently, the only such diagnostic issued by G++ is the one for a name having
multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to warnings.
Thus, using -fpermissive will allow some nonconforming code to compile.
-frepo
Enable automatic template instantiation at link time. This option also implies
-fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual functions for
use by the C++ runtime type identification features (dynamic_cast and typeid).
If you don’t use those parts of the language, you can save some space by using
this flag. Note that exception handling uses the same information, but it will
generate it as needed.
-fstats
Emit statistics about front-end processing at the end of the compilation. This
information is generally only useful to the G++ development team.
-ftemplate-depth-n
Set the maximum instantiation depth for template classes to n. A limit on the
template instantiation depth is needed to detect endless recursions during tem-
plate 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 ("hid-
den")))" 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 tem-
plates. 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 -fvisibil-
ity=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 infe-
rior 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 gen-
erated by other compilers.
The known incompatibilities at this point include:
* Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack
data into the same byte as a base class. For example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte as"A::f1"; other
compilers will not. You can avoid this problem by explicitly padding "A"
so that its size is a multiple of the byte size on your platform; that will
cause G++ and other compilers to layout "B" identically.
* Incorrect handling of tail-padding for virtual bases. G++ does not use
tail padding when laying out virtual bases. For example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for "A"; other
compilers will. You can avoid this problem by explicitly padding "A" so
that its size is a multiple of its alignment (ignoring virtual base
classes); that will cause G++ and other compilers to layout "C" identi-
cally.
* Incorrect handling of bit-fields with declared widths greater than that of
their underlying types, when the bit-fields appear in a union. For exam-
ple:
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 tem-
plate parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable because all the constructors or destructors in
that class are private, and it has neither friends nor public static member
functions.
-Wnon-virtual-dtor (C++ only)
Warn when a class appears to be polymorphic, thereby requiring a virtual
destructor, yet it declares a non-virtual one. This warning is enabled by
-Wall.
-Wreorder (C++ only)
Warn when the order of member initializers given in the code does not match the
order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler will rearrange the member initializers for i and j to match the
declaration order of the members, emitting a warning to that effect. This
warning is enabled by -Wall.
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 sentinel, as "NULL" is defined to "__null". Although
it is a null pointer constant not a null pointer, it is guaranteed to of the
same size as a pointer. But this use is not portable across different compil-
ers.
-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 spec-
ification, unqualified-ids could be interpreted as a particular specialization
of a templatized function. Because this non-conforming behavior is no longer
the default behavior for G++, -Wnon-template-friend allows the compiler to
check existing code for potential trouble spots and is on by default. This new
compiler behavior can be turned off with -Wno-non-template-friend which keeps
the conformant compiler code but disables the helpful warning.
-Wold-style-cast (C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C++
program. The new-style casts (static_cast, reinterpret_cast, and const_cast)
are less vulnerable to unintended effects and much easier to search for.
-Woverloaded-virtual (C++ only)
Warn when a function declaration hides virtual functions from a base class.
For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member function to a
plain pointer.
-Wsign-promo (C++ only)
Warn when overload resolution chooses a promotion from unsigned or enumerated
type to a signed type, over a conversion to an unsigned type of the same size.
Previous versions of G++ would try to preserve unsignedness, but the standard
mandates the current behavior.
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator = (const A&);, while
cfront will use the user-defined operator =.
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 Objec-
tive-C and Objective-C++ programs, but you can also use most of the language-inde-
pendent GNU compiler options. For example, you might compile a file "some_class.m"
like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C and Objec-
tive-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 compila-
tions may also use options specific to the C front-end (e.g., -Wtraditional). Sim-
ilarly, 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 "NXCon-
stantString" if the GNU runtime is being used, and "NSConstantString" if the
NeXT runtime is being used (see below). The -fconstant-cfstrings option, if
also present, will override the -fconstant-string-class setting and cause
"@"..."" literals to be laid out as constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime.
This is the default for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default for
NeXT-based systems, including Darwin and Mac OS X. The macro "__NEXT_RUN-
TIME__" is predefined if (and only if) this option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches (e.g., "[receiver message:arg]")
in this translation unit ensure that the receiver is not "nil". This allows
for more efficient entry points in the runtime to be used. Currently, this
option is only available in conjunction with the NeXT runtime on Mac OS X 10.3
and later.
-fobjc-exceptions
Enable syntactic support for structured exception handling in Objective-C, sim-
ilar to what is offered by C++ and Java. Currently, this option is only avail-
able in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
The @throw statement may appear anywhere in an Objective-C or Objective-C++
program; when used inside of a @catch block, the @throw may appear without an
argument (as shown above), in which case the object caught by the @catch will
be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and caught using
this scheme. When an object is thrown, it will be caught by the nearest @catch
clause capable of handling objects of that type, analogously to how "catch"
blocks work in C++ and Java. A "@catch(id ...)" clause (as shown above) may
also be provided to catch any and all Objective-C exceptions not caught by pre-
vious @catch clauses (if any).
The @finally clause, if present, will be executed upon exit from the immedi-
ately preceding "@try ... @catch" section. This will happen regardless of
whether any exceptions are thrown, caught or rethrown inside the "@try ...
@catch" section, analogously to the behavior of the "finally" clause in Java.
There are several caveats to using the new exception mechanism:
* Although currently designed to be binary compatible with "NS_HANDLER"-style
idioms provided by the "NSException" class, the new exceptions can only be
used on Mac OS X 10.3 (Panther) and later systems, due to additional func-
tionality 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 relin-
quishes 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 @syn-
chronized. Note that throwing exceptions out of @synchronized blocks is
allowed, and will cause the guarding object to be unlocked properly.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the resulting
object file, and allow dyld(1) to load it in at run time instead. This is used
in conjunction with the Fix-and-Continue debugging mode, where the object file
in question may be recompiled and dynamically reloaded in the course of program
execution, without the need to restart the program itself. Currently, Fix-and-
Continue functionality is only available in conjunction with the NeXT runtime
on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls to
"objc_getClass("...")" (when the name of the class is known at compile time)
with static class references that get initialized at load time, which improves
run-time performance. Specifying the -fzero-link flag suppresses this behavior
and causes calls to "objc_getClass("...")" to be retained. This is useful in
Zero-Link debugging mode, since it allows for individual class implementations
to be modified during program execution.
-gen-decls
Dump interface declarations for all classes seen in the source file to a file
named sourcename.decl.
-Wno-protocol
If a class is declared to implement a protocol, a warning is issued for every
method in the protocol that is not implemented by the class. The default
behavior is to issue a warning for every method not explicitly implemented in
the class, even if a method implementation is inherited from the superclass.
If you use the -Wno-protocol option, then methods inherited from the superclass
are considered to be implemented, and no warning is issued for them.
-Wselector
Warn if multiple methods of different types for the same selector are found
during compilation. The check is performed on the list of methods in the final
stage of compilation. Additionally, a check is performed for each selector
appearing in a "@selector(...)" expression, and a corresponding method for
that selector has been found during compilation. Because these checks scan the
method table only at the end of compilation, these warnings are not produced if
the final stage of compilation is not reached, for example because an error is
found during compilation, or because the -fsyntax-only option is being used.
-Wundeclared-selector
Warn if a "@selector(...)" expression referring to an undeclared selector is
found. A selector is considered undeclared if no method with that name has
been declared before the "@selector(...)" expression, either explicitly in an
@interface or @protocol declaration, or implicitly in an @implementation sec-
tion. 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 sup-
ported by GCC. If n is zero, then no line-wrapping will be done; each error
message will appear on a single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages
reporter to emit once source location information; that is, in case the message
is too long to fit on a single physical line and has to be wrapped, the source
location won’t be emitted (as prefix) again, over and over, in subsequent con-
tinuation lines. This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic messages
reporter to emit the same source location information (as prefix) for physical
lines that result from the process of breaking a message which is too long to
fit on a single line.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are not inherently
erroneous but which are risky or suggest there may have been an error.
You can request many specific warnings with options beginning -W, for example -Wim-
plicit to request warnings on implicit declarations. Each of these specific warn-
ing 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 pro-
grams that use forbidden extensions, and some other programs that do not follow
ISO C and ISO C++. For ISO C, follows the version of the ISO C standard speci-
fied by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without this
option (though a rare few will require -ansi or a -std option specifying the
required version of ISO C). However, without this option, certain GNU exten-
sions and traditional C and C++ features are supported as well. With this
option, they are rejected.
-pedantic does not cause warning messages for use of the alternate keywords
whose names begin and end with __. Pedantic warnings are also disabled in the
expression that follows "__extension__". However, only system header files
should use these escape routes; application programs should avoid them.
Some users try to use -pedantic to check programs for strict ISO C conformance.
They soon find that it does not do quite what they want: it finds some non-ISO
practices, but not all---only those for which ISO C requires a diagnostic, and
some others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some
instances, but would require considerable additional work and would be quite
different from -pedantic. We don’t have plans to support such a feature in the
near future.
Where the standard specified with -std represents a GNU extended dialect of C,
such as gnu89 or gnu99, there is a corresponding base standard, the version of
ISO C on which the GNU extended dialect is based. Warnings from -pedantic are
given where they are required by the base standard. (It would not make sense
for such warnings to be given only for features not in the specified GNU 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 sup-
plied have types appropriate to the format string specified, and that the con-
versions 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 for-
mat 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 ver-
sion 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 warn-
ing about features that go beyond a particular library’s limitations. However,
if -pedantic is used with -Wformat, warnings will be given about format fea-
tures 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 -Wformat-y2k, -Wno-format-extra-args, -Wno-for-
mat-zero-length, -Wformat-nonliteral, -Wformat-security, and -Wformat=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 specifications, normally warnings are still given, since the
implementation could not know what type to pass to "va_arg" to skip the unused
arguments. However, in the case of "scanf" formats, this option will suppress
the warning if the unused arguments are all pointers, since the Single Unix
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 stan-
dard specifies that zero-length formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string lit-
eral 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 repre-
sent 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 secu-
rity 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-nonliteral.)
-Wformat=2
Enable -Wformat plus format checks not included in -Wformat. Currently equiva-
lent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a non-null
value by the "nonnull" function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the
-Wno-nonnull option.
-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 snip-
pet 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 warn-
ing 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 exter-
nal linkage, returning int, taking either zero arguments, two, or three argu-
ments of appropriate types. This warning is enabled by -Wall.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the fol-
lowing 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 different interpretation from that of ordinary mathemati-
cal notation.
Also warn about constructions where there may be confusion to which "if" state-
ment 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 warn-
ing 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 evalu-
ated in terms of sequence points, which represent a partial ordering between
the execution of parts of the program: those executed before the sequence
point, and those executed after it. These occur after the evaluation of a full
expression (one which is not part of a larger expression), after the evaluation
of the first operand of a "&&", "││", "? :" or "," (comma) operator, before a
function is called (but after the evaluation of its arguments and the expres-
sion denoting the called function), and in certain other places. Other than as
expressed by the sequence point rules, the order of evaluation of subexpres-
sions of an expression is not specified. All these rules describe only a par-
tial 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 com-
mittee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of
objects take effect. Programs whose behavior depends on this have undefined
behavior; the C standard specifies that ‘‘Between the previous and next
sequence point an object shall have its stored value modified at most once by
the evaluation of an expression. Furthermore, the prior value shall be read
only to determine the value to be stored.’’. If a program breaks these rules,
the results on any particular implementation are entirely unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and
"a[i++] = i;". Some more complicated cases are not diagnosed by this option,
and it may give an occasional false 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 discus-
sions 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 qualifier 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 with-
out 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 enumer-
ation 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 pro-
gram (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 vari-
able in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered elements of
structure, union or array variables as well as for variables which are unini-
tialized or clobbered as a whole. They do not occur for variables 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 signal handler could call it at any point in the
code. As a result, you may get a warning even when there is in fact no problem
because "longjmp" cannot in fact be called at the place which would cause a
problem.
Some spurious warnings can be avoided if you declare all the functions you use
that never return as "noreturn".
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.
-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 con-
structions that some users consider questionable, and that are easy to avoid
(or modify to prevent the warning), even in conjunction with macros. This also
enables some language-specific warnings described in 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 con-
structions that users generally do not consider questionable, but which occasion-
ally you might wish to check for; others warn about constructions that are neces-
sary 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 descriptive.) Print extra warning messages for these
events:
* A function can return either with or without a value. (Falling off the end
of the function body is considered returning without a value.) For exam-
ple, 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 con-
tains no side effects. To suppress the warning, cast the unused expression
to void. For example, an expression such as x[i,j] will cause a warning,
but x[(void)i,j] will not.
* An unsigned value is compared against zero with < or >=.
* Storage-class specifiers like "static" are not the first things in a decla-
ration. 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-initializ-
ers.
* 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 with-
out 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 divi-
sion 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 normally suppressed, on the assumption that they usu-
ally do not indicate real problems and would only make the compiler output
harder to read. Using this command line option tells GCC to emit warnings from
system headers as if they occurred in user code. However, note that using
-Wall in conjunction with this option will not warn about unknown pragmas in
system headers---for that, -Wunknown-pragmas must also be used.
-Wfloat-equal
Warn if floating point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to
consider floating-point values as approximations to infinitely precise real
numbers. If you are doing this, then you need to compute (by analyzing the
code, or in some other way) the maximum or likely maximum error that the compu-
tation introduces, and allow for it when performing comparisons (and when pro-
ducing output, but that’s a different problem). In particular, instead of
testing for equality, you would check to see whether the two values have ranges
that overlap; and this is done with the relational operators, so equality com-
parisons are probably mistaken.
-Wtraditional (C only)
Warn about certain constructs that behave differently in traditional and ISO C.
Also warn about ISO C constructs that have no traditional C equivalent, and/or
problematic constructs which should be avoided.
* Macro parameters that appear within string literals in the macro body. In
traditional C macro replacement takes place within string literals, but
does not in ISO C.
* In traditional C, some preprocessor directives did not exist. Traditional
preprocessors would only consider a line to be a directive if the #
appeared in column 1 on the line. Therefore -Wtraditional warns about
directives that traditional C 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 traditional 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 suf-
fixes. (Traditional C does support the L suffix on integer constants.)
Note, these suffixes appear in macros defined in the system headers of most
modern systems, e.g. the _MIN/_MAX macros in "<limits.h>". Use of these
macros in user code might normally lead to spurious warnings, however GCC’s
integrated preprocessor has enough context to avoid warning in these cases.
* A function declared external in one block and then used after the end of
the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static" one. This construct
is not accepted by some traditional C compilers.
* The ISO type of an integer constant has a different width or signedness
from its traditional type. This warning is only issued if the base of the
constant is ten. I.e. hexadecimal or octal values, which typically repre-
sent 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 omit-
ted. This is done under the assumption that the zero initializer in user
code appears conditioned on e.g. "__STDC__" to avoid missing initializer
warnings and relies on default initialization to zero in the traditional C
case.
* Conversions by prototypes between fixed/floating point values and vice
versa. The absence of these prototypes when compiling with traditional C
would cause serious problems. This is a subset of the possible conversion
warnings, for the full set use -Wconversion.
* Use of ISO C style function definitions. This warning intentionally is not
issued for prototype declarations or variadic functions because these ISO C
features will appear in your code when using libiberty’s traditional C com-
patibility 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 compatibility.
-Wdeclaration-after-statement (C only)
Warn when a declaration is found after a statement in a block. This construct,
known from C++, was introduced 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 an object of larger than len bytes is defined.
-Wpointer-arith
Warn about anything that depends on the ‘‘size of’’ a function type or of
"void". GNU C assigns these types a size of 1, for convenience in calculations
with "void *" pointers and pointers to functions.
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type. For example,
warn if "int malloc()" is cast to "anything *".
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the tar-
get type. For example, warn if a "const char *" is cast to an ordinary "char
*".
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target
is increased. For example, warn if a "char *" is cast to an "int *" on
machines where integers can only be accessed at two- or four-byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const char[length]" so that
copying the address of one into a non-"const" "char *" pointer will get a warn-
ing; when compiling C++, warn about the deprecated conversion from string con-
stants to "char *". These warnings will help you find at compile time code
that can try to write into a string constant, but only if you have been very
careful about using "const" in declarations and prototypes. Otherwise, it will
just be a nuisance; this is why we did not make -Wall request these warnings.
-Wconversion
Warn if a prototype causes a type conversion that is different from what would
happen to the same argument in the absence of a prototype. This includes con-
versions of fixed point to floating and vice versa, and conversions changing
the width or signedness of a fixed point argument except when the same as the
default promotion.
Also, warn if a negative integer constant expression is implicitly converted to
an unsigned type. For example, warn about the assignment "x = -1" if "x" is
unsigned. But do not warn about explicit casts like "(unsigned) -1".
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce an
incorrect result when the signed value is converted to unsigned. This warning
is also enabled by -Wextra; to get the other warnings of -Wextra without this
warning, use -Wextra -Wno-sign-compare.
-Waggregate-return
Warn if any functions that return structures or unions are defined or called.
(In languages where you can return an array, this also elicits a warning.)
-Wstrict-prototypes (C only)
Warn if a function is declared or defined without specifying the argument
types. (An old-style function definition is permitted without a warning if
preceded by a declaration which specifies the argument types.)
-Wold-style-definition (C only)
Warn if an old-style function definition is used. A warning is given even if
there is a previous prototype.
-Wmissing-prototypes (C only)
Warn if a global function is defined without a previous prototype declaration.
This warning is issued even if the definition itself provides a prototype. The
aim is to detect global functions that fail to be declared in header files.
-Wmissing-declarations (C only)
Warn if a global function is defined without a previous declaration. Do so
even if the definition itself provides a prototype. Use this option to detect
global functions that are not declared in header files.
-Wmissing-field-initializers
Warn if a structure’s initializer has some fields missing. For example, the
following code would cause such a warning, because "x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the following modi-
fication would not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra