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 prepro-
cessor and others the compiler itself. Yet other options control the assembler and
linker; most of these are not documented here, since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful for C programs; when
an option is only useful with another language (usually C++), the explanation says so
explicitly. If the description for a particular option does not mention a source lan-
guage, you can use that option with all supported languages.
The gcc program accepts options and file names as operands. Many options have multi-let-
ter names; therefore multiple single-letter options may not be grouped: -dr is very dif-
ferent from -d -r.
You can mix options and other arguments. For the most part, the order you use doesn't
matter. Order does matter when you use several options of the same kind; for example, if
you specify -L more than once, the directories are searched in the order specified.
Many options have long names starting with -f or with -W---for example, -fforce-mem,
-fstrength-reduce, -Wformat and so on. Most of these have both positive and negative
forms; the negative form of -ffoo would be -fno-foo. This manual documents only one of
these two forms, whichever one is not the default.
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 -pipe -pass-exit-codes -x language -v -### --help --tar-
get-help --version
C Language Options
-ansi -std=standard -aux-info filename -fno-asm -fno-builtin -fno-builtin-function
-fhosted -ffreestanding -fms-extensions -trigraphs -no-integrated-cpp -traditional
-traditional-cpp -fallow-single-precision -fcond-mismatch -fsigned-bitfields
-fsigned-char -funsigned-bitfields -funsigned-char -fwritable-strings
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space -fno-const-strings
-fno-elide-constructors -fno-enforce-eh-specs -ffor-scope -fno-for-scope
-fno-gnu-keywords -fno-implicit-templates -fno-implicit-inline-templates -fno-imple-
ment-inlines -fms-extensions -fno-nonansi-builtins -fno-operator-names
-fno-optional-diags -fpermissive -frepo -fno-rtti -fstats -ftemplate-depth-n
-fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline
-fvisibility-inlines-hidden -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder
-Weffc++ -Wno-deprecated -Wno-non-template-friend -Wold-style-cast -Woverloaded-vir-
tual -Wno-pmf-conversions -Wsign-promo
Objective-C Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers
-fobjc-exceptions -freplace-objc-classes -fzero-link -gen-decls -Wno-protocol -Wse-
lector -Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall -Waggregate-return
-Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Wno-depre-
cated-declarations -Wdisabled-optimization -Wno-div-by-zero -Wendif-labels -Werror
-Werror-implicit-function-declaration -Wfloat-equal -Wformat -Wformat=2 -Wno-for-
mat-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k -Wimplicit -Wim-
plicit-function-declaration -Wimplicit-int -Wimport -Wno-import -Winit-self -Win-
line -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than-len -Wlong-long -Wmain
-Wmissing-braces -Wmissing-format-attribute -Wmissing-noreturn -Wno-multichar -Wnon-
null -Wpacked -Wpadded -Wparentheses -Wpointer-arith -Wredundant-decls -Wre-
turn-type -Wsequence-point -Wshadow -Wsign-compare -Wstrict-aliasing -Wswitch
-Wswitch-default -Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef -Wuninitial-
ized -Wunknown-pragmas -Wunreachable-code -Wunused -Wunused-function -Wunused-label
-Wunused-parameter -Wunused-value -Wunused-variable -Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes -Wnested-externs
-Wold-style-definition -Wstrict-prototypes -Wtraditional -Wdeclaration-after-state-
ment
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered -fdump-transla-
tion-unit[-n] -fdump-class-hierarchy[-n] -fdump-tree-original[-n] -fdump-tree-opti-
mized[-n] -fdump-tree-inlined[-n] -feliminate-dwarf2-dups -felimi-
nate-unused-debug-types -feliminate-unused-debug-symbols -fmem-report -fprofile-arcs
-frandom-seed=string -fsched-verbose=n -ftest-coverage -ftime-report -fvar-tracking
-g -glevel -gcoff -gdwarf-2 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ -p
-pg -print-file-name=library -print-libgcc-file-name -print-multi-directory
-print-multi-lib -print-prog-name=program -print-search-dirs -Q -save-temps -time
Optimization Options
-falign-functions=n -falign-jumps=n -falign-labels=n -falign-loops=n -fbranch-proba-
bilities -fprofile-values -fvpt -fbranch-target-load-optimize -fbranch-tar-
get-load-optimize2 -fcaller-saves -fcprop-registers -fcse-follow-jumps
-fcse-skip-blocks -fdata-sections -fdelayed-branch -fdelete-null-pointer-checks
-fexpensive-optimizations -ffast-math -ffloat-store -fforce-addr -fforce-mem
-ffunction-sections -fgcse -fgcse-lm -fgcse-sm -fgcse-las -floop-optimize
-fcrossjumping -fif-conversion -fif-conversion2 -finline-functions -finline-limit=n
-fkeep-inline-functions -fkeep-static-consts -fmerge-constants -fmerge-all-constants
-fmove-all-movables -fnew-ra -fno-branch-count-reg -fno-default-inline
-fno-defer-pop -fno-function-cse -fno-guess-branch-probability -fno-inline
-fno-math-errno -fno-peephole -fno-peephole2 -funsafe-math-optimizations -ffi-
nite-math-only -fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer
-foptimize-register-move -foptimize-sibling-calls -fprefetch-loop-arrays -fpro-
file-generate -fprofile-use -freduce-all-givs -fregmove -frename-registers -fre-
order-blocks -freorder-functions -frerun-cse-after-loop -frerun-loop-opt -fround-
ing-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 -fsignal-
ing-nans -fsingle-precision-constant -fstrength-reduce -fstrict-aliasing -ftracer
-fthread-jumps -funroll-all-loops -funroll-loops -fpeel-loops -funswitch-loops
-fold-unroll-loops -fold-unroll-all-loops --param name=value -O -O0 -O1 -O2 -O3
-Os
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn] -E -H
-idirafter dir -include file -imacros file -iprefix file -iwithprefix dir -iwithpre-
fixbefore dir -isystem dir -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -fwork-
ing-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 sym-
bol
Directory Options
-Bprefix -Idir -I- -Ldir -specs=file
Target Options
-V version -b machine
Machine Dependent Options
M680x0 Options -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060
-mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 -mnobitfield -mrtd -mshort
-msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data
-mshared-library-id=n -mid-shared-library -mno-id-shared-library
M68hc1x Options -m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 -mauto-incdec -minmax
-mlong-calls -mshort -msoft-reg-count=count
VAX Options -mg -mgnu -munix
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -m32 -m64
-mapp-regs -mno-app-regs -mfaster-structs -mno-faster-structs -mflat -mno-flat
-mfpu -mno-fpu -mhard-float -msoft-float -mhard-quad-float -msoft-quad-float -mim-
pure-text -mno-impure-text -mlittle-endian -mstack-bias -mno-stack-bias
-munaligned-doubles -mno-unaligned-doubles -mv8plus -mno-v8plus -mvis -mno-vis
-mcypress -mf930 -mf934 -msparclite -msupersparc -mv8 -threads -pthreads
ARM Options -mapcs-frame -mno-apcs-frame -mapcs-26 -mapcs-32 -mapcs-stack-check
-mno-apcs-stack-check -mapcs-float -mno-apcs-float -mapcs-reentrant -mno-apcs-reen-
trant -msched-prolog -mno-sched-prolog -mlittle-endian -mbig-endian -mwords-lit-
tle-endian -malignment-traps -mno-alignment-traps -msoft-float -mhard-float -mfpe
-mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name -mfpe=name -mstruc-
ture-size-boundary=n -mabort-on-noreturn -mlong-calls -mno-long-calls -msin-
gle-pic-base -mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport -mcir-
rus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-function-name -mthumb
-marm -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking
-mcallee-super-interworking
MN10300 Options -mmult-bug -mno-mult-bug -mam33 -mno-am33 -mam33-2 -mno-am33-2
-mno-crt0 -mrelax
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -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
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mpower -mno-power
-mpower2 -mno-power2 -mpowerpc -mpowerpc64 -mno-powerpc -maltivec -mno-altivec
-mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt
-mnew-mnemonics -mold-mnemonics -mfull-toc -mminimal-toc -mno-fp-in-toc
-mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe -malign-power
-malign-natural -msoft-float -mhard-float -mmultiple -mno-multiple -mstring
-mno-string -mupdate -mno-update -mfused-madd -mno-fused-madd -mbit-align
-mno-bit-align -mstrict-align -mno-strict-align -mrelocatable -mno-relocatable
-mrelocatable-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian
-mbig -mbig-endian -mdynamic-no-pic -mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme -mcall-sysv
-mcall-netbsd -maix-struct-return -msvr4-struct-return -mabi=altivec
-mabi=no-altivec -mabi=spe -mabi=no-spe -misel=yes -misel=no -mspe=yes -mspe=no
-mfloat-gprs=yes -mfloat-gprs=no -mprototype -mno-prototype -msim -mmvme -mads
-myellowknife -memb -msdata -msdata=opt -mvxworks -mwindiss -G num -pthread
Darwin Options -all_load -allowable_client -arch -arch_errors_fatal -arch_only
-bind_at_load -bundle -bundle_loader -client_name -compatibility_version -cur-
rent_version -dependency-file -dylib_file -dylinker_install_name -dynamic -dynami-
clib -exported_symbols_list -filelist -flat_namespace -force_cpusubtype_ALL
-force_flat_namespace -headerpad_max_install_names -image_base -init -install_name
-keep_private_externs -multi_module -multiply_defined -multiply_defined_unused
-noall_load -nofixprebinding -nomultidefs -noprebind -noseglinkedit -pagezero_size
-prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs -sectalign
-sectobjectsymbols -whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder
-seg_addr_table -seg_addr_table_filename -seglinkedit -segprot -segs_read_only_addr
-segs_read_write_addr -single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined -unexported_symbols_list -weak_refer-
ence_mismatches -whatsloaded
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4
-mips32 -mips32r2 -mips64 -mips16 -mno-mips16 -mabi=abi -mabicalls -mno-abicalls
-mxgot -mno-xgot -membedded-pic -mno-embedded-pic -mgp32 -mgp64 -mfp32 -mfp64
-mhard-float -msoft-float -msingle-float -mdouble-float -mint64 -mlong64 -mlong32
-Gnum -membedded-data -mno-embedded-data -muninit-const-in-rodata
-mno-uninit-const-in-rodata -msplit-addresses -mno-split-addresses -mexplicit-relocs
-mno-explicit-relocs -mrnames -mno-rnames -mcheck-zero-division
-mno-check-zero-division -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls -mmad
-mno-mad -mfused-madd -mno-fused-madd -nocpp -mfix-sb1 -mno-fix-sb1
-mflush-func=func -mno-flush-func -mbranch-likely -mno-branch-likely
i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfpmath=unit -masm=dialect
-mno-fancy-math-387 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib -mno-wide-multiply
-mrtd -malign-double -mpreferred-stack-boundary=num -mmmx -msse -msse2 -msse3
-m3dnow -mthreads -mno-align-stringops -minline-all-stringops -mpush-args -maccumu-
late-outgoing-args -m128bit-long-double -m96bit-long-double -mregparm=num
-momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model
-m32 -m64
HPPA Options -march=architecture-type -mbig-switch -mdisable-fpregs -mdisable-index-
ing -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld -mjump-in-delay -mlinker-opt
-mlong-calls -mlong-load-store -mno-big-switch -mno-disable-fpregs -mno-dis-
able-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 -msched-
ule=cpu-type -mspace-regs -msio -mwsio -nolibdld -static -threads
Intel 960 Options -mcpu-type -masm-compat -mclean-linkage -mcode-align -mcom-
plex-addr -mleaf-procedures -mic-compat -mic2.0-compat -mic3.0-compat -mintel-asm
-mno-clean-linkage -mno-code-align -mno-complex-addr -mno-leaf-procedures
-mno-old-align -mno-strict-align -mno-tail-call -mnumerics -mold-align
-msoft-float -mstrict-align -mtail-call
DEC Alpha Options -mno-fp-regs -msoft-float -malpha-as -mgas -mieee
-mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode -mfp-rounding-mode=mode
-mtrap-precision=mode -mbuild-constants -mcpu=cpu-type -mtune=cpu-type -mbwx -mmax
-mfix -mcix -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data -mlarge-data
-msmall-text -mlarge-text -mmemory-latency=time
DEC Alpha/VMS Options -mvms-return-codes
H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300
SH Options -m1 -m2 -m2e -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4
-m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu -m5-compact -m5-com-
pact-nofpu -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mnomacsave
-mieee -misize -mpadstruct -mspace -mprefergot -musermode
System V Options -Qy -Qn -YP,paths -Ym,dir
ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section -mdata=data-section
-mrodata=readonly-data-section
TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm -mmemparm -mfast-fix
-mmpyi -mbk -mti -mdp-isr-reload -mrpts=count -mrptb -mdb -mloop-unsigned -mpar-
allel-insns -mparallel-mpy -mpreserve-float
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-pro-
log-function -mspace -mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs -mdis-
able-callt -mno-disable-callt -mv850e1 -mv850e -mv850 -mbig-switch
NS32K Options -m32032 -m32332 -m32532 -m32081 -m32381 -mmult-add -mnomult-add
-msoft-float -mrtd -mnortd -mregparam -mnoregparam -msb -mnosb -mbitfield -mno-
bitfield -mhimem -mnohimem
AVR Options -mmcu=mcu -msize -minit-stack=n -mno-interrupts -mcall-prologues
-mno-tablejump -mtiny-stack
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates
-mno-relax-immediates -mwide-bitfields -mno-wide-bitfields -m4byte-functions
-mno-4byte-functions -mcallgraph-data -mno-callgraph-data -mslow-bytes
-mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340 -mstack-incre-
ment
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-sin-
gle-exit
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic
-mvolatile-asm-stop -mb-step -mregister-names -mno-sdata -mconstant-gp -mauto-pic
-minline-float-divide-min-latency -minline-float-divide-max-throughput -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
D30V Options -mextmem -mextmemory -monchip -mno-asm-optimize -masm-optimize
-mbranch-cost=n -mcond-exec=n
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float
-mbackchain -mno-backchain -mpacked-stack -mno-packed-stack -msmall-exec
-mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch
-mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size
-mstack-guard
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -melinux-stack-
size=n -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects -mstack-align
-mdata-align -mconst-align -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue
-mno-gotplt -melf -maout -melinux -mlinux -sim -sim2 -mmul-bug-workaround
-mno-mul-bug-workaround
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy
-mbcopy-builtin -mint32 -mno-int16 -mint16 -mno-int32 -mfloat32 -mno-float64
-mfloat64 -mno-float32 -mabshi -mno-abshi -mbranch-expensive -mbranch-cheap
-msplit -mno-split -munix-asm -mdec-asm
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mtext-sec-
tion-literals -mno-text-section-literals -mtarget-align -mno-target-align -mlong-
calls -mno-longcalls
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -mal-
loc-cc -mfixed-cc -mdword -mno-dword -mdouble -mno-double -mmedia -mno-media
-mmuladd -mno-muladd -mlibrary-pic -macc-4 -macc-8 -mpack -mno-pack -mno-eflags
-mcond-move -mno-cond-move -mscc -mno-scc -mcond-exec -mno-cond-exec -mvliw-branch
-mno-vliw-branch -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec
-mno-nested-cond-exec -mtomcat-stats -mcpu=cpu
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions
-funwind-tables -fasynchronous-unwind-tables -finhibit-size-directive -finstru-
ment-functions -fno-common -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE
-freg-struct-return -fshared-data -fshort-enums -fshort-double -fshort-wchar -fver-
bose-asm -fpack-struct -fstack-check -fstack-limit-register=reg -fstack-limit-sym-
bol=sym -fargument-alias -fargument-noalias -fargument-noalias-global -flead-
ing-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 library libobjc.a to make
an Objective-C program work.
file.mi
Objective-C source code which should not be preprocessed.
file.h
C or C++ header file to be turned into a precompiled header.
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code which must be preprocessed. Note that in .cxx, the last two letters
must both be literally x. Likewise, .C refers to a literal capital C.
file.hh
file.H
C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR
Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fortran source code which must be preprocessed (with the traditional preprocessor).
file.r
Fortran source code which must be preprocessed with a RATFOR preprocessor (not
included with GCC).
file.ads
Ada source code file which contains a library unit declaration (a declaration of a
package, subprogram, or generic, or a generic instantiation), or a library unit renam-
ing declaration (a package, generic, or subprogram renaming declaration). Such files
are also called specs.
file.adb
Ada source code file containing a library unit body (a subprogram or package body).
Such files are also called bodies.
file.s
Assembler code.
file.S
Assembler code which must be preprocessed.
other
An object file to be fed straight into linking. Any file name with no recognized suf-
fix is treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files (rather than letting the
compiler choose a default based on the file name suffix). This option applies to all
following input files until the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objc-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
java
treelang
-x none
Turn off any specification of a language, so that subsequent files are handled accord-
ing to their file name suffixes (as they are if -x has not been used at all).
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase of the compiler
returns a non-success return code. If you specify -pass-exit-codes, the gcc program
will instead return with numerically highest error produced by any phase that returned
an error indication.
If you only want some of the stages of compilation, you can use -x (or filename suffixes)
to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to
stop. Note that some combinations (for example, -x cpp-output -E) instruct gcc to do
nothing at all.
-c Compile or assemble the source files, but do not link. The linking stage simply is
not done. The ultimate output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix .c,
.i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S Stop after the stage of compilation proper; do not assemble. The output is in the
form of an assembler code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix
.c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper. The output is in
the form of preprocessed source code, which is sent to the standard output.
Input files which don't require preprocessing are ignored.
-o file
Place output in file file. This applies regardless to whatever sort of output is
being produced, whether it be an executable file, an object file, an assembler file or
preprocessed C code.
If you specify -o when compiling more than one input file, or you are producing an
executable file as output, all the source files on the command line will be compiled
at once.
If -o is not specified, the default is to put an executable file in a.out, the object
file for source.suffix in source.o, its assembler file in source.s, and all prepro-
cessed C source on standard output.
-v Print (on standard error output) the commands executed to run the stages of compila-
tion. Also print the version number of the compiler driver program and of the prepro-
cessor and the compiler proper.
-###
Like -v except the commands are not executed and all command arguments are quoted.
This is useful for shell scripts to capture the driver-generated command lines.
-pipe
Use pipes rather than temporary files for communication between the various stages of
compilation. This fails to work on some systems where the assembler is unable to read
from a pipe; but the GNU assembler has no trouble.
--help
Print (on the standard output) a description of the command line options understood by
gcc. If the -v option is also specified then --help will also be passed on to the
various processes invoked by gcc, so that they can display the command line options
they accept. If the -Wextra option is also specified then command line options which
have no documentation associated with them will also be displayed.
--target-help
Print (on the standard output) a description of target specific command line options
for each tool.
--version
Display the version number and copyrights of the invoked GCC.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or
.cxx; C++ header files often use .hh or .H; and preprocessed C++ files use the suffix .ii.
GCC recognizes files with these names and compiles them as C++ programs even if you call
the compiler the same way as for compiling C programs (usually with the name gcc).
However, C++ programs often require class libraries as well as a compiler that understands
the C++ language---and under some circumstances, you might want to compile programs or
header files from standard input, or otherwise without a suffix that flags them as C++
programs. You might also like to precompile a C header file with a .h extension to be
used in C++ compilations. g++ is a program that calls GCC with the default language set
to C++, and automatically specifies linking against the C++ library. On many systems, g++
is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that
you use for compiling programs in any language; or command-line options meaningful for C
and related languages; or options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++
and Objective-C) that the compiler accepts:
-ansi
In C mode, support all ISO C90 programs. In C++ mode, remove GNU extensions that con-
flict with ISO C++.
This turns off certain features of GCC that are incompatible with ISO C90 (when com-
piling C code), or of standard C++ (when compiling C++ code), such as the "asm" and
"typeof" keywords, and predefined macros such as "unix" and "vax" that identify the
type of system you are using. It also enables the undesirable and rarely used ISO
trigraph feature. For the C compiler, it disables recognition of C++ style // com-
ments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" con-
tinue 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 compi-
lations 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 compil-
ing C or C++. A value for this option must be provided; possible values are
c89
iso9899:1990
ISO C90 (same as -ansi).
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported; see
<http://gcc.gnu.org/gcc-3.4/c99status.html> for more information. The names c9x
and iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including some C99 features).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will
become the default. The name gnu9x is deprecated.
c++98
The 1998 ISO C++ standard plus amendments.
gnu++98
The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
Even when this option is not specified, you can still use some of the features of
newer standards in so far as they do not conflict with previous C standards. For
example, you may use "__restrict__" even when -std=c99 is not specified.
The -std options specifying some version of ISO C have the same effects as -ansi,
except that features that were not in ISO C90 but are in the specified version (for
example, // comments and the "inline" keyword in ISO C99) are not disabled.
-aux-info filename
Output to the given filename prototyped declarations for all functions declared and/or
defined in a translation unit, including those in header files. This option is
silently ignored in any language other than C.
Besides declarations, the file indicates, in comments, the origin of each declaration
(source file and line), whether the declaration was implicit, prototyped or unproto-
typed (I, N for new or O for old, respectively, in the first character after the line
number and the colon), and whether it came from a declaration or a definition (C or F,
respectively, in the following character). In the case of function definitions, a
K&R-style list of arguments followed by their declarations is also provided, inside
comments, after the declaration.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these
words as identifiers. You can use the keywords "__asm__", "__inline__" and
"__typeof__" instead. -ansi implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm" and "inline" are
standard keywords. You may want to use the -fno-gnu-keywords flag instead, which has
the same effect. In C99 mode (-std=c99 or -std=gnu99), this switch only affects the
"asm" and "typeof" keywords, since "inline" is a standard keyword in ISO C99.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with __builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more effi-
ciently; for instance, calls to "alloca" may become single instructions that adjust
the stack directly, and calls to "memcpy" may become inline copy loops. The resulting
code is often both smaller and faster, but since the function calls no longer appear
as such, you cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library.
With the -fno-builtin-function option only the built-in function function is disabled.
function must not begin with __builtin_. If a function is named this is not built-in
in this version of GCC, this option is ignored. There is no corresponding
-fbuiltin-function option; if you wish to enable built-in functions selectively when
using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation takes place in a hosted environment. This implies -fbuiltin.
A hosted environment is one in which the entire standard library is available, and in
which "main" has a return type of "int". Examples are nearly everything except a ker-
nel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding environment. This implies
-fno-builtin. A freestanding environment is one in which the standard library may not
exist, and program startup may not necessarily be at "main". The most obvious example
is an OS kernel. This is equivalent to -fno-hosted.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C confor-
mance) implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling. This option allows
a user supplied "cc1", "cc1plus", or "cc1obj" via the -B option. The user supplied
compilation step can then add in an additional preprocessing step after normal
preprocessing but before compiling. The default is to use the integrated cpp (internal
cpp)
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler.
They are now only supported with the -E switch. The preprocessor continues to support
a pre-standard mode. See the GNU CPP manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments.
The value of such an expression is void. This option is not supported for C++.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It is either like
"unsigned char" by default or like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char" when it
depends on the signedness of an object. But many programs have been written to use
plain "char" and expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let you make such a
program work with the opposite default.
The type "char" is always a distinct type from each of "signed char" or "unsigned
char", even though its behavior is always just like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the negative form of
-funsigned-char. Likewise, the option -fno-signed-char is equivalent to -fun-
signed-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the declaration
does not use either "signed" or "unsigned". By default, such a bit-field is signed,
because this is consistent: the basic integer types such as "int" are signed types.
-fwritable-strings
Store string constants in the writable data segment and don't uniquize them. This is
for compatibility with old programs which assume they can write into string constants.
Writing into string constants is a very bad idea; ''constants'' should be constant.
This option is deprecated.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++ programs;
but you can also use most of the GNU compiler options regardless of what language your
program is in. For example, you might compile a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use the
other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first
appeared in G++ 3.4. Version 1 is the version of the C++ ABI that first appeared in
G++ 3.2. Version 0 will always be the version that conforms most closely to the C++
ABI specification. Therefore, the ABI obtained using version 0 will change as ABI
bugs are fixed.
The default is version 2.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in
the access control code.
-fcheck-new
Check that the pointer returned by "operator new" is non-null before attempting to
modify the storage allocated. This check is normally unnecessary because the C++
standard specifies that "operator new" will only return 0 if it is declared throw(),
in which case the compiler will always check the return value even without this
option. In all other cases, when "operator new" has a non-empty exception specifica-
tion, memory exhaustion is signalled by throwing "std::bad_alloc". See also new
(nothrow).
-fconserve-space
Put uninitialized or runtime-initialized global variables into the common segment, as
C does. This saves space in the executable at the cost of not diagnosing duplicate
definitions. If you compile with this flag and your program mysteriously crashes
after "main()" has completed, you may have an object that is being destroyed twice
because two definitions were merged.
This option is no longer useful on most targets, now that support has been added for
putting variables into BSS without making them common.
-fno-const-strings
Give string constants type "char *" instead of type "const char *". By default, G++
uses type "const char *" as required by the standard. Even if you use
-fno-const-strings, you cannot actually modify the value of a string constant, unless
you also use -fwritable-strings.
This option might be removed in a future release of G++. For maximum portability, you
should structure your code so that it works with string constants that have type
"const char *".
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary which is only
used to initialize another object of the same type. Specifying this option disables
that optimization, and forces G++ to call the copy constructor in all cases.
-fno-enforce-eh-specs
Don't check for violation of exception specifications at runtime. This option vio-
lates the C++ standard, but may be useful for reducing code size in production builds,
much like defining NDEBUG. The compiler will still optimize based on the exception
specifications.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a for-init-statement
is limited to the for loop itself, as specified by the C++ standard. If
-fno-for-scope is specified, the scope of variables declared in a for-init-statement
extends to the end of the enclosing scope, as was the case in old versions of G++, and
other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to allow and give a
warning for old-style code that would otherwise be invalid, or have different behav-
ior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this word as an identi-
fier. 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 get-
ting 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 func-
tions.
-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. Cur-
rently, 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 infor-
mation 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 tem-
plate instantiation depth is needed to detect endless recursions during template class
instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth
greater than 17.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++ ABI for thread-
safe initialization of local statics. You can use this option to reduce code size
slightly in code that doesn't need to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the "__cxa_atexit"
function rather than the "atexit" function. This option is required for fully stan-
dards-compliant handling of static destructors, but will only work if your C library
supports "__cxa_atexit".
-fvisibility-inlines-hidden
Causes all inlined methods to be marked with "__attribute__ ((visibility ("hidden")))"
so that they do not appear in the export table of a DSO and do not require a PLT indi-
rection when used within the DSO. Enabling this option can have a dramatic effect on
load and link times of a DSO as it massively reduces the size of the dynamic export
table when the library makes heavy use of templates. While it can cause bloating
through duplication of code within each DSO where it is used, often the wastage is
less than the considerable space occupied by a long symbol name in the export table
which is typical when using templates and namespaces. For even more savings, combine
with the "-fvisibility=hidden" switch.
-fno-weak
Do not use weak symbol support, even if it is provided by the linker. By default, G++
will use weak symbols if they are available. This option exists only for testing, and
should not be used by end-users; it will result in inferior code and has no benefits.
This option may be removed in a future release of G++.
-nostdinc++
Do not search for header files in the standard directories specific to C++, but do
still search the other standard directories. (This option is used when building the
C++ library.)
In addition, these optimization, warning, and code generation options have meanings only
for C++ programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline functions; they just won't
be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with the vendor-neutral
C++ ABI. Although an effort has been made to warn about all such cases, there are
probably some cases that are not warned about, even though G++ is generating incompat-
ible code. There may also be cases where warnings are emitted even though the code
that is generated will be compatible.
You should rewrite your code to avoid these warnings if you are concerned about the
fact that code generated by G++ may not be binary compatible with code generated by
other compilers.
The known incompatibilities at this point include:
* Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data
into the same byte as a base class. For example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte as"A::f1"; other compilers
will not. You can avoid this problem by explicitly padding "A" so that its size
is a multiple of the byte size on your platform; that will cause G++ and other
compilers to layout "B" identically.
* Incorrect handling of tail-padding for virtual bases. G++ does not use tail
padding when laying out virtual bases. For example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for "A"; other compil-
ers will. You can avoid this problem by explicitly padding "A" so that its size
is a multiple of its alignment (ignoring virtual base classes); that will cause
G++ and other compilers to layout "C" identically.
* Incorrect handling of bit-fields with declared widths greater than that of their
underlying types, when the bit-fields appear in a union. For example:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make the union too small
by the number of bits in an "int".
* Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset; it should be placed
at offset zero. G++ mistakenly believes that the "A" data member of "B" is
already at offset zero.
* Names of template functions whose types involve "typename" or template template
parameters can be mangled incorrectly.
template
void f(typename Q::X) {}
template class Q>
void f(typename Q::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 declara-
tion 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.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared within a template.
Since the advent of explicit template specification support in G++, if the name of the
friend is an unqualified-id (i.e., friend foo(int)), the C++ language specification
demands that the friend declare or define an ordinary, nontemplate function. (Section
14.5.3). Before G++ implemented explicit specification, unqualified-ids could be
interpreted as a particular specialization of a templatized function. Because this
non-conforming behavior is no longer the default behavior for G++, -Wnon-tem-
plate-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-tem-
plate-friend which keeps the conformant compiler code but disables the helpful warn-
ing.
-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 vulnera-
ble 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 exam-
ple, 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 ver-
sions 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 Dialect
(NOTE: This manual does not describe the Objective-C language itself. See
<http://gcc.gnu.org/readings.html> for references.)
This section describes the command-line options that are only meaningful for Objective-C
programs, but you can also use most of the GNU compiler options regardless of what lan-
guage your program is in. 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 programs; you can
use the other options with any language supported by GCC.
Here is a list of options that are only for compiling Objective-C programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each literal string speci-
fied with the syntax "@"..."". The default class name is "NXConstantString" if the
GNU runtime is being used, and "NSConstantString" if the NeXT runtime is being used
(see below). The -fconstant-cfstrings option, if also present, will override the
-fconstant-string-class setting and cause "@"..."" literals to be laid out as constant
CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime. This is
the default for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default for NeXT-based
systems, including Darwin and Mac OS X. The macro "__NEXT_RUNTIME__" is predefined if
(and only if) this option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches (e.g., "[receiver message:arg]") in
this translation unit ensure that the receiver is not "nil". This allows for more
efficient entry points in the runtime to be used. Currently, this option is only
available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-fobjc-exceptions
Enable syntactic support for structured exception handling in Objective-C, similar to
what is offered by C++ and Java. Currently, this option is only available in conjunc-
tion with the NeXT runtime on Mac OS X 10.3 and later.
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
The @throw statement may appear anywhere in an Objective-C or Objective-C++ program;
when used inside of a @catch block, the @throw may appear without an argument (as
shown above), in which case the object caught by the @catch will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and caught using this
scheme. When an object is thrown, it will be caught by the nearest @catch clause
capable of handling objects of that type, analogously to how "catch" blocks work in
C++ and Java. A "@catch(id ...)" clause (as shown above) may also be provided to
catch any and all Objective-C exceptions not caught by previous @catch clauses (if
any).
The @finally clause, if present, will be executed upon exit from the immediately pre-
ceding "@try ... @catch" section. This will happen regardless of whether any excep-
tions are thrown, caught or rethrown inside the "@try ... @catch" section, analogously
to the behavior of the "finally" clause in Java.
There are several caveats to using the new exception mechanism:
* Although currently designed to be binary compatible with "NS_HANDLER"-style idioms
provided by the "NSException" class, the new exceptions can only be used on Mac OS
X 10.3 (Panther) and later systems, due to additional functionality needed in the
(NeXT) Objective-C runtime.
* As mentioned above, the new exceptions do not support handling types other than
Objective-C objects. Furthermore, when used from Objective-C++, the Objective-C
exception model does not interoperate with C++ exceptions at this time. This
means you cannot @throw an exception from Objective-C and "catch" it in C++, or
vice versa (i.e., "throw ... @catch").
The -fobjc-exceptions switch also enables the use of synchronization blocks for
thread-safe execution:
@synchronized (ObjCClass *guard) {
...
}
Upon entering the @synchronized block, a thread of execution shall first check whether
a lock has been placed on the corresponding "guard" object by another thread. If it
has, the current thread shall wait until the other thread relinquishes its lock. Once
"guard" becomes available, the current thread will place its own lock on it, execute
the code contained in the @synchronized block, and finally relinquish the lock
(thereby making "guard" available to other threads).
Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized.
Note that throwing exceptions out of @synchronized blocks is allowed, and will cause
the guarding object to be unlocked properly.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the resulting object
file, and allow dyld(1) to load it in at run time instead. This is used in conjunc-
tion 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 debug-
ging 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-proto-
col" option, then methods inherited from the superclass are considered to be imple-
mented, 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 com-
pilation. 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 dec-
laration, or implicitly in an @implementation section. This option always performs
its checks as soon as a "@selector(...)" expression is found, while "-Wselector" only
performs its checks in the final stage of compilation. This also enforces the coding
style convention that methods and selectors must be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output device's
aspect (e.g. its width, ...). The options described below can be used to control the
diagnostic messages formatting algorithm, e.g. how many characters per line, how often
source location information should be reported. Right now, only the C++ front end can
honor these options. However it is expected, in the near future, that the remaining front
ends would be able to digest them correctly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n characters. The
default is 72 characters for g++ and 0 for the rest of the front ends supported by
GCC. If n is zero, then no line-wrapping will be done; each error message will appear
on a single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to
emit once source location information; that is, in case the message is too long to fit
on a single physical line and has to be wrapped, the source location won't be emitted
(as prefix) again, over and over, in subsequent continuation lines. This is the
default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to
emit the same source location information (as prefix) for physical lines that result
from the process of breaking a message which is too long to fit on a single line.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are not inherently erro-
neous but which are risky or suggest there may have been an error.
You can request many specific warnings with options beginning -W, for example -Wimplicit
to request warnings on implicit declarations. Each of these specific warning options also
has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit.
This manual lists only one of the two forms, whichever is not the default.
The following options control the amount and kinds of warnings produced by GCC; for fur-
ther, language-specific options also refer to C++ Dialect Options and Objective-C Dialect
Options.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that
use forbidden extensions, and some other programs that do not follow ISO C and ISO
C++. For ISO C, follows the version of the ISO C standard specified by any -std
option used.
Valid ISO C and ISO C++ programs should compile properly with or without this option
(though a rare few will require -ansi or a -std option specifying the required version
of ISO C). However, without this option, certain GNU extensions and traditional C and
C++ features are supported as well. With this option, they are rejected.
-pedantic does not cause warning messages for use of the alternate keywords whose
names begin and end with __. Pedantic warnings are also disabled in the expression
that follows "__extension__". However, only system header files should use these
escape routes; application programs should avoid them.
Some users try to use -pedantic to check programs for strict ISO C conformance. They
soon find that it does not do quite what they want: it finds some non-ISO practices,
but not all---only those for which ISO C requires a diagnostic, and some others for
which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some instances,
but would require considerable additional work and would be quite different from
-pedantic. We don't have plans to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended dialect of C, such as
gnu89 or gnu99, there is a corresponding base standard, the version of ISO C on which
the GNU extended dialect is based. Warnings from -pedantic are given where they are
required by the base standard. (It would not make sense for such warnings to be given
only for features not in the specified GNU 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 pro-
grammers often forget that this type is signed on some machines.
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Back-
slash-Newline appears in a // comment.
-Wformat
Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied
have types appropriate to the format string specified, and that the conversions speci-
fied 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.
The formats are checked against the format features supported by GNU libc version 2.2.
These include all ISO C90 and C99 features, as well as features from the Single Unix
Specification and some BSD and GNU extensions. Other library implementations may not
support all these features; GCC does not support warning about features that go beyond
a particular library's limitations. However, if -pedantic is used with -Wformat,
warnings will be given about format features not in the selected standard version (but
not for "strfmon" formats, since those are not in any version of the C standard).
Since -Wformat also checks for null format arguments for several functions, -Wformat
also implies -Wnonnull.
-Wformat is included in -Wall. For more control over some aspects of format checking,
the options -Wformat-y2k, -Wno-format-extra-args, -Wno-format-zero-length, -Wfor-
mat-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 implemen-
tation could not know what type to pass to "va_arg" to skip the unused arguments.
However, in the case of "scanf" formats, this option will suppress the warning if the
unused arguments are all pointers, since the Single Unix Specification says that such
unused arguments are allowed.
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The C standard spec-
ifies that zero-length formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string literal and
so cannot be checked, unless the format function takes its format arguments as a
"va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions that represent pos-
sible security problems. At present, this warns about calls to "printf" and "scanf"
functions where the format string is not a string literal and there are no format
arguments, as in "printf (foo);". This may be a security hole if the format string
came from untrusted input and contains %n. (This is currently a subset of what -Wfor-
mat-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 equivalent 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++, and Objective-C only)
Warn about uninitialized variables which are initialized with themselves. Note this
option can only be used with the -Wuninitialized option, which in turn only works with
-O1 and above.
For example, GCC will warn about "i" being uninitialized in the following snippet only
when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int
Warn when a declaration does not specify a type.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being declared.
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration.
-Wmain
Warn if the type of main is suspicious. main should be a function with external link-
age, returning int, taking either zero arguments, two, or three arguments of appropri-
ate types.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the following
example, the initializer for a is not fully bracketed, but that for b is fully brack-
eted.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assign-
ment in a context where a truth value is expected, or when operators are nested whose
precedence people often get confused about.
Also warn about constructions where there may be confusion to which "if" statement an
"else" branch belongs. Here is an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every "else" branch belongs to the innermost possible "if" statement, which in
this example is "if (b)". This is often not what the programmer expected, as illus-
trated in the above example by indentation the programmer chose. When there is the
potential for this confusion, GCC will issue a warning when this flag is specified.
To eliminate the warning, add explicit braces around the innermost "if" statement so
there is no way the "else" could belong to the enclosing "if". The resulting code
would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
-Wsequence-point
Warn about code that may have undefined semantics because of violations of sequence
point rules in the C standard.
The C standard defines the order in which expressions in a C program are evaluated in
terms of sequence points, which represent a partial ordering between the execution of
parts of the program: those executed before the sequence point, and those executed
after it. These occur after the evaluation of a full 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 evalu-
ation of its arguments and the expression denoting the called function), and in cer-
tain other places. Other than as expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not specified. All these rules
describe only a partial order rather than a total order, since, for example, if two
functions are called within one expression with no sequence point between them, the
order in which the functions are called is not specified. However, the standards 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 implementa-
tion 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 imple-
mentation may also work for C++ programs.
The C standard is worded confusingly, therefore there is some debate over the precise
meaning of the sequence point rules in subtle cases. Links to discussions of the
problem, including proposed formal definitions, may be found on the GCC readings page,
at <http://gcc.gnu.org/readings.html>.
-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++, 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.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case"
for one or more of the named codes of that enumeration. (The presence of a "default"
label prevents this warning.) "case" labels outside the enumeration range also pro-
voke warnings when this option is used.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case"
for one or more of the named codes of that enumeration. "case" labels outside the
enumeration range also provoke warnings when this option is used.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program
(trigraphs within comments are not warned about).
-Wunused-function
Warn whenever a static function is declared but not defined or a non\-inline static
function is unused.
-Wunused-label
Warn whenever a label is declared but not used.
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
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used.
To suppress this warning cast the expression to void.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must either specify
-Wextra -Wunused (note that -Wall implies -Wunused), or separately specify
-Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being initialized or if a variable
may be clobbered by a "setjmp" call.
These warnings are possible only in optimizing compilation, because they require data
flow information that is computed only when optimizing. If you don't specify -O, you
simply won't get these warnings.
If you want to warn about code which uses the uninitialized value of the variable in
its own initializer, use the -Winit-self option.
These warnings occur only for variables that are candidates for register allocation.
Therefore, they do not occur for a variable that is declared "volatile", or whose
address is taken, or whose size is other than 1, 2, 4 or 8 bytes. Also, they do not
occur for structures, unions or arrays, even when they are in registers.
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 rea-
sons 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 compila-
tion.
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".
-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 sys-
tem 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 optimiza-
tion. The warning does not catch all cases, but does attempt to catch the more common
pitfalls. It is included in -Wall.
-Wall
All of the above -W options combined. This enables all the warnings about construc-
tions 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 lan-
guage-specific warnings described in C++ Dialect Options and Objective-C Dialect
Options.
The following -W... options are not implied by -Wall. Some of them warn about construc-
tions that users generally do not consider questionable, but which occasionally you might
wish to check for; others warn about constructions that are necessary or hard to avoid in
some cases, and there is no simple way to modify the code to suppress the warning.
-Wextra
(This option used to be called -W. The older name is still supported, but the newer
name is more 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 example, this func-
tion would evoke such a warning:
foo (a)
{
if (a > 0)
return a;
}
* An expression-statement or the left-hand side of a comma expression contains no
side effects. To 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 >=.
* 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 mathematical notation.
* Storage-class specifiers like "static" are not the first things in a declaration.
According to the C Standard, this usage is obsolescent.
* 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.
(But don't warn about the GNU extension of "volatile void" return types. That
extension will be warned about if -pedantic is specified.)
* 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-com-
pare is also specified.)
* An aggregate has an initializer which does not initialize all members. For exam-
ple, the following code would cause such a warning, because "x.h" would be implic-
itly initialized to zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
* A function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }
* An empty body occurs in an if or else statement.
* A pointer is compared against integer zero with <, <=, >, or >=.
* A variable might be changed by longjmp or vfork.
* Any of several floating-point events that often indicate errors, such as overflow,
underflow, loss of precision, etc.
*<(C++ only)>
An enumerator and a non-enumerator both appear in a conditional expression.
*<(C++ only)>
A non-static reference or non-static const member appears in a class without con-
structors.
*<(C++ only)>
Ambiguous virtual bases.
*<(C++ only)>
Subscripting an array which has been declared register.
*<(C++ only)>
Taking the address of a variable which has been declared register.
*<(C++ only)>
A base class is not initialized in a derived class' copy constructor.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating point division by
zero is not warned about, as it can be a legitimate way of obtaining infinities and
NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from
system headers are normally suppressed, on the assumption that they usually do not
indicate real problems and would only make the compiler output harder to read. Using
this command line option tells GCC to emit warnings from system 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 con-
sider 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 computation introduces, and allow
for it when performing comparisons (and when producing 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 comparisons 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 tradi-
tional C macro replacement takes place within string literals, but does not in ISO
C.
* In traditional C, some preprocessor directives did not exist. Traditional prepro-
cessors 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 rec-
ognize #elif, so it suggests avoiding it altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating point constant suffixes.
(Traditional C does support the L suffix on integer constants.) Note, these suf-
fixes appear in macros defined in the system headers of most modern systems, e.g.
the _MIN/_MAX macros in "". Use of these macros in user code might nor-
mally 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 represent bit patterns, are not
warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a separate namespace for
labels.
* Initialization of unions. If the initializer is zero, the warning is omitted.
This is done under the assumption that the zero initializer in user code appears
conditioned on e.g. "__STDC__" to avoid missing initializer warnings and relies on
default initialization to zero in the traditional C case.
* Conversions by prototypes between fixed/floating point values and vice versa. The
absence of these 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 compatibility macros,
"PARAMS" and "VPARAMS". This warning is also bypassed for nested functions
because that feature is already a GCC extension and thus not relevant to tradi-
tional 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.
-Wendif-labels
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 target 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 warning; when com-
piling C++, warn about the deprecated conversion from string constants 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 decla-
rations 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 conversions of
fixed point to floating and vice versa, and conversions changing the width or signed-
ness 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 declara-
tion 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 func-
tions that are not declared in header files.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute "noreturn". Note these
are only possible candidates, not absolute ones. Care should be taken to manually
verify functions actually do not ever return before adding the "noreturn" attribute,
otherwise subtle code generation bugs could be introduced. You will not get a warning
for "main" in hosted C environments.
-Wmissing-format-attribute
If -Wformat is enabled, also warn about functions which might be candidates for "for-
mat" attributes. Note these are only possible candidates, not absolute ones. GCC
will guess that "format" attributes might be appropriate for any function that calls a
function like "vprintf" or "vscanf", but this might not always be the case, and some
functions for which "format" attributes are appropriate may not be detected. This
option has no effect unless -Wformat is enabled (possibly by -Wall).
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a
typo in the user's code, as they have implementation-defined values, and should not be
used in portable code.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as deprecated by
using the "deprecated" attribute. (@pxref{Function Attributes}, @pxref{Variable
Attributes}, @pxref{Type Attributes}.)
-Wpacked
Warn if a structure is given the packed attribute, but the packed attribute has no
effect on the layout or size of the structure. Such structures may be mis-aligned for
little benefit. For instance, in this code, the variable "f.x" in "struct bar" will
be misaligned even though "struct bar" does not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpadded
Warn if padding is included in a structure, either to align an element of the struc-
ture or to align the whole structure. Sometimes when this happens it is possible to
rearrange the fields of the structure to reduce the padding and so make the structure
smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases where
multiple declaration is valid and changes nothing.
-Wnested-externs (C only)
Warn if an "extern" declaration is encountered within a function.
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at least a whole line
of source code will never be executed, because some condition is never satisfied or
because it is after a procedure that never returns.
It is possible for this option to produce a warning even though there are circum-
stances under which part of the affected line can be executed, so care should be taken
when removing apparently-unreachable code.
For instance, when a function is inlined, a warning may mean that the line is unreach-
able in only one inlined copy of the function.
This option is not made part of -Wall because in a debugging version of a program
there is often substantial code which checks correct functioning of the program and
is, hopefully, unreachable because the program does work. Another common use of
unreachable code is to provide behavior which is selectable at compile-time.
-Winline
Warn if a function can not be inlined and it was declared as inline. Even with this
option, the compiler will not warn about failures to inline functions declared in sys-
tem headers.
The compiler uses a variety of heuristics to determine whether or not to inline a
function. For example, the compiler takes into account the size of the function being
inlined and the the amount of inlining that has already been done in the current func-
tion. Therefore, seemingly insignificant changes in the source program can cause the
warnings produced by -Winline to appear or disappear.
-Wno-invalid-offsetof (C++ only)
Suppress warnings from applying the offsetof macro to a non-POD type. According to
the 1998 ISO C++ standard, applying offsetof to a non-POD type is undefined. In
existing C++ implementations, however, offsetof typically gives meaningful results
even when applied to certain kinds of non-POD types. (Such as a simple struct that
fails to be a POD type only by virtue of having a constructor.) This flag is for
users who are aware that they are writi
|