GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
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] [@file] infile...
Only the most useful options are listed here; see below for the
remainder. g++ accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The "overall options" allow you to stop this
process at an intermediate stage. For example, the -c option says not
to run the linker. Then the output consists of object files output by
the assembler.
Other options are passed on to one stage of processing. Some options
control the preprocessor and others the compiler itself. Yet other
options control the assembler and linker; most of these are not
documented here, since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful
for C programs; when an option is only useful with another language
(usually C++), the explanation says so explicitly. If the description
for a particular option does not mention a source language, you can
use that option with all supported languages.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dr is very different from -d -r.
You can mix options and other arguments. For the most part, the order
you use doesn't matter. Order does matter when you use several
options of the same kind; for example, if you specify -L more than
once, the directories are searched in the order specified.
Many options have long names starting with -f or with -W---for
example, -fmove-loop-invariants, -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
- 1 - Formatted: November 5, 2008
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
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 @file
C Language Options
-ansi -std=standard -fgnu89-inline -aux-info filename -fno-asm
-fno-builtin -fno-builtin-function -fhosted -ffreestanding
-fopenmp -fms-extensions -trigraphs -no-integrated-cpp
-traditional -traditional-cpp -fallow-single-precision
-fcond-mismatch -fsigned-bitfields -fsigned-char
-funsigned-bitfields -funsigned-char
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space
-ffriend-injection -fno-elide-constructors -fno-enforce-eh-specs
-ffor-scope -fno-for-scope -fno-gnu-keywords
-fno-implicit-templates -fno-implicit-inline-templates
-fno-implement-inlines -fms-extensions -fno-nonansi-builtins
-fno-operator-names -fno-optional-diags -fpermissive -frepo
-fno-rtti -fstats -ftemplate-depth-n -fno-threadsafe-statics
-fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline
-fvisibility-inlines-hidden -Wabi -Wctor-dtor-privacy
-Wnon-virtual-dtor -Wreorder -Weffc++ -Wno-deprecated
-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-call-cxx-cdtors -fobjc-direct-dispatch
-fobjc-exceptions -fobjc-gc -freplace-objc-classes -fzero-link
-gen-decls -Wassign-intercept -Wno-protocol -Wselector
-Wstrict-selector-match -Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
-fdiagnostics-show-option
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall
-Waddress -Waggregate-return -Wno-attributes -Wc++-compat
-Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment
-Wconversion -Wno-deprecated-declarations -Wdisabled-optimization
-Wno-div-by-zero -Wno-endif-labels -Werror -Werror=*
-Werror-implicit-function-declaration -Wfatal-errors
- 2 - Formatted: November 5, 2008
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
-Wfloat-equal -Wformat -Wformat=2 -Wno-format-extra-args
-Wformat-nonliteral -Wformat-security -Wformat-y2k -Wimplicit
-Wimplicit-function-declaration -Wimplicit-int -Wimport
-Wno-import -Winit-self -Winline -Wno-int-to-pointer-cast
-Wno-invalid-offsetof -Winvalid-pch -Wlarger-than-len
-Wunsafe-loop-optimizations -Wlong-long -Wmain -Wmissing-braces
-Wmissing-field-initializers -Wmissing-format-attribute
-Wmissing-include-dirs -Wmissing-noreturn -Wno-multichar
-Wnonnull -Wno-overflow -Woverlength-strings -Wpacked -Wpadded
-Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast
-Wredundant-decls -Wreturn-type -Wsequence-point -Wshadow
-Wsign-compare -Wstack-protector -Wstrict-aliasing
-Wstrict-aliasing=2 -Wstrict-overflow -Wstrict-overflow=n -Wswitch
-Wswitch-default -Wswitch-enum -Wsystem-headers -Wtrigraphs
-Wundef -Wuninitialized -Wunknown-pragmas -Wno-pragmas
-Wunreachable-code -Wunused -Wunused-function -Wunused-label
-Wunused-parameter -Wunused-value -Wunused-variable
-Wvariadic-macros -Wvolatile-register-var -Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes
-Wnested-externs -Wold-style-definition -Wstrict-prototypes
-Wtraditional -Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-noaddr
-fdump-unnumbered -fdump-translation-unit[-n]
-fdump-class-hierarchy[-n] -fdump-ipa-all -fdump-ipa-cgraph
-fdump-tree-all -fdump-tree-original[-n] -fdump-tree-optimized[-n]
-fdump-tree-inlined[-n] -fdump-tree-cfg -fdump-tree-vcg
-fdump-tree-alias -fdump-tree-ch -fdump-tree-ssa[-n]
-fdump-tree-pre[-n] -fdump-tree-ccp[-n] -fdump-tree-dce[-n]
-fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n]
-fdump-tree-dom[-n] -fdump-tree-dse[-n] -fdump-tree-phiopt[-n]
-fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n]
-fdump-tree-nrv -fdump-tree-vect -fdump-tree-sink
-fdump-tree-sra[-n] -fdump-tree-salias -fdump-tree-fre[-n]
-fdump-tree-vrp[-n] -ftree-vectorizer-verbose=n
-fdump-tree-storeccp[-n] -feliminate-dwarf2-dups
-feliminate-unused-debug-types -feliminate-unused-debug-symbols
-femit-class-debug-always -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
- 3 - Formatted: November 5, 2008
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
-falign-functions[=n] -falign-jumps[=n] -falign-labels[=n]
-falign-loops[=n] -fmudflap -fmudflapth -fmudflapir
-fbranch-probabilities -fprofile-values -fvpt
-fbranch-target-load-optimize -fbranch-target-load-optimize2
-fbtr-bb-exclusive -fcaller-saves -fcprop-registers
-fcse-follow-jumps -fcse-skip-blocks -fcx-limited-range
-fdata-sections -fdelayed-branch -fdelete-null-pointer-checks
-fearly-inlining -fexpensive-optimizations -ffast-math
-ffloat-store -fforce-addr -ffunction-sections -fgcse -fgcse-lm
-fgcse-sm -fgcse-las -fgcse-after-reload -fcrossjumping
-fif-conversion -fif-conversion2 -finline-functions
-finline-functions-called-once -finline-limit=n
-fkeep-inline-functions -fkeep-static-consts -fmerge-constants
-fmerge-all-constants -fmodulo-sched -fno-branch-count-reg
-fno-default-inline -fno-defer-pop -fmove-loop-invariants
-fno-function-cse -fno-guess-branch-probability -fno-inline
-fno-math-errno -fno-peephole -fno-peephole2
-funsafe-math-optimizations -funsafe-loop-optimizations
-ffinite-math-only -fno-toplevel-reorder -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
-frename-registers -freorder-blocks
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -frounding-math -frtl-abstract-sequences
-fschedule-insns -fschedule-insns2 -fno-sched-interblock
-fno-sched-spec -fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns[=n] -fsched-stalled-insns-dep[=n]
-fsched2-use-superblocks -fsched2-use-traces -fsee
-freschedule-modulo-scheduled-loops -fsection-anchors
-fsignaling-nans -fsingle-precision-constant -fstack-protector
-fstack-protector-all -fstrict-aliasing -fstrict-overflow
-ftracer -fthread-jumps -funroll-all-loops -funroll-loops
-fpeel-loops -fsplit-ivs-in-unroller -funswitch-loops
-fvariable-expansion-in-unroller -ftree-pre -ftree-ccp
-ftree-dce -ftree-loop-optimize -ftree-loop-linear -ftree-loop-im
-ftree-loop-ivcanon -fivopts -ftree-dominator-opts -ftree-dse
-ftree-copyrename -ftree-sink -ftree-ch -ftree-sra -ftree-ter
-ftree-lrs -ftree-fre -ftree-vectorize -ftree-vect-loop-version
-ftree-salias -fipa-pta -fweb -ftree-copy-prop -ftree-store-ccp
-ftree-store-copy-prop -ftree-vrp -funit-at-a-time -fwhole-program
--param name=value -O -O0 -O1 -O2 -O3 -Os
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN
-Dmacro[=defn] -E -H -idirafter dir -include file -imacros file
-iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem
dir -imultilib dir -isysroot dir -M -MM -MF -MG -MP -MQ -MT
-nostdinc -P -fworking-directory -remap -trigraphs -undef
- 4 - Formatted: November 5, 2008
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
-Umacro -Wp,option -Xpreprocessor option
Assembler Option
-Wa,option -Xassembler option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs
-nostdlib -pie -rdynamic -s -static -static-libgcc -shared
-shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -iquotedir -Ldir -specs=file -I- --sysroot=dir
Target Options
-V version -b machine
Machine Dependent Options
ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section
-mdata=data-section -mrodata=readonly-data-section
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name
-mapcs-stack-check -mno-apcs-stack-check -mapcs-float
-mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant
-msched-prolog -mno-sched-prolog -mlittle-endian -mbig-endian
-mwords-little-endian -mfloat-abi=name -msoft-float -mhard-float
-mfpe -mthumb-interwork -mno-thumb-interwork -mcpu=name
-march=name -mfpu=name -mstructure-size-boundary=n
-mabort-on-noreturn -mlong-calls -mno-long-calls
-msingle-pic-base -mno-single-pic-base -mpic-register=reg
-mnop-fun-dllimport -mcirrus-fix-invalid-insns
-mno-cirrus-fix-invalid-insns -mpoke-function-name -mthumb -marm
-mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking
-mcallee-super-interworking -mtp=name
AVR Options -mmcu=mcu -msize -minit-stack=n -mno-interrupts
-mcall-prologues -mno-tablejump -mtiny-stack -mint8
Blackfin Options -momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer -mspecld-anomaly -mno-specld-anomaly
-mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k
-mid-shared-library -mno-id-shared-library -mshared-library-id=n
-mlong-calls -mno-long-calls
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n
-melinux-stacksize=n -metrax4 -metrax100 -mpdebug -mcc-init
-mno-side-effects -mstack-align -mdata-align -mconst-align
-m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt
-melf -maout -melinux -mlinux -sim -sim2 -mmul-bug-workaround
-mno-mul-bug-workaround
- 5 - Formatted: November 5, 2008
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
CRX Options -mmac -mpush-args
Darwin Options -all_load -allowable_client -arch
-arch_errors_fatal -arch_only -bind_at_load -bundle
-bundle_loader -client_name -compatibility_version
-current_version -dead_strip -dependency-file -dylib_file
-dylinker_install_name -dynamic -dynamiclib
-exported_symbols_list -filelist -flat_namespace
-force_cpusubtype_ALL -force_flat_namespace
-headerpad_max_install_names -image_base -init -install_name
-keep_private_externs -multi_module -multiply_defined
-multiply_defined_unused -noall_load
-no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs
-noprebind -noseglinkedit -pagezero_size -prebind
-prebind_all_twolevel_modules -private_bundle -read_only_relocs
-sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate
-sectobjectsymbols -sectorder -segaddr -segs_read_only_addr
-segs_read_write_addr -seg_addr_table -seg_addr_table_filename
-seglinkedit -segprot -segs_read_only_addr -segs_read_write_addr
-single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined
-unexported_symbols_list -weak_reference_mismatches -whatsloaded
-F -gused -gfull -mmacosx-version-min=version -mkernel
-mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -malpha-as -mgas
-mieee -mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode
-mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants
-mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix
-mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data
-mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
DEC Alpha/VMS Options -mvms-return-codes
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float
-msoft-float -malloc-cc -mfixed-cc -mdword -mno-dword -mdouble
-mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic
-minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp
-mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8
-mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move
-moptimize-membar -mno-optimize-membar -mscc -mno-scc
-mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch
-mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec
-mno-nested-cond-exec -mtomcat-stats -mTLS -mtls -mcpu=cpu
GNU/Linux Options -muclibc
H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300
- 6 - Formatted: November 5, 2008
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
HPPA Options -march=architecture-type -mbig-switch
-mdisable-fpregs -mdisable-indexing -mfast-indirect-calls -mgas
-mgnu-ld -mhp-ld -mfixed-range=register-range -mjump-in-delay
-mlinker-opt -mlong-calls -mlong-load-store -mno-big-switch
-mno-disable-fpregs -mno-disable-indexing
-mno-fast-indirect-calls -mno-gas -mno-jump-in-delay
-mno-long-load-store -mno-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-boundary=num -mmmx -msse
-msse2 -msse3 -m3dnow -mthreads -mno-align-stringops
-minline-all-stringops -mpush-args -maccumulate-outgoing-args
-m128bit-long-double -m96bit-long-double -mregparm=num
-msseregparm -mstackrealign -momit-leaf-frame-pointer
-mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model -m32
-m64 -mlarge-data-threshold=num
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld
-mno-pic -mvolatile-asm-stop -mregister-names -mno-sdata
-mconstant-gp -mauto-pic -minline-float-divide-min-latency
-minline-float-divide-max-throughput
-minline-int-divide-min-latency -minline-int-divide-max-throughput
-minline-sqrt-min-latency -minline-sqrt-max-throughput
-mno-dwarf2-asm -mearly-stop-bits -mfixed-range=register-range
-mtls-size=tls-size -mtune=cpu-type -mt -pthread -milp32 -mlp64
-mno-sched-br-data-spec -msched-ar-data-spec
-mno-sched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec -msched-ldc
-mno-sched-control-ldc -mno-sched-spec-verbose
-mno-sched-prefer-non-data-spec-insns
-mno-sched-prefer-non-control-spec-insns
-mno-sched-count-spec-in-critical-path
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops
-mno-align-loops -missue-rate=number -mbranch-cost=number
-mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func
-mflush-func=name -mno-flush-trap -mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -m68000 -m68020 -m68020-40 -m68020-60 -m68030
-m68040 -m68060 -mcpu32 -m5200 -mcfv4e -m68881 -mbitfield
-mc68000 -mc68020 -mnobitfield -mrtd -mshort -msoft-float
- 7 - Formatted: November 5, 2008
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
-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 -mshared -mno-shared -mxgot
-mno-xgot -mgp32 -mgp64 -mfp32 -mfp64 -mhard-float
-msoft-float -msingle-float -mdouble-float -mdsp
-mpaired-single -mips3d -mlong64 -mlong32 -msym32 -mno-sym32
-Gnum -membedded-data -mno-embedded-data
-muninit-const-in-rodata -mno-uninit-const-in-rodata
-msplit-addresses -mno-split-addresses -mexplicit-relocs
-mno-explicit-relocs -mcheck-zero-division
-mno-check-zero-division -mdivide-traps -mdivide-breaks -mmemcpy
-mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad
-mfused-madd -mno-fused-madd -nocpp -mfix-r4000 -mno-fix-r4000
-mfix-r4400 -mno-fix-r4400 -mfix-vr4120 -mno-fix-vr4120
-mfix-vr4130 -mfix-sb1 -mno-fix-sb1 -mflush-func=func
-mno-flush-func -mbranch-likely -mno-branch-likely
-mfp-exceptions -mno-fp-exceptions -mvr4130-align
-mno-vr4130-align
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon
-mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv
-mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict
-mbase-addresses -mno-base-addresses -msingle-exit
-mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mam33 -mno-am33
-mam33-2 -mno-am33-2 -mreturn-pointer-on-d0 -mno-crt0 -mrelax
MT Options -mno-crt0 -mbacc -msim -march=cpu-type
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45
-m10 -mbcopy -mbcopy-builtin -mint32 -mno-int16 -mint16
-mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32
-mabshi -mno-abshi -mbranch-expensive -mbranch-cheap -msplit
-mno-split -munix-asm -mdec-asm
- 8 - Formatted: November 5, 2008
GCC(1) gcc-4.2.3 GCC(1)
GNU GNU
2008-02-01
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-powerpc-gfxopt -mmfcrf
-mno-mfcrf -mpopcntb -mno-popcntb -mfprnd -mno-fprnd
-mnew-mnemonics -mold-mnemonics -mfull-toc -mminimal-toc
-mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-compat
-mno-xl-compat -mpe -malign-power -malign-natural -msoft-float
-mhard-float -mmultiple -mno-multiple -mstring -mno-string
-mupdate -mno-update -mfused-madd -mno-fused-madd -mbit-align
-mno-bit-align -mstrict-align -mno-strict-align -mrelocatable
-mno-relocatable -mrelocatable-lib -mno-relocatable-lib -mtoc
-mno-toc -mlittle -mlittle-endian -mbig -mbig-endian
-mdynamic-no-pic -maltivec -mswdiv
-mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme
-mcall-sysv -mcall-netbsd -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt -misel
-mno-isel -misel=yes -misel=no -mspe -mno-spe -mspe=yes -mspe=no
-mvrsave -mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb
-mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single
-mfloat-gprs=double -mprototype -mno-prototype -msim -mmvme
-mads -myellowknife -memb -msdata -msdata=opt -mvxworks
-mwindiss -G num -pthread
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type
-mhard-float -msoft-float -mlong-double-64 -mlong-double-128
-mbackchain -mno-backchain -mpacked-stack -mno-packed-stack
-msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31
-mdebug -mno-debug -mesa -mzarch -mtpf-trace -mno-tpf-trace
-mfused-madd -mno-fused-madd -mwarn-framesize
-mwarn-dynamicstack -mstack-size -mstack-guard
Score Options -meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u
-mscore7 -mscore7d
SH Options -m1 -m2 -m2e -m3 -m3e -m4-nofpu -m4-single-only
-m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al
-m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu
-m5-compact -m5-compact-nofpu -mb -ml -mdalign -mrelax
-mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave
-mieee -misize -mpadstruct -mspace -mprefergot -musermode
-multcost=number -mdiv=strategy -mdivsi3_libfunc=name
-madjust-unroll -mindexed-addressing -mgettrcost=number -mpt-fixed
-minvalid-symbols
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
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-m32 -m64 -mapp-regs -mno-app-regs -mfaster-structs
-mno-faster-structs -mfpu -mno-fpu -mhard-float -msoft-float
-mhard-quad-float -msoft-quad-float -mimpure-text
-mno-impure-text -mlittle-endian -mstack-bias -mno-stack-bias
-munaligned-doubles -mno-unaligned-doubles -mv8plus -mno-v8plus
-mvis -mno-vis -threads -pthreads -pthread
System V Options -Qy -Qn -YP,paths -Ym,dir
TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm
-mmemparm -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload
-mrpts=count -mrptb -mdb -mloop-unsigned -mparallel-insns
-mparallel-mpy -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-section-literals
-mno-text-section-literals -mtarget-align -mno-target-align
-mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -funwind-tables
-fasynchronous-unwind-tables -finhibit-size-directive
-finstrument-functions -fno-common -fno-ident -fpcc-struct-return
-fpic -fPIC -fpie -fPIE -fno-jump-tables -freg-struct-return
-fshort-enums -fshort-double -fshort-wchar -fverbose-asm
-fpack-struct[=n] -fstack-check -fstack-limit-register=reg
-fstack-limit-symbol=sym -fno-stack-limit -fargument-alias
-fargument-noalias -fargument-noalias-global
-fargument-noalias-anything -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
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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 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.
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file.mm
file.M
Objective-C++ source code which must be preprocessed.
file.mii
Objective-C++ source code which should not be preprocessed.
file.hh
file.H
C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR
Fixed form Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fixed form Fortran source code which must be preprocessed (with
the traditional preprocessor).
file.f90
file.f95
Free form Fortran source code which should not be preprocessed.
file.F90
file.F95
Free form Fortran source code which must be preprocessed (with the
traditional preprocessor).
file.ads
Ada source code file which contains a library unit declaration (a
declaration of a package, subprogram, or generic, or a generic
instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration). Such files are also
called specs.
file.adb
Ada source code file containing a library unit body (a subprogram
or package body). Such files are also called bodies.
file.s
Assembler code.
file.S
Assembler code which must be preprocessed.
other
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An object file to be fed straight into linking. Any file name
with no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on the
file name suffix). This option applies to all following input
files until the next -x option. Possible values for language are:
c c-header c-cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f95 f95-cpp-input
java
treelang
-x none
Turn off any specification of a language, so that subsequent files
are handled according to their file name suffixes (as they are if
-x has not been used at all).
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase
of the compiler returns a non-success return code. If you specify
-pass-exit-codes, the gcc program will instead return with
numerically highest error produced by any phase that returned an
error indication. The C, C++, and Fortran frontends return 4, if
an internal compiler error is encountered.
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.
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-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 -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 command lines.
-pipe
Use pipes rather than temporary files for communication between
the various stages of compilation. This fails to work on some
systems where the assembler is unable to read from a pipe; but the
GNU assembler has no trouble.
-combine
If you are compiling multiple source files, this option tells the
driver to pass all the source files to the compiler at once (for
those languages for which the compiler can handle this). This
will allow intermodule analysis (IMA) to be performed by the
compiler. Currently the only language for which this is supported
is C. If you pass source files for multiple languages to the
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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 (combined) .o or .s file.
--help
Print (on the standard output) a description of the command line
options understood by gcc. If the -v option is also specified
then --help will also be passed on to the various processes
invoked by gcc, so that they can display the command line options
they accept. If the -Wextra option is also specified then command
line options which have no documentation associated with them will
also be displayed.
--target-help
Print (on the standard output) a description of target specific
command line options for each tool.
--version
Display the version number and copyrights of the invoked GCC.
@file
Read command-line options from file. The options read are
inserted in place of the original @file option. If file does not
exist, or cannot be read, then the option will be treated
literally, and not removed.
Options in file are separated by whitespace. A whitespace
character may be included in an option by surrounding the entire
option in either single or double quotes. Any character
(including a backslash) may be included by prefixing the character
to be included with a backslash. The file may itself contain
additional @file options; any such options will be processed
recursively.
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, the use of gcc does not add the C++ library. g++ is a
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program that calls GCC and treats .c, .h and .i files as C++ source
files instead of C source files unless -x is used, and automatically
specifies linking against the C++ library. This program is also
useful when precompiling a C header file with a .h extension for use
in C++ compilations. 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, this is equivalent to -std=c89. In C++ mode, it is
equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with
ISO C90 (when compiling C code), or of standard C++ (when
compiling C++ code), such as the "asm" and "typeof" keywords, and
predefined macros such as "unix" and "vax" that identify the type
of system you are using. It also enables the undesirable and
rarely used ISO trigraph feature. For the C compiler, it disables
recognition of C++ style // comments as well as the "inline"
keyword.
The alternate keywords "__asm__", "__extension__", "__inline__"
and "__typeof__" continue to work despite -ansi. You would not
want to use them in an ISO C program, of course, but it is useful
to put them in header files that might be included in compilations
done with -ansi. Alternate predefined macros such as "__unix__"
and "__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected
gratuitously. For that, -pedantic is required in addition to
-ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is
used. Some header files may notice this macro and refrain from
declaring certain functions or defining certain macros that the
ISO standard doesn't call for; this is to avoid interfering with
any programs that might use these names for other things.
Functions that would normally be built in but do not have
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semantics defined by ISO C (such as "alloca" and "ffs") are not
built-in functions when -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-4.2/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
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"inline" keyword in ISO C99) are not disabled.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU
semantics for "inline" functions when in C99 mode.
Using this option is roughly equivalent to adding the
"gnu_inline" function attribute to all inline functions.
This option is accepted by GCC versions 4.1.3 and up. In GCC
versions prior to 4.3, C99 inline semantics are not supported, and
thus this option is effectively assumed to be present regardless
of whether or not it is specified; the only effect of specifying
it explicitly is to disable warnings about using inline functions
in C99 mode. Likewise, the option -fno-gnu89-inline is not
supported in versions of GCC before 4.3. It will be supported
only in C99 or gnu99 mode, not in C89 or gnu89 mode.
The preprocesor macros "__GNUC_GNU_INLINE__" and
"__GNUC_STDC_INLINE__" may be used to check which semantics are in
effect for "inline" functions.
-aux-info filename
Output to the given filename prototyped declarations for all
functions declared and/or defined in a translation unit, including
those in header files. This option is silently ignored in any
language other than C.
Besides declarations, the file indicates, in comments, the origin
of each declaration (source file and line), whether the
declaration was implicit, prototyped or unprototyped (I, N for new
or O for old, respectively, in the first character after the line
number and the colon), and whether it came from a declaration or a
definition (C or F, respectively, in the following character). In
the case of function definitions, a K&R-style list of arguments
followed by their declarations is also provided, inside comments,
after the declaration.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that
code can use these words as identifiers. You can use the keywords
"__asm__", "__inline__" and "__typeof__" instead. -ansi implies
-fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm"
and "inline" are standard keywords. You may want to use the
-fno-gnu-keywords flag instead, which has the same effect. In C99
mode (-std=c99 or -std=gnu99), this switch only affects the "asm"
and "typeof" keywords, since "inline" is a standard keyword in ISO
C99.
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-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.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and
"!$omp" in Fortran. When -fopenmp is specified, the compiler
generates parallel code according to the OpenMP Application
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Program Interface v2.5 <http://www.openmp.org/>.
-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 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.
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The type "char" is always a distinct type from each of "signed
char" or "unsigned char", even though its behavior is always just
like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the
negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned,
when the declaration does not use either "signed" or "unsigned".
By default, such a bit-field is signed, because this is
consistent: the basic integer types such as "int" are signed
types.
Options Controlling C++ Dialect
This section describes the command-line options that are only
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.
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-fcheck-new
Check that the pointer returned by "operator new" is non-null
before attempting to modify the storage allocated. This check is
normally unnecessary because the C++ standard specifies that
"operator new" will only return 0 if it is declared throw(), in
which case the compiler will always check the return value even
without this option. In all other cases, when "operator new" has
a non-empty exception specification, memory exhaustion is
signalled by throwing "std::bad_alloc". See also new (nothrow).
-fconserve-space
Put uninitialized or runtime-initialized global variables into the
common segment, as C does. This saves space in the executable at
the cost of not diagnosing duplicate definitions. If you compile
with this flag and your program mysteriously crashes after
"main()" has completed, you may have an object that is being
destroyed twice because two definitions were merged.
This option is no longer useful on most targets, now that support
has been added for putting variables into BSS without making them
common.
-ffriend-injection
Inject friend functions into the enclosing namespace, so that they
are visible outside the scope of the class in which they are
declared. Friend functions were documented to work this way in
the old Annotated C++ Reference Manual, and versions of G++ before
4.1 always worked that way. However, in ISO C++ a friend function
which is not declared in an enclosing scope can only be found
using argument dependent lookup. This option causes friends to be
injected as they were in earlier releases.
This option is for compatibility, and may be removed in a future
release of G++.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a
temporary which is only used to initialize another object of the
same type. Specifying this option disables that optimization, and
forces G++ to call the copy constructor in all cases.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception
specifications at runtime. This option violates the C++ standard,
but may be useful for reducing code size in production builds,
much like defining NDEBUG. This does not give user code
permission to throw exceptions in violation of the exception
specifications; the compiler will still optimize based on the
specifications, so throwing an unexpected exception will result in
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undefined behavior.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a
for-init-statement is limited to the for loop itself, as specified
by the C++ standard. If -fno-for-scope is specified, the scope of
variables declared in a for-init-statement extends to the end of
the enclosing scope, as was the case in old versions of G++, and
other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but
to allow and give a warning for old-style code that would
otherwise be invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this
word as an identifier. You can use the keyword "__typeof__"
instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated
implicitly (i.e. by use); only emit code for explicit
instantiations.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates,
either. The default is to handle inlines differently so that
compiles with and without optimization will need the same set of
explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
controlled by #pragma implementation. This will cause linker
errors if these functions are not inlined everywhere they are
called.
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as
implicit int and getting a pointer to member function via non-
standard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated
by ANSI/ISO C. These include "ffs", "alloca", "_exit", "index",
"bzero", "conjf", and other related functions.
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor",
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"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. The dynamic_cast operator can still be used for
casts that do not require runtime type information, i.e. casts to
"void *" or to unambiguous base classes.
-fstats
Emit statistics about front-end processing at the end of the
compilation. This information is generally only useful to the G++
development team.
-ftemplate-depth-n
Set the maximum instantiation depth for template classes to n. A
limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation. ANSI/ISO
C++ conforming programs must not rely on a maximum depth greater
than 17.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the
C++ ABI for thread-safe initialization of local statics. You can
use this option to reduce code size slightly in code that doesn't
need to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with
the "__cxa_atexit" function rather than the "atexit" function.
This option is required for fully standards-compliant handling of
static destructors, but will only work if your C library supports
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"__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This
will cause "std::uncaught_exception" to be incorrect, but is
necessary if the runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare
pointers to inline methods where the addresses of the two
functions were taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline
methods with "__attribute__ ((visibility ("hidden")))" so that
they do not appear in the export table of a DSO and do not require
a PLT indirection when used within the DSO. Enabling this option
can have a dramatic effect on load and link times of a DSO as it
massively reduces the size of the dynamic export table when the
library makes heavy use of templates.
The behaviour of this switch is not quite the same as marking the
methods as hidden directly, because it does not affect static
variables local to the function or cause the compiler to deduce
that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate
the effect of the switch for that method. For example, if you do
want to compare pointers to a particular inline method, you might
mark it as having default visibility. Marking the enclosing class
with explicit visibility will have no effect.
Explicitly instantiated inline methods are unaffected by this
option as their linkage might otherwise cross a shared library
boundary.
-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:
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-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline
functions; they just won't be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with
the vendor-neutral C++ ABI. Although an effort has been made to
warn about all such cases, there are probably some cases that are
not warned about, even though G++ is generating incompatible code.
There may also be cases where warnings are emitted even though the
code that is generated will be compatible.
You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be
binary compatible with code generated by other compilers.
The known incompatibilities at this point include:
* Incorrect handling of tail-padding for bit-fields. G++ may
attempt to pack data into the same byte as a base class. For
example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte
as"A::f1"; other compilers will not. You can avoid this
problem by explicitly padding "A" so that its size is a
multiple of the byte size on your platform; that will cause
G++ and other compilers to layout "B" identically.
* Incorrect handling of tail-padding for virtual bases. G++
does not use tail padding when laying out virtual bases. For
example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for
"A"; other compilers will. You can avoid this problem by
explicitly padding "A" so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++
and other compilers to layout "C" identically.
* Incorrect handling of bit-fields with declared widths greater
than that of their underlying types, when the bit-fields
appear in a union. For example:
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union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make
the union too small by the number of bits in an "int".
* Empty classes can be placed at incorrect offsets. For
example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes
that the "A" data member of "B" is already at offset zero.
* Names of template functions whose types involve "typename" or
template template parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable because all the constructors or
destructors in that class are private, and it has neither friends
nor public static member functions.
-Wnon-virtual-dtor (C++ only)
Warn when a class appears to be polymorphic, thereby requiring a
virtual destructor, yet it declares a non-virtual one. This
warning is also enabled if -Weffc++ is specified.
-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:
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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 compilers.
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-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are
declared within a template. Since the advent of explicit template
specification support in G++, if the name of the friend is an
unqualified-id (i.e., friend foo(int)), the C++ language
specification demands that the friend declare or define an
ordinary, nontemplate function. (Section 14.5.3). Before G++
implemented explicit specification, unqualified-ids could be
interpreted as a particular specialization of a templatized
function. Because this non-conforming behavior is no longer the
default behavior for G++, -Wnon-template-friend allows the
compiler to check existing code for potential trouble spots and is
on by default. This new compiler behavior can be turned off with
-Wno-non-template-friend which keeps the conformant compiler code
but disables the helpful warning.
-Wold-style-cast (C++ only)
Warn if an old-style (C-style) cast to a non-void type is used
within a C++ program. The new-style casts (dynamic_cast,
static_cast, reinterpret_cast, and const_cast) are less vulnerable
to unintended effects and much easier to search for.
-Woverloaded-virtual (C++ only)
Warn when a function declaration hides virtual functions from a
base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
-Wsign-promo (C++ only)
Warn when overload resolution chooses a promotion from unsigned or
enumerated type to a signed type, over a conversion to an unsigned
type of the same size. Previous versions of G++ would try to
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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 Objective-C and Objective-C++ programs, but you can
also use most of the language-independent GNU compiler options. For
example, you might compile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C
and Objective-C++ programs; you can use the other options with any
language supported by GCC.
Note that since Objective-C is an extension of the C language,
Objective-C compilations may also use options specific to the C
front-end (e.g., -Wtraditional). Similarly, Objective-C++
compilations may use C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and
Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each
literal string specified with the syntax "@"..."". The default
class name is "NXConstantString" if the GNU runtime is being used,
and "NSConstantString" if the NeXT runtime is being used (see
below). The -fconstant-cfstrings option, if also present, will
override the -fconstant-string-class setting and cause "@"...""
literals to be laid out as constant CoreFoundation strings.
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-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-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables
is a C++ object with a non-trivial default constructor. If so,
synthesize a special "- (id) .cxx_construct" instance method that
will run non-trivial default constructors on any such instance
variables, in order, and then return "self". Similarly, check if
any instance variable is a C++ object with a non-trivial
destructor, and if so, synthesize a special "- (void)
.cxx_destruct" method that will run all such default destructors,
in reverse order.
The "- (id) .cxx_construct" and/or "- (void) .cxx_destruct"
methods thusly generated will only operate on instance variables
declared in the current Objective-C class, and not those inherited
from superclasses. It is the responsibility of the Objective-C
runtime to invoke all such methods in an object's inheritance
hierarchy. The "- (id) .cxx_construct" methods will be invoked by
the runtime immediately after a new object instance is allocated;
the "- (void) .cxx_destruct" methods will be invoked immediately
before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and
later has support for invoking the "- (id) .cxx_construct" and "-
(void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is
accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++ and Java. This
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option is unavailable in conjunction with the NeXT runtime on Mac
OS X 10.2 and earlier.
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
The @throw statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a @catch block, the
@throw may appear without an argument (as shown above), in which
case the object caught by the @catch will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and
caught using this scheme. When an object is thrown, it will be
caught by the nearest @catch clause capable of handling objects of
that type, analogously to how "catch" blocks work in C++ and Java.
A "@catch(id ...)" clause (as shown above) may also be provided to
catch any and all Objective-C exceptions not caught by previous
@catch clauses (if any).
The @finally clause, if present, will be executed upon exit from
the immediately preceding "@try ... @catch" section. This will
happen regardless of whether any exceptions are thrown, caught or
rethrown inside the "@try ... @catch" section, analogously to the
behavior of the "finally" clause in Java.
There are several caveats to using the new exception mechanism:
* Although currently designed to be binary compatible with
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"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.
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++
programs.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in
the resulting object file, and allow dyld(1) to load it in at run
time instead. This is used in conjunction with the Fix-and-
Continue debugging mode, where the object file in question may be
recompiled and dynamically reloaded in the course of program
execution, without the need to restart the program itself.
Currently, Fix-and-Continue functionality is only available in
conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily
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replaces calls to "objc_getClass("...")" (when the name of the
class is known at compile time) with static class references that
get initialized at load time, which improves run-time performance.
Specifying the -fzero-link flag suppresses this behavior and
causes calls to "objc_getClass("...")" to be retained. This is
useful in Zero-Link debugging mode, since it allows for individual
class implementations to be modified during program execution.
-gen-decls
Dump interface declarations for all classes seen in the source
file to a file named sourcename.decl.
-Wassign-intercept
Warn whenever an Objective-C assignment is being intercepted by
the garbage collector.
-Wno-protocol
If a class is declared to implement a protocol, a warning is
issued for every method in the protocol that is not implemented by
the class. The default behavior is to issue a warning for every
method not explicitly implemented in the class, even if a method
implementation is inherited from the superclass. If you use the
-Wno-protocol option, then methods inherited from the superclass
are considered to be implemented, and no warning is issued for
them.
-Wselector
Warn if multiple methods of different types for the same selector
are found during compilation. The check is performed on the list
of methods in the final stage of compilation. Additionally, a
check is performed for each selector appearing in a
"@selector(...)" expression, and a corresponding method for that
selector has been found during compilation. Because these checks
scan the method table only at the end of compilation, these
warnings are not produced if the final stage of compilation is not
reached, for example because an error is found during compilation,
or because the -fsyntax-only option is being used.
-Wstrict-selector-match
Warn if multiple methods with differing argument and/or return
types are found for a given selector when attempting to send a
message using this selector to a receiver of type "id" or "Class".
When this flag is off (which is the default behavior), the
compiler will omit such warnings if any differences found are
confined to types which share the same size and alignment.
-Wundeclared-selector
Warn if a "@selector(...)" expression referring to an undeclared
selector is found. A selector is considered undeclared if no
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method with that name has been declared before the
"@selector(...)" expression, either explicitly in an @interface or
@protocol declaration, or implicitly in an @implementation
section. This option always performs its checks as soon as a
"@selector(...)" expression is found, while -Wselector only
performs its checks in the final stage of compilation. This also
enforces the coding style convention that methods and selectors
must be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed
by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of
the output device's aspect (e.g. its width, ...). The options
described below can be used to control the diagnostic messages
formatting algorithm, e.g. how many characters per line, how often
source location information should be reported. Right now, only the
C++ front end can honor these options. However it is expected, in the
near future, that the remaining front ends would be able to digest
them correctly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n
characters. The default is 72 characters for g++ and 0 for the
rest of the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a
single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit once source location information; that
is, in case the message is too long to fit on a single physical
line and has to be wrapped, the source location won't be emitted
(as prefix) again, over and over, in subsequent continuation
lines. This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location information (as
prefix) for physical lines that result from the process of
breaking a message which is too long to fit on a single line.
-fdiagnostics-show-option
This option instructs the diagnostic machinery to add text to each
diagnostic emitted, which indicates which command line option
directly controls that diagnostic, when such an option is known to
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the diagnostic machinery.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are
not inherently erroneous but which are risky or suggest there may have
been an error.
You can request many specific warnings with options beginning -W, for
example -Wimplicit to request warnings on implicit declarations. Each
of these specific warning options also has a negative form beginning
-Wno- to turn off warnings; for example, -Wno-implicit. This manual
lists only one of the two forms, whichever is not the default.
The following options control the amount and kinds of warnings
produced by GCC; for further, language-specific options also refer to
C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond
that.
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++;
reject all programs that use forbidden extensions, and some other
programs that do not follow ISO C and ISO C++. For ISO C, follows
the 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
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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 supplied have types appropriate to the format string
specified, and that the conversions specified in the format string
make sense. This includes standard functions, and others
specified by format attributes, in the "printf", "scanf",
"strftime" and "strfmon" (an X/Open extension, not in the C
standard) families (or other target-specific families). Which
functions are checked without format attributes having been
specified depends on the standard version selected, and such
checks of functions without the attribute specified are disabled
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by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by
GNU libc version 2.2. These include all ISO C90 and C99 features,
as well as features from the Single Unix Specification and some
BSD and GNU extensions. Other library implementations may not
support all these features; GCC does not support warning about
features that go beyond a particular library's limitations.
However, if -pedantic is used with -Wformat, warnings will be
given about format features not in the selected standard version
(but not for "strfmon" formats, since those are not in any version
of the C standard).
Since -Wformat also checks for null format arguments for several
functions, -Wformat also implies -Wnonnull.
-Wformat is included in -Wall. For more control over some aspects
of format checking, the options -Wformat-y2k,
-Wno-format-extra-args, -Wno-format-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 standard specifies that zero-length formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a
string literal and so cannot be checked, unless the format
function takes its format arguments as a "va_list".
-Wformat-security
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If -Wformat is specified, also warn about uses of format functions
that represent possible security problems. At present, this warns
about calls to "printf" and "scanf" functions where the format
string is not a string literal and there are no format arguments,
as in "printf (foo);". This may be a security hole if the format
string came from untrusted input and contains %n. (This is
currently a subset of what -Wformat-nonliteral warns about, but in
future warnings may be added to -Wformat-security that are not
included in -Wformat-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++, Objective-C and Objective-C++ only)
Warn about uninitialized variables which are initialized with
themselves. Note this option can only be used with the
-Wuninitialized option, which in turn only works with -O1 and
above.
For example, GCC will warn about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int
Warn when a declaration does not specify a type. This warning is
enabled by -Wall.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being
declared. The form -Wno-error-implicit-function-declaration is
not supported. This warning is enabled by -Wall (as a warning,
not an error).
-Wimplicit
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Same as -Wimplicit-int and -Wimplicit-function-declaration. This
warning is enabled by -Wall.
-Wmain
Warn if the type of main is suspicious. main should be a function
with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types. This
warning is enabled by -Wall.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed.
In the following example, the initializer for a is not fully
bracketed, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is
expected, or when operators are nested whose precedence people
often get confused about. Only the warning for an assignment used
as a truth value is supported when compiling C++; the other
warnings are only supported when compiling C.
Also warn if a comparison like x<=y<=z appears; this is equivalent
to (x<=y ? 1 : 0) <= z, which is a different interpretation from
that of ordinary mathematical notation.
Also warn about constructions where there may be confusion to
which "if" statement an "else" branch belongs. Here is an example
of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every "else" branch belongs to the innermost possible "if"
statement, which in this example is "if (b)". This is often not
what the programmer expected, as illustrated in the above example
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by indentation the programmer chose. When there is the potential
for this confusion, GCC will issue a warning when this flag is
specified. To eliminate the warning, add explicit braces around
the innermost "if" statement so there is no way the "else" could
belong to the enclosing "if". The resulting code would look like
this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of
violations of sequence point rules in the C and C++ standards.
The C and C++ standards defines the order in which expressions in
a C/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
evaluation of its arguments and the expression denoting the called
function), and in certain other places. Other than as expressed
by the sequence point rules, the order of evaluation of
subexpressions of an expression is not specified. All these rules
describe only a partial order rather than a total order, since,
for example, if two functions are called within one expression
with no sequence point between them, the order in which the
functions are called is not specified. However, the standards
committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to
the values of objects take effect. Programs whose behavior
depends on this have undefined behavior; the C and C++ standards
specify 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
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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 standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle
cases. Links to discussions of the problem, including proposed
formal definitions, may be found on the GCC readings page, at
<http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
-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 without this
option.
For C++, a function without return type always produces a
diagnostic message, even when -Wno-return-type is specified. The
only exceptions are main and functions defined in system headers.
This warning is enabled by -Wall.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type
and lacks a "case" for one or more of the named codes of that
enumeration. (The presence of a "default" label prevents this
warning.) "case" labels outside the enumeration range also
provoke warnings when this option is used. This warning is
enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type
and lacks a "case" for one or more of the named codes of that
enumeration. "case" labels outside the enumeration range also
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provoke warnings when this option is used.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the
meaning of the program (trigraphs within comments are not warned
about). This warning is enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a
non-inline static function is unused. This warning is enabled by
-Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is
enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its
declaration.
To suppress this warning use the unused attribute.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is
unused aside from its declaration. This warning is enabled by
-Wall.
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not
used. This warning is enabled by -Wall.
To suppress this warning cast the expression to void.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you
must either specify -Wextra -Wunused (note that -Wall implies
-Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being
initialized or if a variable may be clobbered by a "setjmp" call.
These warnings are possible only in optimizing compilation,
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because they require data flow information that is computed only
when optimizing. If you do not specify -O, you will not get these
warnings. Instead, GCC will issue a warning about -Wuninitialized
requiring -O.
If you want to warn about code which uses the uninitialized value
of the variable in its own initializer, use the -Winit-self
option.
These warnings occur for individual uninitialized or clobbered
elements of structure, union or array variables as well as for
variables which are uninitialized 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:
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{
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.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect
parameters, invalid syntax, or conflicts between pragmas. See
also -Wunknown-pragmas.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It
warns about code which might break the strict aliasing rules that
the compiler is using for optimization. The warning does not
catch all cases, but does attempt to catch the more common
pitfalls. It is included in -Wall.
-Wstrict-aliasing=2
This option is only active when -fstrict-aliasing is active. It
warns about code which might break the strict aliasing rules that
the compiler is using for optimization. This warning catches more
cases than -Wstrict-aliasing, but it will also give a warning for
some ambiguous cases that are safe.
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-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when -fstrict-overflow is active. It
warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it does
not warn about all cases where the code might overflow: it only
warns about cases where the compiler implements some optimization.
Thus this warning depends on the optimization level.
An optimization which assumes that signed overflow does not occur
is perfectly safe if the values of the variables involved are such
that overflow never does, in fact, occur. Therefore this warning
can easily give a false positive: a warning about code which is
not actually a problem. To help focus on important issues,
several warning levels are defined. No warnings are issued for
the use of undefined signed overflow when estimating how many
iterations a loop will require, in particular when determining
whether a loop will be executed at all.
-Wstrict-overflow=1
Warn about cases which are both questionable and easy to
avoid. For example: "x + 1 > x"; with -fstrict-overflow, the
compiler will simplify this to 1. This level of
-Wstrict-overflow is enabled by -Wall; higher levels are not,
and must be explicitly requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified
to a constant. For example: "abs (x) >= 0". This can only be
simplified when -fstrict-overflow is in effect, because "abs
(INT_MIN)" overflows to "INT_MIN", which is less than zero.
-Wstrict-overflow (with no level) is the same as
-Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified.
For example: "x + 1 > 1" will be simplified to "x > 0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above
cases. For example: "(x * 10) / 5" will be simplified to "x *
2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude
of a constant involved in a comparison. For example: "x + 2 >
y" will be simplified to "x + 1 >= y". This is reported only
at the highest warning level because this simplification
applies to many comparisons, so this warning level will give a
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very large number of false positives.
-Wall
All of the above -W options combined. This enables all the
warnings about constructions 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 constructions that users generally do not consider
questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary |
