gcc; man page
Man Pages Index
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below for the remain‐
der. g++ accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The "overall options" allow you to stop this
process at an intermediate stage. For example, the -c option says not
to run the linker. Then the output consists of object files output by
the assembler.
Other options are passed on to one stage of processing. Some options
control the preprocessor and others the compiler itself. Yet other
options control the assembler and linker; most of these are not docu‐
mented here, since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful
for C programs; when an option is only useful with another language
(usually C++), the explanation says so explicitly. If the description
for a particular option does not mention a source language, you can use
that option with all supported languages.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dr is very different from -d -r.
You can mix options and other arguments. For the most part, the order
you use doesn’t matter. Order does matter when you use several options
of the same kind; for example, if you specify -L more than once, the
directories are searched in the order specified.
Many options have long names starting with -f or with -W---for example,
-fstrength-reduce, -Wformat and so on. Most of these have both posi‐
tive and negative forms; the negative form of -ffoo would be -fno-foo.
This manual documents only one of these two forms, whichever one is not
the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations
are in the following sections.
Overall Options
words -fno-implicit-templates -fno-implicit-inline-templates
-fno-implement-inlines -fms-extensions -fno-nonansi-builtins
-fno-operator-names -fno-optional-diags -fpermissive -frepo
-fno-rtti -fstats -ftemplate-depth-n -fno-threadsafe-statics
-fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline
-fvisibility-inlines-hidden -Wabi -Wctor-dtor-privacy -Wnon-vir‐
tual-dtor -Wreorder -Weffc++ -Wno-deprecated -Wstrict-null-sen‐
tinel -Wno-non-template-friend -Wold-style-cast -Woverloaded-vir‐
tual -Wno-pmf-conversions -Wsign-promo
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers -fobjc-call-cxx-cdtors -fobjc-direct-dispatch
-fobjc-exceptions -fobjc-gc -freplace-objc-classes -fzero-link
-gen-decls -Wassign-intercept -Wno-protocol -Wselector
-Wstrict-selector-match -Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
-fdiagnostics-show-options
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall
-Waggregate-return -Wno-attributes -Wc++-compat -Wcast-align
-Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Wno-depre‐
cated-declarations -Wdisabled-optimization -Wno-div-by-zero
-Wno-endif-labels -Werror -Werror-implicit-function-declaration
-Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 -Wno-for‐
mat-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k
-Wimplicit -Wimplicit-function-declaration -Wimplicit-int -Wim‐
port -Wno-import -Winit-self -Winline -Wno-int-to-pointer-cast
-Wno-invalid-offsetof -Winvalid-pch -Wlarger-than-len -Wun‐
safe-loop-optimizations -Wlong-long -Wmain -Wmissing-braces
-Wmissing-field-initializers -Wmissing-format-attribute -Wmiss‐
ing-include-dirs -Wmissing-noreturn -Wno-multichar -Wnonnull
-Wpacked -Wpadded -Wparentheses -Wpointer-arith
-Wno-pointer-to-int-cast -Wredundant-decls -Wreturn-type -Wse‐
quence-point -Wshadow -Wsign-compare -Wstack-protector
-Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch -Wswitch-default
-Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef -Wuninitial‐
ized -Wunknown-pragmas -Wno-pragmas -Wunreachable-code -Wunused
-Wunused-function -Wunused-label -Wunused-parameter
-Wunused-value -Wunused-variable -Wvariadic-macros
-Wvolatile-register-var -Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes
-Wnested-externs -Wold-style-definition -Wstrict-prototypes
-Wtraditional -Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered
-ftest-coverage -ftime-report -fvar-tracking -g -glevel -gcoff
-gdwarf-2 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ -p
-pg -print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib -print-prog-name=program
-print-search-dirs -Q -save-temps -time
Optimization Options
-falign-functions=n -falign-jumps=n -falign-labels=n
-falign-loops=n -fbounds-check -fmudflap -fmudflapth -fmudflapir
-fbranch-probabilities -fprofile-values -fvpt -fbranch-tar‐
get-load-optimize -fbranch-target-load-optimize2 -fbtr-bb-exclusive
-fcaller-saves -fcprop-registers -fcse-follow-jumps
-fcse-skip-blocks -fcx-limited-range -fdata-sections -fde‐
layed-branch -fdelete-null-pointer-checks -fearly-inlining -fex‐
pensive-optimizations -ffast-math -ffloat-store -fforce-addr
-ffunction-sections -fgcse -fgcse-lm -fgcse-sm -fgcse-las
-fgcse-after-reload -floop-optimize -fcrossjumping -fif-conversion
-fif-conversion2 -finline-functions -finline-functions-called-once
-finline-limit=n -fkeep-inline-functions -fkeep-static-consts
-fmerge-constants -fmerge-all-constants -fmodulo-sched
-fno-branch-count-reg -fno-default-inline -fno-defer-pop
-floop-optimize2 -fmove-loop-invariants -fno-function-cse
-fno-guess-branch-probability -fno-inline -fno-math-errno
-fno-peephole -fno-peephole2 -funsafe-math-optimizations -fun‐
safe-loop-optimizations -ffinite-math-only -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-reg‐
ister-move -foptimize-sibling-calls -fprefetch-loop-arrays -fpro‐
file-generate -fprofile-use -fregmove -frename-registers -fre‐
order-blocks -freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -frerun-loop-opt -frounding-math -fsched‐
ule-insns -fschedule-insns2 -fno-sched-interblock -fno-sched-spec
-fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns=n -fsched-stalled-insns-dep=n
-fsched2-use-superblocks -fsched2-use-traces -freschedule-mod‐
ulo-scheduled-loops -fsignaling-nans -fsingle-precision-constant
-fstack-protector -fstack-protector-all -fstrength-reduce
-fstrict-aliasing -ftracer -fthread-jumps -funroll-all-loops
-funroll-loops -fpeel-loops -fsplit-ivs-in-unroller
-funswitch-loops -fvariable-expansion-in-unroller -ftree-pre
-ftree-ccp -ftree-dce -ftree-loop-optimize -ftree-loop-linear
-ftree-loop-im -ftree-loop-ivcanon -fivopts -ftree-dominator-opts
-ftree-dse -ftree-copyrename -ftree-sink -ftree-ch -ftree-sra
-ftree-ter -ftree-lrs -ftree-fre -ftree-vectorize
-ftree-vect-loop-version -ftree-salias -fweb -ftree-copy-prop
-ftree-store-ccp -ftree-store-copy-prop -fwhole-program --param
name=value -O -O0 -O1 -O2 -O3 -Os
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN
-Dmacro[=defn] -E -H -idirafter dir -include file -imacros file
-iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem
dir -imultilib dir -isysroot dir -M -MM -MF -MG -MP -MQ -MT
Target Options
-V version -b machine
Machine Dependent Options
ARC Options -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section
-mdata=data-section -mrodata=readonly-data-section
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name
-mapcs-stack-check -mno-apcs-stack-check -mapcs-float
-mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant -msched-pro‐
log -mno-sched-prolog -mlittle-endian -mbig-endian -mwords-lit‐
tle-endian -mfloat-abi=name -msoft-float -mhard-float -mfpe
-mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name
-mfpu=name -mstructure-size-boundary=n -mabort-on-noreturn
-mlong-calls -mno-long-calls -msingle-pic-base -mno-sin‐
gle-pic-base -mpic-register=reg -mnop-fun-dllimport -mcir‐
rus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-func‐
tion-name -mthumb -marm -mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking -mtp=name
AVR Options -mmcu=mcu -msize -minit-stack=n -mno-interrupts
-mcall-prologues -mno-tablejump -mtiny-stack -mint8
Blackfin Options -momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer -mspecld-anomaly -mno-specld-anomaly
-mcsync-anomaly -mno-csync-anomaly -mlow-64k -mno-low64k
-mid-shared-library -mno-id-shared-library -mshared-library-id=n
-mlong-calls -mno-long-calls
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n
-melinux-stacksize=n -metrax4 -metrax100 -mpdebug -mcc-init
-mno-side-effects -mstack-align -mdata-align -mconst-align
-m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt
-melf -maout -melinux -mlinux -sim -sim2 -mmul-bug-workaround
-mno-mul-bug-workaround
CRX Options -mmac -mpush-args
Darwin Options -all_load -allowable_client -arch
-arch_errors_fatal -arch_only -bind_at_load -bundle -bun‐
dle_loader -client_name -compatibility_version -current_version
-dead_strip -dependency-file -dylib_file -dylinker_install_name
-dynamic -dynamiclib -exported_symbols_list -filelist
-flat_namespace -force_cpusubtype_ALL -force_flat_namespace
-headerpad_max_install_names -image_base -init -install_name
-keep_private_externs -multi_module -multiply_defined -multi‐
ply_defined_unused -noall_load -no_dead_strip_inits_and_terms
-nofixprebinding -nomultidefs -noprebind -noseglinkedit
-pagezero_size -prebind -prebind_all_twolevel_modules -pri‐
vate_bundle -read_only_relocs -sectalign -sectobjectsymbols
-whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder
-segaddr -segs_read_only_addr -segs_read_write_addr -seg_addr_table
DEC Alpha/VMS Options -mvms-return-codes
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float
-msoft-float -malloc-cc -mfixed-cc -mdword -mno-dword -mdouble
-mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic
-minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp
-mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8 -mpack
-mno-pack -mno-eflags -mcond-move -mno-cond-move -moptimize-mem‐
bar -mno-optimize-membar -mscc -mno-scc -mcond-exec
-mno-cond-exec -mvliw-branch -mno-vliw-branch -mmulti-cond-exec
-mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec
-mtomcat-stats -mTLS -mtls -mcpu=cpu
H8/300 Options -mrelax -mh -ms -mn -mint32 -malign-300
HPPA Options -march=architecture-type -mbig-switch -mdis‐
able-fpregs -mdisable-indexing -mfast-indirect-calls -mgas
-mgnu-ld -mhp-ld -mfixed-range=register-range -mjump-in-delay
-mlinker-opt -mlong-calls -mlong-load-store -mno-big-switch
-mno-disable-fpregs -mno-disable-indexing -mno-fast-indirect-calls
-mno-gas -mno-jump-in-delay -mno-long-load-store
-mno-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 -mfp‐
math=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387
-msoft-float -msvr3-shlib -mno-wide-multiply -mrtd -malign-dou‐
ble -mpreferred-stack-boundary=num -mmmx -msse -msse2 -msse3
-m3dnow -mthreads -mno-align-stringops -minline-all-stringops
-mpush-args -maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mregparm=num -msseregparm
-momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs
-mcmodel=code-model -m32 -m64 -mlarge-data-threshold=num
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld
-mno-pic -mvolatile-asm-stop -mregister-names -mno-sdata -mcon‐
stant-gp -mauto-pic -minline-float-divide-min-latency -min‐
line-float-divide-max-throughput -minline-int-divide-min-latency
-minline-int-divide-max-throughput -minline-sqrt-min-latency -min‐
line-sqrt-max-throughput -mno-dwarf2-asm -mearly-stop-bits
-mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-type
-mt -pthread -milp32 -mlp64
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops
-mno-align-loops -missue-rate=number -mbranch-cost=number
-mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func
-mflush-func=name -mno-flush-trap -mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
-mno-lsim -mlittle-endian -mbig-endian -m210 -m340
-mstack-increment
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2
-mips3 -mips4 -mips32 -mips32r2 -mips64 -mips16 -mno-mips16
-mabi=abi -mabicalls -mno-abicalls -mxgot -mno-xgot -mgp32
-mgp64 -mfp32 -mfp64 -mhard-float -msoft-float -msingle-float
-mdouble-float -mdsp -mpaired-single -mips3d -mlong64 -mlong32
-msym32 -mno-sym32 -Gnum -membedded-data -mno-embedded-data
-muninit-const-in-rodata -mno-uninit-const-in-rodata
-msplit-addresses -mno-split-addresses -mexplicit-relocs
-mno-explicit-relocs -mcheck-zero-division -mno-check-zero-divi‐
sion -mdivide-traps -mdivide-breaks -mmemcpy -mno-memcpy
-mlong-calls -mno-long-calls -mmad -mno-mad -mfused-madd
-mno-fused-madd -nocpp -mfix-r4000 -mno-fix-r4000 -mfix-r4400
-mno-fix-r4400 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130
-mfix-sb1 -mno-fix-sb1 -mflush-func=func -mno-flush-func
-mbranch-likely -mno-branch-likely -mfp-exceptions -mno-fp-excep‐
tions -mvr4130-align -mno-vr4130-align
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon
-mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv -mto‐
plevel-symbols -melf -mbranch-predict -mno-branch-predict
-mbase-addresses -mno-base-addresses -msingle-exit -mno-sin‐
gle-exit
MN10300 Options -mmult-bug -mno-mult-bug -mam33 -mno-am33
-mam33-2 -mno-am33-2 -mreturn-pointer-on-d0 -mno-crt0 -mrelax
MT Options -mno-crt0 -mbacc -msim -march=cpu-type
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45
-m10 -mbcopy -mbcopy-builtin -mint32 -mno-int16 -mint16
-mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32 -mab‐
shi -mno-abshi -mbranch-expensive -mbranch-cheap -msplit
-mno-split -munix-asm -mdec-asm
PowerPC Options See RS/6000 and PowerPC Options.
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mpower
-mno-power -mpower2 -mno-power2 -mpowerpc -mpowerpc64 -mno-pow‐
erpc -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt
-mpowerpc-gfxopt -mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopc‐
ntb -mno-popcntb -mfprnd -mno-fprnd -mnew-mnemonics
-mold-mnemonics -mfull-toc -mminimal-toc -mno-fp-in-toc
-mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe
-malign-power -malign-natural -msoft-float -mhard-float -mmulti‐
ple -mno-multiple -mstring -mno-string -mupdate -mno-update
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable -mno-relocatable
-mrelocatable-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle
-mlittle-endian -mbig -mbig-endian -mdynamic-no-pic -maltivec
bug -mno-debug -mesa -mzarch -mtpf-trace -mno-tpf-trace
-mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack
-mstack-size -mstack-guard
SH Options -m1 -m2 -m2e -m3 -m3e -m4-nofpu -m4-single-only
-m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al
-m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu
-m5-compact -m5-compact-nofpu -mb -ml -mdalign -mrelax
-mbigtable -mfmovd -mhitachi -mrenesas -mno-renesas -mnomacsave
-mieee -misize -mpadstruct -mspace -mprefergot -musermode
-multcost=number -mdiv=strategy -mdivsi3_libfunc=name -mad‐
just-unroll -mindexed-addressing -mgettrcost=number -mpt-fixed
-minvalid-symbols
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
-m32 -m64 -mapp-regs -mno-app-regs -mfaster-structs
-mno-faster-structs -mfpu -mno-fpu -mhard-float -msoft-float
-mhard-quad-float -msoft-quad-float -mimpure-text
-mno-impure-text -mlittle-endian -mstack-bias -mno-stack-bias
-munaligned-doubles -mno-unaligned-doubles -mv8plus -mno-v8plus
-mvis -mno-vis -threads -pthreads -pthread
System V Options -Qy -Qn -YP,paths -Ym,dir
TMS320C3x/C4x Options -mcpu=cpu -mbig -msmall -mregparm -mmem‐
parm -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload -mrpts=count
-mrptb -mdb -mloop-unsigned -mparallel-insns -mparallel-mpy
-mpreserve-float
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep -mpro‐
log-function -mno-prolog-function -mspace -mtda=n -msda=n
-mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-dis‐
able-callt -mv850e1 -mv850e -mv850 -mbig-switch
VAX Options -mg -mgnu -munix
x86-64 Options See i386 and x86-64 Options.
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd
-mtext-section-literals -mno-text-section-literals -mtarget-align
-mno-target-align -mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -funwind-tables -fasynchronous-unwind-tables
-finhibit-size-directive -finstrument-functions -fno-common
-fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE
-fno-jump-tables -freg-struct-return -fshared-data -fshort-enums
cutable file.
For any given input file, the file name suffix determines what kind of
compilation is done:
file.c
C source code which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc
library to make an Objective-C program work.
file.mi
Objective-C source code which should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the
libobjc library to make an Objective-C++ program work. Note that
.M refers to a literal capital M.
file.mii
Objective-C++ source code which should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into
a precompiled header.
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code which must be preprocessed. Note that in .cxx, the
last two letters must both be literally x. Likewise, .C refers to
a literal capital C.
file.mm
file.M
Objective-C++ source code which must be preprocessed.
file.mii
Objective-C++ source code which should not be preprocessed.
file.f90
file.f95
Free form Fortran source code which should not be preprocessed.
file.F90
file.F95
Free form Fortran source code which must be preprocessed (with the
traditional preprocessor).
file.ads
Ada source code file which contains a library unit declaration (a
declaration of a package, subprogram, or generic, or a generic
instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration). Such files are also
called specs.
file.adb
Ada source code file containing a library unit body (a subprogram
or package body). Such files are also called bodies.
file.s
Assembler code.
file.S
Assembler code which must be preprocessed.
other
An object file to be fed straight into linking. Any file name with
no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on the
file name suffix). This option applies to all following input
files until the next -x option. Possible values for language are:
c c-header c-cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
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
-c Compile or assemble the source files, but do not link. The linking
stage simply is not done. The ultimate output is in the form of an
object file for each source file.
By default, the object file name for a source file is made by
replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly,
are ignored.
-S Stop after the stage of compilation proper; do not assemble. The
output is in the form of an assembler code file for each non-assem‐
bler input file specified.
By default, the assembler file name for a source file is made by
replacing the suffix .c, .i, etc., with .s.
Input files that don’t require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper.
The output is in the form of preprocessed source code, which is
sent to the standard output.
Input files which don’t require preprocessing are ignored.
-o file
Place output in file file. This applies regardless to whatever
sort of output is being produced, whether it be an executable file,
an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in
a.out, the object file for source.suffix in source.o, its assembler
file in source.s, a precompiled header file in source.suffix.gch,
and all preprocessed C source on standard output.
-v Print (on standard error output) the commands executed to run the
stages of compilation. Also print the version number of the com‐
piler driver program and of the preprocessor and the compiler
proper.
-###
Like -v except the commands are not executed and all command argu‐
ments are quoted. This is useful for shell scripts to capture the
driver-generated command lines.
-pipe
Use pipes rather than temporary files for communication between the
various stages of compilation. This fails to work on some systems
where the assembler is unable to read from a pipe; but the GNU
assembler has no trouble.
-combine
--help
Print (on the standard output) a description of the command line
options understood by gcc. If the -v option is also specified then
--help will also be passed on to the various processes invoked by
gcc, so that they can display the command line options they accept.
If the -Wextra option is also specified then command line options
which have no documentation associated with them will also be dis‐
played.
--target-help
Print (on the standard output) a description of target specific
command line options for each tool.
--version
Display the version number and copyrights of the invoked GCC.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp,
.CPP, .c++, .cp, or .cxx; C++ header files often use .hh or .H; and
preprocessed C++ files use the suffix .ii. GCC recognizes files with
these names and compiles them as C++ programs even if you call the com‐
piler the same way as for compiling C programs (usually with the name
gcc).
However, C++ programs often require class libraries as well as a com‐
piler that understands the C++ language---and under some circumstances,
you might want to compile programs or header files from standard input,
or otherwise without a suffix that flags them as C++ programs. You
might also like to precompile a C header file with a .h extension to be
used in C++ compilations. g++ is a program that calls GCC with the
default language set to C++, and automatically specifies linking
against the C++ library. On many systems, g++ is also installed with
the name c++.
When you compile C++ programs, you may specify many of the same com‐
mand-line options that you use for compiling programs in any language;
or command-line options meaningful for C and related languages; or
options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived
from C, such as C++, Objective-C and Objective-C++) that the compiler
accepts:
-ansi
In C mode, support all ISO C90 programs. In C++ mode, remove GNU
extensions that conflict with ISO C++.
This turns off certain features of GCC that are incompatible with
The -ansi option does not cause non-ISO programs to be rejected
gratuitously. For that, -pedantic is required in addition to
-ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is
used. Some header files may notice this macro and refrain from
declaring certain functions or defining certain macros that the ISO
standard doesn’t call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions which would normally be built in but do not have
semantics defined by ISO C (such as "alloca" and "ffs") are not
built-in functions with -ansi is used.
-std=
Determine the language standard. This option is currently only
supported when compiling C or C++. A value for this option must be
provided; possible values are
c89
iso9899:1990
ISO C90 (same as -ansi).
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported;
see <http://gcc.gnu.org/gcc-4.1/c99status.html> for more infor‐
mation. The names c9x and iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including some C99 fea‐
tures).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented
in GCC, this will become the default. The name gnu9x is depre‐
cated.
c++98
The 1998 ISO C++ standard plus amendments.
gnu++98
The same as -std=c++98 plus GNU extensions. This is the
default for C++ code.
Even when this option is not specified, you can still use some of
Besides declarations, the file indicates, in comments, the origin
of each declaration (source file and line), whether the declaration
was implicit, prototyped or unprototyped (I, N for new or O for
old, respectively, in the first character after the line number and
the colon), and whether it came from a declaration or a definition
(C or F, respectively, in the following character). In the case of
function definitions, a K&R-style list of arguments followed by
their declarations is also provided, inside comments, after the
declaration.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that
code can use these words as identifiers. You can use the keywords
"__asm__", "__inline__" and "__typeof__" instead. -ansi implies
-fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm"
and "inline" are standard keywords. You may want to use the
-fno-gnu-keywords flag instead, which has the same effect. In C99
mode (-std=c99 or -std=gnu99), this switch only affects the "asm"
and "typeof" keywords, since "inline" is a standard keyword in ISO
C99.
-fno-builtin
-fno-builtin-function
Don’t recognize built-in functions that do not begin with
__builtin_ as prefix.
GCC normally generates special code to handle certain built-in
functions more efficiently; for instance, calls to "alloca" may
become single instructions that adjust the stack directly, and
calls to "memcpy" may become inline copy loops. The resulting code
is often both smaller and faster, but since the function calls no
longer appear as such, you cannot set a breakpoint on those calls,
nor can you change the behavior of the functions by linking with a
different library. In addition, when a function is recognized as a
built-in function, GCC may use information about that function to
warn about problems with calls to that function, or to generate
more efficient code, even if the resulting code still contains
calls to that function. For example, warnings are given with
-Wformat for bad calls to "printf", when "printf" is built in, and
"strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function
function is disabled. function must not begin with __builtin_. If
a function is named this is not built-in in this version of GCC,
this option is ignored. There is no corresponding -fbuiltin-func‐
tion option; if you wish to enable built-in functions selectively
when using -fno-builtin or -ffreestanding, you may define macros
such as:
not necessarily be at "main". The most obvious example is an OS
kernel. This is equivalent to -fno-hosted.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
Some cases of unnamed fields in structures and unions are only
accepted with this option.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for
strict ISO C conformance) implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling.
This option allows a user supplied "cc1", "cc1plus", or "cc1obj"
via the -B option. The user supplied compilation step can then add
in an additional preprocessing step after normal preprocessing but
before compiling. The default is to use the integrated cpp (inter‐
nal cpp)
The semantics of this option will change if "cc1", "cc1plus", and
"cc1obj" are merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-
standard C compiler. They are now only supported with the -E
switch. The preprocessor continues to support a pre-standard mode.
See the GNU CPP manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second
and third arguments. The value of such an expression is void.
This option is not supported for C++.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It
is either like "unsigned char" by default or like "signed char" by
default.
Ideally, a portable program should always use "signed char" or
"unsigned char" when it depends on the signedness of an object.
But many programs have been written to use plain "char" and expect
it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let
you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed
char" or "unsigned char", even though its behavior is always just
when the declaration does not use either "signed" or "unsigned".
By default, such a bit-field is signed, because this is consistent:
the basic integer types such as "int" are signed types.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaning‐
ful for C++ programs; but you can also use most of the GNU compiler
options regardless of what language your program is in. For example,
you might compile a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs;
you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++
ABI that first appeared in G++ 3.4. Version 1 is the version of
the C++ ABI that first appeared in G++ 3.2. Version 0 will always
be the version that conforms most closely to the C++ ABI specifica‐
tion. Therefore, the ABI obtained using version 0 will change as
ABI bugs are fixed.
The default is version 2.
-fno-access-control
Turn off all access checking. This switch is mainly useful for
working around bugs in the access control code.
-fcheck-new
Check that the pointer returned by "operator new" is non-null
before attempting to modify the storage allocated. This check is
normally unnecessary because the C++ standard specifies that "oper‐
ator new" will only return 0 if it is declared 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
-fno-const-strings
Give string constants type "char *" instead of type "const char *".
By default, G++ uses type "const char *" as required by the stan‐
dard. Even if you use -fno-const-strings, you cannot actually mod‐
ify the value of a string constant.
This option might be removed in a future release of G++. For maxi‐
mum portability, you should structure your code so that it works
with string constants that have type "const char *".
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a tempo‐
rary which is only used to initialize another object of the same
type. Specifying this option disables that optimization, and
forces G++ to call the copy constructor in all cases.
-fno-enforce-eh-specs
Don’t generate code to check for violation of exception specifica‐
tions at runtime. This option violates the C++ standard, but may
be useful for reducing code size in production builds, much like
defining NDEBUG. This does not give user code permission to throw
exceptions in violation of the exception specifications; the com‐
piler will still optimize based on the specifications, so throwing
an unexpected exception will result in undefined behavior.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a
for-init-statement is limited to the for loop itself, as specified
by the C++ standard. If -fno-for-scope is specified, the scope of
variables declared in a for-init-statement extends to the end of
the enclosing scope, as was the case in old versions of G++, and
other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to
allow and give a warning for old-style code that would otherwise be
invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this
word as an identifier. You can use the keyword "__typeof__"
instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated
implicitly (i.e. by use); only emit code for explicit instantia‐
tions.
-fno-implicit-inline-templates
Don’t emit code for implicit instantiations of inline templates,
either. The default is to handle inlines differently so that com‐
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C. These include "ffs", "alloca", "_exit", "index",
"bzero", "conjf", and other related functions.
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor",
"compl", "not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need
to issue. Currently, the only such diagnostic issued by G++ is the
one for a name having multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to
warnings. Thus, using -fpermissive will allow some nonconforming
code to compile.
-frepo
Enable automatic template instantiation at link time. This option
also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual
functions for use by the C++ runtime type identification features
(dynamic_cast and typeid). If you don’t use those parts of the
language, you can save some space by using this flag. Note that
exception handling uses the same information, but it will generate
it as needed.
-fstats
Emit statistics about front-end processing at the end of the compi‐
lation. This information is generally only useful to the G++
development team.
-ftemplate-depth-n
Set the maximum instantiation depth for template classes to n. A
limit on the template instantiation depth is needed to detect end‐
less recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than
17.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++
ABI for thread-safe initialization of local statics. You can use
this option to reduce code size slightly in code that doesn’t need
to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with
the "__cxa_atexit" function rather than the "atexit" function.
typical when using templates and namespaces. For even more sav‐
ings, combine with the -fvisibility=hidden switch.
-fno-weak
Do not use weak symbol support, even if it is provided by the
linker. By default, G++ will use weak symbols if they are avail‐
able. This option exists only for testing, and should not be used
by end-users; it will result in inferior code and has no benefits.
This option may be removed in a future release of G++.
-nostdinc++
Do not search for header files in the standard directories specific
to C++, but do still search the other standard directories. (This
option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline func‐
tions; they just won’t be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with
the vendor-neutral C++ ABI. Although an effort has been made to
warn about all such cases, there are probably some cases that are
not warned about, even though G++ is generating incompatible code.
There may also be cases where warnings are emitted even though the
code that is generated will be compatible.
You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be
binary compatible with code generated by other compilers.
The known incompatibilities at this point include:
* Incorrect handling of tail-padding for bit-fields. G++ may
attempt to pack data into the same byte as a base class. For
example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte
as"A::f1"; other compilers will not. You can avoid this prob‐
lem by explicitly padding "A" so that its size is a multiple of
the byte size on your platform; that will cause G++ and other
compilers to layout "B" identically.
* Incorrect handling of tail-padding for virtual bases. G++ does
not use tail padding when laying out virtual bases. For exam‐
in a union. For example:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make
the union too small by the number of bits in an "int".
* Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes
that the "A" data member of "B" is already at offset zero.
* Names of template functions whose types involve "typename" or
template template parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable because all the constructors or
destructors in that class are private, and it has neither friends
nor public static member functions.
-Wnon-virtual-dtor (C++ only)
Warn when a class appears to be polymorphic, thereby requiring a
virtual destructor, yet it declares a non-virtual one. This warn‐
ing is enabled by -Wall.
-Wreorder (C++ only)
Warn when the order of member initializers given in the code does
not match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
* Item 12: Prefer initialization to assignment in constructors.
* Item 14: Make destructors virtual in base classes.
* Item 15: Have "operator=" return a reference to *this.
* Item 23: Don’t try to return a reference when you must return
an object.
Also warn about violations of the following style guidelines from
Scott Meyers’ More Effective C++ book:
* Item 6: Distinguish between prefix and postfix forms of incre‐
ment and decrement operators.
* Item 7: Never overload "&&", "||", or ",".
When selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use grep -v to filter
out those warnings.
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features.
-Wstrict-null-sentinel (C++ only)
Warn also about the use of an uncasted "NULL" as sentinel. When
compiling only with GCC this is a valid sentinel, as "NULL" is
defined to "__null". Although it is a null pointer constant not a
null pointer, it is guaranteed to of the same size as a pointer.
But this use is not portable across different compilers.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared
within a template. Since the advent of explicit template specifi‐
cation support in G++, if the name of the friend is an unqualified-
id (i.e., friend foo(int)), the C++ language specification demands
that the friend declare or define an ordinary, nontemplate func‐
tion. (Section 14.5.3). Before G++ implemented explicit specifi‐
cation, unqualified-ids could be interpreted as a particular spe‐
cialization of a templatized function. Because this non-conforming
behavior is no longer the default behavior for G++,
-Wnon-template-friend allows the compiler to check existing code
for potential trouble spots and is on by default. This new com‐
piler 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.
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
-Wsign-promo (C++ only)
Warn when overload resolution chooses a promotion from unsigned or
enumerated type to a signed type, over a conversion to an unsigned
type of the same size. Previous versions of G++ would try to pre‐
serve unsignedness, but the standard mandates the current behavior.
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator = (const
A&);, while cfront will use the user-defined operator =.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++
languages themselves. See
This section describes the command-line options that are only meaning‐
ful for Objective-C and Objective-C++ programs, but you can also use
most of the language-independent GNU compiler options. For example,
you might compile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C
and Objective-C++ programs; you can use the other options with any lan‐
guage supported by GCC.
Note that since Objective-C is an extension of the C language, Objec‐
tive-C compilations may also use options specific to the C front-end
(e.g., -Wtraditional). Similarly, Objective-C++ compilations may use
C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the
default for NeXT-based systems, including Darwin and Mac OS X. The
macro "__NEXT_RUNTIME__" is predefined if (and only if) this option
is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches (e.g., "[receiver
message:arg]") in this translation unit ensure that the receiver is
not "nil". This allows for more efficient entry points in the run‐
time to be used. Currently, this option is only available in con‐
junction with the NeXT runtime on Mac OS X 10.3 and later.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables
is a C++ object with a non-trivial default constructor. If so,
synthesize a special "- (id) .cxx_construct" instance method that
will run non-trivial default constructors on any such instance
variables, in order, and then return "self". Similarly, check if
any instance variable is a C++ object with a non-trivial destruc‐
tor, and if so, synthesize a special "- (void) .cxx_destruct"
method that will run all such default destructors, in reverse
order.
The "- (id) .cxx_construct" and/or "- (void) .cxx_destruct" methods
thusly generated will only operate on instance variables declared
in the current Objective-C class, and not those inherited from
superclasses. It is the responsibility of the Objective-C runtime
to invoke all such methods in an object’s inheritance hierarchy.
The "- (id) .cxx_construct" methods will be invoked by the runtime
immediately after a new object instance is allocated; the "- (void)
.cxx_destruct" methods will be invoked immediately before the run‐
time deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and
later has support for invoking the "- (id) .cxx_construct" and "-
(void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is
accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++ and Java. Cur‐
rently, this option is only available in conjunction with the NeXT
runtime on Mac OS X 10.3 and later.
@try {
...
@throw expr;
}
@finally {
...
@throw expr;
...
}
The @throw statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a @catch block, the
@throw may appear without an argument (as shown above), in which
case the object caught by the @catch will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and
caught using this scheme. When an object is thrown, it will be
caught by the nearest @catch clause capable of handling objects of
that type, analogously to how "catch" blocks work in C++ and Java.
A "@catch(id ...)" clause (as shown above) may also be provided to
catch any and all Objective-C exceptions not caught by previous
@catch clauses (if any).
The @finally clause, if present, will be executed upon exit from
the immediately preceding "@try ... @catch" section. This will
happen regardless of whether any exceptions are thrown, caught or
rethrown inside the "@try ... @catch" section, analogously to the
behavior of the "finally" clause in Java.
There are several caveats to using the new exception mechanism:
* Although currently designed to be binary compatible with
"NS_HANDLER"-style idioms provided by the "NSException" class,
the new exceptions can only be used on Mac OS X 10.3 (Panther)
and later systems, due to additional functionality needed in
the (NeXT) Objective-C runtime.
* As mentioned above, the new exceptions do not support handling
types other than Objective-C objects. Furthermore, when used
from Objective-C++, the Objective-C exception model does not
interoperate with C++ exceptions at this time. This means you
cannot @throw an exception from Objective-C and "catch" it in
C++, or vice versa (i.e., "throw ... @catch").
The -fobjc-exceptions switch also enables the use of synchroniza‐
tion blocks for thread-safe execution:
@synchronized (ObjCClass *guard) {
...
}
Upon entering the @synchronized block, a thread of execution shall
first check whether a lock has been placed on the corresponding
"guard" object by another thread. If it has, the current thread
shall wait until the other thread relinquishes its lock. Once
-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-Con‐
tinue debugging mode, where the object file in question may be
recompiled and dynamically reloaded in the course of program execu‐
tion, without the need to restart the program itself. Currently,
Fix-and-Continue functionality is only available in conjunction
with the NeXT runtime on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily
replaces calls to "objc_getClass("...")" (when the name of the
class is known at compile time) with static class references that
get initialized at load time, which improves run-time performance.
Specifying the -fzero-link flag suppresses this behavior and causes
calls to "objc_getClass("...")" to be retained. This is useful in
Zero-Link debugging mode, since it allows for individual class
implementations to be modified during program execution.
-gen-decls
Dump interface declarations for all classes seen in the source file
to a file named sourcename.decl.
-Wassign-intercept
Warn whenever an Objective-C assignment is being intercepted by the
garbage collector.
-Wno-protocol
If a class is declared to implement a protocol, a warning is issued
for every method in the protocol that is not implemented by the
class. The default behavior is to issue a warning for every method
not explicitly implemented in the class, even if a method implemen‐
tation is inherited from the superclass. If you use the -Wno-pro‐
tocol option, then methods inherited from the superclass are con‐
sidered to be implemented, and no warning is issued for them.
-Wselector
Warn if multiple methods of different types for the same selector
are found during compilation. The check is performed on the list
of methods in the final stage of compilation. Additionally, a
check is performed for each selector appearing in a "@selec‐
tor(...)" expression, and a corresponding method for that selector
has been found during compilation. Because these checks scan the
method table only at the end of compilation, these warnings are not
produced if the final stage of compilation is not reached, for
example because an error is found during compilation, or because
the -fsyntax-only option is being used.
-Wstrict-selector-match
Warn if multiple methods with differing argument and/or return
types are found for a given selector when attempting to send a mes‐
tion that methods and selectors must be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed
by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of
the output device’s aspect (e.g. its width, ...). The options
described below can be used to control the diagnostic messages format‐
ting algorithm, e.g. how many characters per line, how often source
location information should be reported. Right now, only the C++ front
end can honor these options. However it is expected, in the near
future, that the remaining front ends would be able to digest them cor‐
rectly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n
characters. The default is 72 characters for g++ and 0 for the
rest of the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a
single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit once source location information; that
is, in case the message is too long to fit on a single physical
line and has to be wrapped, the source location won’t be emitted
(as prefix) again, over and over, in subsequent continuation lines.
This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location information (as
prefix) for physical lines that result from the process of breaking
a message which is too long to fit on a single line.
-fdiagnostics-show-options
This option instructs the diagnostic machinery to add text to each
diagnostic emitted, which indicates which command line option
directly controls that diagnostic, when such an option is known to
the diagnostic machinery.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are
not inherently erroneous but which are risky or suggest there may have
been an error.
You can request many specific warnings with options beginning -W, for
example -Wimplicit to request warnings on implicit declarations. Each
all programs that use forbidden extensions, and some other programs
that do not follow ISO C and ISO C++. For ISO C, follows the ver‐
sion of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or
without this option (though a rare few will require -ansi or a -std
option specifying the required version of ISO C). However, without
this option, certain GNU extensions and traditional C and C++ fea‐
tures are supported as well. With this option, they are rejected.
-pedantic does not cause warning messages for use of the alternate
keywords whose names begin and end with __. Pedantic warnings are
also disabled in the expression that follows "__extension__". How‐
ever, only system header files should use these escape routes;
application programs should avoid them.
Some users try to use -pedantic to check programs for strict ISO C
conformance. They soon find that it does not do quite what they
want: it finds some non-ISO practices, but not all---only those for
which ISO C requires a diagnostic, and some others for which diag‐
nostics have been added.
A feature to report any failure to conform to ISO C might be useful
in some instances, but would require considerable additional work
and would be quite different from -pedantic. We don’t have plans
to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended
dialect of C, such as gnu89 or gnu99, there is a corresponding base
standard, the version of ISO C on which the GNU extended dialect is
based. Warnings from -pedantic are given where they are required
by the base standard. (It would not make sense for such warnings
to be given only for features not in the specified GNU C dialect,
since by definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be nothing
to warn about.)
-pedantic-errors
Like -pedantic, except that errors are produced rather than warn‐
ings.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause
of error, as programmers often forget that this type is signed on
some machines. This warning is enabled by -Wall.
-Wcomment
by format attributes, in the "printf", "scanf", "strftime" and
"strfmon" (an X/Open extension, not in the C standard) families (or
other target-specific families). Which functions are checked
without format attributes having been specified depends on the
standard version selected, and such checks of functions without the
attribute specified are disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by
GNU libc version 2.2. These include all ISO C90 and C99 features,
as well as features from the Single Unix Specification and some BSD
and GNU extensions. Other library implementations may not support
all these features; GCC does not support warning about features
that go beyond a particular library’s limitations. However, if
-pedantic is used with -Wformat, warnings will be given about for‐
mat 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-for‐
mat-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 Speci‐
fication 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".
rently 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 them‐
selves. Note this option can only be used with the -Wuninitialized
option, which in turn only works with -O1 and above.
For example, GCC will warn about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int
Warn when a declaration does not specify a type. This warning is
enabled by -Wall.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being
declared. The form -Wno-error-implicit-function-declaration is not
supported. This warning is enabled by -Wall (as a warning, not an
error).
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration. This
warning is enabled by -Wall.
-Wmain
Warn if the type of main is suspicious. main should be a function
with external linkage, returning int, 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 brack‐
eted, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
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
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 viola‐
tions of sequence point rules in the C standard.
The C standard defines the order in which expressions in a C pro‐
gram 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 argu‐
ments and the expression denoting the called function), and in cer‐
tain other places. Other than as expressed by the sequence point
determine the value to be stored.". If a program breaks these
rules, the results on any particular implementation are entirely
unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] =
b[n++]" and "a[i++] = i;". Some more complicated cases are not
diagnosed by this option, and it may give an occasional false posi‐
tive result, but in general it has been found fairly effective at
detecting this sort of problem in programs.
The present implementation of this option only works for C pro‐
grams. A future implementation may also work for C++ programs.
The C standard is worded confusingly, therefore there is some
debate over the precise meaning of the sequence point rules in sub‐
tle cases. Links to discussions of the problem, including proposed
formal definitions, may be found on the GCC readings page, at
<http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall.
-Wreturn-type
Warn whenever a function is defined with a return-type that
defaults to "int". Also warn about any "return" statement with no
return-value in a function whose return-type is not "void".
For C, also warn if the return type of a function has a type quali‐
fier 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 diagnos‐
tic 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 enu‐
meration. (The presence of a "default" label prevents this warn‐
ing.) "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 enu‐
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 decla‐
ration.
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 initial‐
ized or if a variable may be clobbered by a "setjmp" call.
These warnings are possible only in optimizing compilation, because
they require data flow information that is computed only when opti‐
mizing. If you don’t specify -O, you simply won’t get these warn‐
ings.
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 ele‐
ments of structure, union or array variables as well as for vari‐
ables 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 optimiza‐
tion options and version of GCC used.
Note that there may be no warning about a variable that is used
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 ini‐
tialized, but GCC doesn’t know this. Here is another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because "save_y" is used only if it is set.
This option also warns when a non-volatile automatic variable might
be changed by a call to "longjmp". These warnings as well are pos‐
sible 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 func‐
tions 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 under‐
stood 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 -Wun‐
known-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.
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 question‐
able, but which occasionally you might wish to check for; others warn
about constructions that are necessary or hard to avoid in some cases,
and there is no simple way to modify the code to suppress the warning.
-Wextra
(This option used to be called -W. The older name is still sup‐
ported, but the newer name is more descriptive.) Print extra warn‐
ing messages for these events:
* A function can return either with or without a value. (Falling
off the end of the function body is considered returning with‐
out a value.) For example, this function would evoke such a
warning:
foo (a)
{
if (a > 0)
return a;
}
* An expression-statement or the left-hand side of a comma
expression contains no side effects. To suppress the warning,
cast the unused expression to void. For example, an expression
such as x[i,j] will cause a warning, but x[(void)i,j] will not.
* An unsigned value is compared against zero with < or >=.
* Storage-class specifiers like "static" are not the first things
in a declaration. According to the C Standard, this usage is
obsolescent.
* If -Wall or -Wunused is also specified, warn about unused argu‐
ments.
* A comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to
unsigned. (But don’t warn if -Wno-sign-compare is also speci‐
fied.)
* An aggregate has an initializer which does not initialize all
members. This warning can be independently controlled by
-Wmissing-field-initializers.
* A function parameter is declared without a type specifier in
K&R-style functions:
void foo(bar) { }
*<(C++ only)>
A non-static reference or non-static const member appears in a
class without constructors.
*<(C++ only)>
Ambiguous virtual bases.
*<(C++ only)>
Subscripting an array which has been declared register.
*<(C++ only)>
Taking the address of a variable which has been declared regis‐
ter.
*<(C++ only)>
A base class is not initialized in a derived class’ copy con‐
structor.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating
point division by zero is not warned about, as it can be a legiti‐
mate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the
assumption that they usually do not indicate real problems and
would only make the compiler output harder to read. Using this
command line option tells GCC to emit warnings from system headers
as if they occurred in user code. However, note that using -Wall
in conjunction with this option will not warn about unknown pragmas
in system headers---for that, -Wunknown-pragmas must also be used.
-Wfloat-equal
Warn if floating point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers. If you are doing this, then you
need to compute (by analyzing the code, or in some other way) the
maximum or likely maximum error that the computation introduces,
and allow for it when performing comparisons (and when producing
output, but that’s a different problem). In particular, instead of
testing for equality, you would check to see whether the two values
have ranges that overlap; and this is done with the relational
operators, so equality comparisons are probably mistaken.
-Wtraditional (C only)
Warn about certain constructs that behave differently in tradi‐
tional and ISO C. Also warn about ISO C constructs that have no
traditional C equivalent, and/or problematic constructs which
should be avoided.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating point
constant suffixes. (Traditional C does support the L suffix on
integer constants.) Note, these suffixes appear in macros
defined in the system headers of most modern systems, e.g. the
_MIN/_MAX macros in "<limits.h>". Use of these macros in user
code might normally lead to spurious warnings, however GCC’s
integrated preprocessor has enough context to avoid warning in
these cases.
* A function declared external in one block and then used after
the end of the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static" one.
This construct is not accepted by some traditional C compilers.
* The ISO type of an integer constant has a different width or
signedness from its traditional type. This warning is only
issued if the base of the constant is ten. I.e. hexadecimal or
octal values, which typically represent bit patterns, are not
warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a sepa‐
rate namespace for labels.
* Initialization of unions. If the initializer is zero, the
warning is omitted. This is done under the assumption that the
zero initializer in user code appears conditioned on e.g.
"__STDC__" to avoid missing initializer warnings and relies on
default initialization to zero in the traditional C case.
* Conversions by prototypes between fixed/floating point values
and vice versa. The absence of these prototypes when compiling
with traditional C would cause serious problems. This is a
subset of the possible conversion warnings, for the full set
use -Wconversion.
* Use of ISO C style function definitions. This warning inten‐
tionally is not issued for prototype declarations or variadic
functions because these ISO C features will appear in your code
when using libiberty’s traditional C compatibility macros,
"PARAMS" and "VPARAMS". This warning is also bypassed for
-Wshadow
Warn whenever a local variable shadows another local variable,
parameter or global variable or whenever a built-in function is
shadowed.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler could not
assume anything on the bounds of the loop indices. With -fun‐
safe-loop-optimizations warn if the compiler made such assumptions.
-Wpointer-arith
Warn about anything that depends on the "size of" a function type
or of "void". GNU C assigns these types a size of 1, for conve‐
nience in calculations with "void *" pointers and pointers to func‐
tions.
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type. For
example, warn if "int malloc()" is cast to "anything *".
-Wc++-compat
Warn about ISO C constructs that are outside of the common subset
of ISO C and ISO C++, e.g. request for implicit conversion from
"void *" to a pointer to non-"void" type.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if a "const char *" is
cast to an ordinary "char *".
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of
the target is increased. For example, warn if a "char *" is cast
to an "int *" on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const
char[length]" so that copying the address of one into a non-"const"
"char *" pointer will get a warning; when compiling C++, warn about
the deprecated conversion from string constants to "char *". These
warnings will help you find at compile time code that can try to
write into a string constant, but only if you have been very care‐
ful about using "const" in declarations and prototypes. Otherwise,
it will just be a nuisance; this is why we did not make -Wall
request these warnings.
-Wconversion
unsigned. This warning is also enabled by -Wextra; to get the
other warnings of -Wextra without this warning, use -Wextra
-Wno-sign-compare.
-Waggregate-return
Warn if any functions that return structures or unions are defined
or called. (In languages where you can return an array, this also
elicits a warning.)
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This will not stop errors for incorrect use of supported
attributes.
-Wstrict-prototypes (C only)
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration which specifies the
argument types.)
-Wold-style-definition (C only)
Warn if an old-style function definition is used. A warning is
given even if there is a previous prototype.
-Wmissing-prototypes (C only)
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. The aim is to detect global functions that
fail to be declared in header files.
-Wmissing-declarations (C only)
Warn if a global function is defined without a previous declara‐
tion. Do so even if the definition itself provides a prototype.
Use this option to detect global functions that are not declared in
header files.
-Wmissing-field-initializers
Warn if a structure’s initializer has some fields missing. For
example, the following code would cause such a warning, because
"x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the
following modification would not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra warnings
attributes that are used in assignment, initialization, parameter
passing or return statements should have a corresponding "format"
attribute in the resulting type. I.e. the left-hand side of the
assignment or initialization, the type of the parameter variable,
or the return type of the containing function respectively should
also have a "format" attribute to avoid the warning.
GCC will also warn about function definitions which might be candi‐
dates for "format" attributes. Again, these are only possible can‐
didates. GCC will guess that "format" attributes might be appro‐
priate for any function that calls a function like "vprintf" or
"vscanf", but this might not always be the case, and some functions
for which "format" attributes are appropriate may not be detected.
-Wno-multichar
Do not warn if a multicharacter constant (’FOOF’) is used. Usually
they indicate a typo in the user’s code, as they have implementa‐
tion-defined values, and should not be used in portable code.
-Wnormalized=<none|id|nfc|nfkc>
In ISO C and ISO C++, two identifiers are different if they are
different sequences of characters. However, sometimes when charac‐
ters outside the basic ASCII character set are used, you can have
two different character sequences that look the same. To avoid
confusion, the ISO 10646 standard sets out some normalization rules
which when applied ensure that two sequences that look the same are
turned into the same sequence. GCC can warn you if you are using
identifiers which have not been normalized; this option controls
that warning.
There are four levels of warning that GCC supports. The default is
-Wnormalized=nfc, which warns about any identifier which is not in
the ISO 10646 "C" normalized form, NFC. NFC is the recommended
form for most uses.
Unfortunately, there are some characters which ISO C and ISO C++
allow in identifiers that when turned into NFC aren’t allowable as
identifiers. That is, there’s no way to use these symbols in
portable ISO C or C++ and have all your identifiers in NFC. -Wnor‐
malized=id suppresses the warning for these characters. It is
hoped that future versions of the standards involved will correct
this, which is why this option is not the default.
You can switch the warning off for all characters by writing -Wnor‐
malized=none. You would only want to do this if you were using
some other normalization scheme (like "D"), because otherwise you
can easily create bugs that are literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look iden‐
tical in some fonts or display methodologies, especially once for‐
matting has been applied. For instance "\u207F", "SUPERSCRIPT
LATIN SMALL LETTER N", will display just like a regular "n" which
-Wpacked
Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable "f.x" in "struct bar" will be
misaligned even though "struct bar" does not itself have the packed
attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpadded
Warn if padding is included in a structure, either to align an ele‐
ment of the structure or to align the whole structure. Sometimes
when this happens it is possible to rearrange the fields of the
structure to reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even
in cases where multiple declaration is valid and changes nothing.
-Wnested-externs (C only)
Warn if an "extern" declaration is encountered within a function.
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at
least a whole line of source code will never be executed, because
some condition is never satisfied or because it is after a proce‐
dure that never returns.
It is possible for this option to produce a warning even though
there are circumstances under which part of the affected line can
be executed, so care should be taken when removing apparently-
unreachable code.
For instance, when a function is inlined, a warning may mean that
the line is unreachable in only one inlined copy of the function.
This option is not made part of -Wall because in a debugging ver‐
sion of a program there is often substantial code which checks cor‐
rect functioning of the program and is, hopefully, unreachable
because the program does work. Another common use of unreachable
code is to provide behavior which is selectable at compile-time.
type. According to the 1998 ISO C++ standard, applying offsetof to
a non-POD type is undefined. In existing C++ implementations, how‐
ever, offsetof typically gives meaningful results even when applied
to certain kinds of non-POD types. (Such as a simple struct that
fails to be a POD type only by virtue of having a constructor.)
This flag is for users who are aware that they are writing non‐
portable code and who have deliberately chosen to ignore the warn‐
ing about it.
The restrictions on offsetof may be relaxed in a future version of
the C++ standard.
-Wno-int-to-pointer-cast (C only)
Suppress warnings from casts to pointer type of an integer of a
different size.
-Wno-pointer-to-int-cast (C only)
Suppress warnings from casts from a pointer to an integer type of a
different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but can’t
be used.
-Wlong-long
Warn if long long type is used. This is default. To inhibit the
warning messages, use -Wno-long-long. Flags -Wlong-long and
-Wno-long-long are taken into account only when -pedantic flag is
used.
-Wvariadic-macros
Warn if variadic macros are used in pedantic ISO C90 mode, or the
GNU alternate syntax when in pedantic ISO C99 mode. This is
default. To inhibit the warning messages, use -Wno-vari‐
adic-macros.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may eliminate
reads and/or writes to register variables.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning
does not generally indicate that there is anything wrong with your
code; it merely indicates that GCC’s optimizers were unable to han‐
dle the code effectively. Often, the problem is that your code is
too big or too complex; GCC will refuse to optimize programs when
the optimization itself is likely to take inordinate amounts of
time.
-Wpointer-sign
Warn for pointer argument passing or assignment with different
GCC has various special options that are used for debugging either your
program or GCC:
-g Produce debugging information in the operating system’s native for‐
mat (stabs, COFF, XCOFF, or DWARF 2). GDB can work with this
debugging information.
On most systems that use stabs format, -g enables use of extra
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but will probably make other
debuggers crash or refuse to read the program. If you want to con‐
trol for certain whether to generate the extra information, use
-gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
GCC allows you to use -g with -O. The shortcuts taken by optimized
code may occasionally produce surprising results: some variables
you declared may not exist at all; flow of control may briefly move
where you did not expect it; some statements may not be executed
because they compute constant results or their values were already
at hand; some statements may execute in different places because
they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This
makes it reasonable to use the optimizer for programs that might
have bugs.
The following options are useful when GCC is generated with the
capability for more than one debugging format.
-ggdb
Produce debugging information for use by GDB. This means to use
the most expressive format available (DWARF 2, stabs, or the native
format if neither of those are supported), including GDB extensions
if at all possible.
-gstabs
Produce debugging information in stabs format (if that is sup‐
ported), without GDB extensions. This is the format used by DBX on
most BSD systems. On MIPS, Alpha and System V Release 4 systems
this option produces stabs debugging output which is not understood
by DBX or SDB. On System V Release 4 systems this option requires
the GNU assembler.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is sup‐
ported), for only symbols that are actually used.
-gstabs+
Produce debugging information in stabs format (if that is sup‐
ported), using GNU extensions understood only by the GNU debugger
(GDB). The use of these extensions is likely to make other debug‐
gers crash or refuse to read the program.
(GDB). The use of these extensions is likely to make other debug‐
gers crash or refuse to read the program, and may cause assemblers
other than the GNU assembler (GAS) to fail with an error.
-gdwarf-2
Produce debugging information in DWARF version 2 format (if that is
supported). This is the format used by DBX on IRIX 6. With this
option, GCC uses features of DWARF version 3 when they are useful;
version 3 is upward compatible with version 2, but may still cause
problems for older debuggers.
-gvms
Produce debugging information in VMS debug format (if that is sup‐
ported). This is the format used by DEBUG on VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how
much information. The default level is 2.
Level 1 produces minimal information, enough for making backtraces
in parts of the program that you don’t plan to debug. This
includes descriptions of functions and external variables, but no
information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro
definitions present in the program. Some debuggers support macro
expansion when you use -g3.
-gdwarf-2 does not accept a concatenated debug level, because GCC
used to support an option -gdwarf that meant to generate debug
information in version 1 of the DWARF format (which is very differ‐
ent from version 2), and it would have been too confusing. That
debug format is long obsolete, but the option cannot be changed
now. Instead use an additional -glevel option to change the debug
level for DWARF2.
-feliminate-dwarf2-dups
Compress DWARF2 debugging information by eliminating duplicated
information about each symbol. This option only makes sense when
generating DWARF2 debugging information with -gdwarf-2.
-p Generate extra code to write profile information suitable for the
analysis program prof. You must use this option when compiling the
source files you want data about, and you must also use it when
linking.
-pg Generate extra code to write profile information suitable for the
-fprofile-arcs
Add code so that program flow arcs are instrumented. During execu‐
tion the program records how many times each branch and call is
executed and how many times it is taken or returns. When the com‐
piled program exits it saves this data to a file called aux‐
name.gcda for each source file. The data may be used for profile-
directed optimizations (-fbranch-probabilities), or for test cover‐
age analysis (-ftest-coverage). Each object file’s auxname is gen‐
erated from the name of the output file, if explicitly specified
and it is not the final executable, otherwise it is the basename of
the source file. In both cases any suffix is removed (e.g.
foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file
specified as -o dir/foo.o).
--coverage
This option is used to compile and link code instrumented for cov‐
erage analysis. The option is a synonym for -fprofile-arcs
-ftest-coverage (when compiling) and -lgcov (when linking). See
the documentation for those options for more details.
@bullet
Compile the source files with -fprofile-arcs plus optimization
and code generation options. For test coverage analysis, use
the additional -ftest-coverage option. You do not need to pro‐
file every source file in a program.
@cvmmfu
Link your object files with -lgcov or -fprofile-arcs (the lat‐
ter implies the former).
@dwnngv
Run the program on a representative workload to generate the
arc profile information. This may be repeated any number of
times. You can run concurrent instances of your program, and
provided that the file system supports locking, the data files
will be correctly updated. Also "fork" calls are detected and
correctly handled (double counting will not happen).
@exoohw
For profile-directed optimizations, compile the source files
again with the same optimization and code generation options
plus -fbranch-probabilities.
@fyppix
For test coverage analysis, use gcov to produce human readable
information from the .gcno and .gcda files. Refer to the gcov
documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates
a program flow graph, then finds a spanning tree for the graph.
Only arcs that are not on the spanning tree have to be instru‐
-dletters
-fdump-rtl-pass
Says to make debugging dumps during compilation at times specified
by letters. This is used for debugging the RTL-based passes of
the compiler. The file names for most of the dumps are made by
appending a pass number and a word to the dumpname. dumpname is
generated from the name of the output file, if explicitly specified
and it is not an executable, otherwise it is the basename of the
source file.
Most debug dumps can be enabled either passing a letter to the -d
option, or with a long -fdump-rtl switch; here are the possible
letters for use in letters and pass, and their meanings:
-dA Annotate the assembler output with miscellaneous debugging
information.
-db
-fdump-rtl-bp
Dump after computing branch probabilities, to file.09.bp.
-dB
-fdump-rtl-bbro
Dump after block reordering, to file.30.bbro.
-dc
-fdump-rtl-combine
Dump after instruction combination, to the file file.17.com‐
bine.
-dC
-fdump-rtl-ce1
-fdump-rtl-ce2
-dC and -fdump-rtl-ce1 enable dumping after the first if con‐
version, to the file file.11.ce1. -dC and -fdump-rtl-ce2
enable dumping after the second if conversion, to the file
file.18.ce2.
-dd
-fdump-rtl-btl
-fdump-rtl-dbr
-dd and -fdump-rtl-btl enable dumping after branch target load
optimization, to file.31.btl. -dd and -fdump-rtl-dbr enable
dumping after delayed branch scheduling, to file.36.dbr.
-dD Dump all macro definitions, at the end of preprocessing, in
addition to normal output.
-dE
-fdump-rtl-ce3
Dump after the third if conversion, to file.28.ce3.
-dG and -fdump-rtl-gcse enable dumping after GCSE, to
file.05.gcse. -dG and -fdump-rtl-bypass enable dumping after
jump bypassing and control flow optimizations, to
file.07.bypass.
-dh
-fdump-rtl-eh
Dump after finalization of EH handling code, to file.02.eh.
-di
-fdump-rtl-sibling
Dump after sibling call optimizations, to file.01.sibling.
-dj
-fdump-rtl-jump
Dump after the first jump optimization, to file.03.jump.
-dk
-fdump-rtl-stack
Dump after conversion from registers to stack, to
file.33.stack.
-dl
-fdump-rtl-lreg
Dump after local register allocation, to file.22.lreg.
-dL
-fdump-rtl-loop
-fdump-rtl-loop2
-dL and -fdump-rtl-loop enable dumping after the first loop
optimization pass, to file.06.loop. -dL and -fdump-rtl-loop2
enable dumping after the second pass, to file.13.loop2.
-dm
-fdump-rtl-sms
Dump after modulo scheduling, to file.20.sms.
-dM
-fdump-rtl-mach
Dump after performing the machine dependent reorganization
pass, to file.35.mach.
-dn
-fdump-rtl-rnreg
Dump after register renumbering, to file.29.rnreg.
-dN
-fdump-rtl-regmove
Dump after the register move pass, to file.19.regmove.
-do
-fdump-rtl-postreload
-dS
-fdump-rtl-sched
Dump after the first scheduling pass, to file.21.sched.
-dt
-fdump-rtl-cse2
Dump after the second CSE pass (including the jump optimization
that sometimes follows CSE), to file.15.cse2.
-dT
-fdump-rtl-tracer
Dump after running tracer, to file.12.tracer.
-dV
-fdump-rtl-vpt
-fdump-rtl-vartrack
-dV and -fdump-rtl-vpt enable dumping after the value profile
transformations, to file.10.vpt. -dV and -fdump-rtl-vartrack
enable dumping after variable tracking, to file.34.vartrack.
-dw
-fdump-rtl-flow2
Dump after the second flow pass, to file.26.flow2.
-dz
-fdump-rtl-peephole2
Dump after the peephole pass, to file.27.peephole2.
-dZ
-fdump-rtl-web
Dump after live range splitting, to file.14.web.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dH Produce a core dump whenever an error occurs.
-dm Print statistics on memory usage, at the end of the run, to
standard error.
-dp Annotate the assembler output with a comment indicating which
pattern and alternative was used. The length of each instruc‐
tion is also printed.
-dP Dump the RTL in the assembler output as a comment before each
instruction. Also turns on -dp annotation.
-dv For each of the other indicated dump files (either with -d or
-fdump-rtl-pass), dump a representation of the control flow
graph suitable for viewing with VCG to file.pass.vcg.
Dump a representation of the tree structure for the entire transla‐
tion unit to a file. The file name is made by appending .tu to the
source file name. If the -options form is used, options controls
the details of the dump as described for the -fdump-tree options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class’s hierarchy and virtual func‐
tion table layout to a file. The file name is made by appending
.class to the source file name. If the -options form is used,
options controls the details of the dump as described for the
-fdump-tree options.
-fdump-ipa-switch
Control the dumping at various stages of inter-procedural analysis
language tree to a file. The file name is generated by appending a
switch specific suffix to the source file name. The following
dumps are possible:
all Enables all inter-procedural analysis dumps; currently the only
produced dump is the cgraph dump.
cgraph
Dumps information about call-graph optimization, unused func‐
tion removal, and inlining decisions.
-fdump-tree-switch
-fdump-tree-switch-options
Control the dumping at various stages of processing the intermedi‐
ate language tree to a file. The file name is generated by append‐
ing a switch specific suffix to the source file name. If the
-options form is used, options is a list of - separated options
that control the details of the dump. Not all options are applica‐
ble to all dumps, those which are not meaningful will be ignored.
The following options are available
address
Print the address of each node. Usually this is not meaningful
as it changes according to the environment and source file.
Its primary use is for tying up a dump file with a debug envi‐
ronment.
slim
Inhibit dumping of members of a scope or body of a function
merely because that scope has been reached. Only dump such
items when they are directly reachable by some other path.
When dumping pretty-printed trees, this option inhibits dumping
the bodies of control structures.
raw Print a raw representation of the tree. By default, trees are
pretty-printed into a C-like representation.
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each variable.
all Turn on all options, except raw, slim and lineno.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
inlined
Dump after function inlining, to file.inlined.
gimple
Dump each function before and after the gimplification pass to
a file. The file name is made by appending .gimple to the
source file name.
cfg Dump the control flow graph of each function to a file. The
file name is made by appending .cfg to the source file name.
vcg Dump the control flow graph of each function to a file in VCG
format. The file name is made by appending .vcg to the source
file name. Note that if the file contains more than one func‐
tion, the generated file cannot be used directly by VCG. You
will need to cut and paste each function’s graph into its own
separate file first.
ch Dump each function after copying loop headers. The file name
is made by appending .ch to the source file name.
ssa Dump SSA related information to a file. The file name is made
by appending .ssa to the source file name.
salias
Dump structure aliasing variable information to a file. This
file name is made by appending .salias to the source file name.
alias
Dump aliasing information for each function. The file name is
made by appending .alias to the source file name.
ccp Dump each function after CCP. The file name is made by append‐
ing .ccp to the source file name.
storeccp
Dump each function after STORE-CCP. The file name is made by
appending .storeccp to the source file name.
dce Dump each function after dead code elimination. The file name
is made by appending .dce to the source file name.
mudflap
Dump each function after adding mudflap instrumentation. The
file name is made by appending .mudflap to the source file
name.
sra Dump each function after performing scalar replacement of
aggregates. The file name is made by appending .sra to the
source file name.
sink
Dump each function after performing code sinking. The file
name is made by appending .sink to the source file name.
dom Dump each function after applying dominator tree optimizations.
The file name is made by appending .dom to the source file
name.
dse Dump each function after applying dead store elimination. The
file name is made by appending .dse to the source file name.
phiopt
Dump each function after optimizing PHI nodes into straightline
code. The file name is made by appending .phiopt to the source
file name.
forwprop
Dump each function after forward propagating single use vari‐
ables. The file name is made by appending .forwprop to the
source file name.
copyrename
Dump each function after applying the copy rename optimization.
The file name is made by appending .copyrename to the source
file name.
nrv Dump each function after applying the named return value opti‐
mization on generic trees. The file name is made by appending
.nrv to the source file name.
vect
Dump each function after applying vectorization of loops. The
file name is made by appending .vect to the source file name.
vrp Dump each function after Value Range Propagation (VRP). The
file name is made by appending .vrp to the source file name.
all Enable all the available tree dumps with the flags provided in
this option.
The string should be different for every file you compile.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls
the amount of debugging output the scheduler prints. This informa‐
tion is written to standard error, unless -dS or -dR is specified,
in which case it is output to the usual dump listing file, .sched
or .sched2 respectively. However for n greater than nine, the out‐
put is always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same informa‐
tion as -dRS. For n greater than one, it also output basic block
probabilities, detailed ready list information and unit/insn info.
For n greater than two, it includes RTL at abort point, control-
flow and regions info. And for n over four, -fsched-verbose also
includes dependence info.
-save-temps
Store the usual "temporary" intermediate files permanently; place
them in the current directory and name them based on the source
file. Thus, compiling foo.c with -c -save-temps would produce
files foo.i and foo.s, as well as foo.o. This creates a prepro‐
cessed foo.i output file even though the compiler now normally uses
an integrated preprocessor.
When used in combination with the -x command line option,
-save-temps is sensible enough to avoid over writing an input
source file with the same extension as an intermediate file. The
corresponding intermediate file may be obtained by renaming the
source file before using -save-temps.
-time
Report the CPU time taken by each subprocess in the compilation
sequence. For C source files, this is the compiler proper and
assembler (plus the linker if linking is done). The output looks
like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time
spent executing the program itself. The second number is "system
time", time spent executing operating system routines on behalf of
the program. Both numbers are in seconds.
-fvar-tracking
Run variable tracking pass. It computes where variables are stored
at each position in code. Better debugging information is then
generated (if the debugging information format supports this infor‐
mation).
-print-multi-lib
Print the mapping from multilib directory names to compiler
switches that enable them. The directory name is separated from
the switches by ;, and each switch starts with an @} instead of the
@samp{-, without spaces between multiple switches. This is sup‐
posed to ease shell-processing.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do
want to link with libgcc.a. You can do
gcc -nostdlib <files>... ‘gcc -print-libgcc-file-name‘
-print-search-dirs
Print the name of the configured installation directory and a list
of program and library directories gcc will search---and don’t do
anything else.
This is useful when gcc prints the error message installation prob‐
lem, cannot exec cpp0: No such file or directory. To resolve this
you either need to put cpp0 and the other compiler components where
gcc expects to find them, or you can set the environment variable
GCC_EXEC_PREFIX to the directory where you installed them. Don’t
forget the trailing /.
-dumpmachine
Print the compiler’s target machine (for example,
i686-pc-linux-gnu)---and don’t do anything else.
-dumpversion
Print the compiler version (for example, 3.0)---and don’t do any‐
thing else.
-dumpspecs
Print the compiler’s built-in specs---and don’t do anything else.
(This is used when GCC itself is being built.)
-feliminate-unused-debug-types
Normally, when producing DWARF2 output, GCC will emit debugging
information for all types declared in a compilation unit, regard‐
less of whether or not they are actually used in that compilation
unit. Sometimes