GCC

NAME

gcc − GNU project C and C++ compiler

SYNOPSIS

gcc [−c|−S|−E] [−std=standard]
[−g] [−pg] [−Olevel]
[−Wwarn...] [−Wpedantic]
[−Idir...] [−Ldir...]
[−Dmacro[=defn]...] [−Umacro]
[−foption...] [−mmachine-option...]
[−o outfile] [@file] infile...

Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.

DESCRIPTION

When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" allow you to stop this process at an intermediate stage. For example, the −c option says not to run the linker. Then the output consists of object files output by the assembler.

Other options are passed on to one stage of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.

Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C ++ ), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.

The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: −dv is very different from −d −v.

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. Also, the placement of the −l option is significant.

Many options have long names starting with −f or with −W−−−for example, −fmove−loop−invariants, −Wformat and so on. Most of these have both positive and negative forms; the negative form of −ffoo is −fno−foo. This manual documents only one of these two forms, whichever one is not the default.

OPTIONS

Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
Overall Options

−c −S −E −o file −no−canonical−prefixes −pipe −pass−exit−codes −x language −v −### −−help[=class[,...]] −−target−help −−version −wrapper @file −fplugin=file −fplugin−arg−name=arg −fdump−ada−spec[−slim] −fada−spec−parent=unit −fdump−go−spec=file

C Language Options

−ansi −std=standard −fgnu89−inline −aux−info filename −fallow−parameterless−variadic−functions −fno−asm −fno−builtin −fno−builtin−function −fhosted −ffreestanding −fopenmp −fms−extensions −fplan9−extensions −trigraphs −traditional −traditional−cpp −fallow−single−precision −fcond−mismatch −flax−vector−conversions −fsigned−bitfields −fsigned−char −funsigned−bitfields −funsigned−char

C ++ Language Options

−fabi−version=n −fno−access−control −fcheck−new −fconstexpr−depth=n −ffriend−injection −fno−elide−constructors −fno−enforce−eh−specs −ffor−scope −fno−for−scope −fno−gnu−keywords −fno−implicit−templates −fno−implicit−inline−templates −fno−implement−inlines −fms−extensions −fno−nonansi−builtins −fnothrow−opt −fno−operator−names −fno−optional−diags −fpermissive −fno−pretty−templates −frepo −fno−rtti −fstats −ftemplate−backtrace−limit=n −ftemplate−depth=n −fno−threadsafe−statics −fuse−cxa−atexit −fno−weak −nostdinc++ −fno−default−inline −fvisibility−inlines−hidden −fvisibility−ms−compat −fext−numeric−literals −Wabi −Wconversion−null −Wctor−dtor−privacy −Wdelete−non−virtual−dtor −Wliteral−suffix −Wnarrowing −Wnoexcept −Wnon−virtual−dtor −Wreorder −Weffc++ −Wstrict−null−sentinel −Wno−non−template−friend −Wold−style−cast −Woverloaded−virtual −Wno−pmf−conversions −Wsign−promo

Objective-C and Objective−C ++ Language Options

−fconstant−string−class=class-name −fgnu−runtime −fnext−runtime −fno−nil−receivers −fobjc−abi−version=n −fobjc−call−cxx−cdtors −fobjc−direct−dispatch −fobjc−exceptions −fobjc−gc −fobjc−nilcheck −fobjc−std=objc1 −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−color=[auto|never|always] −fno−diagnostics−show−option −fno−diagnostics−show−caret

Warning Options

−fsyntax−only −fmax−errors=n −Wpedantic −pedantic−errors −w −Wextra −Wall −Waddress −Waggregate−return −Waggressive−loop−optimizations −Warray−bounds −Wno−attributes −Wno−builtin−macro−redefined −Wc++−compat −Wc++11−compat −Wcast−align −Wcast−qual −Wchar−subscripts −Wclobbered −Wcomment −Wconversion −Wcoverage−mismatch −Wno−cpp −Wno−deprecated −Wno−deprecated−declarations −Wdisabled−optimization −Wno−div−by−zero −Wdouble−promotion −Wempty−body −Wenum−compare −Wno−endif−labels −Werror −Werror=* −Wfatal−errors −Wfloat−equal −Wformat −Wformat=2 −Wno−format−contains−nul −Wno−format−extra−args −Wformat−nonliteral −Wformat−security −Wformat−y2k −Wframe−larger−than=len −Wno−free−nonheap−object −Wjump−misses−init −Wignored−qualifiers −Wimplicit −Wimplicit−function−declaration −Wimplicit−int −Winit−self −Winline −Wmaybe−uninitialized −Wno−int−to−pointer−cast −Wno−invalid−offsetof −Winvalid−pch −Wlarger−than=len −Wunsafe−loop−optimizations −Wlogical−op −Wlong−long −Wmain −Wmaybe−uninitialized −Wmissing−braces −Wmissing−field−initializers −Wmissing−include−dirs −Wno−mudflap −Wno−multichar −Wnonnull −Wno−overflow −Woverlength−strings −Wpacked −Wpacked−bitfield−compat −Wpadded −Wparentheses −Wpedantic−ms−format −Wno−pedantic−ms−format −Wpointer−arith −Wno−pointer−to−int−cast −Wredundant−decls −Wno−return−local−addr −Wreturn−type −Wsequence−point −Wshadow −Wsign−compare −Wsign−conversion −Wsizeof−pointer−memaccess −Wstack−protector −Wstack−usage=len −Wstrict−aliasing −Wstrict−aliasing=n −Wstrict−overflow −Wstrict−overflow=n −Wsuggest−attribute=[pure|const|noreturn|format] −Wmissing−format−attribute −Wswitch −Wswitch−default −Wswitch−enum −Wsync−nand −Wsystem−headers −Wtrampolines −Wtrigraphs −Wtype−limits −Wundef −Wuninitialized −Wunknown−pragmas −Wno−pragmas −Wunsuffixed−float−constants −Wunused −Wunused−function −Wunused−label −Wunused−local−typedefs −Wunused−parameter −Wno−unused−result −Wunused−value −Wunused−variable −Wunused−but−set−parameter −Wunused−but−set−variable −Wuseless−cast −Wvariadic−macros −Wvector−operation−performance −Wvla −Wvolatile−register−var −Wwrite−strings −Wzero−as−null−pointer−constant

C and Objective-C-only Warning Options

−Wbad−function−cast −Wmissing−declarations −Wmissing−parameter−type −Wmissing−prototypes −Wnested−externs −Wold−style−declaration −Wold−style−definition −Wstrict−prototypes −Wtraditional −Wtraditional−conversion −Wdeclaration−after−statement −Wpointer−sign

Debugging Options

−dletters −dumpspecs −dumpmachine −dumpversion −fsanitize=style −fdbg−cnt−list −fdbg−cnt=counter-value-list −fdisable−ipa−pass_name −fdisable−rtl−pass_name −fdisable−rtl−pass-name=range-list −fdisable−tree−pass_name −fdisable−tree−pass-name=range-list −fdump−noaddr −fdump−unnumbered −fdump−unnumbered−links −fdump−translation−unit[−n] −fdump−class−hierarchy[−n] −fdump−ipa−all −fdump−ipa−cgraph −fdump−ipa−inline −fdump−passes −fdump−statistics −fdump−tree−all −fdump−tree−original[−n] −fdump−tree−optimized[−n] −fdump−tree−cfg −fdump−tree−alias −fdump−tree−ch −fdump−tree−ssa[−n] −fdump−tree−pre[−n] −fdump−tree−ccp[−n] −fdump−tree−dce[−n] −fdump−tree−gimple[−raw] −fdump−tree−mudflap[−n] −fdump−tree−dom[−n] −fdump−tree−dse[−n] −fdump−tree−phiprop[−n] −fdump−tree−phiopt[−n] −fdump−tree−forwprop[−n] −fdump−tree−copyrename[−n] −fdump−tree−nrv −fdump−tree−vect −fdump−tree−sink −fdump−tree−sra[−n] −fdump−tree−forwprop[−n] −fdump−tree−fre[−n] −fdump−tree−vrp[−n] −ftree−vectorizer−verbose=n −fdump−tree−storeccp[−n] −fdump−final−insns=file −fcompare−debug[=opts] −fcompare−debug−second −feliminate−dwarf2−dups −fno−eliminate−unused−debug−types −feliminate−unused−debug−symbols −femit−class−debug−always −fenable−kind−pass −fenable−kind−pass=range-list −fdebug−types−section −fmem−report−wpa −fmem−report −fpre−ipa−mem−report −fpost−ipa−mem−report −fprofile−arcs −fopt−info −fopt−info−options[=file] −frandom−seed=string −fsched−verbose=n −fsel−sched−verbose −fsel−sched−dump−cfg −fsel−sched−pipelining−verbose −fstack−usage −ftest−coverage −ftime−report −fvar−tracking −fvar−tracking−assignments −fvar−tracking−assignments−toggle −g −glevel −gtoggle −gcoff −gdwarf−version −ggdb −grecord−gcc−switches −gno−record−gcc−switches −gstabs −gstabs+ −gstrict−dwarf −gno−strict−dwarf −gvms −gxcoff −gxcoff+ −fno−merge−debug−strings −fno−dwarf2−cfi−asm −fdebug−prefix−map=old=new −femit−struct−debug−baseonly −femit−struct−debug−reduced −femit−struct−debug−detailed[=spec-list] −p −pg −print−file−name=library −print−libgcc−file−name −print−multi−directory −print−multi−lib −print−multi−os−directory −print−prog−name=program −print−search−dirs −Q −print−sysroot −print−sysroot−headers−suffix −save−temps −save−temps=cwd −save−temps=obj −time[=file]

Optimization Options

−faggressive−loop−optimizations −falign−functions[=n] −falign−jumps[=n] −falign−labels[=n] −falign−loops[=n] −fassociative−math −fauto−inc−dec −fbranch−probabilities −fbranch−target−load−optimize −fbranch−target−load−optimize2 −fbtr−bb−exclusive −fcaller−saves −fcheck−data−deps −fcombine−stack−adjustments −fconserve−stack −fcompare−elim −fcprop−registers −fcrossjumping −fcse−follow−jumps −fcse−skip−blocks −fcx−fortran−rules −fcx−limited−range −fdata−sections −fdce −fdelayed−branch −fdelete−null−pointer−checks −fdevirtualize −fdse −fearly−inlining −fipa−sra −fexpensive−optimizations −ffat−lto−objects −ffast−math −ffinite−math−only −ffloat−store −fexcess−precision=style −fforward−propagate −ffp−contract=style −ffunction−sections −fgcse −fgcse−after−reload −fgcse−las −fgcse−lm −fgraphite−identity −fgcse−sm −fhoist−adjacent−loads −fif−conversion −fif−conversion2 −findirect−inlining −finline−functions −finline−functions−called−once −finline−limit=n −finline−small−functions −fipa−cp −fipa−cp−clone −fipa−pta −fipa−profile −fipa−pure−const −fipa−reference −fira−algorithm=algorithm −fira−region=region −fira−hoist−pressure −fira−loop−pressure −fno−ira−share−save−slots −fno−ira−share−spill−slots −fira−verbose=n −fivopts −fkeep−inline−functions −fkeep−static−consts −floop−block −floop−interchange −floop−strip−mine −floop−nest−optimize −floop−parallelize−all −flto −flto−compression−level −flto−partition=alg −flto−report −fmerge−all−constants −fmerge−constants −fmodulo−sched −fmodulo−sched−allow−regmoves −fmove−loop−invariants fmudflap −fmudflapir −fmudflapth −fno−branch−count−reg −fno−default−inline −fno−defer−pop −fno−function−cse −fno−guess−branch−probability −fno−inline −fno−math−errno −fno−peephole −fno−peephole2 −fno−sched−interblock −fno−sched−spec −fno−signed−zeros −fno−toplevel−reorder −fno−trapping−math −fno−zero−initialized−in−bss −fomit−frame−pointer −foptimize−register−move −foptimize−sibling−calls −fpartial−inlining −fpeel−loops −fpredictive−commoning −fprefetch−loop−arrays −fprofile−report −fprofile−correction −fprofile−dir=path −fprofile−generate −fprofile−generate=path −fprofile−use −fprofile−use=path −fprofile−values −freciprocal−math −free −fregmove −frename−registers −freorder−blocks −freorder−blocks−and−partition −freorder−functions −frerun−cse−after−loop −freschedule−modulo−scheduled−loops −frounding−math −fsched2−use−superblocks −fsched−pressure −fsched−spec−load −fsched−spec−load−dangerous −fsched−stalled−insns−dep[=n] −fsched−stalled−insns[=n] −fsched−group−heuristic −fsched−critical−path−heuristic −fsched−spec−insn−heuristic −fsched−rank−heuristic −fsched−last−insn−heuristic −fsched−dep−count−heuristic −fschedule−insns −fschedule−insns2 −fsection−anchors −fselective−scheduling −fselective−scheduling2 −fsel−sched−pipelining −fsel−sched−pipelining−outer−loops −fshrink−wrap −fsignaling−nans −fsingle−precision−constant −fsplit−ivs−in−unroller −fsplit−wide−types −fstack−protector −fstack−protector−all −fstack−protector−strong −fstrict−aliasing −fstrict−overflow −fthread−jumps −ftracer −ftree−bit−ccp −ftree−builtin−call−dce −ftree−ccp −ftree−ch −ftree−coalesce−inline−vars −ftree−coalesce−vars −ftree−copy−prop −ftree−copyrename −ftree−dce −ftree−dominator−opts −ftree−dse −ftree−forwprop −ftree−fre −ftree−loop−if−convert −ftree−loop−if−convert−stores −ftree−loop−im −ftree−phiprop −ftree−loop−distribution −ftree−loop−distribute−patterns −ftree−loop−ivcanon −ftree−loop−linear −ftree−loop−optimize −ftree−parallelize−loops=n −ftree−pre −ftree−partial−pre −ftree−pta −ftree−reassoc −ftree−sink −ftree−slsr −ftree−sra −ftree−switch−conversion −ftree−tail−merge −ftree−ter −ftree−vect−loop−version −ftree−vectorize −ftree−vrp −funit−at−a−time −funroll−all−loops −funroll−loops −funsafe−loop−optimizations −funsafe−math−optimizations −funswitch−loops −fvariable−expansion−in−unroller −fvect−cost−model −fvpt −fweb −fwhole−program −fwpa −fuse−ld=linker −fuse−linker−plugin −−param name=value −O −O0 −O1 −O2 −O3 −Os −Ofast −Og

Preprocessor Options

−Aquestion=answer −A−question[=answer] −C −dD −dI −dM −dN −Dmacro[=defn] −E −H −idirafter dir −include file −imacros file −iprefix file −iwithprefix dir −iwithprefixbefore dir −isystem dir −imultilib dir −isysroot dir −M −MM −MF −MG −MP −MQ −MT −nostdinc −P −fdebug−cpp −ftrack−macro−expansion −fworking−directory −remap −trigraphs −undef −Umacro −Wp,option −Xpreprocessor option −no−integrated−cpp

Assembler Option

−Wa,option −Xassembler option

Linker Options

object-file-name −llibrary −nostartfiles −nodefaultlibs −nostdlib −pie −rdynamic −s −static −static−libgcc −static−libstdc++ −static−libasan −static−libtsan −shared −shared−libgcc −symbolic −T script −Wl,option −Xlinker option −u symbol

Directory Options

−Bprefix −Idir −iplugindir=dir −iquotedir −Ldir −specs=file −I− −−sysroot=dir −−no−sysroot−suffix

Machine Dependent Options

AArch64 Options −mbig−endian −mlittle−endian −mgeneral−regs−only −mcmodel=tiny −mcmodel=small −mcmodel=large −mstrict−align −momit−leaf−frame−pointer −mno−omit−leaf−frame−pointer −mtls−dialect=desc −mtls−dialect=traditional −march=name −mcpu=name −mtune=name

Adapteva Epiphany Options −mhalf−reg−file −mprefer−short−insn−regs −mbranch−cost=num −mcmove −mnops=num −msoft−cmpsf −msplit−lohi −mpost−inc −mpost−modify −mstack−offset=num −mround−nearest −mlong−calls −mshort−calls −msmall16 −mfp−mode=mode −mvect−double −max−vect−align=num −msplit−vecmove−early −m1reg−reg

ARM Options −mapcs−frame −mno−apcs−frame −mabi=name −mapcs−stack−check −mno−apcs−stack−check −mapcs−float −mno−apcs−float −mapcs−reentrant −mno−apcs−reentrant −msched−prolog −mno−sched−prolog −mlittle−endian −mbig−endian −mwords−little−endian −mfloat−abi=name −mfp16−format=name −mthumb−interwork −mno−thumb−interwork −mcpu=name −march=name −mfpu=name −mstructure−size−boundary=n −mabort−on−noreturn −mlong−calls −mno−long−calls −msingle−pic−base −mno−single−pic−base −mpic−register=reg −mnop−fun−dllimport −mpoke−function−name −mthumb −marm −mtpcs−frame −mtpcs−leaf−frame −mcaller−super−interworking −mcallee−super−interworking −mtp=name −mtls−dialect=dialect −mword−relocations −mfix−cortex−m3−ldrd −munaligned−access

AVR Options −mmcu=mcu −maccumulate−args −mbranch−cost=cost −mcall−prologues −mint8 −mno−interrupts −mrelax −mstrict−X −mtiny−stack −Waddr−space−convert

Blackfin Options −mcpu=cpu[−sirevision] −msim −momit−leaf−frame−pointer −mno−omit−leaf−frame−pointer −mspecld−anomaly −mno−specld−anomaly −mcsync−anomaly −mno−csync−anomaly −mlow−64k −mno−low64k −mstack−check−l1 −mid−shared−library −mno−id−shared−library −mshared−library−id=n −mleaf−id−shared−library −mno−leaf−id−shared−library −msep−data −mno−sep−data −mlong−calls −mno−long−calls −mfast−fp −minline−plt −mmulticore −mcorea −mcoreb −msdram −micplb

C6X Options −mbig−endian −mlittle−endian −march=cpu −msim −msdata=sdata-type

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

CR16 Options −mmac −mcr16cplus −mcr16c −msim −mint32 −mbit−ops −mdata−model=model

Darwin Options −all_load −allowable_client −arch −arch_errors_fatal −arch_only −bind_at_load −bundle −bundle_loader −client_name −compatibility_version −current_version −dead_strip −dependency−file −dylib_file −dylinker_install_name −dynamic −dynamiclib −exported_symbols_list −filelist −flat_namespace −force_cpusubtype_ALL −force_flat_namespace −headerpad_max_install_names −iframework −image_base −init −install_name −keep_private_externs −multi_module −multiply_defined −multiply_defined_unused −noall_load −no_dead_strip_inits_and_terms −nofixprebinding −nomultidefs −noprebind −noseglinkedit −pagezero_size −prebind −prebind_all_twolevel_modules −private_bundle −read_only_relocs −sectalign −sectobjectsymbols −whyload −seg1addr −sectcreate −sectobjectsymbols −sectorder −segaddr −segs_read_only_addr −segs_read_write_addr −seg_addr_table −seg_addr_table_filename −seglinkedit −segprot −segs_read_only_addr −segs_read_write_addr −single_module −static −sub_library −sub_umbrella −twolevel_namespace −umbrella −undefined −unexported_symbols_list −weak_reference_mismatches −whatsloaded −F −gused −gfull −mmacosx−version−min=version −mkernel −mone−byte−bool

DEC Alpha Options −mno−fp−regs −msoft−float −mieee −mieee−with−inexact −mieee−conformant −mfp−trap−mode=mode −mfp−rounding−mode=mode −mtrap−precision=mode −mbuild−constants −mcpu=cpu-type −mtune=cpu-type −mbwx −mmax −mfix −mcix −mfloat−vax −mfloat−ieee −mexplicit−relocs −msmall−data −mlarge−data −msmall−text −mlarge−text −mmemory−latency=time

FR30 Options −msmall−model −mno−lsim

FRV Options −mgpr−32 −mgpr−64 −mfpr−32 −mfpr−64 −mhard−float −msoft−float −malloc−cc −mfixed−cc −mdword −mno−dword −mdouble −mno−double −mmedia −mno−media −mmuladd −mno−muladd −mfdpic −minline−plt −mgprel−ro −multilib−library−pic −mlinked−fp −mlong−calls −malign−labels −mlibrary−pic −macc−4 −macc−8 −mpack −mno−pack −mno−eflags −mcond−move −mno−cond−move −moptimize−membar −mno−optimize−membar −mscc −mno−scc −mcond−exec −mno−cond−exec −mvliw−branch −mno−vliw−branch −mmulti−cond−exec −mno−multi−cond−exec −mnested−cond−exec −mno−nested−cond−exec −mtomcat−stats −mTLS −mtls −mcpu=cpu

GNU/Linux Options −mglibc −muclibc −mbionic −mandroid −tno−android−cc −tno−android−ld

H8/300 Options −mrelax −mh −ms −mn −mexr −mno−exr −mint32 −malign−300

HPPA Options −march=architecture-type −mbig−switch −mdisable−fpregs −mdisable−indexing −mfast−indirect−calls −mgas −mgnu−ld −mhp−ld −mfixed−range=register-range −mjump−in−delay −mlinker−opt −mlong−calls −mlong−load−store −mno−big−switch −mno−disable−fpregs −mno−disable−indexing −mno−fast−indirect−calls −mno−gas −mno−jump−in−delay −mno−long−load−store −mno−portable−runtime −mno−soft−float −mno−space−regs −msoft−float −mpa−risc−1−0 −mpa−risc−1−1 −mpa−risc−2−0 −mportable−runtime −mschedule=cpu-type −mspace−regs −msio −mwsio −munix=unix-std −nolibdld −static −threads

i386 and x86−64 Options −mtune=cpu-type −march=cpu-type −mfpmath=unit −masm=dialect −mno−fancy−math−387 −mno−fp−ret−in−387 −msoft−float −mno−wide−multiply −mrtd −malign−double −mpreferred−stack−boundary=num −mincoming−stack−boundary=num −mcld −mcx16 −msahf −mmovbe −mcrc32 −mrecip −mrecip=opt −mvzeroupper −mprefer−avx128 −mmmx −msse −msse2 −msse3 −mssse3 −msse4.1 −msse4.2 −msse4 −mavx −mavx2 −maes −mpclmul −mfsgsbase −mrdrnd −mf16c −mfma −msse4a −m3dnow −mpopcnt −mabm −mbmi −mtbm −mfma4 −mxop −mlzcnt −mbmi2 −mrtm −mlwp −mthreads −mno−align−stringops −minline−all−stringops −minline−stringops−dynamically −mstringop−strategy=alg −mpush−args −maccumulate−outgoing−args −m128bit−long−double −m96bit−long−double −mlong−double−64 −mlong−double−80 −mregparm=num −msseregparm −mveclibabi=type −mvect8−ret−in−mem −mpc32 −mpc64 −mpc80 −mstackrealign −momit−leaf−frame−pointer −mno−red−zone −mno−tls−direct−seg−refs −mcmodel=code-model −mabi=name −maddress−mode=mode −m32 −m64 −mx32 −mlarge−data−threshold=num −msse2avx −mfentry −m8bit−idiv −mavx256−split−unaligned−load −mavx256−split−unaligned−store

i386 and x86−64 Windows Options −mconsole −mcygwin −mno−cygwin −mdll −mnop−fun−dllimport −mthread −municode −mwin32 −mwindows −fno−set−stack−executable

IA−64 Options −mbig−endian −mlittle−endian −mgnu−as −mgnu−ld −mno−pic −mvolatile−asm−stop −mregister−names −msdata −mno−sdata −mconstant−gp −mauto−pic −mfused−madd −minline−float−divide−min−latency −minline−float−divide−max−throughput −mno−inline−float−divide −minline−int−divide−min−latency −minline−int−divide−max−throughput −mno−inline−int−divide −minline−sqrt−min−latency −minline−sqrt−max−throughput −mno−inline−sqrt −mdwarf2−asm −mearly−stop−bits −mfixed−range=register-range −mtls−size=tls-size −mtune=cpu-type −milp32 −mlp64 −msched−br−data−spec −msched−ar−data−spec −msched−control−spec −msched−br−in−data−spec −msched−ar−in−data−spec −msched−in−control−spec −msched−spec−ldc −msched−spec−control−ldc −msched−prefer−non−data−spec−insns −msched−prefer−non−control−spec−insns −msched−stop−bits−after−every−cycle −msched−count−spec−in−critical−path −msel−sched−dont−check−control−spec −msched−fp−mem−deps−zero−cost −msched−max−memory−insns−hard−limit −msched−max−memory−insns=max-insns

LM32 Options −mbarrel−shift−enabled −mdivide−enabled −mmultiply−enabled −msign−extend−enabled −muser−enabled

M32R/D Options −m32r2 −m32rx −m32r −mdebug −malign−loops −mno−align−loops −missue−rate=number −mbranch−cost=number −mmodel=code-size-model-type −msdata=sdata-type −mno−flush−func −mflush−func=name −mno−flush−trap −mflush−trap=number −G num

M32C Options −mcpu=cpu −msim −memregs=number

M680x0 Options −march=arch −mcpu=cpu −mtune=tune −m68000 −m68020 −m68020−40 −m68020−60 −m68030 −m68040 −m68060 −mcpu32 −m5200 −m5206e −m528x −m5307 −m5407 −mcfv4e −mbitfield −mno−bitfield −mc68000 −mc68020 −mnobitfield −mrtd −mno−rtd −mdiv −mno−div −mshort −mno−short −mhard−float −m68881 −msoft−float −mpcrel −malign−int −mstrict−align −msep−data −mno−sep−data −mshared−library−id=n −mid−shared−library −mno−id−shared−library −mxgot −mno−xgot

MCore Options −mhardlit −mno−hardlit −mdiv −mno−div −mrelax−immediates −mno−relax−immediates −mwide−bitfields −mno−wide−bitfields −m4byte−functions −mno−4byte−functions −mcallgraph−data −mno−callgraph−data −mslow−bytes −mno−slow−bytes −mno−lsim −mlittle−endian −mbig−endian −m210 −m340 −mstack−increment

MeP Options −mabsdiff −mall−opts −maverage −mbased=n −mbitops −mc=n −mclip −mconfig=name −mcop −mcop32 −mcop64 −mivc2 −mdc −mdiv −meb −mel −mio−volatile −ml −mleadz −mm −mminmax −mmult −mno−opts −mrepeat −ms −msatur −msdram −msim −msimnovec −mtf −mtiny=n

MicroBlaze Options −msoft−float −mhard−float −msmall−divides −mcpu=cpu −mmemcpy −mxl−soft−mul −mxl−soft−div −mxl−barrel−shift −mxl−pattern−compare −mxl−stack−check −mxl−gp−opt −mno−clearbss −mxl−multiply−high −mxl−float−convert −mxl−float−sqrt −mbig−endian −mlittle−endian −mxl−reorder −mxl−mode−app-model

MIPS Options −EL −EB −march=arch −mtune=arch −mips1 −mips2 −mips3 −mips4 −mips32 −mips32r2 −mips64 −mips64r2 −mips16 −mno−mips16 −mflip−mips16 −minterlink−mips16 −mno−interlink−mips16 −mabi=abi −mabicalls −mno−abicalls −mshared −mno−shared −mplt −mno−plt −mxgot −mno−xgot −mgp32 −mgp64 −mfp32 −mfp64 −mhard−float −msoft−float −mno−float −msingle−float −mdouble−float −mdsp −mno−dsp −mdspr2 −mno−dspr2 −mmcu −mmno−mcu −mfpu=fpu-type −msmartmips −mno−smartmips −mpaired−single −mno−paired−single −mdmx −mno−mdmx −mips3d −mno−mips3d −mmt −mno−mt −mllsc −mno−llsc −mlong64 −mlong32 −msym32 −mno−sym32 −Gnum −mlocal−sdata −mno−local−sdata −mextern−sdata −mno−extern−sdata −mgpopt −mno−gopt −membedded−data −mno−embedded−data −muninit−const−in−rodata −mno−uninit−const−in−rodata −mcode−readable=setting −msplit−addresses −mno−split−addresses −mexplicit−relocs −mno−explicit−relocs −mcheck−zero−division −mno−check−zero−division −mdivide−traps −mdivide−breaks −mmemcpy −mno−memcpy −mlong−calls −mno−long−calls −mmad −mno−mad −mfused−madd −mno−fused−madd −nocpp −mfix−24k −mno−fix−24k −mfix−r4000 −mno−fix−r4000 −mfix−r4400 −mno−fix−r4400 −mfix−r10000 −mno−fix−r10000 −mfix−vr4120 −mno−fix−vr4120 −mfix−vr4130 −mno−fix−vr4130 −mfix−sb1 −mno−fix−sb1 −mflush−func=func −mno−flush−func −mbranch−cost=num −mbranch−likely −mno−branch−likely −mfp−exceptions −mno−fp−exceptions −mvr4130−align −mno−vr4130−align −msynci −mno−synci −mrelax−pic−calls −mno−relax−pic−calls −mmcount−ra−address

MMIX Options −mlibfuncs −mno−libfuncs −mepsilon −mno−epsilon −mabi=gnu −mabi=mmixware −mzero−extend −mknuthdiv −mtoplevel−symbols −melf −mbranch−predict −mno−branch−predict −mbase−addresses −mno−base−addresses −msingle−exit −mno−single−exit

MN10300 Options −mmult−bug −mno−mult−bug −mno−am33 −mam33 −mam33−2 −mam34 −mtune=cpu-type −mreturn−pointer−on−d0 −mno−crt0 −mrelax −mliw −msetlb

Moxie Options −meb −mel −mno−crt0

PDP−11 Options −mfpu −msoft−float −mac0 −mno−ac0 −m40 −m45 −m10 −mbcopy −mbcopy−builtin −mint32 −mno−int16 −mint16 −mno−int32 −mfloat32 −mno−float64 −mfloat64 −mno−float32 −mabshi −mno−abshi −mbranch−expensive −mbranch−cheap −munix−asm −mdec−asm

picoChip Options −mae=ae_type −mvliw−lookahead=N −msymbol−as−address −mno−inefficient−warnings

PowerPC Options See RS/6000 and PowerPC Options.

RL78 Options −msim −mmul=none −mmul=g13 −mmul=rl78

RS/6000 and PowerPC Options −mcpu=cpu-type −mtune=cpu-type −mcmodel=code-model −mpowerpc64 −maltivec −mno−altivec −mpowerpc−gpopt −mno−powerpc−gpopt −mpowerpc−gfxopt −mno−powerpc−gfxopt −mmfcrf −mno−mfcrf −mpopcntb −mno−popcntb −mpopcntd −mno−popcntd −mfprnd −mno−fprnd −mcmpb −mno−cmpb −mmfpgpr −mno−mfpgpr −mhard−dfp −mno−hard−dfp −mfull−toc −mminimal−toc −mno−fp−in−toc −mno−sum−in−toc −m64 −m32 −mxl−compat −mno−xl−compat −mpe −malign−power −malign−natural −msoft−float −mhard−float −mmultiple −mno−multiple −msingle−float −mdouble−float −msimple−fpu −mstring −mno−string −mupdate −mno−update −mavoid−indexed−addresses −mno−avoid−indexed−addresses −mfused−madd −mno−fused−madd −mbit−align −mno−bit−align −mstrict−align −mno−strict−align −mrelocatable −mno−relocatable −mrelocatable−lib −mno−relocatable−lib −mtoc −mno−toc −mlittle −mlittle−endian −mbig −mbig−endian −mdynamic−no−pic −maltivec −mswdiv −msingle−pic−base −mprioritize−restricted−insns=priority −msched−costly−dep=dependence_type −minsert−sched−nops=scheme −mcall−sysv −mcall−netbsd −maix−struct−return −msvr4−struct−return −mabi=abi-type −msecure−plt −mbss−plt −mblock−move−inline−limit=num −misel −mno−isel −misel=yes −misel=no −mspe −mno−spe −mspe=yes −mspe=no −mpaired −mgen−cell−microcode −mwarn−cell−microcode −mvrsave −mno−vrsave −mmulhw −mno−mulhw −mdlmzb −mno−dlmzb −mfloat−gprs=yes −mfloat−gprs=no −mfloat−gprs=single −mfloat−gprs=double −mprototype −mno−prototype −msim −mmvme −mads −myellowknife −memb −msdata −msdata=opt −mvxworks −G num −pthread −mrecip −mrecip=opt −mno−recip −mrecip−precision −mno−recip−precision −mveclibabi=type −mfriz −mno−friz −mpointers−to−nested−functions −mno−pointers−to−nested−functions −msave−toc−indirect −mno−save−toc−indirect −mpower8−fusion −mno−mpower8−fusion −mpower8−vector −mno−power8−vector −mcrypto −mno−crypto −mdirect−move −mno−direct−move −mquad−memory −mno−quad−memory −mquad−memory−atomic −mno−quad−memory−atomic −mcompat−align−parm −mno−compat−align−parm

RX Options −m64bit−doubles −m32bit−doubles −fpu −nofpu −mcpu= −mbig−endian−data −mlittle−endian−data −msmall−data −msim −mno−sim −mas100−syntax −mno−as100−syntax −mrelax −mmax−constant−size= −mint−register= −mpid −mno−warn−multiple−fast−interrupts −msave−acc−in−interrupts

S/390 and zSeries Options −mtune=cpu-type −march=cpu-type −mhard−float −msoft−float −mhard−dfp −mno−hard−dfp −mlong−double−64 −mlong−double−128 −mbackchain −mno−backchain −mpacked−stack −mno−packed−stack −msmall−exec −mno−small−exec −mmvcle −mno−mvcle −m64 −m31 −mdebug −mno−debug −mesa −mzarch −mtpf−trace −mno−tpf−trace −mfused−madd −mno−fused−madd −mwarn−framesize −mwarn−dynamicstack −mstack−size −mstack−guard −mhotpatch[=halfwords] −mno−hotpatch

Score Options −meb −mel −mnhwloop −muls −mmac −mscore5 −mscore5u −mscore7 −mscore7d

SH Options −m1 −m2 −m2e −m2a−nofpu −m2a−single−only −m2a−single −m2a −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 −mno−ieee −mbitops −misize −minline−ic_invalidate −mpadstruct −mspace −mprefergot −musermode −multcost=number −mdiv=strategy −mdivsi3_libfunc=name −mfixed−range=register-range −mindexed−addressing −mgettrcost=number −mpt−fixed −maccumulate−outgoing−args −minvalid−symbols −matomic−model=atomic-model −mbranch−cost=num −mzdcbranch −mno−zdcbranch −mcbranchdi −mcmpeqdi −mfused−madd −mno−fused−madd −mfsca −mno−fsca −mfsrra −mno−fsrra −mpretend−cmove −mtas

Solaris 2 Options −mimpure−text −mno−impure−text −pthreads −pthread

SPARC Options −mcpu=cpu-type −mtune=cpu-type −mcmodel=code-model −mmemory−model=mem-model −m32 −m64 −mapp−regs −mno−app−regs −mfaster−structs −mno−faster−structs −mflat −mno−flat −mfpu −mno−fpu −mhard−float −msoft−float −mhard−quad−float −msoft−quad−float −mstack−bias −mno−stack−bias −munaligned−doubles −mno−unaligned−doubles −muser−mode −mno−user−mode −mv8plus −mno−v8plus −mvis −mno−vis −mvis2 −mno−vis2 −mvis3 −mno−vis3 −mcbcond −mno−cbcond −mfmaf −mno−fmaf −mpopc −mno−popc −mfix−at697f −mfix−ut699

SPU Options −mwarn−reloc −merror−reloc −msafe−dma −munsafe−dma −mbranch−hints −msmall−mem −mlarge−mem −mstdmain −mfixed−range=register-range −mea32 −mea64 −maddress−space−conversion −mno−address−space−conversion −mcache−size=cache-size −matomic−updates −mno−atomic−updates

System V Options −Qy −Qn −YP,paths −Ym,dir

TILE-Gx Options −mcpu=cpu −m32 −m64 −mcmodel=code-model

TILEPro Options −mcpu=cpu −m32

V850 Options −mlong−calls −mno−long−calls −mep −mno−ep −mprolog−function −mno−prolog−function −mspace −mtda=n −msda=n −mzda=n −mapp−regs −mno−app−regs −mdisable−callt −mno−disable−callt −mv850e2v3 −mv850e2 −mv850e1 −mv850es −mv850e −mv850 −mv850e3v5 −mloop −mrelax −mlong−jumps −msoft−float −mhard−float −mgcc−abi −mrh850−abi −mbig−switch

VAX Options −mg −mgnu −munix

VMS Options −mvms−return−codes −mdebug−main=prefix −mmalloc64 −mpointer−size=size

VxWorks Options −mrtp −non−static −Bstatic −Bdynamic −Xbind−lazy −Xbind−now

x86−64 Options See i386 and x86−64 Options.

Xstormy16 Options −msim

Xtensa Options −mconst16 −mno−const16 −mfused−madd −mno−fused−madd −mforce−no−pic −mserialize−volatile −mno−serialize−volatile −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 −fdelete−dead−exceptions −funwind−tables −fasynchronous−unwind−tables −fno−gnu−unique −finhibit−size−directive −finstrument−functions −finstrument−functions−exclude−function−list=sym,sym,... −finstrument−functions−exclude−file−list=file,file,... −fno−common −fno−ident −fpcc−struct−return −fpic −fPIC −fpie −fPIE −fno−jump−tables −frecord−gcc−switches −freg−struct−return −fshort−enums −fshort−double −fshort−wchar −fverbose−asm −fpack−struct[=n] −fstack−check −fstack−limit−register=reg −fstack−limit−symbol=sym −fno−stack−limit −fsplit−stack −fleading−underscore −ftls−model=model −fstack−reuse=reuse_level −ftrapv −fwrapv −fbounds−check −fvisibility −fstrict−volatile−bitfields −fsync−libcalls

Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file.

For any given input file, the file name suffix determines what kind of compilation is done:
file.c

C source code that must be preprocessed.

file.i

C source code that should not be preprocessed.

file.ii

C ++ source code that 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 that 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 that should not be preprocessed.

file.h

C, C ++ , Objective-C or Objective−C ++ header file to be turned into a precompiled header (default), or C, C ++ header file to be turned into an Ada spec (via the −fdump−ada−spec switch).

file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C

C ++ source code that 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 that must be preprocessed.

file.mii

Objective−C ++ source code that should not be preprocessed.

file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc

C ++ header file to be turned into a precompiled header or Ada spec.

file.f
file.for
file.ftn

Fixed form Fortran source code that should not be preprocessed.

file.F
file.FOR
file.fpp
file.FPP
file.FTN

Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).

file.f90
file.f95
file.f03
file.f08

Free form Fortran source code that should not be preprocessed.

file.F90
file.F95
file.F03
file.F08

Free form Fortran source code that must be preprocessed (with the traditional preprocessor).

file.go

Go source code.

file.ads

Ada source code file that 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
file.sx

Assembler code that 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  cpp−output
        c++  c++−header  c++−cpp−output
        objective−c  objective−c−header  objective−c−cpp−output
        objective−c++ objective−c++−header objective−c++−cpp−output
        assembler  assembler−with−cpp
        ada
        f77  f77−cpp−input f95  f95−cpp−input
        go
        java

−x none

Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if −x has not been used at all).

−pass−exit−codes

Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify −pass−exit−codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C ++ , and Fortran front ends return 4 if an internal compiler error is encountered.

If you only want some of the stages of compilation, you can use −x (or filename suffixes) to tell gcc where to start, and one of the options −c, −S, or −E to say where gcc is to stop. Note that some combinations (for example, −x cpp-output −E) instruct gcc to do nothing at all.

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.

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.

Input files that don’t require preprocessing are ignored.

−o file

Place output in file file. This applies 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.

−###

Like −v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or "./−_". This is useful for shell scripts to capture the driver-generated command lines.

−pipe

Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble.

−−help

Print (on the standard output) a description of the command-line options understood by gcc. If the −v option is also specified then −−help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. If the −Wextra option has also been specified (prior to the −−help option), then command-line options that have no documentation associated with them are also displayed.

−−target−help

Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed.

−−help={class|[^]qualifier}[,...]

Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes:
optimizers

Display all of the optimization options supported by the compiler.

warnings

Display all of the options controlling warning messages produced by the compiler.

target

Display target-specific options. Unlike the −−target−help option however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended −−help= syntax.

params

Display the values recognized by the −−param option.

language

Display the options supported for language, where language is the name of one of the languages supported in this version of GCC.

common

Display the options that are common to all languages.

These are the supported qualifiers:
undocumented

Display only those options that are undocumented.

joined

Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as: −−help=target.

separate

Display options taking an argument that appears as a separate word following the original option, such as: −o output-file.

Thus for example to display all the undocumented target-specific switches supported by the compiler, use:

        −−help=target,undocumented

The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:

        −−help=warnings,^joined,^undocumented

The argument to −−help= should not consist solely of inverted qualifiers.

Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:

        −−help=target,optimizers

The −−help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed.

If the −Q option appears on the command line before the −−help= option, then the descriptive text displayed by −−help= is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the −−help= option is used).

Here is a truncated example from the ARM port of gcc:

          % gcc −Q −mabi=2 −−help=target −c
          The following options are target specific:
          −mabi=                                2
          −mabort−on−noreturn                   [disabled]
          −mapcs                                [disabled]

The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at −O2 by using:

        −Q −O2 −−help=optimizers

Alternatively you can discover which binary optimizations are enabled by −O3 by using:

        gcc −c −Q −O3 −−help=optimizers > /tmp/O3−opts
        gcc −c −Q −O2 −−help=optimizers > /tmp/O2−opts
        diff /tmp/O2−opts /tmp/O3−opts | grep enabled

−no−canonical−prefixes

Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix.

−−version

Display the version number and copyrights of the invoked GCC.

−wrapper

Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.

        gcc −c t.c −wrapper gdb,−−args

This invokes all subprograms of gcc under gdb −−args, thus the invocation of cc1 is gdb −−args cc1 ....

−fplugin=name.so

Load the plugin code in file name.so, assumed to be a shared object to be dlopen’d by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (See −fplugin−arg−name−key=value below). Each plugin should define the callback functions specified in the Plugins API.

−fplugin−arg−name−key=value

Define an argument called key with a value of value for the plugin called name.

−fdump−ada−spec[−slim]

For C and C ++ source and include files, generate corresponding Ada specs.

−fada−spec−parent=unit

In conjunction with −fdump−ada−spec[−slim] above, generate Ada specs as child units of parent unit.

−fdump−go−spec=file

For input files in any language, generate corresponding Go declarations in file. This generates Go "const", "type", "var", and "func" declarations which may be a useful way to start writing a Go interface to code written in some other language.

@file

Read command-line options from file. The options read are inserted in place of the original @file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed.

Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively.

Compiling C ++ Programs
C ++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C ++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C ++ files use the suffix .ii. GCC recognizes files with these names and compiles them as C ++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).

However, the use of gcc does not add the C ++ library. g++ is a program that calls GCC and automatically specifies linking against the C ++ library. It treats .c, .h and .i files as C ++ source files instead of C source files unless −x is used. This program is also useful when precompiling a C header file with a .h extension for use in C ++ compilations. On many systems, g++ is also installed with the name c++.

When you compile C ++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C ++ programs.

Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C ++ , Objective-C and Objective−C ++ ) that the compiler accepts:
−ansi

In C mode, this is equivalent to −std=c90. In C ++ mode, it is equivalent to −std=c++98.

This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C ++ (when compiling C ++ code), such as the "asm" and "typeof" keywords, and predefined macros such as "unix" and "vax" that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C ++ style // comments as well as the "inline" keyword.

The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" continue to work despite −ansi. You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with −ansi. Alternate predefined macros such as "__unix__" and "__vax__" are also available, with or without −ansi.

The −ansi option does not cause non-ISO programs to be rejected gratuitously. For that, −Wpedantic is required in addition to −ansi.

The macro "__STRICT_ANSI__" is predefined when the −ansi option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn’t call for; this is to avoid interfering with any programs that might use these names for other things.

Functions that are normally built in but do not have semantics defined by ISO C (such as "alloca" and "ffs") are not built-in functions when −ansi is used.

−std=

Determine the language standard. This option is currently only supported when compiling C or C ++ .

The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, −std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the "asm" and "typeof" keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a "?:" expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by −Wpedantic to identify which features are GNU extensions given that version of the standard. For example −std=gnu90 −Wpedantic warns about C ++ style // comments, while −std=gnu99 −Wpedantic does not.

A value for this option must be provided; possible values are

iso9899:1990

Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as −ansi for C code.

iso9899:199409

ISO C90 as modified in amendment 1.

iso9899:1999
iso9899:199x

ISO C99. Note that this standard is not yet fully supported; see <http://gcc.gnu.org/c99status.html> for more information. The names c9x and iso9899:199x are deprecated.

iso9899:2011

ISO C11, the 2011 revision of the ISO C standard. Support is incomplete and experimental. The name c1x is deprecated.

gnu90
gnu89

GNU dialect of ISO C90 (including some C99 features). This is the default for C code.

gnu99
gnu9x

GNU dialect of ISO C99. When ISO C99 is fully implemented in GCC, this will become the default. The name gnu9x is deprecated.

gnu11
gnu1x

GNU dialect of ISO C11. Support is incomplete and experimental. The name gnu1x is deprecated.

c++98
c++03

The 1998 ISO C ++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as −ansi for C ++ code.

gnu++98
gnu++03

GNU dialect of −std=c++98. This is the default for C ++ code.

c++11
c++0x

The 2011 ISO C ++ standard plus amendments. Support for C ++ 11 is still experimental, and may change in incompatible ways in future releases. The name c++0x is deprecated.

gnu++11
gnu++0x

GNU dialect of −std=c++11. Support for C ++ 11 is still experimental, and may change in incompatible ways in future releases. The name gnu++0x is deprecated.

c++1y

The next revision of the ISO C ++ standard, tentatively planned for 2017. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.

gnu++1y

GNU dialect of −std=c++1y. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.

−fgnu89−inline

The option −fgnu89−inline tells GCC to use the traditional GNU semantics for "inline" functions when in C99 mode.
This option is accepted and ignored by GCC versions 4.1.3 up to but not including 4.3. In GCC versions 4.3 and later it changes the behavior of GCC in C99 mode. Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline functions.

The option −fno−gnu89−inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option was first supported in GCC 4.3. This option is not supported in −std=c90 or −std=gnu90 mode.

The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are in effect for "inline" functions.

−aux−info filename

Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C.

Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character). In the case of function definitions, a K&R−style list of arguments followed by their declarations is also provided, inside comments, after the declaration.

−fallow−parameterless−variadic−functions

Accept variadic functions without named parameters.

Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments. This is only supported for C as this construct is allowed by C ++ .

−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 which 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 that is not built-in in this version of GCC, this option is ignored. There is no corresponding −fbuiltin−function option; if you wish to enable built-in functions selectively when using −fno−builtin or −ffreestanding, you may define macros such as:

        #define abs(n)          __builtin_abs ((n))
        #define strcpy(d, s)    __builtin_strcpy ((d), (s))

−fhosted

Assert that compilation targets a hosted environment. This implies −fbuiltin. A hosted environment is one in which the entire standard library is available, and in which "main" has a return type of "int". Examples are nearly everything except a kernel. This is equivalent to −fno−freestanding.

−ffreestanding

Assert that compilation targets a freestanding environment. This implies −fno−builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at "main". The most obvious example is an OS kernel. This is equivalent to −fno−hosted.

−fopenmp

Enable handling of OpenMP directives "#pragma omp" in C/C ++ and "!$omp" in Fortran. When −fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v3.0 <http://www.openmp.org/>. This option implies −pthread, and thus is only supported on targets that have support for −pthread.

−fgnu−tm

When the option −fgnu−tm is specified, the compiler generates code for the Linux variant of Intel’s current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature.

For more information on GCC ’s support for transactional memory,

Note that the transactional memory feature is not supported with non-call exceptions (−fnon−call−exceptions).

−fms−extensions

Accept some non-standard constructs used in Microsoft header files.

In C ++ code, this allows member names in structures to be similar to previous types declarations.

        typedef int UOW;
        struct ABC {
          UOW UOW;
        };

Some cases of unnamed fields in structures and unions are only accepted with this option.

−fplan9−extensions

Accept some non-standard constructs used in Plan 9 code.

This enables −fms−extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. This is only supported for C, not C ++ .

−trigraphs

Support ISO C trigraphs. The −ansi option (and −std options for strict ISO C conformance) implies −trigraphs.

−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 ++ .

−flax−vector−conversions

Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code.

−funsigned−char

Let the type "char" be unsigned, like "unsigned char".

Each kind of machine has a default for what "char" should be. It is either like "unsigned char" by default or like "signed char" by default.

Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on the signedness of an object. But many programs have been written to use plain "char" and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default.

The type "char" is always a distinct type from each of "signed char" or "unsigned char", even though its behavior is always just like one of those two.

−fsigned−char

Let the type "char" be signed, like "signed char".

Note that this is equivalent to −fno−unsigned−char, which is the negative form of −funsigned−char. Likewise, the option −fno−signed−char is equivalent to −funsigned−char.

−fsigned−bitfields
−funsigned−bitfields
−fno−signed−bitfields
−fno−unsigned−bitfields

These options control whether a bit-field is signed or unsigned, when the declaration does not use either "signed" or "unsigned". By default, such a bit-field is signed, because this is consistent: the basic integer types such as "int" are signed types.

Options Controlling C ++ Dialect
This section describes the command-line options that are only meaningful for C ++ programs. 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. The default is version 2.

Version 0 refers to the version conforming most closely to the C ++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed.

Version 1 is the version of the C ++ ABI that first appeared in G++ 3.2.

Version 2 is the version of the C ++ ABI that first appeared in G++ 3.4.

Version 3 corrects an error in mangling a constant address as a template argument.

Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.

Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.

Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C ++ 11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and −−, and a class scope function used as a template argument.

See also −Wabi.

−fno−access−control

Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.

−fcheck−new

Check that the pointer returned by "operator new" is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C ++ standard specifies that "operator new" only returns 0 if it is declared throw(), in which case the compiler always checks 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).

−fconstexpr−depth=n

Set the maximum nested evaluation depth for C ++ 11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512.

−fdeduce−init−list

Enable deduction of a template type parameter as "std::initializer_list" from a brace-enclosed initializer list, i.e.

        template <class T> auto forward(T t) −> decltype (realfn (t))
        {
          return realfn (t);
        }
        void f()
        {
          forward({1,2}); // call forward<std::initializer_list<int>>
        }

This deduction was implemented as a possible extension to the originally proposed semantics for the C ++ 11 standard, but was not part of the final standard, so it is disabled by default. This option is deprecated, and may be removed in a future version of G++.

−ffriend−injection

Inject friend functions into the enclosing namespace, so that they are visible outside the scope of the class in which they are declared. Friend functions were documented to work this way in the old Annotated C ++ Reference Manual, and versions of G++ before 4.1 always worked that way. However, in ISO C ++ a friend function that is not declared in an enclosing scope can only be found using argument dependent lookup. This option causes friends to be injected as they were in earlier releases.

This option is for compatibility, and may be removed in a future release of G++.

−fno−elide−constructors

The C ++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases.

−fno−enforce−eh−specs

Don’t generate code to check for violation of exception specifications at run time. This option violates the C ++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG . This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time.

−fextern−tls−init
−fno−extern−tls−init

The C ++ 11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn’t actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU ), they can avoid this overhead with the −fno−extern−tls−init option.

On targets that support symbol aliases, the default is −fextern−tls−init. On targets that do not support symbol aliases, the default is −fno−extern−tls−init.

−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 ++ .

If neither flag is given, the default is 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 that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations.

−fno−implicit−inline−templates

Don’t emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations.

−fno−implement−inlines

To save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation. This causes linker errors if these functions are not inlined everywhere they are called.

−fms−extensions

Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.

−fno−nonansi−builtins

Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include "ffs", "alloca", "_exit", "index", "bzero", "conjf", and other related functions.

−fnothrow−opt

Treat a "throw()" exception specification as if it were a "noexcept" specification to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to "terminate" rather than "unexpected".

−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 allows some nonconforming code to compile.

−fno−pretty−templates

When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. "void f(T) [with T = int]" rather than "void f(int)") so that it’s clear which template is involved. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use −fno−pretty−templates to disable them.

−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 ++ run-time 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 G++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to "void *" or to unambiguous base classes.

−fstats

Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++ development team.

−fstrict−enums

Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C ++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type.

−ftemplate−backtrace−limit=n

Set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10.

−ftemplate−depth=n

Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C ++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C ++ 11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations.

−fno−threadsafe−statics

Do not emit the extra code to use the routines specified in the C ++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn’t need to be thread-safe.

−fuse−cxa−atexit

Register destructors for objects with static storage duration with the "__cxa_atexit" function rather than the "atexit" function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports "__cxa_atexit".

−fno−use−cxa−get−exception−ptr

Don’t use the "__cxa_get_exception_ptr" runtime routine. This causes "std::uncaught_exception" to be incorrect, but is necessary if the runtime routine is not available.

−fvisibility−inlines−hidden

This switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

The effect of this is that GCC may, effectively, mark inline methods with "__attribute__ ((visibility ("hidden")))" so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates.

The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.

You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect.

Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary.

−fvisibility−ms−compat

This flag attempts to use visibility settings to make GCC ’s C ++ linkage model compatible with that of Microsoft Visual Studio.

The flag makes these changes to GCC ’s linkage model:

In new code it is better to use −fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.

Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.

−fno−weak

Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results 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 have linkage like inline functions; they just aren’t inlined by default.

−Wabi (C, Objective-C, C ++ and Objective−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 is 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 in −fabi−version=2 (the default) include:

        extern int N;
        template <int &> struct S {};
        void n (S<N>) {2}

This is fixed in −fabi−version=3.

The mangling is changed in −fabi−version=4.

The known incompatibilities in −fabi−version=1 include:

        struct A { virtual void f(); int f1 : 1; };
        struct B : public A { int f2 : 1; };

In this case, G++ places "B::f2" into the same byte as "A::f1"; other compilers do not. You can avoid this problem by explicitly padding "A" so that its size is a multiple of the byte size on your platform; that causes G++ and other compilers to lay out "B" identically.

        struct A { virtual void f(); char c1; };
        struct B { B(); char c2; };
        struct C : public A, public virtual B {};

In this case, G++ does not place "B" into the tail-padding for "A"; other compilers do. You can avoid this problem by explicitly padding "A" so that its size is a multiple of its alignment (ignoring virtual base classes); that causes G++ and other compilers to lay out "C" identically.

        union U { int i : 4096; };

Assuming that an "int" does not have 4096 bits, G++ makes the union too small by the number of bits in an "int".

        struct A {};
        struct B {
          A a;
          virtual void f ();
        };
        struct C : public B, public A {};

G++ places 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.

        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.

It also warns about psABI-related changes. The known psABI changes at this point include:

        union U {
          long double ld;
          int i;
        };

"union U" is always passed in memory.

−Wctor−dtor−privacy (C ++ and Objective−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. Also warn if there are no non-private methods, and there’s at least one private member function that isn’t a constructor or destructor.

−Wdelete−non−virtual−dtor (C ++ and Objective−C ++ only)

Warn when delete is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by −Wall.

−Wliteral−suffix (C ++ and Objective−C ++ only)

Warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from "<inttypes.h>". For example:

        #define __STDC_FORMAT_MACROS
        #include <inttypes.h>
        #include <stdio.h>
        int main() {
          int64_t i64 = 123;
          printf("My int64: %"PRId64"\n", i64);
        }

In this case, "PRId64" is treated as a separate preprocessing token.

This warning is enabled by default.

−Wnarrowing (C ++ and Objective−C ++ only)

Warn when a narrowing conversion prohibited by C ++ 11 occurs within { }, e.g.

        int i = { 2.2 }; // error: narrowing from double to int

This flag is included in −Wall and −Wc++11−compat.

With −std=c++11, −Wno−narrowing suppresses the diagnostic required by the standard. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE context.

−Wnoexcept (C ++ and Objective−C ++ only)

Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. throw() or noexcept) but is known by the compiler to never throw an exception.

−Wnon−virtual−dtor (C ++ and Objective−C ++ only)

Warn when a class has virtual functions and an accessible non-virtual destructor, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the base class. This warning is also enabled if −Weffc++ is specified.

−Wreorder (C ++ and Objective−C ++ only)

Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance:

        struct A {
          int i;
          int j;
          A(): j (0), i (1) { }
        };

The compiler rearranges the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by −Wall.

−fext−numeric−literals (C ++ and Objective−C ++ only)

Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C ++ 11 user-defined literal numeric suffixes. This is on by default for all pre−C ++ 11 dialects and all GNU dialects: −std=c++98, −std=gnu++98, −std=gnu++11, −std=gnu++1y. This option is off by default for ISO C ++ 11 onwards (−std=c++11, ...).

The following −W... options are not affected by −Wall.
−Weffc++ (C ++ and Objective−C ++ only)

Warn about violations of the following style guidelines from Scott Meyers’ Effective C ++ , Second Edition book:

Also warn about violations of the following style guidelines from Scott Meyers’ More Effective C ++ book:

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.

−Wstrict−null−sentinel (C ++ and Objective−C ++ only)

Warn 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 rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers.

−Wno−non−template−friend (C ++ and Objective−C ++ only)

Disable warnings when non-templatized friend functions are declared within a template. Since the advent of explicit template specification support in G++, if the name of the friend is an unqualified-id (i.e., friend foo(int)), the C ++ language specification demands that the friend declare or define an ordinary, nontemplate function. (Section 14.5.3). Before G++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a templatized function. Because this non-conforming behavior is no longer the default behavior for G++, −Wnon−template−friend allows the compiler to check existing code for potential trouble spots and is on by default. This new compiler behavior can be turned off with −Wno−non−template−friend, which keeps the conformant compiler code but disables the helpful warning.

−Wold−style−cast (C ++ and Objective−C ++ only)

Warn if an old-style (C−style) cast to a non-void type is used within a C ++ program. The new-style casts (dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for.

−Woverloaded−virtual (C ++ and Objective−C ++ only)

Warn when a function declaration hides virtual functions from a base class. For example, in:

        struct A {
          virtual void f();
        };
        struct B: public A {
          void f(int);
        };

the "A" class version of "f" is hidden in "B", and code like:

        B* b;
        b−>f();

fails to compile.

−Wno−pmf−conversions (C ++ and Objective−C ++ only)

Disable the diagnostic for converting a bound pointer to member function to a plain pointer.

−Wsign−promo (C ++ and Objective−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++ tried to preserve unsignedness, but the standard mandates the current behavior.

Options Controlling Objective-C and Objective−C ++ Dialects
( NOTE: This manual does not describe the Objective-C and Objective−C ++ languages themselves.

This section describes the command-line options that are only meaningful for Objective-C and Objective−C ++ programs. You can also use most of the language-independent GNU compiler options. For example, you might compile a file "some_class.m" like this:

        gcc −g −fgnu−runtime −O −c some_class.m

In this example, −fgnu−runtime is an option meant only for Objective-C and Objective−C ++ programs; you can use the other options with any language supported by GCC.

Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., −Wtraditional). Similarly, Objective−C ++ compilations may use C ++ −specific options (e.g., −Wabi).

Here is a list of options that are only for compiling Objective-C and Objective−C ++ programs:
−fconstant−string−class=class-name

Use class-name as the name of the class to instantiate for each literal string specified with the syntax "@"..."". The default class name is "NXConstantString" if the GNU runtime is being used, and "NSConstantString" if the NeXT runtime is being used (see below). The −fconstant−cfstrings option, if also present, overrides the −fconstant−string−class setting and cause "@"..."" literals to be laid out as constant CoreFoundation strings.

−fgnu−runtime

Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems.

−fnext−runtime

Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin and Mac OS X. The macro "__NEXT_RUNTIME__" is predefined if (and only if) this option is used.

−fno−nil−receivers

Assume that all Objective-C message dispatches ("[receiver message:arg]") in this translation unit ensure that the receiver is not "nil". This allows for more efficient entry points in the runtime to be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.

−fobjc−abi−version=n

Use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case, Version 0 is the traditional (32−bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32−bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64−bit) ABI. If nothing is specified, the default is Version 0 on 32−bit target machines, and Version 2 on 64−bit target machines.

−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 which runs non-trivial default constructors on any such instance variables, in order, and then return "self". Similarly, check if any instance variable is a C ++ object with a non-trivial destructor, and if so, synthesize a special "− (void) .cxx_destruct" method which runs all such default destructors, in reverse order.

The "− (id) .cxx_construct" and "− (void) .cxx_destruct" methods thusly generated 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 are invoked by the runtime immediately after a new object instance is allocated; the "− (void) .cxx_destruct" methods are invoked immediately before the runtime deallocates an object instance.

As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the "− (id) .cxx_construct" and "− (void) .cxx_destruct" methods.

−fobjc−direct−dispatch

Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.

−fobjc−exceptions

Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C ++ and Java. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).

−fobjc−gc

Enable garbage collection ( GC ) in Objective-C and Objective−C ++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags.

−fobjc−nilcheck

For the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using −fno−objc−nilcheck. Class methods and super calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.

−fobjc−std=objc1

Conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C ++ language; it does not affect conformance to C/C ++ standards, which is controlled by the separate C/C ++ dialect option flags. When this option is used with the Objective-C or Objective−C ++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC.

−freplace−objc−classes

Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

−fzero−link

When compiling for the NeXT runtime, the compiler ordinarily replaces calls to "objc_getClass("...")" (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance. Specifying the −fzero−link flag suppresses this behavior and causes calls to "objc_getClass("...")" to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls to "objc_get_class("...")" regardless of command-line options.

−gen−decls

Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.

−Wassign−intercept (Objective-C and Objective−C ++ only)

Warn whenever an Objective-C assignment is being intercepted by the garbage collector.

−Wno−protocol (Objective-C and Objective−C ++ only)

If a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the −Wno−protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them.

−Wselector (Objective-C and Objective−C ++ only)

Warn if multiple methods of different types for the same selector are found during compilation. The check is performed on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing in a "@selector(...)" expression, and a corresponding method for that selector has been found during compilation. Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached, for example because an error is found during compilation, or because the −fsyntax−only option is being used.

−Wstrict−selector−match (Objective-C and Objective−C ++ only)

Warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to send a message using this selector to a receiver of type "id" or "Class". When this flag is off (which is the default behavior), the compiler omits such warnings if any differences found are confined to types that share the same size and alignment.

−Wundeclared−selector (Objective-C and Objective−C ++ only)

Warn if a "@selector(...)" expression referring to an undeclared selector is found. A selector is considered undeclared if no method with that name has been declared before the "@selector(...)" expression, either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. This option always performs its checks as soon as a "@selector(...)" expression is found, while −Wselector only performs its checks in the final stage of compilation. This also enforces the coding style convention that methods and selectors must be declared before being used.

−print−objc−runtime−info

Generate C header describing the largest structure that is passed by value, if any.

Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output device’s aspect (e.g. its width, ...). You can use the options described below to control the formatting algorithm for diagnostic messages, e.g. how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options.
−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 is done; each error message appears on a single line.

−fdiagnostics−show−location=once

Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once; 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−color[= WHEN ]
−fno−diagnostics−color

Use color in diagnostics. WHEN is never, always, or auto. The default is auto. auto means to use color only when the standard error is a terminal. The forms −fdiagnostics−color and −fno−diagnostics−color are aliases for −fdiagnostics−color=always and −fdiagnostics−color=never, respectively.

The colors are defined by the environment variable GCC_COLORS . Its value is a colon-separated list of capabilities and Select Graphic Rendition ( SGR ) substrings. SGR commands are interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16−color mode foreground colors, 38;5;0 to 38;5;255 for 88−color and 256−color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16−color mode background colors, and 48;5;0 to 48;5;255 for 88−color and 256−color modes background colors.

The default GCC_COLORS is error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01 where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 01;32 is bold green and 01 is bold. Setting GCC_COLORS to the empty string disables colors. Supported capabilities are as follows.
"error="

SGR substring for error: markers.

"warning="

SGR substring for warning: markers.

"note="

SGR substring for note: markers.

"caret="

SGR substring for caret line.

"locus="

SGR substring for location information, file:line or file:line:column etc.

"quote="

SGR substring for information printed within quotes.

−fno−diagnostics−show−option

By default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the −fno−diagnostics−show−option flag suppresses that behavior.

−fno−diagnostics−show−caret

By default, each diagnostic emitted includes the original source line and a caret ’^’ indicating the column. This option suppresses this information.

Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error.

The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.
−fsyntax−only

Check the code for syntax errors, but don’t do anything beyond that.

−fmax−errors=n

Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If −Wfatal−errors is also specified, then −Wfatal−errors takes precedence over this option.

−Werror

Make all warnings into errors.

−Werror=

Make the specified warning into an error. The specifier for a warning is appended; for example −Werror=switch turns the warnings controlled by −Wswitch into errors. This switch takes a negative form, to be used to negate −Werror for specific warnings; for example −Wno−error=switch makes −Wswitch warnings not be errors, even when −Werror is in effect.

The warning message for each controllable warning includes the option that controls the warning. That option can then be used with −Werror= and −Wno−error= as described above. (Printing of the option in the warning message can be disabled using the −fno−diagnostics−show−option flag.)

Note that specifying −Werror=foo automatically implies −Wfoo. However, −Wno−error=foo does not imply anything.

−Wfatal−errors

This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages.

You can request many specific warnings with options beginning with −W, for example −Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning −Wno− to turn off warnings; for example, −Wno−implicit. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C ++ Dialect Options and Objective-C and Objective−C ++ Dialect Options.

When an unrecognized warning option is requested (e.g., −Wunknown−warning), GCC emits a diagnostic stating that the option is not recognized. However, if the −Wno− form is used, the behavior is slightly different: no diagnostic is produced for −Wno−unknown−warning unless other diagnostics are being produced. This allows the use of new −Wno− options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present.
−Wpedantic
−pedantic

Issue all the warnings demanded by strict ISO C and ISO C ++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C ++ . For ISOC, followstheversionof the ISOC standardspecifiedbyany−stdoptionused.

Valid ISOC and ISOC ++programsshouldcompileproperlywithorwithoutthis option(thoughararefewrequire−ansiora−stdoptionspecifyingtherequired versionofISOC).However,withoutthisoption,certainGNUextensionsand traditionalCandC++featuresaresupportedaswell.Withthisoption,theyare rejected.

−Wpedanticdoesnotcausewarningmessagesforuseofthealternatekeywords whosenamesbeginandendwith__.Pedanticwarningsarealsodisabledinthe expressionthatfollows"__extension__".However,onlysystemheaderfiles shouldusetheseescaperoutes;applicationprogramsshouldavoidthem.

Someuserstrytouse−WpedantictocheckprogramsforstrictISOCconformance. Theysoonfindthatitdoesnotdoquitewhattheywant:itfindssomenon-ISO practices,butnotall−−−onlythoseforwhichISOCrequiresadiagnostic,and someothersforwhichdiagnosticshavebeenadded.

AfeaturetoreportanyfailuretoconformtoISOCmightbeusefulinsome instances,butwouldrequireconsiderableadditionalworkandwouldbequite differentfrom−Wpedantic.Wedon’thaveplanstosupportsuchafeatureinthe nearfuture.

Wherethestandardspecifiedwith−stdrepresentsaGNUextendeddialectofC, suchasgnu90orgnu99,thereisacorrespondingbasestandard,theversionof ISOConwhichtheGNUextendeddialectisbased.Warningsfrom−Wpedanticare givenwheretheyarerequiredbythebasestandard.(Itdoesnotmakesensefor suchwarningstobegivenonlyforfeaturesnotinthespecifiedGNUCdialect, sincebydefinitiontheGNUdialectsofCincludeallfeaturesthecompiler supportswiththegivenoption,andtherewouldbenothingtowarnabout.)

−pedantic−errors

Like −Wpedantic, except that errors are produced rather than warnings.

−Wall

This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables some language-specific warnings described in C ++ Dialect Options and Objective-C and Objective−C ++ Dialect Options.

−Wall turns on the following warning flags:

−Waddress −Warray−bounds (only with −O2) −Wc++11−compat −Wchar−subscripts −Wenum−compare (in C/ObjC; this is on by default in C ++ ) −Wimplicit−int (C and Objective-C only) −Wimplicit−function−declaration (C and Objective-C only) −Wcomment −Wformat −Wmain (only for C/ObjC and unless −ffreestanding) −Wmaybe−uninitialized −Wmissing−braces (only for C/ObjC) −Wnonnull −Wparentheses −Wpointer−sign −Wreorder −Wreturn−type −Wsequence−point −Wsign−compare (only in C ++ ) −Wstrict−aliasing −Wstrict−overflow=1 −Wswitch −Wtrigraphs −Wuninitialized −Wunknown−pragmas −Wunused−function −Wunused−label −Wunused−value −Wunused−variable −Wvolatile−register−var

Note that some warning flags are not implied by −Wall. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of them are enabled by −Wextra but many of them must be enabled individually.

−Wextra

This enables some extra warning flags that are not enabled by −Wall. (This option used to be called −W. The older name is still supported, but the newer name is more descriptive.)

−Wclobbered −Wempty−body −Wignored−qualifiers −Wmissing−field−initializers −Wmissing−parameter−type (C only) −Wold−style−declaration (C only) −Woverride−init −Wsign−compare −Wtype−limits −Wuninitialized −Wunused−parameter (only with −Wunused or −Wall) −Wunused−but−set−parameter (only with −Wunused or −Wall)

The option −Wextra also prints warning messages for the following cases:

−Wchar−subscripts

Warn if an array subscript has type "char". This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by −Wall.

−Wcomment

Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline appears in a // comment. This warning is enabled by −Wall.

−Wno−coverage−mismatch

Warn if feedback profiles do not match when using the −fprofile−use option. If a source file is changed between compiling with −fprofile−gen and with −fprofile−use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error. −Wno−coverage−mismatch can be used to disable the warning or −Wno−error=coverage−mismatch can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended.

−Wno−cpp

(C, Objective-C, C ++ , Objective−C ++ and Fortran only)

Suppress warning messages emitted by "#warning" directives.

−Wdouble−promotion (C, C ++ , Objective-C and Objective−C ++ only)

Give a warning when a value of type "float" is implicitly promoted to "double". CPUs with a 32−bit "single-precision" floating-point unit implement "float" in hardware, but emulate "double" in software. On such a machine, doing computations using "double" values is much more expensive because of the overhead required for software emulation.

It is easy to accidentally do computations with "double" because floating-point literals are implicitly of type "double". For example, in:

        float area(float radius)
        {
           return 3.14159 * radius * radius;
        }

the compiler performs the entire computation with "double" because the floating-point literal is a "double".

−Wformat
−Wformat=n

Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes, in the "printf", "scanf", "strftime" and "strfmon" (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by −ffreestanding or −fno−builtin.

The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library’s limitations. However, if −Wpedantic is used with −Wformat, warnings are given about format features not in the selected standard version (but not for "strfmon" formats, since those are not in any version of the C standard).
−Wformat=1
−Wformat

Option −Wformat is equivalent to −Wformat=1, and −Wno−format is equivalent to −Wformat=0. Since −Wformat also checks for null format arguments for several functions, −Wformat also implies −Wnonnull. Some aspects of this level of format checking can be disabled by the options: −Wno−format−contains−nul, −Wno−format−extra−args, and −Wno−format−zero−length. −Wformat is enabled by −Wall.

−Wno−format−contains−nul

If −Wformat is specified, do not warn about format strings that contain NUL bytes.

−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 suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed.

−Wno−format−zero−length

If −Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed.

−Wformat=2

Enable −Wformat plus additional format checks. Currently equivalent to −Wformat −Wformat−nonliteral −Wformat−security −Wformat−y2k.

−Wformat−nonliteral

If −Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a "va_list".

−Wformat−security

If −Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to "printf" and "scanf" functions where the format string is not a string literal and there are no format arguments, as in "printf (foo);". This may be a security hole if the format string came from untrusted input and contains %n. (This is currently a subset of what −Wformat−nonliteral warns about, but in future warnings may be added to −Wformat−security that are not included in −Wformat−nonliteral.)

−Wformat−y2k

If −Wformat is specified, also warn about "strftime" formats that may yield only a two-digit year.

−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 that are initialized with themselves. Note this option can only be used with the −Wuninitialized option.

For example, GCC warns about "i" being uninitialized in the following snippet only when −Winit−self has been specified:

        int f()
        {
          int i = i;
          return i;
        }

This warning is enabled by −Wall in C ++ .

−Wimplicit−int (C and Objective-C only)

Warn when a declaration does not specify a type. This warning is enabled by −Wall.

−Wimplicit−function−declaration (C and Objective-C only)

Give a warning whenever a function is used before being declared. In C99 mode (−std=c99 or −std=gnu99), this warning is enabled by default and it is made into an error by −pedantic−errors. This warning is also enabled by −Wall.

−Wimplicit (C and Objective-C only)

Same as −Wimplicit−int and −Wimplicit−function−declaration. This warning is enabled by −Wall.

−Wignored−qualifiers (C and C ++ only)

Warn if the return type of a function has a type qualifier such as "const". For ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C ++ , the warning is only emitted for scalar types or "void". ISO C prohibits qualified "void" return types on function definitions, so such return types always receive a warning even without this option.

This warning is also enabled by −Wextra.

−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 default in C ++ and is enabled by either −Wall or −Wpedantic.

−Wmissing−braces

Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed. This warning is enabled by −Wall in C.

        int a[2][2] = { 0, 1, 2, 3 };
        int b[2][2] = { { 0, 1 }, { 2, 3 } };

This warning is enabled by −Wall.

−Wmissing−include−dirs (C, C ++ , Objective-C and Objective−C ++ only)

Warn if a user-supplied include directory does not exist.

−Wparentheses

Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about.

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/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 issues 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" can belong to the enclosing "if". The resulting code looks like this:

        {
          if (a)
            {
              if (b)
                foo ();
              else
                bar ();
            }
        }

Also warn for dangerous uses of the GNU extension to "?:" with omitted middle operand. When the condition in the "?": operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead.

This warning is enabled by −Wall.

−Wsequence−point

Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C ++ standards.

The C and C ++ standards define the order in which expressions in a C/C ++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a "&&", "||", "? :" or "," (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap.

It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C ++ standards specify that "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.". If a program breaks these rules, the results on any particular implementation are entirely unpredictable.

Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and "a[i++] = i;". Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.

The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at <http://gcc.gnu.org/readings.html>.

This warning is enabled by −Wall for C and C ++ .

−Wno−return−local−addr

Do not warn about returning a pointer (or in C ++ , a reference) to a variable that goes out of scope after the function returns.

−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" (falling off the end of the function body is considered returning without a value), and about a "return" statement with an expression in a function whose return type is "void".

For C ++ , a function without return type always produces a diagnostic message, even when −Wno−return−type is specified. The only exceptions are main and functions defined in system headers.

This warning is enabled by −Wall.

−Wswitch

Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the named codes of that enumeration. (The presence of a "default" label prevents this warning.) "case" labels outside the enumeration range also provoke warnings when this option is used (even if there is a "default" label). This warning is enabled by −Wall.

−Wswitch−default

Warn whenever a "switch" statement does not have a "default" case.

−Wswitch−enum

Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the named codes of that enumeration. "case" labels outside the enumeration range also provoke warnings when this option is used. The only difference between −Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there is a "default" label.

−Wsync−nand (C and C ++ only)

Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch" built-in functions are used. These functions changed semantics in GCC 4.4.

−Wtrigraphs

Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs within comments are not warned about). This warning is enabled by −Wall.

−Wunused−but−set−parameter

Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).

To suppress this warning use the unused attribute.

This warning is also enabled by −Wunused together with −Wextra.

−Wunused−but−set−variable

Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by −Wall.

To suppress this warning use the unused attribute.

This warning is also enabled by −Wunused, which is enabled by −Wall.

−Wunused−function

Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by −Wall.

−Wunused−label

Warn whenever a label is declared but not used. This warning is enabled by −Wall.

To suppress this warning use the unused attribute.

−Wunused−local−typedefs (C, Objective-C, C ++ and Objective−C ++ only)

Warn when a typedef locally defined in a function is not used. This warning is enabled by −Wall.

−Wunused−parameter

Warn whenever a function parameter is unused aside from its declaration.

To suppress this warning use the unused attribute.

−Wno−unused−result

Do not warn if a caller of a function marked with attribute "warn_unused_result" does not use its return value. The default is −Wunused−result.

−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. To suppress this warning cast the unused expression to void. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as x[i,j] causes a warning, while x[(void)i,j] does not.

This warning is enabled by −Wall.

−Wunused

All the above −Wunused options combined.

In order to get a warning about an unused function parameter, you must either specify −Wextra −Wunused (note that −Wall implies −Wunused), or separately specify −Wunused−parameter.

−Wuninitialized

Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a "setjmp" call. In C ++ , warn if a non-static reference or non-static const member appears in a class without constructors.

If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the −Winit−self option.

These warnings occur for individual uninitialized or clobbered elements of structure, union or array variables as well as for variables that 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 depends on the precise optimization options and version of GCC used.

Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.

−Wmaybe−uninitialized

For an automatic variable, if there exists a path from the function entry to a use of the variable that is initialized, but there exist some other paths for which the variable is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time. These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct in spite of appearing to have an error. Here is one example of how this can happen:

        {
          int x;
          switch (y)
            {
            case 1: x = 1;
              break;
            case 2: x = 4;
              break;
            case 3: x = 5;
            }
          foo (x);
        }

If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn’t know this. To suppress the warning, you need to provide a default case with assert(0) or similar code.

This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp". These warnings as well are possible only in optimizing compilation.

The compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because "longjmp" cannot in fact be called at the place that would cause a problem.

Some spurious warnings can be avoided if you declare all the functions you use that never return as "noreturn".

This warning is enabled by −Wall or −Wextra.

−Wunknown−pragmas

Warn when a "#pragma" directive is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the −Wall command-line option.

−Wno−pragmas

Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also −Wunknown−pragmas.

−Wstrict−aliasing

This option is only active when −fstrict−aliasing is active. It warns about code that 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. It is equivalent to −Wstrict−aliasing=3

−Wstrict−aliasing=n

This option is only active when −fstrict−aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way −O works. −Wstrict−aliasing is equivalent to −Wstrict−aliasing=3.

Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but −fstrict−aliasing still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only.

Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.

Level 3 (default for −Wstrict−aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: "*(int*)&some_float". If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.

−Wstrict−overflow
−Wstrict−overflow=n

This option is only active when −fstrict−overflow is active. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. Thus this warning depends on the optimization level.

An optimization that assumes that signed overflow does not occur is perfectly safe if the values of the variables involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false positive: a warning about code that is not actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all.
−Wstrict−overflow=1

Warn about cases that are both questionable and easy to avoid. For example, with −fstrict−overflow, the compiler simplifies "x + 1 > x" to 1. This level of −Wstrict−overflow is enabled by −Wall; higher levels are not, and must be explicitly requested.

−Wstrict−overflow=2

Also warn about other cases where a comparison is simplified to a constant. For example: "abs (x) >= 0". This can only be simplified when −fstrict−overflow is in effect, because "abs (INT_MIN)" overflows to "INT_MIN", which is less than zero. −Wstrict−overflow (with no level) is the same as −Wstrict−overflow=2.

−Wstrict−overflow=3

Also warn about other cases where a comparison is simplified. For example: "x + 1 > 1" is simplified to "x > 0".

−Wstrict−overflow=4

Also warn about other simplifications not covered by the above cases. For example: "(x * 10) / 5" is simplified to "x * 2".

−Wstrict−overflow=5

Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: "x + 2 > y" is simplified to "x + 1 >= y". This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives.

−Wsuggest−attribute=[pure|const|noreturn|format]

Warn for cases where adding an attribute may be beneficial. The attributes currently supported are listed below.
−Wsuggest−attribute=pure
−Wsuggest−attribute=const
−Wsuggest−attribute=noreturn

Warn about functions that might be candidates for attributes "pure", "const" or "noreturn". The compiler only warns for functions visible in other compilation units or (in the case of "pure" and "const") if it cannot prove that the function returns normally. A function returns normally if it doesn’t contain an infinite loop or return abnormally by throwing, calling "abort()" or trapping. This analysis requires option −fipa−pure−const, which is enabled by default at −O and higher. Higher optimization levels improve the accuracy of the analysis.

−Wsuggest−attribute=format
−Wmissing−format−attribute

Warn about function pointers that might be candidates for "format" attributes. Note these are only possible candidates, not absolute ones. GCC guesses that function pointers with "format" 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 also warns about function definitions that might be candidates for "format" attributes. Again, these are only possible candidates. GCC guesses that "format" attributes might be appropriate for any function that calls a function like "vprintf" or "vscanf", but this might not always be the case, and some functions for which "format" attributes are appropriate may not be detected.

−Warray−bounds

This option is only active when −ftree−vrp is active (default for −O2 and above). It warns about subscripts to arrays that are always out of bounds. This warning is enabled by −Wall.

−Wno−div−by−zero

Do not warn about compile-time integer division by zero. Floating-point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs.

−Wsystem−headers

Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. However, note that using −Wall in conjunction with this option does not warn about unknown pragmas in system headers−−−for that, −Wunknown−pragmas must also be used.

−Wtrampolines

 Warn about trampolines generated for pointers to nested functions.
 A trampoline is a small piece of data or code that is created at run
 time on the stack when the address of a nested function is taken, and
 is used to call the nested function indirectly.  For some targets, it
 is made up of data only and thus requires no special treatment.  But,
 for most targets, it is made up of code and thus requires the stack
 to be made executable in order for the program to work properly.

−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 should 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 and Objective-C only)

Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided.

−Wtraditional−conversion (C and Objective-C only)

Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion.

−Wdeclaration−after−statement (C and Objective-C only)

Warn when a declaration is found after a statement in a block. This construct, known from C ++ , was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90 and was not supported by GCC versions before GCC 3.0.

−Wundef

Warn if an undefined identifier is evaluated in an #if directive.

−Wno−endif−labels

Do not warn whenever an #else or an #endif are followed by text.

−Wshadow

Warn whenever a local variable or type declaration shadows another variable, parameter, type, or class member (in C ++ ), or whenever a built-in function is shadowed. Note that in C ++ , the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum.

−Wlarger−than=len

Warn whenever an object of larger than len bytes is defined.

−Wframe−larger−than=len

Warn if the size of a function frame is larger than len bytes. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than len even if you do not get a warning. In addition, any space allocated via "alloca", variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning.

−Wno−free−nonheap−object

Do not warn when attempting to free an object that was not allocated on the heap.

−Wstack−usage=len

Warn if the stack usage of a function might be larger than len bytes. The computation done to determine the stack usage is conservative. Any space allocated via "alloca", variable-length arrays, or related constructs is included by the compiler when determining whether or not to issue a warning.

The message is in keeping with the output of −fstack−usage.

          warning: stack usage is 1120 bytes

          warning: stack usage might be 1648 bytes

          warning: stack usage might be unbounded

−Wunsafe−loop−optimizations

Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop indices. With −funsafe−loop−optimizations warn if the compiler makes such assumptions.

−Wno−pedantic−ms−format (MinGW targets only)

When used in combination with −Wformat and −pedantic without GNU extensions, this option disables the warnings about non-ISO "printf" / "scanf" format width specifiers "I32", "I64", and "I" used on Windows targets, which depend on the MS runtime.

−Wpointer−arith

Warn about anything that depends on the "size of" a function type or of "void". GNU C assigns these types a size of 1, for convenience in calculations with "void *" pointers and pointers to functions. In C ++ , warn also when an arithmetic operation involves "NULL". This warning is also enabled by −Wpedantic.

−Wtype−limits

Warn if a comparison is always true or always false due to the limited range of the data type, but do not warn for constant expressions. For example, warn if an unsigned variable is compared against zero with < or >=. This warning is also enabled by −Wextra.

−Wbad−function−cast (C and Objective-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 (C and Objective-C only)

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.

−Wc++11−compat (C ++ and Objective−C ++ only)

Warn about C ++ constructs whose meaning differs between ISO C ++ 1998 and ISO C ++ 2011, e.g.,identifiersin ISOC ++ 1998 thatarekeywordsin ISOC++2011.Thiswarningturnson−Wnarrowingandisenabledby−Wall.

−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 *".

Also warn when making a cast that introduces a type qualifier in an unsafe way. For example, casting "char **" to "const char **" is unsafe, as in this example:

          /* p is char ** value.  */
          const char **q = (const char **) p;
          /* Assignment of readonly string to const char * is OK.  */
          *q = "string";
          /* Now char** pointer points to read−only memory.  */
          **p = 'b';

−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 produces a warning. These warnings help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using "const" in declarations and prototypes. Otherwise, it is just a nuisance. This is why we did not make −Wall request these warnings.

When compiling C ++ , warn about the deprecated conversion from string literals to "char *". This warning is enabled by default for C ++ programs.

−Wclobbered

Warn for variables that might be changed by longjmp or vfork. This warning is also enabled by −Wextra.

−Wconversion

Warn for implicit conversions that may alter a value. This includes conversions between real and integer, like "abs (x)" when "x" is "double"; conversions between signed and unsigned, like "unsigned ui = −1"; and conversions to smaller types, like "sqrtf (M_PI)". Do not warn for explicit casts like "abs ((int) x)" and "ui = (unsigned) −1", or if the value is not changed by the conversion like in "abs (2.0)". Warnings about conversions between signed and unsigned integers can be disabled by using −Wno−sign−conversion.

For C ++ , also warn for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator: conversions to "void", the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C ++ unless −Wsign−conversion is explicitly enabled.

−Wno−conversion−null (C ++ and Objective−C ++ only)

Do not warn for conversions between "NULL" and non-pointer types. −Wconversion−null is enabled by default.

−Wzero−as−null−pointer−constant (C ++ and Objective−C ++ only)

Warn when a literal ’0’ is used as null pointer constant. This can be useful to facilitate the conversion to "nullptr" in C ++ 11.

−Wuseless−cast (C ++ and Objective−C ++ only)

Warn when an expression is casted to its own type.

−Wempty−body

Warn if an empty body occurs in an if, else or do while statement. This warning is also enabled by −Wextra.

−Wenum−compare

Warn about a comparison between values of different enumerated types. In C ++ enumeral mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C this warning is enabled by −Wall.

−Wjump−misses−init (C, Objective-C only)

Warn if a "goto" statement or a "switch" statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. This only warns about variables that are initialized when they are declared. This warning is only supported for C and Objective-C; in C ++ this sort of branch is an error in any case.

−Wjump−misses−init is included in −Wc++−compat. It can be disabled with the −Wno−jump−misses−init option.

−Wsign−compare

Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. This warning is also enabled by −Wextra; to get the other warnings of −Wextra without this warning, use −Wextra −Wno−sign−compare.

−Wsign−conversion

Warn for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is enabled also by −Wconversion.

−Wsizeof−pointer−memaccess

Warn for suspicious length parameters to certain string and memory built-in functions if the argument uses "sizeof". This warning warns e.g. about "memset (ptr, 0, sizeof (ptr));" if "ptr" is not an array, but a pointer, and suggests a possible fix, or about "memcpy (&foo, ptr, sizeof (&foo));". This warning is enabled by −Wall.

−Waddress

Warn about suspicious uses of memory addresses. These include using the address of a function in a conditional expression, such as "void func(void); if (func)", and comparisons against the memory address of a string literal, such as "if (x == "abc")". Such uses typically indicate a programmer error: the address of a function always evaluates to true, so their use in a conditional usually indicate that the programmer forgot the parentheses in a function call; and comparisons against string literals result in unspecified behavior and are not portable in C, so they usually indicate that the programmer intended to use "strcmp". This warning is enabled by −Wall.

−Wlogical−op

Warn about suspicious uses of logical operators in expressions. This includes using logical operators in contexts where a bit-wise operator is likely to be expected.

−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−aggressive−loop−optimizations

Warn if in a loop with constant number of iterations the compiler detects undefined behavior in some statement during one or more of the iterations.

−Wno−attributes

Do not warn if an unexpected "__attribute__" is used, such as unrecognized attributes, function attributes applied to variables, etc. This does not stop errors for incorrect use of supported attributes.

−Wno−builtin−macro−redefined

Do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of "__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".

−Wstrict−prototypes (C and Objective-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 that specifies the argument types.)

−Wold−style−declaration (C and Objective-C only)

Warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like "static" are not the first things in a declaration. This warning is also enabled by −Wextra.

−Wold−style−definition (C and Objective-C only)

Warn if an old-style function definition is used. A warning is given even if there is a previous prototype.

−Wmissing−parameter−type (C and Objective-C only)

A function parameter is declared without a type specifier in K&R−style functions:

        void foo(bar) { }

This warning is also enabled by −Wextra.

−Wmissing−prototypes (C and Objective-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. Use this option to detect global functions that do not have a matching prototype declaration in a header file. This option is not valid for C ++ because all function declarations provide prototypes and a non-matching declaration will declare an overload rather than conflict with an earlier declaration. Use −Wmissing−declarations to detect missing declarations in C ++ .

−Wmissing−declarations

Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use −Wmissing−prototype to detect missing prototypes. In C ++ , no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces.

−Wmissing−field−initializers

Warn if a structure’s initializer has some fields missing. For example, the following code causes 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 does 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 without this one, use −Wextra −Wno−missing−field−initializers.

−Wno−multichar

Do not warn if a multicharacter constant (’ FOOF ’) is used. Usually they indicate a typo in the user’s code, as they have implementation-defined values, and should not be used in portable code.

−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 characters 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 that have not been normalized; this option controls that warning.

There are four levels of warning supported by GCC. The default is −Wnormalized=nfc, which warns about any identifier that is not in the ISO 10646 "C" normalized form, NFC . NFC is the recommended form for most uses.

Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C ++that,whenturnedinto NFC, arenotallowedinidentifiers. Thatis,there’snowaytousethesesymbolsinportable ISOC orC ++ andhaveall youridentifiersin NFC. −Wnormalized=idsuppressesthewarningforthese characters.Itishopedthatfutureversionsofthestandardsinvolvedwill correctthis,whichiswhythisoptionisnotthedefault.

Youcanswitchthewarningoffforallcharactersbywriting −Wnormalized=none.Youshouldonlydothisifyouareusingsomeother normalizationscheme(like"D"),becauseotherwiseyoucaneasilycreatebugs thatareliterallyimpossibletosee.

Somecharactersin ISO10646 havedistinctmeaningsbutlookidenticalinsome fontsordisplaymethodologies,especiallyonceformattinghasbeenapplied. Forinstance"\u207F"," SUPERSCRIPTLATINSMALLLETTERN", displaysjustlike aregular"n"thathasbeenplacedinasuperscript. ISO10646 definesthe NFKC normalizationschemetoconvertalltheseintoastandardformaswell,and GCC warnsifyourcodeisnotin NFKC ifyouuse−Wnormalized=nfkc.Thiswarningis comparabletowarningabouteveryidentifierthatcontainstheletterO becauseitmightbeconfusedwiththedigit0,andsoisnotthedefault,butmaybe usefulasalocalcodingconventioniftheprogrammingenvironmentcannotbe fixedtodisplaythesecharactersdistinctly.

−Wno−deprecated

Do not warn about usage of deprecated features.

−Wno−deprecated−declarations

Do not warn about uses of functions, variables, and types marked as deprecated by using the "deprecated" attribute.

−Wno−overflow

Do not warn about compile-time overflow in constant expressions.

−Woverride−init (C and Objective-C only)

Warn if an initialized field without side effects is overridden when using designated initializers.

This warning is included in −Wextra. To get other −Wextra warnings without this one, use −Wextra −Wno−override−init.

−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" is 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;
        };

−Wpacked−bitfield−compat

The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". This has been fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4−bit padding between field "a" and "b" in this structure:

        struct foo
        {
          char a:4;
          char b:8;
        } __attribute__ ((packed));

This warning is enabled by default. Use −Wno−packed−bitfield−compat to disable this warning.

−Wpadded

Warn if padding is included in a structure, either to align an element 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 and Objective-C only)

Warn if an "extern" declaration is encountered within a function.

−Wno−inherited−variadic−ctor

Suppress warnings about use of C ++ 11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited.

−Winline

Warn if a function that is declared as inline cannot be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers.

The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by −Winline to appear or disappear.

−Wno−invalid−offsetof (C ++ and Objective−C ++ only)

Suppress warnings from applying the offsetof macro to a non-POD type. According to the 1998 ISO C ++ standard, applying offsetof to a non-POD type is undefined. In existing C ++ implementations, however, offsetof typically gives meaningful results even when applied to certain kinds of non-POD types (such as a simple struct that fails to be a POD type only by virtue of having a constructor). This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.

The restrictions on offsetof may be relaxed in a future version of the C ++ standard.

−Wno−int−to−pointer−cast

Suppress warnings from casts to pointer type of an integer of a different size. In C ++ , casting to a pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default.

−Wno−pointer−to−int−cast (C and Objective-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 enabled by either −Wpedantic or −Wtraditional in ISO C90 and C ++ 98 modes. To inhibit the warning messages, use −Wno−long−long.

−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−variadic−macros.

−Wvarargs

Warn upon questionable usage of the macros used to handle variable arguments like va_start. This is default. To inhibit the warning messages, use −Wno−varargs.

−Wvector−operation−performance

Warn if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance tuning. Vector operation can be implemented "piecewise", which means that the scalar operation is performed on every vector element; "in parallel", which means that the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and "as a single scalar", which means that vector fits into a scalar type.

−Wno−virtual−move−assign

Suppress warnings about inheriting from a virtual base with a non-trivial C ++ 11 move assignment operator. This is dangerous because if the virtual base is reachable along more than one path, it will be moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled.

−Wvla

Warn if variable length array is used in the code. −Wno−vla prevents the −Wpedantic warning of the variable length array.

−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. This warning is enabled by −Wall.

−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 are unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time.

−Wpointer−sign (C and Objective-C only)

Warn for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by −Wall and by −Wpedantic, which can be disabled with −Wno−pointer−sign.

−Wstack−protector

This option is only active when −fstack−protector is active. It warns about functions that are not protected against stack smashing.

−Wno−mudflap

Suppress warnings about constructs that cannot be instrumented by −fmudflap.

−Woverlength−strings

Warn about string constants that are longer than the "minimum maximum" length specified in the C standard. Modern compilers generally allow string constants that are much longer than the standard’s minimum limit, but very portable programs should avoid using longer strings.

The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C ++ 98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C ++ .

This option is implied by −Wpedantic, and can be disabled with −Wno−overlength−strings.

−Wunsuffixed−float−constants (C and Objective-C only)

Issue a warning for any floating constant that does not have a suffix. When used together with −Wsystem−headers it warns about such constants in system header files. This can be useful when preparing code to use with the "FLOAT_CONST_DECIMAL64" pragma from the decimal floating-point extension to C99.

Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program or GCC:

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 probably makes other debuggers crash or refuse to read the program. If you want to control 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 are already at hand; some statements may execute in different places because they have been 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.

−gsplit−dwarf

Separate as much dwarf debugging information as possible into a separate output file with the extension .dwo. This option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading .dwo files.

−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.

−gpubnames

Generate dwarf .debug_pubnames and .debug_pubtypes sections.

−gstabs

Produce debugging information in stabs format (if that is supported), 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 that 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 supported), for only symbols that are actually used.

−femit−class−debug−always

Instead of emitting debugging information for a C ++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes because using this option increases the size of debugging information by as much as a factor of two.

−fdebug−types−section

When using DWARF Version 4 or higher, type DIEs can be put into their own ".debug_types" section instead of making them part of the ".debug_info" section. It is more efficient to put them in a separate comdat sections since the linker can then remove duplicates. But not all DWARF consumers support ".debug_types" sections yet and on some objects ".debug_types" produces larger instead of smaller debugging information.

−gstabs+

Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger ( GDB ). The use of these extensions is likely to make other debuggers crash or refuse to read the program.

−gcoff

Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4.

−gxcoff

Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.

−gxcoff+

Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger ( GDB ). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler ( GAS ) to fail with an error.

−gdwarf−version

Produce debugging information in DWARF format (if that is supported). The value of version may be either 2, 3 or 4; the default version for most targets is 4.

Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.

Version 4 may require GDB 7.0 and −fvar−tracking−assignments for maximum benefit.

−grecord−gcc−switches

This switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. See also −frecord−gcc−switches for another way of storing compiler options into the object file. This is the default.

−gno−record−gcc−switches

Disallow appending command-line options to the DW_AT_producer attribute in DWARF debugging information.

−gstrict−dwarf

Disallow using extensions of later DWARF standard version than selected with −gdwarf−version. On most targets using non-conflicting DWARF extensions from later standard versions is allowed.

−gno−strict−dwarf

Allow using extensions of later DWARF standard version than selected with −gdwarf−version.

−gvms

Produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/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 0 produces no debug information at all. Thus, −g0 negates −g.

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 different 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 DWARF.

−gtoggle

Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used with −fcompare−debug.

−fsanitize=address

Enable AddressSanitizer, a fast memory error detector. Memory access instructions will be instrumented to detect out-of-bounds and use-after-free bugs. See <http://code.google.com/p/address−sanitizer/> for more details.

−fsanitize=thread

Enable ThreadSanitizer, a fast data race detector. Memory access instructions will be instrumented to detect data race bugs. See <http://code.google.com/p/data−race−test/wiki/ThreadSanitizer> for more details.

−fdump−final−insns[=file]

Dump the final internal representation ( RTL ) to file. If the optional argument is omitted (or if file is "."), the name of the dump file is determined by appending ".gkd" to the compilation output file name.

−fcompare−debug[=opts]

If no error occurs during compilation, run the compiler a second time, adding opts and −fcompare−debug−second to the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ.

If the equal sign is omitted, the default −gtoggle is used.

The environment variable GCC_COMPARE_DEBUG , if defined, non-empty and nonzero, implicitly enables −fcompare−debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used for opts, otherwise the default −gtoggle is used.

−fcompare−debug=, with the equal sign but without opts, is equivalent to −fno−compare−debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.

To verify full coverage during −fcompare−debug testing, set GCC_COMPARE_DEBUG to say −fcompare−debug−not−overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG to −w%n−fcompare−debug not overridden will do.

−fcompare−debug−second

This option is implicitly passed to the compiler for the second compilation requested by −fcompare−debug, along with options to silence warnings, and omitting other options that would cause side-effect compiler outputs to files or to the standard output. Dump files and preserved temporary files are renamed so as to contain the ".gk" additional extension during the second compilation, to avoid overwriting those generated by the first.

When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper.

−feliminate−dwarf2−dups

Compress DWARF 2 debugging information by eliminating duplicated information about each symbol. This option only makes sense when generating DWARF 2 debugging information with −gdwarf−2.

−femit−struct−debug−baseonly

Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the struct is defined.

This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger. See −femit−struct−debug−reduced for a less aggressive option. See −femit−struct−debug−detailed for more detailed control.

This option works only with DWARF 2.

−femit−struct−debug−reduced

Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the type is defined, unless the struct is a template or defined in a system header.

This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See −femit−struct−debug−baseonly for a more aggressive option. See −femit−struct−debug−detailed for more detailed control.

This option works only with DWARF 2.

−femit−struct−debug−detailed[=spec-list]

Specify the struct-like types for which the compiler generates debug information. The intent is to reduce duplicate struct debug information between different object files within the same program.

This option is a detailed version of −femit−struct−debug−reduced and −femit−struct−debug−baseonly, which serves for most needs.

A specification has the syntax[dir:|ind:][ord:|gen:](any|sys|base|none)

The optional first word limits the specification to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;.

The optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain. For C ++ , these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but −femit−struct−debug−detailed does not yet implement them.

The third word specifies the source files for those structs for which the compiler should emit debug information. The values none and any have the normal meaning. The value base means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.

You may need to experiment to determine the best settings for your application.

The default is −femit−struct−debug−detailed=all.

This option works only with DWARF 2.

−fno−merge−debug−strings

Direct the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default.

−fdebug−prefix−map=old=new

When compiling files in directory old, record debugging information describing them as in new instead.

−fno−dwarf2−cfi−asm

Emit DWARF 2 unwind info as compiler generated ".eh_frame" section instead of using GAS ".cfi_*" directives.

−ftime−report

Makes the compiler print some statistics about the time consumed by each pass when it finishes.

−fmem−report

Makes the compiler print some statistics about permanent memory allocation when it finishes.

−fmem−report−wpa

Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.

−fpre−ipa−mem−report
−fpost−ipa−mem−report

Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.

−fprofile−report

Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes.

−fstack−usage

Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname 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. An entry is made up of three fields:

The qualifier "static" means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.

The qualifier "dynamic" means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier "bounded" is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part.

−fprofile−arcs

Add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (−fbranch−probabilities), or for test coverage analysis (−ftest−coverage). Each object file’s auxname is generated 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 coverage 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.