Manuál NetBSD

GCC

NAME

gcc − GNU project C and C++ compiler

SYNOPSIS

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

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

DESCRIPTION

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

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

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

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

You can mix options and other arguments. For the most part, the order you use doesn’t matter. Order does matter when you use several options of the same kind; for example, if you specify −L more than once, the directories are searched in the order specified.

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

OPTIONS

Option Summary

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

−c −S −E −o file −combine −pipe −pass−exit−codes −x language −v −### −−help −−target−help −−version

C Language Options

−ansi −std=standard −aux−info filename −fno−asm −fno−builtin −fno−builtin−function −fhosted −ffreestanding −fms−extensions −trigraphs −no−integrated−cpp −traditional −traditional−cpp −fallow−single−precision −fcond−mismatch −fsigned−bitfields −fsigned−char −funsigned−bitfields −funsigned−char

C ++ Language Options

−fabi−version=n −fno−access−control −fcheck−new −fconserve−space −ffriend−injection −fno−const−strings −fno−elide−constructors −fno−enforce−eh−specs −ffor−scope −fno−for−scope −fno−gnu−keywords −fno−implicit−templates −fno−implicit−inline−templates −fno−implement−inlines −fms−extensions −fno−nonansi−builtins −fno−operator−names −fno−optional−diags −fpermissive −frepo −fno−rtti −fstats −ftemplate−depth−n −fno−threadsafe−statics −fuse−cxa−atexit −fno−weak −nostdinc++ −fno−default−inline −fvisibility−inlines−hidden −Wabi −Wctor−dtor−privacy −Wnon−virtual−dtor −Wreorder −Weffc++ −Wno−deprecated −Wstrict−null−sentinel −Wno−non−template−friend −Wold−style−cast −Woverloaded−virtual −Wno−pmf−conversions −Wsign−promo

Objective-C and Objective−C ++ Language Options

−fconstant−string−class=class-name −fgnu−runtime −fnext−runtime −fno−nil−receivers −fobjc−call−cxx−cdtors −fobjc−direct−dispatch −fobjc−exceptions −fobjc−gc −freplace−objc−classes −fzero−link −gen−decls −Wassign−intercept −Wno−protocol −Wselector −Wstrict−selector−match −Wundeclared−selector

Language Independent Options

−fmessage−length=n −fdiagnostics−show−location=[once⎪every-line] −fdiagnostics−show−options

Warning Options

−fsyntax−only −pedantic −pedantic−errors −w −Wextra −Wall −Waggregate−return −Wno−attributes −Wc++−compat −Wcast−align −Wcast−qual −Wchar−subscripts −Wcomment −Wconversion −Wno−deprecated−declarations −Wdisabled−optimization −Wno−div−by−zero −Wno−endif−labels −Werror −Werror−implicit−function−declaration −Wfatal−errors −Wfloat−equal −Wformat −Wformat=2 −Wno−format−extra−args −Wformat−nonliteral −Wformat−security −Wformat−y2k −Wimplicit −Wimplicit−function−declaration −Wimplicit−int −Wimport −Wno−import −Winit−self −Winline −Wno−int−to−pointer−cast −Wno−invalid−offsetof −Winvalid−pch −Wlarger−than−len −Wunsafe−loop−optimizations −Wlong−long −Wmain −Wmissing−braces −Wmissing−field−initializers −Wmissing−format−attribute −Wmissing−include−dirs −Wmissing−noreturn −Wno−multichar −Wnonnull −Wpacked −Wpadded −Wparentheses −Wpointer−arith −Wno−pointer−to−int−cast −Wredundant−decls −Wreturn−type −Wsequence−point −Wshadow −Wsign−compare −Wstack−protector −Wstrict−aliasing −Wstrict−aliasing=2 −Wswitch −Wswitch−default −Wswitch−enum −Wsystem−headers −Wtrigraphs −Wundef −Wuninitialized −Wunknown−pragmas −Wno−pragmas −Wunreachable−code −Wunused −Wunused−function −Wunused−label −Wunused−parameter −Wunused−value −Wunused−variable −Wvariadic−macros −Wvolatile−register−var −Wwrite−strings

C−only Warning Options

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

Debugging Options

−dletters −dumpspecs −dumpmachine −dumpversion −fdump−unnumbered −fdump−translation−unit[−n] −fdump−class−hierarchy[−n] −fdump−ipa−all −fdump−ipa−cgraph −fdump−tree−all −fdump−tree−original[−n] −fdump−tree−optimized[−n] −fdump−tree−inlined[−n] −fdump−tree−cfg −fdump−tree−vcg −fdump−tree−alias −fdump−tree−ch −fdump−tree−ssa[−n] −fdump−tree−pre[−n] −fdump−tree−ccp[−n] −fdump−tree−dce[−n] −fdump−tree−gimple[−raw] −fdump−tree−mudflap[−n] −fdump−tree−dom[−n] −fdump−tree−dse[−n] −fdump−tree−phiopt[−n] −fdump−tree−forwprop[−n] −fdump−tree−copyrename[−n] −fdump−tree−nrv −fdump−tree−vect −fdump−tree−sink −fdump−tree−sra[−n] −fdump−tree−salias −fdump−tree−fre[−n] −fdump−tree−vrp[−n] −ftree−vectorizer−verbose=n −fdump−tree−storeccp[−n] −feliminate−dwarf2−dups −feliminate−unused−debug−types −feliminate−unused−debug−symbols −fmem−report −fprofile−arcs −frandom−seed=string −fsched−verbose=n −ftest−coverage −ftime−report −fvar−tracking −g −glevel −gcoff −gdwarf−2 −ggdb −gstabs −gstabs+ −gvms −gxcoff −gxcoff+ −p −pg −print−file−name=library −print−libgcc−file−name −print−multi−directory −print−multi−lib −print−prog−name=program −print−search−dirs −Q −save−temps −time

Optimization Options

−falign−functions=n −falign−jumps=n −falign−labels=n −falign−loops=n −fbounds−check −fmudflap −fmudflapth −fmudflapir −fbranch−probabilities −fprofile−values −fvpt −fbranch−target−load−optimize −fbranch−target−load−optimize2 −fbtr−bb−exclusive −fcaller−saves −fcprop−registers −fcse−follow−jumps −fcse−skip−blocks −fcx−limited−range −fdata−sections −fdelayed−branch −fdelete−null−pointer−checks −fearly−inlining −fexpensive−optimizations −ffast−math −ffloat−store −fforce−addr −ffunction−sections −fgcse −fgcse−lm −fgcse−sm −fgcse−las −fgcse−after−reload −floop−optimize −fcrossjumping −fif−conversion −fif−conversion2 −finline−functions −finline−functions−called−once −finline−limit=n −fkeep−inline−functions −fkeep−static−consts −fmerge−constants −fmerge−all−constants −fmodulo−sched −fno−branch−count−reg −fno−default−inline −fno−defer−pop −floop−optimize2 −fmove−loop−invariants −fno−function−cse −fno−guess−branch−probability −fno−inline −fno−math−errno −fno−peephole −fno−peephole2 −funsafe−math−optimizations −funsafe−loop−optimizations −ffinite−math−only −fno−trapping−math −fno−zero−initialized−in−bss −fomit−frame−pointer −foptimize−register−move −foptimize−sibling−calls −fprefetch−loop−arrays −fprofile−generate −fprofile−use −fregmove −frename−registers −freorder−blocks −freorder−blocks−and−partition −freorder−functions −frerun−cse−after−loop −frerun−loop−opt −frounding−math −fschedule−insns −fschedule−insns2 −fno−sched−interblock −fno−sched−spec −fsched−spec−load −fsched−spec−load−dangerous −fsched−stalled−insns=n −fsched−stalled−insns−dep=n −fsched2−use−superblocks −fsched2−use−traces −freschedule−modulo−scheduled−loops −fsignaling−nans −fsingle−precision−constant −fstack−protector −fstack−protector−all −fstrength−reduce −fstrict−aliasing −ftracer −fthread−jumps −funroll−all−loops −funroll−loops −fpeel−loops −fsplit−ivs−in−unroller −funswitch−loops −fvariable−expansion−in−unroller −ftree−pre −ftree−ccp −ftree−dce −ftree−loop−optimize −ftree−loop−linear −ftree−loop−im −ftree−loop−ivcanon −fivopts −ftree−dominator−opts −ftree−dse −ftree−copyrename −ftree−sink −ftree−ch −ftree−sra −ftree−ter −ftree−lrs −ftree−fre −ftree−vectorize −ftree−vect−loop−version −ftree−salias −fweb −ftree−copy−prop −ftree−store−ccp −ftree−store−copy−prop −fwhole−program −−param name=value −O −O0 −O1 −O2 −O3 −Os

Preprocessor Options

−Aquestion=answer −A−question[=answer] −C −dD −dI −dM −dN −Dmacro[=defn] −E −H −idirafter dir −include file −imacros file −iprefix file −iwithprefix dir −iwithprefixbefore dir −isystem dir −isysroot dir −M −MM −MF −MG −MP −MQ −MT −nostdinc −P −fworking−directory −remap −trigraphs −undef −Umacro −Wp,option −Xpreprocessor option

Assembler Option

−Wa,option −Xassembler option

Linker Options

object-file-name −llibrary −nostartfiles −nodefaultlibs −nostdlib −pie −rdynamic −s −static −static−libgcc −shared −shared−libgcc −symbolic −Wl,option −Xlinker option −u symbol

Directory Options

−Bprefix −Idir −iquotedir −Ldir −specs=file −I− −−sysroot=dir

Target Options

−V version −b machine

Machine Dependent Options

ARC Options −EB −EL −mmangle−cpu −mcpu=cpu −mtext=text-section −mdata=data-section −mrodata=readonly-data-section

ARM Options −mapcs−frame −mno−apcs−frame −mabi=name −mapcs−stack−check −mno−apcs−stack−check −mapcs−float −mno−apcs−float −mapcs−reentrant −mno−apcs−reentrant −msched−prolog −mno−sched−prolog −mlittle−endian −mbig−endian −mwords−little−endian −mfloat−abi=name −msoft−float −mhard−float −mfpe −mthumb−interwork −mno−thumb−interwork −mcpu=name −march=name −mfpu=name −mstructure−size−boundary=n −mabort−on−noreturn −mlong−calls −mno−long−calls −msingle−pic−base −mno−single−pic−base −mpic−register=reg −mnop−fun−dllimport −mcirrus−fix−invalid−insns −mno−cirrus−fix−invalid−insns −mpoke−function−name −mthumb −marm −mtpcs−frame −mtpcs−leaf−frame −mcaller−super−interworking −mcallee−super−interworking −mtp=name

AVR Options −mmcu=mcu −msize −minit−stack=n −mno−interrupts −mcall−prologues −mno−tablejump −mtiny−stack −mint8

Blackfin Options −momit−leaf−frame−pointer −mno−omit−leaf−frame−pointer −mspecld−anomaly −mno−specld−anomaly −mcsync−anomaly −mno−csync−anomaly −mlow−64k −mno−low64k −mid−shared−library −mno−id−shared−library −mshared−library−id=n −mlong−calls −mno−long−calls

CRIS Options −mcpu=cpu −march=cpu −mtune=cpu −mmax−stack−frame=n −melinux−stacksize=n −metrax4 −metrax100 −mpdebug −mcc−init −mno−side−effects −mstack−align −mdata−align −mconst−align −m32−bit −m16−bit −m8−bit −mno−prologue−epilogue −mno−gotplt −melf −maout −melinux −mlinux −sim −sim2 −mmul−bug−workaround −mno−mul−bug−workaround

CRX Options −mmac −mpush−args

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

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

DEC Alpha/VMS Options −mvms−return−codes

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

H8/300 Options −mrelax −mh −ms −mn −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 −msvr3−shlib −mno−wide−multiply −mrtd −malign−double −mpreferred−stack−boundary=num −mmmx −msse −msse2 −msse3 −m3dnow −mthreads −mno−align−stringops −minline−all−stringops −mpush−args −maccumulate−outgoing−args −m128bit−long−double −m96bit−long−double −mregparm=num −msseregparm −momit−leaf−frame−pointer −mno−red−zone −mno−tls−direct−seg−refs −mcmodel=code-model −m32 −m64 −mlarge−data−threshold=num

IA−64 Options −mbig−endian −mlittle−endian −mgnu−as −mgnu−ld −mno−pic −mvolatile−asm−stop −mregister−names −mno−sdata −mconstant−gp −mauto−pic −minline−float−divide−min−latency −minline−float−divide−max−throughput −minline−int−divide−min−latency −minline−int−divide−max−throughput −minline−sqrt−min−latency −minline−sqrt−max−throughput −mno−dwarf2−asm −mearly−stop−bits −mfixed−range=register-range −mtls−size=tls-size −mtune=cpu-type −mt −pthread −milp32 −mlp64

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

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

M680x0 Options −m68000 −m68020 −m68020−40 −m68020−60 −m68030 −m68040 −m68060 −mcpu32 −m5200 −m68881 −mbitfield −mc68000 −mc68020 −mnobitfield −mrtd −mshort −msoft−float −mpcrel −malign−int −mstrict−align −msep−data −mno−sep−data −mshared−library−id=n −mid−shared−library −mno−id−shared−library

M68hc1x Options −m6811 −m6812 −m68hc11 −m68hc12 −m68hcs12 −mauto−incdec −minmax −mlong−calls −mshort −msoft−reg−count=count

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

MIPS Options −EL −EB −march=arch −mtune=arch −mips1 −mips2 −mips3 −mips4 −mips32 −mips32r2 −mips64 −mips16 −mno−mips16 −mabi=abi −mabicalls −mno−abicalls −mxgot −mno−xgot −mgp32 −mgp64 −mfp32 −mfp64 −mhard−float −msoft−float −msingle−float −mdouble−float −mdsp −mpaired−single −mips3d −mlong64 −mlong32 −msym32 −mno−sym32 −Gnum −membedded−data −mno−embedded−data −muninit−const−in−rodata −mno−uninit−const−in−rodata −msplit−addresses −mno−split−addresses −mexplicit−relocs −mno−explicit−relocs −mcheck−zero−division −mno−check−zero−division −mdivide−traps −mdivide−breaks −mmemcpy −mno−memcpy −mlong−calls −mno−long−calls −mmad −mno−mad −mfused−madd −mno−fused−madd −nocpp −mfix−r4000 −mno−fix−r4000 −mfix−r4400 −mno−fix−r4400 −mfix−vr4120 −mno−fix−vr4120 −mfix−vr4130 −mfix−sb1 −mno−fix−sb1 −mflush−func=func −mno−flush−func −mbranch−likely −mno−branch−likely −mfp−exceptions −mno−fp−exceptions −mvr4130−align −mno−vr4130−align

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

MN10300 Options −mmult−bug −mno−mult−bug −mam33 −mno−am33 −mam33−2 −mno−am33−2 −mreturn−pointer−on−d0 −mno−crt0 −mrelax

MT Options −mno−crt0 −mbacc −msim −march=cpu-type

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

PowerPC Options See RS/6000 and PowerPC Options.

RS/6000 and PowerPC Options −mcpu=cpu-type −mtune=cpu-type −mpower −mno−power −mpower2 −mno−power2 −mpowerpc −mpowerpc64 −mno−powerpc −maltivec −mno−altivec −mpowerpc−gpopt −mno−powerpc−gpopt −mpowerpc−gfxopt −mno−powerpc−gfxopt −mmfcrf −mno−mfcrf −mpopcntb −mno−popcntb −mfprnd −mno−fprnd −mnew−mnemonics −mold−mnemonics −mfull−toc −mminimal−toc −mno−fp−in−toc −mno−sum−in−toc −m64 −m32 −mxl−compat −mno−xl−compat −mpe −malign−power −malign−natural −msoft−float −mhard−float −mmultiple −mno−multiple −mstring −mno−string −mupdate −mno−update −mfused−madd −mno−fused−madd −mbit−align −mno−bit−align −mstrict−align −mno−strict−align −mrelocatable −mno−relocatable −mrelocatable−lib −mno−relocatable−lib −mtoc −mno−toc −mlittle −mlittle−endian −mbig −mbig−endian −mdynamic−no−pic −maltivec −mswdiv −mprioritize−restricted−insns=priority −msched−costly−dep=dependence_type −minsert−sched−nops=scheme −mcall−sysv −mcall−netbsd −maix−struct−return −msvr4−struct−return −mabi=abi-type −msecure−plt −mbss−plt −misel −mno−isel −misel=yes −misel=no −mspe −mno−spe −mspe=yes −mspe=no −mvrsave −mno−vrsave −mfloat−gprs=yes −mfloat−gprs=no −mfloat−gprs=single −mfloat−gprs=double −mprototype −mno−prototype −msim −mmvme −mads −myellowknife −memb −msdata −msdata=opt −mvxworks −mwindiss −G num −pthread

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

SH Options −m1 −m2 −m2e −m3 −m3e −m4−nofpu −m4−single−only −m4−single −m4 −m4a−nofpu −m4a−single−only −m4a−single −m4a −m4al −m5−64media −m5−64media−nofpu −m5−32media −m5−32media−nofpu −m5−compact −m5−compact−nofpu −mb −ml −mdalign −mrelax −mbigtable −mfmovd −mhitachi −mrenesas −mno−renesas −mnomacsave −mieee −misize −mpadstruct −mspace −mprefergot −musermode −multcost=number −mdiv=strategy −mdivsi3_libfunc=name −madjust−unroll −mindexed−addressing −mgettrcost=number −mpt−fixed
−minvalid−symbols

SPARC Options −mcpu=cpu-type −mtune=cpu-type −mcmodel=code-model −m32 −m64 −mapp−regs −mno−app−regs −mfaster−structs −mno−faster−structs −mfpu −mno−fpu −mhard−float −msoft−float −mhard−quad−float −msoft−quad−float −mimpure−text −mno−impure−text −mlittle−endian −mstack−bias −mno−stack−bias −munaligned−doubles −mno−unaligned−doubles −mv8plus −mno−v8plus −mvis −mno−vis −threads −pthreads −pthread

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

TMS320C3x/C4x Options −mcpu=cpu −mbig −msmall −mregparm −mmemparm −mfast−fix −mmpyi −mbk −mti −mdp−isr−reload −mrpts=count −mrptb −mdb −mloop−unsigned −mparallel−insns −mparallel−mpy −mpreserve−float

V850 Options −mlong−calls −mno−long−calls −mep −mno−ep −mprolog−function −mno−prolog−function −mspace −mtda=n −msda=n −mzda=n −mapp−regs −mno−app−regs −mdisable−callt −mno−disable−callt −mv850e1 −mv850e −mv850 −mbig−switch

VAX Options −mg −mgnu −munix

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

Xstormy16 Options −msim

Xtensa Options −mconst16 −mno−const16 −mfused−madd −mno−fused−madd −mtext−section−literals −mno−text−section−literals −mtarget−align −mno−target−align −mlongcalls −mno−longcalls

zSeries Options See S/390 and zSeries Options.

Code Generation Options

−fcall−saved−reg −fcall−used−reg −ffixed−reg −fexceptions −fnon−call−exceptions −funwind−tables −fasynchronous−unwind−tables −finhibit−size−directive −finstrument−functions −fno−common −fno−ident −fpcc−struct−return −fpic −fPIC −fpie −fPIE −fno−jump−tables −freg−struct−return −fshared−data −fshort−enums −fshort−double −fshort−wchar −fverbose−asm −fpack−struct[=n] −fstack−check −fstack−limit−register=reg −fstack−limit−symbol=sym −fargument−alias −fargument−noalias −fargument−noalias−global −fleading−underscore −ftls−model=model −ftrapv −fwrapv −fbounds−check −fvisibility

Options Controlling the Kind of Output

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

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

C source code which must be preprocessed.

file.i

C source code which should not be preprocessed.

file.ii

C ++ source code which should not be preprocessed.

file.m

Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program work.

file.mi

Objective-C source code which should not be preprocessed.

file.mm
file.M

Objective−C ++ source code. Note that you must link with the libobjc library to make an Objective−C ++ program work. Note that .M refers to a literal capital M.

file.mii

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

file.h

C, C ++ , Objective-C or Objective−C ++ header file to be turned into a precompiled header.

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

C ++ source code which must be preprocessed. Note that in .cxx, the last two letters must both be literally x. Likewise, .C refers to a literal capital C.

file.mm
file.M

Objective−C ++ source code which must be preprocessed.

file.mii

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

file.hh
file.H

C ++ header file to be turned into a precompiled header.

file.f
file.for
file.FOR

Fixed form Fortran source code which should not be preprocessed.

file.F
file.fpp
file.FPP

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

file.f90
file.f95

Free form Fortran source code which should not be preprocessed.

file.F90
file.F95

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

file.ads

Ada source code file which contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.

file.adb

Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.

file.s

Assembler code.

file.S

Assembler code which must be preprocessed.

other

An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

You can specify the input language explicitly with the −x option:
−x language

Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next −x option. Possible values for language are:

        c  c-header  c-cpp-output
        c++  c++-header  c++-cpp-output
        objective-c  objective-c-header  objective-c-cpp-output
        objective-c++ objective-c++-header objective-c++-cpp-output
        assembler  assembler-with-cpp
        ada
        f77  f77-cpp-input
        f95  f95-cpp-input
        java
        treelang

−x none

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

−pass−exit−codes

Normally the gcc program will exit with the code of 1 if any phase of the compiler returns a non-success return code. If you specify −pass−exit−codes, the gcc program will instead return with numerically highest error produced by any phase that returned an error indication.

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

−c

Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.

By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.

Unrecognized input files, not requiring compilation or assembly, are ignored.

−S

Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified.

By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.

Input files that don’t require compilation are ignored.

−E

Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.

Input files which don’t require preprocessing are ignored.

−o file

Place output in file file. This applies regardless to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.

If −o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.

−v

Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.

−###

Like −v except the commands are not executed and all command arguments are quoted. This is useful for shell scripts to capture the driver-generated command lines.

−pipe

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

−combine

If you are compiling multiple source files, this option tells the driver to pass all the source files to the compiler at once (for those languages for which the compiler can handle this). This will allow intermodule analysis ( IMA ) to be performed by the compiler. Currently the only language for which this is supported is C. If you pass source files for multiple languages to the driver, using this option, the driver will invoke the compiler(s) that support IMA once each, passing each compiler all the source files appropriate for it. For those languages that do not support IMA this option will be ignored, and the compiler will be invoked once for each source file in that language. If you use this option in conjunction with −save−temps, the compiler will generate multiple pre-processed files (one for each source file), but only one (combined) .o or .s file.

−−help

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

−−target−help

Print (on the standard output) a description of target specific command line options for each tool.

−−version

Display the version number and copyrights of the invoked GCC .

Compiling C ++ Programs

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

However, C ++ programs often require class libraries as well as a compiler that understands the C ++ language−−−and under some circumstances, you might want to compile programs or header files from standard input, or otherwise without a suffix that flags them as C ++ programs. You might also like to precompile a C header file with a .h extension to be used in C ++ compilations. g++ is a program that calls GCC with the default language set to C ++ , and automatically specifies linking against the C ++ library. On many systems, g++ is also installed with the name c++.

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

Options Controlling C Dialect

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

In C mode, support all ISO C90 programs. In C ++ mode, remove GNU extensions that conflict with ISO C ++ .

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

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

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

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

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

−std=

Determine the language standard. This option is currently only supported when compiling C or C ++ . A value for this option must be provided; possible values are

c89

iso9899:1990

ISO C90 (same as −ansi).

iso9899:199409

ISO C90 as modified in amendment 1.

	c99
	c9x

iso9899:1999
iso9899:199x

ISO C99. Note that this standard is not yet fully supported; see <http://gcc.gnu.org/gcc−4.1/c99status.html> for more information. The names c9x and iso9899:199x are deprecated.

gnu89

Default, ISO C90 plus GNU extensions (including some C99 features).

gnu99
gnu9x

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

c++98

The 1998 ISO C ++ standard plus amendments.

gnu++98

The same as −std=c++98 plus GNU extensions. This is the default for C ++ code.

Even when this option is not specified, you can still use some of the features of newer standards in so far as they do not conflict with previous C standards. For example, you may use "__restrict__" even when −std=c99 is not specified.

The −std options specifying some version of ISO C have the same effects as −ansi, except that features that were not in ISO C90 but are in the specified version (for example, // comments and the "inline" keyword in ISO C99) are not disabled.

−aux−info filename

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

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

−fno−asm

Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these words as identifiers. You can use the keywords "__asm__", "__inline__" and "__typeof__" instead. −ansi implies −fno−asm.

In C ++ , this switch only affects the "typeof" keyword, since "asm" and "inline" are standard keywords. You may want to use the −fno−gnu−keywords flag instead, which has the same effect. In C99 mode (−std=c99 or −std=gnu99), this switch only affects the "asm" and "typeof" keywords, since "inline" is a standard keyword in ISO C99.

−fno−builtin
−fno−builtin−function

Don’t recognize built-in functions that do not begin with __builtin_ as prefix.

GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to "alloca" may become single instructions that adjust the stack directly, and calls to "memcpy" may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with −Wformat for bad calls to "printf", when "printf" is built in, and "strlen" is known not to modify global memory.

With the −fno−builtin−function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named this is not built-in in this version of GCC , this option is ignored. There is no corresponding −fbuiltin−function option; if you wish to enable built-in functions selectively when using −fno−builtin or −ffreestanding, you may define macros such as:

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

−fhosted

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

−ffreestanding

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

−fms−extensions

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

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

−trigraphs

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

−no−integrated−cpp

Performs a compilation in two passes: preprocessing and compiling. This option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the −B option. The user supplied compilation step can then add in an additional preprocessing step after normal preprocessing but before compiling. The default is to use the integrated cpp (internal cpp)

The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.

−traditional
−traditional−cpp

Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are now only supported with the −E switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP manual for details.

−fcond−mismatch

Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C ++ .

−funsigned−char

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

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

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

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

−fsigned−char

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

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

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

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

Options Controlling C ++ Dialect

This section describes the command-line options that are only meaningful for C ++ programs; but you can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file "firstClass.C" like this:

        g++ -g -frepo -O -c firstClass.C

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

Here is a list of options that are only for compiling C ++ programs:
−fabi−version=n

Use version n of the C ++ ABI . Version 2 is the version of the C ++ ABI that first appeared in G++ 3.4. Version 1 is the version of the C ++ ABI that first appeared in G++ 3.2. Version 0 will always be the version that conforms most closely to the C ++ ABI specification. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed.

The default is version 2.

−fno−access−control

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

−fcheck−new

Check that the pointer returned by "operator new" is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C ++ standard specifies that "operator new" will only return 0 if it is declared throw(), in which case the compiler will always check the return value even without this option. In all other cases, when "operator new" has a non-empty exception specification, memory exhaustion is signalled by throwing "std::bad_alloc". See also new (nothrow).

−fconserve−space

Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves space in the executable at the cost of not diagnosing duplicate definitions. If you compile with this flag and your program mysteriously crashes after "main()" has completed, you may have an object that is being destroyed twice because two definitions were merged.

This option is no longer useful on most targets, now that support has been added for putting variables into BSS without making them common.

−ffriend−injection

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

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

−fno−const−strings

Give string constants type "char *" instead of type "const char *". By default, G++ uses type "const char *" as required by the standard. Even if you use −fno−const−strings, you cannot actually modify the value of a string constant.

This option might be removed in a future release of G++. For maximum portability, you should structure your code so that it works with string constants that have type "const char *".

−fno−elide−constructors

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

−fno−enforce−eh−specs

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

−ffor−scope
−fno−for−scope

If −ffor−scope is specified, the scope of variables declared in a for-init-statement is limited to the for loop itself, as specified by the C ++ standard. If −fno−for−scope is specified, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions of G++, and other (traditional) implementations of C ++ .

The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.

−fno−gnu−keywords

Do not recognize "typeof" as a keyword, so that code can use this word as an identifier. You can use the keyword "__typeof__" instead. −ansi implies −fno−gnu−keywords.

−fno−implicit−templates

Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations.

−fno−implicit−inline−templates

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

−fno−implement−inlines

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

−fms−extensions

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

−fno−nonansi−builtins

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

−fno−operator−names

Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as synonyms as keywords.

−fno−optional−diags

Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.

−fpermissive

Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using −fpermissive will allow some nonconforming code to compile.

−frepo

Enable automatic template instantiation at link time. This option also implies −fno−implicit−templates.

−fno−rtti

Disable generation of information about every class with virtual functions for use by the C ++ runtime type identification features (dynamic_cast and typeid). If you don’t use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but it will generate it as needed.

−fstats

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

−ftemplate−depth−n

Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C ++ conforming programs must not rely on a maximum depth greater than 17.

−fno−threadsafe−statics

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

−fuse−cxa−atexit

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

−fvisibility−inlines−hidden

Causes all inlined methods to be marked with "__attribute__ ((visibility ("hidden")))" so that they do not appear in the export table of a DSO and do not require a PLT 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. While it can cause bloating through duplication of code within each DSO where it is used, often the wastage is less than the considerable space occupied by a long symbol name in the export table which is typical when using templates and namespaces. For even more savings, combine with the −fvisibility=hidden switch.

−fno−weak

Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak symbols if they are available. This option exists only for testing, and should not be used by end−users; it will result in inferior code and has no benefits. This option may be removed in a future release of G++.

−nostdinc++

Do not search for header files in the standard directories specific to C ++ , but do still search the other standard directories. (This option is used when building the C ++ library.)

In addition, these optimization, warning, and code generation options have meanings only for C ++ programs:
−fno−default−inline

Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline functions; they just won’t be inlined by default.

−Wabi (C ++ only)

Warn when G++ generates code that is probably not compatible with the vendor-neutral C ++ ABI . Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated will be compatible.

You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

The known incompatibilities at this point include:

*

Incorrect handling of tail-padding for bit−fields. G++ may attempt to pack data into the same byte as a base class. For example:

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

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

*

Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out virtual bases. For example:

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

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

*

Incorrect handling of bit-fields with declared widths greater than that of their underlying types, when the bit-fields appear in a union. For example:

        union U { int i : 4096; };

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

*

Empty classes can be placed at incorrect offsets. For example:

        struct A {};
        struct B {
          A a;
          virtual void f ();
        };

        struct C : public B, public A {};

G++ will place the "A" base class of "C" at a nonzero offset; it should be placed at offset zero. G++ mistakenly believes that the "A" data member of "B" is already at offset zero.

*

Names of template functions whose types involve "typename" or template template parameters can be mangled incorrectly.

        template <typename Q>
        void f(typename Q::X) {}

        template <template <typename> class Q>
        void f(typename Q<int>::X) {}

Instantiations of these templates may be mangled incorrectly.

−Wctor−dtor−privacy (C ++ only)

Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions.

−Wnon−virtual−dtor (C ++ only)

Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it declares a non-virtual one. This warning is enabled by −Wall.

−Wreorder (C ++ only)

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

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

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

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

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

	*		Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated memory.
	*		Item 12: Prefer initialization to assignment in constructors.
	*		Item 14: Make destructors virtual in base classes.
	*		Item 15: Have "operator=" return a reference to *this.
	*		Item 23: Don’t try to return a reference when you must return an object.

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

	*		Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.
	*		Item 7: Never overload "&&", "⎪⎪", or ",".

When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep −v to filter out those warnings.

−Wno−deprecated (C ++ only)

Do not warn about usage of deprecated features.

−Wstrict−null−sentinel (C ++ only)

Warn also about the use of an uncasted "NULL" as sentinel. When compiling only with GCC this is a valid sentinel, as "NULL" is defined to "__null". Although it is a null pointer constant not a null pointer, it is guaranteed to of the same size as a pointer. But this use is not portable across different compilers.

−Wno−non−template−friend (C ++ only)

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

−Wold−style−cast (C ++ only)

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

−Woverloaded−virtual (C ++ only)

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

        struct A {
          virtual void f();
        };

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

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

        B* b;
        b->f();

will fail to compile.

−Wno−pmf−conversions (C ++ only)

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

−Wsign−promo (C ++ only)

Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ would try to preserve unsignedness, but the standard mandates the current behavior.

        struct A {
          operator int ();
          A& operator = (int);
        };

        main ()
        {
          A a,b;
          a = b;
        }

In this example, G++ will synthesize a default A& operator = (const A&);, while cfront will use the user-defined operator =.

Options Controlling Objective-C and Objective−C ++ Dialects

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

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

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

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

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

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

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

−fgnu−runtime

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

−fnext−runtime

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

−fno−nil−receivers

Assume that all Objective-C message dispatches (e.g., "[receiver message:arg]") in this translation unit ensure that the receiver is not "nil". This allows for more efficient entry points in the runtime to be used. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

−fobjc−call−cxx−cdtors

For each Objective-C class, check if any of its instance variables is a C ++ object with a non-trivial default constructor. If so, synthesize a special "− (id) .cxx_construct" instance method that will run non-trivial default constructors on any such instance variables, in order, and then return "self". Similarly, check if any instance variable is a C ++ object with a non-trivial destructor, and if so, synthesize a special "− (void) .cxx_destruct" method that will run all such default destructors, in reverse order.

The "− (id) .cxx_construct" and/or "− (void) .cxx_destruct" methods thusly generated will only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object’s inheritance hierarchy. The "− (id) .cxx_construct" methods will be invoked by the runtime immediately after a new object instance is allocated; the "− (void) .cxx_destruct" methods will be invoked immediately before the runtime deallocates an object instance.

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

−fobjc−direct−dispatch

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

−fobjc−exceptions

Enable syntactic support for structured exception handling in Objective−C, similar to what is offered by C ++ and Java. Currently, this option is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.

          @try {
            ...
               @throw expr;
            ...
          }
          @catch (AnObjCClass *exc) {
            ...
              @throw expr;
            ...
              @throw;
            ...
          }
          @catch (AnotherClass *exc) {
            ...
          }
          @catch (id allOthers) {
            ...
          }
          @finally {
            ...
              @throw expr;
            ...
          }

The @throw statement may appear anywhere in an Objective-C or Objective−C ++ program; when used inside of a @catch block, the @throw may appear without an argument (as shown above), in which case the object caught by the @catch will be rethrown.

Note that only (pointers to) Objective-C objects may be thrown and caught using this scheme. When an object is thrown, it will be caught by the nearest @catch clause capable of handling objects of that type, analogously to how "catch" blocks work in C ++ and Java. A "@catch(id ...)" clause (as shown above) may also be provided to catch any and all Objective-C exceptions not caught by previous @catch clauses (if any).

The @finally clause, if present, will be executed upon exit from the immediately preceding "@try ... @catch" section. This will happen regardless of whether any exceptions are thrown, caught or rethrown inside the "@try ... @catch" section, analogously to the behavior of the "finally" clause in Java.

There are several caveats to using the new exception mechanism:

	*		Although currently designed to be binary compatible with "NS_HANDLER"−style idioms provided by the "NSException" class, the new exceptions can only be used on Mac OS X 10.3 (Panther) and later systems, due to additional functionality needed in the (NeXT) Objective-C runtime.
	*		As mentioned above, the new exceptions do not support handling types other than Objective-C objects. Furthermore, when used from Objective−C ++ , the Objective-C exception model does not interoperate with C ++ exceptions at this time. This means you cannot @throw an exception from Objective-C and "catch" it in C ++ , or vice versa (i.e., "throw ... @catch").

The −fobjc−exceptions switch also enables the use of synchronization blocks for thread-safe execution:

          @synchronized (ObjCClass *guard) {
            ...
          }

Upon entering the @synchronized block, a thread of execution shall first check whether a lock has been placed on the corresponding "guard" object by another thread. If it has, the current thread shall wait until the other thread relinquishes its lock. Once "guard" becomes available, the current thread will place its own lock on it, execute the code contained in the @synchronized block, and finally relinquish the lock (thereby making "guard" available to other threads).

Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized. Note that throwing exceptions out of @synchronized blocks is allowed, and will cause the guarding object to be unlocked properly.

−fobjc−gc

Enable garbage collection ( GC ) in Objective-C and Objective−C ++ programs.

−freplace−objc−classes

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

−fzero−link

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

−gen−decls

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

−Wassign−intercept

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

−Wno−protocol

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

−Wselector

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

−Wstrict−selector−match

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

−Wundeclared−selector

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

−print−objc−runtime−info

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

Options to Control Diagnostic Messages Formatting

Traditionally, diagnostic messages have been formatted irrespective of the output device’s aspect (e.g. its width, ...). The options described below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per line, how often source location information should be reported. Right now, only the C ++ front end can honor these options. However it is expected, in the near future, that the remaining front ends would be able to digest them correctly.
−fmessage−length=n

Try to format error messages so that they fit on lines of about n characters. The default is 72 characters for g++ and 0 for the rest of the front ends supported by GCC . If n is zero, then no line-wrapping will be done; each error message will appear on a single line.

−fdiagnostics−show−location=once

Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once source location information; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won’t be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior.

−fdiagnostics−show−location=every−line

Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line.

−fdiagnostics−show−options

This option instructs the diagnostic machinery to add text to each diagnostic emitted, which indicates which command line option directly controls that diagnostic, when such an option is known to the diagnostic machinery.

Options to Request or Suppress Warnings

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