x86_64-linux-gnu-g++-4.9 Man page



gcc – GNU project C and C++ compiler


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.


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.


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
-fopenmp-simd -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++
-fvisibility-inlines-hidden -fvtable-verify=std|preinit|none
-fvtv-counts -fvtv-debug -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
-Wconditionally-supported -Wconversion -Wcoverage-mismatch
-Wdate-time -Wdelete-incomplete -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 -Wmemset-transposed-args -Wmissing-braces
-Wmissing-field-initializers -Wmissing-include-dirs -Wno-multichar
-Wnonnull -Wno-overflow -Wopenmp-simd -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 -Wfloat-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-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-vtable-verify -fdump-tree-vrp[-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 -fdevirtualize-speculatively -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
-fisolate-erroneous-paths-attribute -fivopts
-fkeep-inline-functions -fkeep-static-consts -flive-range-shrinkage
-floop-block -floop-interchange -floop-strip-mine
-floop-nest-optimize -floop-parallelize-all -flto
-flto-compression-level -flto-partition=alg -flto-report
-flto-report-wpa -fmerge-all-constants -fmerge-constants
-fmodulo-sched -fmodulo-sched-allow-regmoves -fmove-loop-invariants
-fno-branch-count-reg -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-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 -fprofile-reorder-functions -freciprocal-math
-free -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-loop-vectorize -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-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 -static-liblsan
-static-libubsan -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 -mabi=name -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 -mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769 -mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419 -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

ARC Options -mbarrel-shifter -mcpu=cpu -mA6 -mARC600 -mA7 -mARC700
-mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr -mea -mno-mpy
-mmul32x16 -mmul64 -mnorm -mspfp -mspfp-compact -mspfp-fast -msimd
-msoft-float -mswap -mcrc -mdsp-packa -mdvbf -mlock -mmac-d16
-mmac-24 -mrtsc -mswape -mtelephony -mxy -misize -mannotate-align
-marclinux -marclinux_prof -mepilogue-cfi -mlong-calls
-mmedium-calls -msdata -mucb-mcount -mvolatile-cache -malign-call
-mauto-modify-reg -mbbit-peephole -mno-brcc -mcase-vector-pcrel
-mcompact-casesi -mno-cond-exec -mearly-cbranchsi -mexpand-adddi
-mindexed-loads -mlra -mlra-priority-none -mlra-priority-compact
mlra-priority-noncompact -mno-millicode -mmixed-code -mq-class
-mRcq -mRcw -msize-level=level -mtune=cpu -mmultcost=num

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 -mneon-for-64bits -mslow-flash-data

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

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

CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops

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

HPPA Options -march=architecture-type -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-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
-mtune-ctrl=feature-list -mdump-tune-features -mno-default
-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 -mavx512f -mavx512pf -mavx512er
-mavx512cd -msha -maes -mpclmul -mfsgsbase -mrdrnd -mf16c -mfma
-mprefetchwt1 -msse4a -m3dnow -mpopcnt -mabm -mbmi -mtbm -mfma4
-mxop -mlzcnt -mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mlwp
-mthreads -mno-align-stringops -minline-all-stringops
-minline-stringops-dynamically -mstringop-strategy=alg
-mmemcpy-strategy=strategy -mmemset-strategy=strategy -mpush-args
-maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mlong-double-64 -mlong-double-80
-mlong-double-128 -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
-m16 -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

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-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
-msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit -msched-max-memory-insns=max-

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

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-compressed
-mno-interlink-compressed -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 -mabs=mode -mnan=encoding -mdsp -mno-dsp -mdspr2
-mno-dspr2 -mmcu -mmno-mcu -meva -mno-eva -mvirt -mno-virt
-mmicromips -mno-micromips -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 -mimadd
-mno-imadd -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-rm7000
-mno-fix-rm7000 -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

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

MSP430 Options -msim -masm-hex -mmcu= -mcpu= -mlarge -msmall
-mrelax -mhwmult= -minrt

NDS32 Options -mbig-endian -mlittle-endian -mreduced-regs
-mfull-regs -mcmov -mno-cmov -mperf-ext -mno-perf-ext -mv3push
-mno-v3push -m16bit -mno-16bit -mgp-direct -mno-gp-direct
-misr-vector-size=num -mcache-block-size=num -march=arch
-mforce-fp-as-gp -mforbid-fp-as-gp -mex9 -mctor-dtor -mrelax

Nios II Options -G num -mgpopt -mno-gpopt -mel -meb
-mno-bypass-cache -mbypass-cache -mno-cache-volatile
-mcache-volatile -mno-fast-sw-div -mfast-sw-div -mhw-mul
-mno-hw-mul -mhw-mulx -mno-hw-mulx -mno-hw-div -mhw-div
-mcustom-insn=N -mno-custom-insn -mcustom-fpu-cfg=name -mhal
-msmallc -msys-crt0=name -msys-lib=name

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

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

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

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

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 -mfused-madd
-mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra -mpretend-cmove

Solaris 2 Options -mclear-hwcap -mno-clear-hwcap -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 -mbig-endian -mlittle-endian

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

VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy

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-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:

C source code that must be preprocessed.

C source code that should not be preprocessed.

C++ source code that should not be preprocessed.

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

Objective-C source code that should not be preprocessed.

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.

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

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).

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.

Objective-C++ source code that must be preprocessed.

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

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

Fixed form Fortran source code that should not be preprocessed.

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

Free form Fortran source code that should not be preprocessed.

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

Go source code.

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.

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

D source code file.

D interface code file.

D documentation code file.

Assembler code.

Assembler code that must be preprocessed.

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
f77 f77-cpp-input f95 f95-cpp-input

-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).

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.

-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 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.

-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

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.

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.

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.

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:

Display all of the optimization options supported by the

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

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.

Display the values recognized by the –param option.

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

Display the options that are common to all languages.

These are the supported qualifiers:

Display only those options that are undocumented.

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

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:


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:


The argument to –help= should not consist solely of inverted

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:


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

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

Display the version number and copyrights of the invoked GCC.

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

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 ….

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.

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

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

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.

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

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

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

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

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.

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

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

ISO C90 as modified in amendment 1.

ISO C99. This standard is substantially completely supported,
modulo bugs, extended identifiers (supported except for corner
cases when -fextended-identifiers is used) and floating-point
issues (mainly but not entirely relating to optional C99
features from Annexes F and G). See
for more information. The
names c9x and iso9899:199x are deprecated.

ISO C11, the 2011 revision of the ISO C standard. This
standard is substantially completely supported, modulo bugs,
extended identifiers (supported except for corner cases when
-fextended-identifiers is used), floating-point issues (mainly
but not entirely relating to optional C11 features from Annexes
F and G) and the optional Annexes K (Bounds-checking
interfaces) and L (Analyzability). The name c1x is deprecated.

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

GNU dialect of ISO C99. The name gnu9x is deprecated.

GNU dialect of ISO C11. This is intended to become the default
in a future release of GCC. The name gnu1x is deprecated.

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

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

The 2011 ISO C++ standard plus amendments. The name c++0x is

GNU dialect of -std=c++11. The name gnu++0x is deprecated.

The next revision of the ISO C++ standard, tentatively planned
for 2014. Support is highly experimental, and will almost
certainly change in incompatible ways in future releases.

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

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

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

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++.

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

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

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))

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.

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.

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 v4.0 . This option implies
-pthread, and thus is only supported on targets that have support
for -pthread. -fopenmp implies -fopenmp-simd.

Enable handling of OpenMP’s SIMD directives with “#pragma omp” in
C/C++ and “!$omp” in Fortran. Other OpenMP directives are ignored.

Enable the usage of Cilk Plus language extension features for
C/C++. When the option -fcilkplus is specified, enable the usage
of the Cilk Plus Language extension features for C/C++. The
present implementation follows ABI version 1.2. This is an
experimental feature that is only partially complete, and whose
interface may change in future versions of GCC as the official
specification changes. Currently, all features but “_Cilk_for”
have been implemented.

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).

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 {

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

Note that this option is off for all targets but i?86 and x86_64
targets using ms-abi.

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++.

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

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.

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++.

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

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

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.

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.

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:

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++

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

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.

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

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).

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.

Enable deduction of a template type parameter as
“std::initializer_list” from a brace-enclosed initializer list,

template auto forward(T t) -> decltype (realfn (t))
return realfn (t);

void f()
forward({1,2}); // call forward>

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

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++.

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.

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.

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.

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.

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.

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

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.

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.

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

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.

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”.

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

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.

Downgrade some diagnostics about nonconformant code from errors to
warnings. Thus, using -fpermissive allows some nonconforming code
to compile.

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.

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

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.

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

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

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

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

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.

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

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.

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

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:

1. It sets the default visibility to “hidden”, like

2. Types, but not their members, are not hidden by default.

3. The One Definition Rule is relaxed for types without explicit
visibility specifications that are defined in more than one
shared object: those declarations are permitted if they are
permitted when this option is not used.

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.

Turn on (or off, if using -fvtable-verify=none) the security
feature that verifies at runtime, for every virtual call that is
made, that the vtable pointer through which the call is made is
valid for the type of the object, and has not been corrupted or
overwritten. If an invalid vtable pointer is detected (at
runtime), an error is reported and execution of the program is
immediately halted.

This option causes runtime data structures to be built, at program
start up, for verifying the vtable pointers. The options “std” and
“preinit” control the timing of when these data structures are
built. In both cases the data structures are built before
execution reaches ‘main’. The -fvtable-verify=std causes these
data structure to be built after the shared libraries have been
loaded and initialized. -fvtable-verify=preinit causes them to be
built before the shared libraries have been loaded and initialized.

If this option appears multiple times in the compiler line, with
different values specified, ‘none’ will take highest priority over
both ‘std’ and ‘preinit’; ‘preinit’ will take priority over ‘std’.

Causes debug versions of the runtime functions for the vtable
verification feature to be called. This assumes the
-fvtable-verify=std or -fvtable-verify=preinit has been used. This
flag will also cause the compiler to keep track of which vtable
pointers it found for each class, and record that information in
the file “vtv_set_ptr_data.log”, in the dump file directory on the
user’s machine.

Note: This feature APPENDS data to the log file. If you want a
fresh log file, be sure to delete any existing one.

This is a debugging flag. When used in conjunction with
-fvtable-verify=std or -fvtable-verify=preinit, this causes the
compiler to keep track of the total number of virtual calls it
encountered and the number of verifications it inserted. It also
counts the number of calls to certain runtime library functions
that it inserts. This information, for each compilation unit, is
written to a file named “vtv_count_data.log”, in the dump_file
directory on the user’s machine. It also counts the size of the
vtable pointer sets for each class, and writes this information to
“vtv_class_set_sizes.log” in the same directory.

Note: This feature APPENDS data to the log files. To get a fresh
log files, be sure to delete any existing ones.

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++.

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:

-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)

· A template with a non-type template parameter of reference type
is mangled incorrectly:

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

This is fixed in -fabi-version=3.

· SIMD vector types declared using “__attribute ((vector_size))”
are mangled in a non-standard way that does not allow for
overloading of functions taking vectors of different sizes.

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

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

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

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.

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

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.

· 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++ makes 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++ 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.

· Names of template functions whose types involve “typename” or
template template parameters can be mangled incorrectly.

void f(typename Q::X) {}