man gcc
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GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for the
remainder. g++ accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The "overall options" allow you to stop this
process at an intermediate stage. For example, the -c option says not
to run the linker. Then the output consists of object files output by
the assembler.
Other options are passed on to one stage of processing. Some options
control the preprocessor and others the compiler itself. Yet other
options control the assembler and linker; most of these are not
documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful
for C programs; when an option is only useful with another language
(usually C++), the explanation says so explicitly. If the description
for a particular option does not mention a source language, you can use
that option with all supported languages.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the order
you use doesn't matter. Order does matter when you use several options
of the same kind; for example, if you specify -L more than once, the
directories are searched in the order specified. Also, the placement
of the -l option is significant.
Many options have long names starting with -f or with -W---for example,
-fmove-loop-invariants, -Wformat and so on. Most of these have both
positive and negative forms; the negative form of -ffoo is -fno-foo.
This manual documents only one of these two forms, whichever one is not
the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations
are in the following sections.
Overall Options
-c -S -E -o file -no-canonical-prefixes -pipe -pass-exit-codes
-x language -v -### --help[=class[,...]] --target-help
--version -wrapper @file -fplugin=file -fplugin-arg-name=arg
-fdump-ada-spec[-slim] -fada-spec-parent=unit -fdump-go-spec=file
C Language Options
-ansi -std=standard -fgnu89-inline -aux-info filename
-fallow-parameterless-variadic-functions -fno-asm -fno-builtin
-fno-builtin-function -fhosted -ffreestanding -fopenacc -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 -fsized-deallocation
-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=n -Wabi-tag -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 -fno-local-ivars
-fivar-visibility=[public|protected|private|package]
-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 -Warray-bounds=n
-Wbool-compare -Wno-attributes -Wno-builtin-macro-redefined
-Wc90-c99-compat -Wc99-c11-compat -Wc++-compat -Wc++11-compat
-Wc++14-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 -Wno-designated-init
-Wdisabled-optimization -Wno-discarded-qualifiers
-Wno-discarded-array-qualifiers -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-signedness -Wformat-y2k
-Wframe-larger-than=len -Wno-free-nonheap-object -Wjump-misses-init
-Wignored-qualifiers -Wincompatible-pointer-types -Wimplicit
-Wimplicit-function-declaration -Wimplicit-int -Winit-self
-Winline -Wno-int-conversion -Wno-int-to-pointer-cast
-Wno-invalid-offsetof -Winvalid-pch -Wlarger-than=len
-Wunsafe-loop-optimizations -Wlogical-op -Wlogical-not-parentheses
-Wlong-long -Wmain -Wmaybe-uninitialized -Wmemset-transposed-args
-Wmissing-braces -Wmissing-field-initializers
-Wmissing-include-dirs -Wno-multichar -Wnonnull
-Wnormalized=[none|id|nfc|nfkc]
-Wodr -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 -Wno-shadow-ivar
-Wshift-count-negative -Wshift-count-overflow -Wsign-compare
-Wsign-conversion -Wfloat-conversion -Wsizeof-pointer-memaccess
-Wsizeof-array-argument -Wstack-protector -Wstack-usage=len
-Wstrict-aliasing -Wstrict-aliasing=n -Wstrict-overflow
-Wstrict-overflow=n
-Wsuggest-attribute=[pure|const|noreturn|format]
-Wsuggest-final-types -Wsuggest-final-methods -Wsuggest-override
-Wmissing-format-attribute -Wswitch -Wswitch-default
-Wswitch-enum -Wswitch-bool -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
-fsanitize-recover -fsanitize-recover=style
-fasan-shadow-offset=number -fsanitize-undefined-trap-on-error
-fcheck-pointer-bounds -fchkp-check-incomplete-type
-fchkp-first-field-has-own-bounds -fchkp-narrow-bounds
-fchkp-narrow-to-innermost-array -fchkp-optimize
-fchkp-use-fast-string-functions -fchkp-use-nochk-string-functions
-fchkp-use-static-bounds -fchkp-use-static-const-bounds
-fchkp-treat-zero-dynamic-size-as-infinite -fchkp-check-read
-fchkp-check-read -fchkp-check-write -fchkp-store-bounds
-fchkp-instrument-calls -fchkp-instrument-marked-only
-fchkp-use-wrappers -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+ -gz[=type]
-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-profile -fauto-profile[=path]
-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
-fdevirtualize-at-ltrans -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-cp-alignment
-fipa-pta -fipa-profile -fipa-pure-const -fipa-reference -fipa-icf
-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-dereference
-fisolate-erroneous-paths-attribute -fivopts
-fkeep-inline-functions -fkeep-static-consts -flive-range-shrinkage
-floop-block -floop-interchange -floop-strip-mine
-floop-unroll-and-jam -floop-nest-optimize -floop-parallelize-all
-flra-remat -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-fusion -fschedule-insns -fschedule-insns2
-fsection-anchors -fselective-scheduling -fselective-scheduling2
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops
-fsemantic-interposition -fshrink-wrap -fsignaling-nans
-fsingle-precision-constant -fsplit-ivs-in-unroller
-fsplit-wide-types -fssa-phiopt -fstack-protector
-fstack-protector-all -fstack-protector-strong
-fstack-protector-explicit -fstdarg-opt -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 -fipa-ra
-fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb
-fwhole-program -fwpa -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 -fuse-ld=linker -llibrary -nostartfiles
-nodefaultlibs -nostdlib -pie -rdynamic -s -static -static-libgcc
-static-libstdc++ -static-libasan -static-libtsan -static-liblsan
-static-libubsan -static-libmpx -static-libmpxwrappers -shared
-shared-libgcc -symbolic -T script -Wl,option -Xlinker option -u
symbol -z keyword
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
-munalign-prob-threshold=probability
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
-mfloat-abi=name -mfp16-format=name -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name -mfpu=name
-mtune=name -mprint-tune-info -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
-masm-syntax-unified -mrestrict-it
AVR Options -mmcu=mcu -maccumulate-args -mbranch-cost=cost
-mcall-prologues -mint8 -mn_flash=size -mno-interrupts -mrelax
-mrmw -mstrict-X -mtiny-stack -nodevicelib -Waddr-space-convert
Blackfin Options -mcpu=cpu[-sirevision] -msim
-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly
-mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1
-mid-shared-library -mno-id-shared-library -mshared-library-id=n
-mleaf-id-shared-library -mno-leaf-id-shared-library -msep-data
-mno-sep-data -mlong-calls -mno-long-calls -mfast-fp -minline-plt
-mmulticore -mcorea -mcoreb -msdram -micplb
C6X Options -mbig-endian -mlittle-endian -march=cpu -msim
-msdata=sdata-type
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n
-melinux-stacksize=n -metrax4 -metrax100 -mpdebug -mcc-init
-mno-side-effects -mstack-align -mdata-align -mconst-align
-m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt
-melf -maout -melinux -mlinux -sim -sim2 -mmul-bug-workaround
-mno-mul-bug-workaround
CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops
-mdata-model=model
Darwin Options -all_load -allowable_client -arch
-arch_errors_fatal -arch_only -bind_at_load -bundle
-bundle_loader -client_name -compatibility_version
-current_version -dead_strip -dependency-file -dylib_file
-dylinker_install_name -dynamic -dynamiclib
-exported_symbols_list -filelist -flat_namespace
-force_cpusubtype_ALL -force_flat_namespace
-headerpad_max_install_names -iframework -image_base -init
-install_name -keep_private_externs -multi_module
-multiply_defined -multiply_defined_unused -noall_load
-no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs
-noprebind -noseglinkedit -pagezero_size -prebind
-prebind_all_twolevel_modules -private_bundle -read_only_relocs
-sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate
-sectobjectsymbols -sectorder -segaddr -segs_read_only_addr
-segs_read_write_addr -seg_addr_table -seg_addr_table_filename
-seglinkedit -segprot -segs_read_only_addr -segs_read_write_addr
-single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined -unexported_symbols_list
-weak_reference_mismatches -whatsloaded -F -gused -gfull
-mmacosx-version-min=version -mkernel -mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -mieee
-mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode
-mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants
-mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix
-mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data
-mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
FR30 Options -msmall-model -mno-lsim
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float
-msoft-float -malloc-cc -mfixed-cc -mdword -mno-dword -mdouble
-mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic
-minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp
-mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8 -mpack
-mno-pack -mno-eflags -mcond-move -mno-cond-move
-moptimize-membar -mno-optimize-membar -mscc -mno-scc -mcond-exec
-mno-cond-exec -mvliw-branch -mno-vliw-branch -mmulti-cond-exec
-mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec
-mtomcat-stats -mTLS -mtls -mcpu=cpu
GNU/Linux Options -mglibc -muclibc -mbionic -mandroid
-tno-android-cc -tno-android-ld
H8/300 Options -mrelax -mh -ms -mn -mexr -mno-exr -mint32
-malign-300
HPPA Options -march=architecture-type -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
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld
-mno-pic -mvolatile-asm-stop -mregister-names -msdata -mno-sdata
-mconstant-gp -mauto-pic -mfused-madd
-minline-float-divide-min-latency
-minline-float-divide-max-throughput -mno-inline-float-divide
-minline-int-divide-min-latency -minline-int-divide-max-throughput
-mno-inline-int-divide -minline-sqrt-min-latency
-minline-sqrt-max-throughput -mno-inline-sqrt -mdwarf2-asm
-mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size
-mtune=cpu-type -milp32 -mlp64 -msched-br-data-spec
-msched-ar-data-spec -msched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc
-msched-spec-control-ldc -msched-prefer-non-data-spec-insns
-msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path
-msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit -msched-max-memory-insns=max-
insns
LM32 Options -mbarrel-shift-enabled -mdivide-enabled
-mmultiply-enabled -msign-extend-enabled -muser-enabled
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops
-mno-align-loops -missue-rate=number -mbranch-cost=number
-mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func
-mflush-func=name -mno-flush-trap -mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -march=arch -mcpu=cpu -mtune=tune -m68000 -m68020
-m68020-40 -m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200
-m5206e -m528x -m5307 -m5407 -mcfv4e -mbitfield -mno-bitfield
-mc68000 -mc68020 -mnobitfield -mrtd -mno-rtd -mdiv -mno-div
-mshort -mno-short -mhard-float -m68881 -msoft-float -mpcrel
-malign-int -mstrict-align -msep-data -mno-sep-data
-mshared-library-id=n -mid-shared-library -mno-id-shared-library
-mxgot -mno-xgot
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div
-mrelax-immediates -mno-relax-immediates -mwide-bitfields
-mno-wide-bitfields -m4byte-functions -mno-4byte-functions
-mcallgraph-data -mno-callgraph-data -mslow-bytes -mno-slow-bytes
-mno-lsim -mlittle-endian -mbig-endian -m210 -m340
-mstack-increment
MeP Options -mabsdiff -mall-opts -maverage -mbased=n -mbitops -mc=n
-mclip -mconfig=name -mcop -mcop32 -mcop64 -mivc2 -mdc -mdiv -meb
-mel -mio-volatile -ml -mleadz -mm -mminmax -mmult -mno-opts
-mrepeat -ms -msatur -msdram -msim -msimnovec -mtf -mtiny=n
MicroBlaze Options -msoft-float -mhard-float -msmall-divides
-mcpu=cpu -mmemcpy -mxl-soft-mul -mxl-soft-div -mxl-barrel-shift
-mxl-pattern-compare -mxl-stack-check -mxl-gp-opt -mno-clearbss
-mxl-multiply-high -mxl-float-convert -mxl-float-sqrt -mbig-endian
-mlittle-endian -mxl-reorder -mxl-mode-app-model
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2
-mips3 -mips4 -mips32 -mips32r2 -mips32r3 -mips32r5 -mips32r6
-mips64 -mips64r2 -mips64r3 -mips64r5 -mips64r6 -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 -mfpxx -mfp64
-mhard-float -msoft-float -mno-float -msingle-float
-mdouble-float -modd-spreg -mno-odd-spreg -mabs=mode
-mnan=encoding -mdsp -mno-dsp -mdspr2 -mno-dspr2 -mmcu -mmno-mcu
-meva -mno-eva -mvirt -mno-virt -mxpa -mno-xpa -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
-mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mno-am33 -mam33 -mam33-2
-mam34 -mtune=cpu-type -mreturn-pointer-on-d0 -mno-crt0 -mrelax
-mliw -msetlb
Moxie Options -meb -mel -mmul.x -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 -misr-vector-size=num
-mcache-block-size=num -march=arch -mcmodel=code-model -mctor-dtor
-mrelax
Nios II Options -G num -mgpopt=option -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
Nvidia PTX Options -m32 -m64 -mmainkernel
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45
-m10 -mbcopy -mbcopy-builtin -mint32 -mno-int16 -mint16
-mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32
-mabshi -mno-abshi -mbranch-expensive -mbranch-cheap -munix-asm
-mdec-asm
picoChip Options -mae=ae_type -mvliw-lookahead=N
-msymbol-as-address -mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
RL78 Options -msim -mmul=none -mmul=g13 -mmul=rl78 -m64bit-doubles
-m32bit-doubles
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type
-mcmodel=code-model -mpowerpc64 -maltivec -mno-altivec
-mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt
-mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb
-mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb -mno-cmpb
-mmfpgpr -mno-mfpgpr -mhard-dfp -mno-hard-dfp -mfull-toc
-mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32
-mxl-compat -mno-xl-compat -mpe -malign-power -malign-natural
-msoft-float -mhard-float -mmultiple -mno-multiple
-msingle-float -mdouble-float -msimple-fpu -mstring -mno-string
-mupdate -mno-update -mavoid-indexed-addresses
-mno-avoid-indexed-addresses -mfused-madd -mno-fused-madd
-mbit-align -mno-bit-align -mstrict-align -mno-strict-align
-mrelocatable -mno-relocatable -mrelocatable-lib
-mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian
-mbig -mbig-endian -mdynamic-no-pic -maltivec -mswdiv
-msingle-pic-base -mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme
-mcall-sysv -mcall-netbsd -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-mblock-move-inline-limit=num -misel -mno-isel -misel=yes
-misel=no -mspe -mno-spe -mspe=yes -mspe=no -mpaired
-mgen-cell-microcode -mwarn-cell-microcode -mvrsave -mno-vrsave
-mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mfloat-gprs=yes
-mfloat-gprs=no -mfloat-gprs=single -mfloat-gprs=double -mprototype
-mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata
-msdata=opt -mvxworks -G num -pthread -mrecip -mrecip=opt
-mno-recip -mrecip-precision -mno-recip-precision -mveclibabi=type
-mfriz -mno-friz -mpointers-to-nested-functions
-mno-pointers-to-nested-functions -msave-toc-indirect
-mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion
-mpower8-vector -mno-power8-vector -mcrypto -mno-crypto
-mdirect-move -mno-direct-move -mquad-memory -mno-quad-memory
-mquad-memory-atomic -mno-quad-memory-atomic -mcompat-align-parm
-mno-compat-align-parm -mupper-regs-df -mno-upper-regs-df
-mupper-regs-sf -mno-upper-regs-sf -mupper-regs -mno-upper-regs
RX Options -m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu=
-mbig-endian-data -mlittle-endian-data -msmall-data -msim -mno-sim
-mas100-syntax -mno-as100-syntax -mrelax -mmax-constant-size=
-mint-register= -mpid -mno-warn-multiple-fast-interrupts
-msave-acc-in-interrupts
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type
-mhard-float -msoft-float -mhard-dfp -mno-hard-dfp
-mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain
-mpacked-stack -mno-packed-stack -msmall-exec -mno-small-exec
-mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch
-mtpf-trace -mno-tpf-trace -mfused-madd -mno-fused-madd
-mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard
-mhotpatch=halfwords,halfwords
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
-mcbranch-force-delay-slot -mfused-madd -mno-fused-madd -mfsca
-mno-fsca -mfsrra -mno-fsrra -mpretend-cmove -mtas
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
-mcmodel=code-model
TILEPro Options -mcpu=cpu -m32
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep
-mprolog-function -mno-prolog-function -mspace -mtda=n -msda=n
-mzda=n -mapp-regs -mno-app-regs -mdisable-callt
-mno-disable-callt -mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e
-mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps -msoft-float
-mhard-float -mgcc-abi -mrh850-abi -mbig-switch
VAX Options -mg -mgnu -munix
Visium Options -mdebug -msim -mfpu -mno-fpu -mhard-float
-msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode -muser-mode
VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64
-mpointer-size=size
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy
-Xbind-now
x86 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 -mclflushopt -mxsavec -mxsaves -msse4a -m3dnow
-mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -mlzcnt -mbmi2 -mfxsr
-mxsave -mxsaveopt -mrtm -mlwp -mmpx -mmwaitx -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 -mrecord-mcount
-mnop-mcount -m8bit-idiv -mavx256-split-unaligned-load
-mavx256-split-unaligned-store -malign-data=type
-mstack-protector-guard=guard -mindirect-branch=choice
-mfunction-return=choice -mindirect-branch-register
x86 Windows Options -mconsole -mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread -municode -mwin32 -mwindows
-fno-set-stack-executable
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd -mno-fused-madd
-mforce-no-pic -mserialize-volatile -mno-serialize-volatile
-mtext-section-literals -mno-text-section-literals -mtarget-align
-mno-target-align -mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -fdelete-dead-exceptions -funwind-tables
-fasynchronous-unwind-tables -fno-gnu-unique
-finhibit-size-directive -finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,... -fno-common
-fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE
-fno-jump-tables -frecord-gcc-switches -freg-struct-return
-fshort-enums -fshort-double -fshort-wchar -fverbose-asm
-fpack-struct[=n] -fstack-check -fstack-limit-register=reg
-fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack
-fleading-underscore -ftls-model=model -fstack-reuse=reuse_level
-ftrapv -fwrapv -fbounds-check
-fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields -fsync-libcalls
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order. GCC is capable of
preprocessing and compiling several files either into several assembler
input files, or into one assembler input file; then each assembler
input file produces an object file, and linking combines all the object
files (those newly compiled, and those specified as input) into an
executable file.
For any given input file, the file name suffix determines what kind of
compilation is done:
file.c
C source code that must be preprocessed.
file.i
C source code that should not be preprocessed.
file.ii
C++ source code that should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc
library to make an Objective-C program work.
file.mi
Objective-C source code that should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the
libobjc library to make an Objective-C++ program work. Note that
.M refers to a literal capital M.
file.mii
Objective-C++ source code that should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into
a precompiled header (default), or C, C++ header file to be turned
into an Ada spec (via the -fdump-ada-spec switch).
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code that must be preprocessed. Note that in .cxx, the
last two letters must both be literally x. Likewise, .C refers to
a literal capital C.
file.mm
file.M
Objective-C++ source code that must be preprocessed.
file.mii
Objective-C++ source code that should not be preprocessed.
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
C++ header file to be turned into a precompiled header or Ada spec.
file.f
file.for
file.ftn
Fixed form Fortran source code that should not be preprocessed.
file.F
file.FOR
file.fpp
file.FPP
file.FTN
Fixed form Fortran source code that must be preprocessed (with the
traditional preprocessor).
file.f90
file.f95
file.f03
file.f08
Free form Fortran source code that should not be preprocessed.
file.F90
file.F95
file.F03
file.F08
Free form Fortran source code that must be preprocessed (with the
traditional preprocessor).
file.go
Go source code.
file.ads
Ada source code file that contains a library unit declaration (a
declaration of a package, subprogram, or generic, or a generic
instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration). Such files are also
called specs.
file.adb
Ada source code file containing a library unit body (a subprogram
or package body). Such files are also called bodies.
file.s
Assembler code.
file.S
file.sx
Assembler code that must be preprocessed.
other
An object file to be fed straight into linking. Any file name with
no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on the
file name suffix). This option applies to all following input
files until the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input f95 f95-cpp-input
go
java
-x none
Turn off any specification of a language, so that subsequent files
are handled according to their file name suffixes (as they are if
-x has not been used at all).
-pass-exit-codes
Normally the gcc program exits with the code of 1 if any phase of
the compiler returns a non-success return code. If you specify
-pass-exit-codes, the gcc program instead returns with the
numerically highest error produced by any phase returning an error
indication. The C, C++, and Fortran front ends return 4 if an
internal compiler error is encountered.
If you only want some of the stages of compilation, you can use -x (or
filename suffixes) to tell gcc where to start, and one of the options
-c, -S, or -E to say where gcc is to stop. Note that some combinations
(for example, -x cpp-output -E) instruct gcc to do nothing at all.
-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
proper.
-###
Like -v except the commands are not executed and arguments are
quoted unless they contain only alphanumeric characters or "./-_".
This is useful for shell scripts to capture the driver-generated
command lines.
-pipe
Use pipes rather than temporary files for communication between the
various stages of compilation. This fails to work on some systems
where the assembler is unable to read from a pipe; but the GNU
assembler has no trouble.
--help
Print (on the standard output) a description of the command-line
options understood by gcc. If the -v option is also specified then
--help is also passed on to the various processes invoked by gcc,
so that they can display the command-line options they accept. If
the -Wextra option has also been specified (prior to the --help
option), then command-line options that have no documentation
associated with them are also displayed.
--target-help
Print (on the standard output) a description of target-specific
command-line options for each tool. For some targets extra target-
specific information may also be printed.
--help={class|[^]qualifier}[,...]
Print (on the standard output) a description of the command-line
options understood by the compiler that fit into all specified
classes and qualifiers. These are the supported classes:
optimizers
Display all of the optimization options supported by the
compiler.
warnings
Display all of the options controlling warning messages
produced by the compiler.
target
Display target-specific options. Unlike the --target-help
option however, target-specific options of the linker and
assembler are not displayed. This is because those tools do
not currently support the extended --help= syntax.
params
Display the values recognized by the --param option.
language
Display the options supported for language, where language is
the name of one of the languages supported in this version of
GCC.
common
Display the options that are common to all languages.
These are the supported qualifiers:
undocumented
Display only those options that are undocumented.
joined
Display options taking an argument that appears after an equal
sign in the same continuous piece of text, such as:
--help=target.
separate
Display options taking an argument that appears as a separate
word following the original option, such as: -o output-file.
Thus for example to display all the undocumented target-specific
switches supported by the compiler, use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the ^
character, so for example to display all binary warning options
(i.e., ones that are either on or off and that do not take an
argument) that have a description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted
qualifiers.
Combining several classes is possible, although this usually
restricts the output so much that there is nothing to display. One
case where it does work, however, is when one of the classes is
target. For example, to display all the target-specific
optimization options, use:
--help=target,optimizers
The --help= option can be repeated on the command line. Each
successive use displays its requested class of options, skipping
those that have already been displayed.
If the -Q option appears on the command line before the --help=
option, then the descriptive text displayed by --help= is changed.
Instead of describing the displayed options, an indication is given
as to whether the option is enabled, disabled or set to a specific
value (assuming that the compiler knows this at the point where the
--help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous command-line
options, so for example it is possible to find out which
optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are
enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or
/./, or make the path absolute when generating a relative prefix.
--version
Display the version number and copyrights of the invoked GCC.
-wrapper
Invoke all subcommands under a wrapper program. The name of the
wrapper program and its parameters are passed as a comma separated
list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under gdb --args, thus the
invocation of cc1 is gdb --args cc1 ....
-fplugin=name.so
Load the plugin code in file name.so, assumed to be a shared object
to be dlopen'd by the compiler. The base name of the shared object
file is used to identify the plugin for the purposes of argument
parsing (See -fplugin-arg-name-key=value below). Each plugin
should define the callback functions specified in the Plugins API.
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the plugin
called name.
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate corresponding Ada
specs.
-fada-spec-parent=unit
In conjunction with -fdump-ada-spec[-slim] above, generate Ada
specs as child units of parent unit.
-fdump-go-spec=file
For input files in any language, generate corresponding Go
declarations in file. This generates Go "const", "type", "var",
and "func" declarations which may be a useful way to start writing
a Go interface to code written in some other language.
@file
Read command-line options from file. The options read are inserted
in place of the original @file option. If file does not exist, or
cannot be read, then the option will be treated literally, and not
removed.
Options in file are separated by whitespace. A whitespace
character may be included in an option by surrounding the entire
option in either single or double quotes. Any character (including
a backslash) may be included by prefixing the character to be
included with a backslash. The file may itself contain additional
@file options; any such options will be processed recursively.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp,
.CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or
(for shared template code) .tcc; and preprocessed C++ files use the
suffix .ii. GCC recognizes files with these names and compiles them as
C++ programs even if you call the compiler the same way as for
compiling C programs (usually with the name gcc).
However, the use of gcc does not add the C++ library. g++ is a program
that calls GCC and automatically specifies linking against the C++
library. It treats .c, .h and .i files as C++ source files instead of
C source files unless -x is used. This program is also useful when
precompiling a C header file with a .h extension for use in C++
compilations. On many systems, g++ is also installed with the name
c++.
When you compile C++ programs, you may specify many of the same
command-line options that you use for compiling programs in any
language; or command-line options meaningful for C and related
languages; or options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived
from C, such as C++, Objective-C and Objective-C++) that the compiler
accepts:
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is
equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with
ISO C90 (when compiling C code), or of standard C++ (when compiling
C++ code), such as the "asm" and "typeof" keywords, and predefined
macros such as "unix" and "vax" that identify the type of system
you are using. It also enables the undesirable and rarely used ISO
trigraph feature. For the C compiler, it disables recognition of
C++ style // comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and
"__typeof__" continue to work despite -ansi. You would not want to
use them in an ISO C program, of course, but it is useful to put
them in header files that might be included in compilations done
with -ansi. Alternate predefined macros such as "__unix__" and
"__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected
gratuitously. For that, -Wpedantic is required in addition to
-ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is
used. Some header files may notice this macro and refrain from
declaring certain functions or defining certain macros that the ISO
standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions that are normally built in but do not have semantics
defined by ISO C (such as "alloca" and "ffs") are not built-in
functions when -ansi is used.
-std=
Determine the language standard. This option is currently only
supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or
c++98, and GNU dialects of those standards, such as gnu90 or
gnu++98. When a base standard is specified, the compiler accepts
all programs following that standard plus those using GNU
extensions that do not contradict it. For example, -std=c90 turns
off certain features of GCC that are incompatible with ISO C90,
such as the "asm" and "typeof" keywords, but not other GNU
extensions that do not have a meaning in ISO C90, such as omitting
the middle term of a "?:" expression. On the other hand, when a GNU
dialect of a standard is specified, all features supported by the
compiler are enabled, even when those features change the meaning
of the base standard. As a result, some strict-conforming programs
may be rejected. The particular standard is used by -Wpedantic to
identify which features are GNU extensions given that version of
the standard. For example -std=gnu90 -Wpedantic warns about C++
style // comments, while -std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
c90
c89
iso9899:1990
Support all ISO C90 programs (certain GNU extensions that
conflict with ISO C90 are disabled). Same as -ansi for C code.
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. This standard is substantially completely supported,
modulo bugs 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.
c11
c1x
iso9899:2011
ISO C11, the 2011 revision of the ISO C standard. This
standard is substantially completely supported, modulo bugs,
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.
gnu90
gnu89
GNU dialect of ISO C90 (including some C99 features).
gnu99
gnu9x
GNU dialect of ISO C99. The name gnu9x is deprecated.
gnu11
gnu1x
GNU dialect of ISO C11. This is the default for C code. The
name gnu1x is deprecated.
c++98
c++03
The 1998 ISO C++ standard plus the 2003 technical corrigendum
and some additional defect reports. Same as -ansi for C++ code.
gnu++98
gnu++03
GNU dialect of -std=c++98. This is the default for C++ code.
c++11
c++0x
The 2011 ISO C++ standard plus amendments. The name c++0x is
deprecated.
gnu++11
gnu++0x
GNU dialect of -std=c++11. The name gnu++0x is deprecated.
c++14
c++1y
The 2014 ISO C++ standard plus amendments. The name c++1y is
deprecated.
gnu++14
gnu++1y
GNU dialect of -std=c++14. The name gnu++1y is deprecated.
c++1z
The next revision of the ISO C++ standard, tentatively planned
for 2017. Support is highly experimental, and will almost
certainly change in incompatible ways in future releases.
gnu++1z
GNU dialect of -std=c++1z. Support is highly experimental, and
will almost certainly change in incompatible ways in future
releases.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU
semantics for "inline" functions when in C99 mode.
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 is not supported in
-std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and
"__GNUC_STDC_INLINE__" may be used to check which semantics are in
effect for "inline" functions.
-aux-info filename
Output to the given filename prototyped declarations for all
functions declared and/or defined in a translation unit, including
those in header files. This option is silently ignored in any
language other than C.
Besides declarations, the file indicates, in comments, the origin
of each declaration (source file and line), whether the declaration
was implicit, prototyped or unprototyped (I, N for new or O for
old, respectively, in the first character after the line number and
the colon), and whether it came from a declaration or a definition
(C or F, respectively, in the following character). In the case of
function definitions, a K&R-style list of arguments followed by
their declarations is also provided, inside comments, after the
declaration.
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very
useful as it is not possible to read the arguments. This is only
supported for C as this construct is allowed by C++.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that
code can use these words as identifiers. You can use the keywords
"__asm__", "__inline__" and "__typeof__" instead. -ansi implies
-fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm"
and "inline" are standard keywords. You may want to use the
-fno-gnu-keywords flag instead, which has the same effect. In C99
mode (-std=c99 or -std=gnu99), this switch only affects the "asm"
and "typeof" keywords, since "inline" is a standard keyword in ISO
C99.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with
__builtin_ as prefix.
GCC normally generates special code to handle certain built-in
functions more efficiently; for instance, calls to "alloca" may
become single instructions which adjust the stack directly, and
calls to "memcpy" may become inline copy loops. The resulting code
is often both smaller and faster, but since the function calls no
longer appear as such, you cannot set a breakpoint on those calls,
nor can you change the behavior of the functions by linking with a
different library. In addition, when a function is recognized as a
built-in function, GCC may use information about that function to
warn about problems with calls to that function, or to generate
more efficient code, even if the resulting code still contains
calls to that function. For example, warnings are given with
-Wformat for bad calls to "printf" when "printf" is built in and
"strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function
function is disabled. function must not begin with __builtin_. If
a function is named that is not built-in in this version of GCC,
this option is ignored. There is no corresponding
-fbuiltin-function option; if you wish to enable built-in functions
selectively when using -fno-builtin or -ffreestanding, you may
define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation targets a hosted environment. This implies
-fbuiltin. A hosted environment is one in which the entire
standard library is available, and in which "main" has a return
type of "int". Examples are nearly everything except a kernel.
This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation targets a freestanding environment. This
implies -fno-builtin. A freestanding environment is one in which
the standard library may not exist, and program startup may not
necessarily be at "main". The most obvious example is an OS
kernel. This is equivalent to -fno-hosted.
-fopenacc
Enable handling of OpenACC directives "#pragma acc" in C/C++ and
"!$acc" in Fortran. When -fopenacc is specified, the compiler
generates accelerated code according to the OpenACC Application
Programming Interface v2.0 for more information.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and
"!$omp" in Fortran. When -fopenmp is specified, the compiler
generates parallel code according to the OpenMP Application Program
Interface v4.0 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
G++.
-ffriend-injection
Inject friend functions into the enclosing namespace, so that they
are visible outside the scope of the class in which they are
declared. Friend functions were documented to work this way in the
old Annotated C++ Reference Manual. However, in ISO C++ a friend
function that is not declared in an enclosing scope can only be
found using argument dependent lookup. GCC defaults to the
standard behavior.
This option is for compatibility, and may be removed in a future
release of G++.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a
temporary that is only used to initialize another object of the
same type. Specifying this option disables that optimization, and
forces G++ to call the copy constructor in all cases.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception
specifications at run time. This option violates the C++ standard,
but may be useful for reducing code size in production builds, much
like defining "NDEBUG". This does not give user code permission to
throw exceptions in violation of the exception specifications; the
compiler still optimizes based on the specifications, so throwing
an unexpected exception results in undefined behavior at run time.
-fextern-tls-init
-fno-extern-tls-init
The C++11 and OpenMP standards allow "thread_local" and
"threadprivate" variables to have dynamic (runtime) initialization.
To support this, any use of such a variable goes through a wrapper
function that performs any necessary initialization. When the use
and definition of the variable are in the same translation unit,
this overhead can be optimized away, but when the use is in a
different translation unit there is significant overhead even if
the variable doesn't actually need dynamic initialization. If the
programmer can be sure that no use of the variable in a non-
defining TU needs to trigger dynamic initialization (either because
the variable is statically initialized, or a use of the variable in
the defining TU will be executed before any uses in another TU),
they can avoid this overhead with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is
-fextern-tls-init. On targets that do not support symbol aliases,
the default is -fno-extern-tls-init.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a
for-init-statement is limited to the "for" loop itself, as
specified by the C++ standard. If -fno-for-scope is specified, the
scope of variables declared in a for-init-statement extends to the
end of the enclosing scope, as was the case in old versions of G++,
and other (traditional) implementations of C++.
If neither flag is given, the default is to follow the standard,
but to allow and give a warning for old-style code that would
otherwise be invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this
word as an identifier. You can use the keyword "__typeof__"
instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates that are instantiated
implicitly (i.e. by use); only emit code for explicit
instantiations.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates,
either. The default is to handle inlines differently so that
compiles with and without optimization need the same set of
explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
controlled by "#pragma implementation". This causes linker errors
if these functions are not inlined everywhere they are called.
-fms-extensions
Disable Wpedantic warnings about constructs used in MFC, such as
implicit int and getting a pointer to member function via non-
standard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C. These include "ffs", "alloca", "_exit", "index",
"bzero", "conjf", and other related functions.
-fnothrow-opt
Treat a "throw()" exception specification as if it were a
"noexcept" specification to reduce or eliminate the text size
overhead relative to a function with no exception specification.
If the function has local variables of types with non-trivial
destructors, the exception specification actually makes the
function smaller because the EH cleanups for those variables can be
optimized away. The semantic effect is that an exception thrown
out of a function with such an exception specification results in a
call to "terminate" rather than "unexpected".
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor",
"compl", "not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need
to issue. Currently, the only such diagnostic issued by G++ is the
one for a name having multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to
warnings. Thus, using -fpermissive allows some nonconforming code
to compile.
-fno-pretty-templates
When an error message refers to a specialization of a function
template, the compiler normally prints the signature of the
template followed by the template arguments and any typedefs or
typenames in the signature (e.g. "void f(T) [with T = int]" rather
than "void f(int)") so that it's clear which template is involved.
When an error message refers to a specialization of a class
template, the compiler omits any template arguments that match the
default template arguments for that template. If either of these
behaviors make it harder to understand the error message rather
than easier, you can use -fno-pretty-templates to disable them.
-frepo
Enable automatic template instantiation at link time. This option
also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual
functions for use by the C++ run-time type identification features
("dynamic_cast" and "typeid"). If you don't use those parts of the
language, you can save some space by using this flag. Note that
exception handling uses the same information, but G++ generates it
as needed. The "dynamic_cast" operator can still be used for casts
that do not require run-time type information, i.e. casts to "void
*" or to unambiguous base classes.
-fsized-deallocation
Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined
replacement deallocation functions that, for example, use the size
of the object to make deallocation faster. Enabled by default
under -std=c++14 and above. The flag -Wsized-deallocation warns
about places that might want to add a definition.
-fstats
Emit statistics about front-end processing at the end of the
compilation. This information is generally only useful to the G++
development team.
-fstrict-enums
Allow the compiler to optimize using the assumption that a value of
enumerated type can only be one of the values of the enumeration
(as defined in the C++ standard; basically, a value that can be
represented in the minimum number of bits needed to represent all
the enumerators). This assumption may not be valid if the program
uses a cast to convert an arbitrary integer value to the enumerated
type.
-ftemplate-backtrace-limit=n
Set the maximum number of template instantiation notes for a single
warning or error to n. The default value is 10.
-ftemplate-depth=n
Set the maximum instantiation depth for template classes to n. A
limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation. ANSI/ISO
C++ conforming programs must not rely on a maximum depth greater
than 17 (changed to 1024 in C++11). The default value is 900, as
the compiler can run out of stack space before hitting 1024 in some
situations.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++
ABI for thread-safe initialization of local statics. You can use
this option to reduce code size slightly in code that doesn't need
to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with
the "__cxa_atexit" function rather than the "atexit" function.
This option is required for fully standards-compliant handling of
static destructors, but only works if your C library supports
"__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This
causes "std::uncaught_exception" to be incorrect, but is necessary
if the runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare
pointers to inline functions or methods where the addresses of the
two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline
methods with "__attribute__ ((visibility ("hidden")))" so that they
do not appear in the export table of a DSO and do not require a PLT
indirection when used within the DSO. Enabling this option can
have a dramatic effect on load and link times of a DSO as it
massively reduces the size of the dynamic export table when the
library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the
methods as hidden directly, because it does not affect static
variables local to the function or cause the compiler to deduce
that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate
the effect of the switch for that method. For example, if you do
want to compare pointers to a particular inline method, you might
mark it as having default visibility. Marking the enclosing class
with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this
option as their linkage might otherwise cross a shared library
boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++
linkage model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like
-fvisibility=hidden.
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.
-fvtable-verify=[std|preinit|none]
Turn on (or off, if using -fvtable-verify=none) the security
feature that verifies at run time, for every virtual call, 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 run time, an error is
reported and execution of the program is immediately halted.
This option causes run-time data structures to be built at program
startup, which are used 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". Using -fvtable-verify=std causes
the data structures to be built after shared libraries have been
loaded and initialized. -fvtable-verify=preinit causes them to be
built before shared libraries have been loaded and initialized.
If this option appears multiple times in the command line with
different values specified, none takes highest priority over both
std and preinit; preinit takes priority over std.
-fvtv-debug
When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, causes debug versions of the runtime
functions for the vtable verification feature to be called. This
flag also causes the compiler to log information about which vtable
pointers it finds for each class. This information is written to a
file named vtv_set_ptr_data.log in the directory named by the
environment variable VTV_LOGS_DIR if that is defined or the current
working directory otherwise.
Note: This feature appends data to the log file. If you want a
fresh log file, be sure to delete any existing one.
-fvtv-counts
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
encounters and the number of verifications it inserts. It also
counts the number of calls to certain run-time library functions
that it inserts and logs this information for each compilation
unit. The compiler writes this information to a file named
vtv_count_data.log in the directory named by the environment
variable VTV_LOGS_DIR if that is defined or the current working
directory otherwise. 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 fresh
log files, be sure to delete any existing ones.
-fno-weak
Do not use weak symbol support, even if it is provided by the
linker. By default, G++ uses weak symbols if they are available.
This option exists only for testing, and should not be used by end-
users; it results in inferior code and has no benefits. This
option may be removed in a future release of G++.
-nostdinc++
Do not search for header files in the standard directories specific
to C++, but do still search the other standard directories. (This
option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:
-Wabi (C, Objective-C, C++ and Objective-C++ only)
When an explicit -fabi-version=n option is used, causes G++ to warn
when it generates code that is probably not compatible with the
vendor-neutral C++ ABI. Since G++ now defaults to -fabi-version=0,
-Wabi has no effect unless either an older ABI version is selected
(with -fabi-version=n) or an older compatibility version is
selected (with -Wabi=n or -fabi-compat-version=n).
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.
-Wabi can also be used with an explicit version number to warn
about compatibility with a particular -fabi-version level, e.g.
-Wabi=2 to warn about changes relative to -fabi-version=2.
Specifying a version number also sets -fabi-compat-version=n.
The known incompatibilities in -fabi-version=2 (which was the
default from GCC 3.4 to 4.9) include:
* A template with a non-type template parameter of reference type
was mangled incorrectly:
extern int N;
template struct S {};
void n (S) {2}
This was fixed in -fabi-version=3.
* SIMD vector types declared using "__attribute ((vector_size))"
were mangled in a non-standard way that does not allow for
overloading of functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
* "__attribute ((const))" and "noreturn" were mangled as type
qualifiers, and "decltype" of a plain declaration was folded
away.
These mangling issues were fixed in -fabi-version=5.
* Scoped enumerators passed as arguments to a variadic function
are promoted like unscoped enumerators, causing "va_arg" to
complain. On most targets this does not actually affect the
parameter passing ABI, as there is no way to pass an argument
smaller than "int".
Also, the ABI changed the mangling of template argument packs,
"const_cast", "static_cast", prefix increment/decrement, and a
class scope function used as a template argument.
These issues were corrected in -fabi-version=6.
* Lambdas in default argument scope were mangled incorrectly, and
the ABI changed the mangling of "nullptr_t".
These issues were corrected in -fabi-version=7.
* When mangling a function type with function-cv-qualifiers, the
un-qualified function type was incorrectly treated as a
substitution candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
* "decltype(nullptr)" incorrectly had an alignment of 1, leading
to unaligned accesses. Note that this did not affect the ABI
of a function with a "nullptr_t" parameter, as parameters have
a minimum alignment.
This was fixed in -fabi-version=9, the default for GCC 5.2.
It also warns about psABI-related changes. The known psABI changes
at this point include:
* For SysV/x86-64, unions with "long double" members are passed
in memory as specified in psABI. For example:
union U {
long double ld;
int i;
};
"union U" is always passed in memory.
-Wabi-tag (C++ and Objective-C++ only)
Warn when a type with an ABI tag is used in a context that does not
have that ABI tag. See C++ Attributes for more information about
ABI tags.
-Wctor-dtor-privacy (C++ and Objective-C++ only)
Warn when a class seems unusable because all the constructors or
destructors in that class are private, and it has neither friends
nor public static member functions. Also warn if there are no non-
private methods, and there's at least one private member function
that isn't a constructor or destructor.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when "delete" is used to destroy an instance of a class that
has virtual functions and non-virtual destructor. It is unsafe to
delete an instance of a derived class through a pointer to a base
class if the base class does not have a virtual destructor. This
warning is enabled by -Wall.
-Wliteral-suffix (C++ and Objective-C++ only)
Warn when a string or character literal is followed by a ud-suffix
which does not begin with an underscore. As a conforming
extension, GCC treats such suffixes as separate preprocessing
tokens in order to maintain backwards compatibility with code that
uses formatting macros from "". For example:
#define __STDC_FORMAT_MACROS
#include
#include
int main() {
int64_t i64 = 123;
printf("My int64: %"PRId64"\n", i64);
}
In this case, "PRId64" is treated as a separate preprocessing
token.
This warning is enabled by default.
-Wnarrowing (C++ and Objective-C++ only)
Warn when a narrowing conversion prohibited by C++11 occurs within
{ }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
With -std=c++11, -Wno-narrowing suppresses the diagnostic required
by the standard. Note that this does not affect the meaning of
well-formed code; narrowing conversions are still considered ill-
formed in SFINAE context.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false because of a
call to a function that does not have a non-throwing exception
specification (i.e. "throw()" or "noexcept") but is known by the
compiler to never throw an exception.
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and an accessible non-
virtual destructor itself or in an accessible polymorphic base
class, in which case it is possible but unsafe to delete an
instance of a derived class through a pointer to the class itself
or base class. This warning is automatically enabled if -Weffc++
is specified.
-Wreorder (C++ and Objective-C++ only)
Warn when the order of member initializers given in the code does
not match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler rearranges the member initializers for "i" and "j" to
match the declaration order of the members, emitting a warning to
that effect. This warning is enabled by -Wall.
-fext-numeric-literals (C++ and Objective-C++ only)
Accept imaginary, fixed-point, or machine-defined literal number
suffixes as GNU extensions. When this option is turned off these
suffixes are treated as C++11 user-defined literal numeric
suffixes. This is on by default for all pre-C++11 dialects and all
GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14.
This option is off by default for ISO C++11 onwards (-std=c++11,
...).
The following -W... options are not affected by -Wall.
-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from Scott
Meyers' Effective C++ series of books:
* Define a copy constructor and an assignment operator for
classes with dynamically-allocated memory.
* Prefer initialization to assignment in constructors.
* Have "operator=" return a reference to *this.
* Don't try to return a reference when you must return an object.
* Distinguish between prefix and postfix forms of increment and
decrement operators.
* Never overload "&&", "||", or ",".
This option also enables -Wnon-virtual-dtor, which is also one of
the effective C++ recommendations. However, the check is extended
to warn about the lack of virtual destructor in accessible non-
polymorphic bases classes too.
When selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use grep -v to filter
out those warnings.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn about the use of an uncasted "NULL" as sentinel. When
compiling only with GCC this is a valid sentinel, as "NULL" is
defined to "__null". Although it is a null pointer constant rather
than a null pointer, it is guaranteed to be of the same size as a
pointer. But this use is not portable across different compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-templatized friend functions are declared
within a template. Since the advent of explicit template
specification support in G++, if the name of the friend is an
unqualified-id (i.e., friend foo(int)), the C++ language
specification demands that the friend declare or define an
ordinary, nontemplate function. (Section 14.5.3). Before G++
implemented explicit specification, unqualified-ids could be
interpreted as a particular specialization of a templatized
function. Because this non-conforming behavior is no longer the
default behavior for G++, -Wnon-template-friend allows the compiler
to check existing code for potential trouble spots and is on by
default. This new compiler behavior can be turned off with
-Wno-non-template-friend, which keeps the conformant compiler code
but disables the helpful warning.
-Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is used
within a C++ program. The new-style casts ("dynamic_cast",
"static_cast", "reinterpret_cast", and "const_cast") are less
vulnerable to unintended effects and much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only)
Warn when a function declaration hides virtual functions from a
base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
fails to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
-Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from unsigned or
enumerated type to a signed type, over a conversion to an unsigned
type of the same size. Previous versions of G++ tried to preserve
unsignedness, but the standard mandates the current behavior.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++
languages themselves.
This section describes the command-line options that are only
meaningful for Objective-C and Objective-C++ programs. You can also
use most of the language-independent GNU compiler options. For
example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C
and Objective-C++ programs; you can use the other options with any
language supported by GCC.
Note that since Objective-C is an extension of the C language,
Objective-C compilations may also use options specific to the C front-
end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may
use C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and
Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each
literal string specified with the syntax "@"..."". The default
class name is "NXConstantString" if the GNU runtime is being used,
and "NSConstantString" if the NeXT runtime is being used (see
below). The -fconstant-cfstrings option, if also present,
overrides the -fconstant-string-class setting and cause "@"...""
literals to be laid out as constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C
runtime. This is the default for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the
default for NeXT-based systems, including Darwin and Mac OS X. The
macro "__NEXT_RUNTIME__" is predefined if (and only if) this option
is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches ("[receiver
message:arg]") in this translation unit ensure that the receiver is
not "nil". This allows for more efficient entry points in the
runtime to be used. This option is only available in conjunction
with the NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected runtime.
This option is currently supported only for the NeXT runtime. In
that case, Version 0 is the traditional (32-bit) ABI without
support for properties and other Objective-C 2.0 additions.
Version 1 is the traditional (32-bit) ABI with support for
properties and other Objective-C 2.0 additions. Version 2 is the
modern (64-bit) ABI. If nothing is specified, the default is
Version 0 on 32-bit target machines, and Version 2 on 64-bit target
machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables
is a C++ object with a non-trivial default constructor. If so,
synthesize a special "- (id) .cxx_construct" instance method which
runs non-trivial default constructors on any such instance
variables, in order, and then return "self". Similarly, check if
any instance variable is a C++ object with a non-trivial
destructor, and if so, synthesize a special "- (void)
.cxx_destruct" method which runs all such default destructors, in
reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods
thusly generated only operate on instance variables declared in the
current Objective-C class, and not those inherited from
superclasses. It is the responsibility of the Objective-C runtime
to invoke all such methods in an object's inheritance hierarchy.
The "- (id) .cxx_construct" methods are invoked by the runtime
immediately after a new object instance is allocated; the "- (void)
.cxx_destruct" methods are invoked immediately before the runtime
deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and
later has support for invoking the "- (id) .cxx_construct" and "-
(void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is
accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++ and Java. This
option is required to use the Objective-C keywords @try, @throw,
@catch, @finally and @synchronized. This option is available with
both the GNU runtime and the NeXT runtime (but not available in
conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++
programs. This option is only available with the NeXT runtime; the
GNU runtime has a different garbage collection implementation that
does not require special compiler flags.
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a nil
receiver in method invocations before doing the actual method call.
This is the default and can be disabled using -fno-objc-nilcheck.
Class methods and super calls are never checked for nil in this way
no matter what this flag is set to. Currently this flag does
nothing when the GNU runtime, or an older version of the NeXT
runtime ABI, is used.
-fobjc-std=objc1
Conform to the language syntax of Objective-C 1.0, the language
recognized by GCC 4.0. This only affects the Objective-C additions
to the C/C++ language; it does not affect conformance to C/C++
standards, which is controlled by the separate C/C++ dialect option
flags. When this option is used with the Objective-C or
Objective-C++ compiler, any Objective-C syntax that is not
recognized by GCC 4.0 is rejected. This is useful if you need to
make sure that your Objective-C code can be compiled with older
versions of GCC.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in
the resulting object file, and allow dyld(1) to load it in at run
time instead. This is used in conjunction with the Fix-and-
Continue debugging mode, where the object file in question may be
recompiled and dynamically reloaded in the course of program
execution, without the need to restart the program itself.
Currently, Fix-and-Continue functionality is only available in
conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily
replaces calls to "objc_getClass("...")" (when the name of the
class is known at compile time) with static class references that
get initialized at load time, which improves run-time performance.
Specifying the -fzero-link flag suppresses this behavior and causes
calls to "objc_getClass("...")" to be retained. This is useful in
Zero-Link debugging mode, since it allows for individual class
implementations to be modified during program execution. The GNU
runtime currently always retains calls to "objc_get_class("...")"
regardless of command-line options.
-fno-local-ivars
By default instance variables in Objective-C can be accessed as if
they were local variables from within the methods of the class
they're declared in. This can lead to shadowing between instance
variables and other variables declared either locally inside a
class method or globally with the same name. Specifying the
-fno-local-ivars flag disables this behavior thus avoiding variable
shadowing issues.
-fivar-visibility=[public|protected|private|package]
Set the default instance variable visibility to the specified
option so that instance variables declared outside the scope of any
access modifier directives default to the specified visibility.
-gen-decls
Dump interface declarations for all classes seen in the source file
to a file named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only)
Warn whenever an Objective-C assignment is being intercepted by the
garbage collector.
-Wno-protocol (Objective-C and Objective-C++ only)
If a class is declared to implement a protocol, a warning is issued
for every method in the protocol that is not implemented by the
class. The default behavior is to issue a warning for every method
not explicitly implemented in the class, even if a method
implementation is inherited from the superclass. If you use the
-Wno-protocol option, then methods inherited from the superclass
are considered to be implemented, and no warning is issued for
them.
-Wselector (Objective-C and Objective-C++ only)
Warn if multiple methods of different types for the same selector
are found during compilation. The check is performed on the list
of methods in the final stage of compilation. Additionally, a
check is performed for each selector appearing in a
"@selector(...)" expression, and a corresponding method for that
selector has been found during compilation. Because these checks
scan the method table only at the end of compilation, these
warnings are not produced if the final stage of compilation is not
reached, for example because an error is found during compilation,
or because the -fsyntax-only option is being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
Warn if multiple methods with differing argument and/or return
types are found for a given selector when attempting to send a
message using this selector to a receiver of type "id" or "Class".
When this flag is off (which is the default behavior), the compiler
omits such warnings if any differences found are confined to types
that share the same size and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
Warn if a "@selector(...)" expression referring to an undeclared
selector is found. A selector is considered undeclared if no
method with that name has been declared before the "@selector(...)"
expression, either explicitly in an @interface or @protocol
declaration, or implicitly in an @implementation section. This
option always performs its checks as soon as a "@selector(...)"
expression is found, while -Wselector only performs its checks in
the final stage of compilation. This also enforces the coding
style convention that methods and selectors must be declared before
being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed
by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of
the output device's aspect (e.g. its width, ...). You can use the
options described below to control the formatting algorithm for
diagnostic messages, e.g. how many characters per line, how often
source location information should be reported. Note that some
language front ends may not honor these options.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n
characters. If n is zero, then no line-wrapping is done; each
error message appears on a single line. This is the default for
all front ends.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit source location information once; that
is, in case the message is too long to fit on a single physical
line and has to be wrapped, the source location won't be emitted
(as prefix) again, over and over, in subsequent continuation lines.
This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location information (as
prefix) for physical lines that result from the process of breaking
a message which is too long to fit on a single line.
-fdiagnostics-color[=WHEN]
-fno-diagnostics-color
Use color in diagnostics. WHEN is never, always, or auto. The
default depends on how the compiler has been configured, it can be
any of the above WHEN options or also never if GCC_COLORS
environment variable isn't present in the environment, and auto
otherwise. auto means to use color only when the standard error is
a terminal. The forms -fdiagnostics-color and
-fno-diagnostics-color are aliases for -fdiagnostics-color=always
and -fdiagnostics-color=never, respectively.
The colors are defined by the environment variable GCC_COLORS. Its
value is a colon-separated list of capabilities and Select Graphic
Rendition (SGR) substrings. SGR commands are interpreted by the
terminal or terminal emulator. (See the section in the
documentation of your text terminal for permitted values and their
meanings as character attributes.) These substring values are
integers in decimal representation and can be concatenated with
semicolons. Common values to concatenate include 1 for bold, 4 for
underline, 5 for blink, 7 for inverse, 39 for default foreground
color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode
foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color
modes foreground colors, 49 for default background color, 40 to 47
for background colors, 100 to 107 for 16-color mode background
colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes
background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01
where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan,
01;32 is bold green and 01 is bold. Setting GCC_COLORS to the empty
string disables colors. Supported capabilities are as follows.
"error="
SGR substring for error: markers.
"warning="
SGR substring for warning: markers.
"note="
SGR substring for note: markers.
"caret="
SGR substring for caret line.
"locus="
SGR substring for location information, file:line or
file:line:column etc.
"quote="
SGR substring for information printed within quotes.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the
command-line option that directly controls the diagnostic (if such
an option is known to the diagnostic machinery). Specifying the
-fno-diagnostics-show-option flag suppresses that behavior.
-fno-diagnostics-show-caret
By default, each diagnostic emitted includes the original source
line and a caret '^' indicating the column. This option suppresses
this information. The source line is truncated to n characters, if
the -fmessage-length=n option is given. When the output is done to
the terminal, the width is limited to the width given by the
COLUMNS environment variable or, if not set, to the terminal width.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are not
inherently erroneous but that are risky or suggest there may have been
an error.
The following language-independent options do not enable specific
warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond
that.
-fmax-errors=n
Limits the maximum number of error messages to n, at which point
GCC bails out rather than attempting to continue processing the
source code. If n is 0 (the default), there is no limit on the
number of error messages produced. If -Wfatal-errors is also
specified, then -Wfatal-errors takes precedence over this option.
-w Inhibit all warning messages.
-Werror
Make all warnings into errors.
-Werror=
Make the specified warning into an error. The specifier for a
warning is appended; for example -Werror=switch turns the warnings
controlled by -Wswitch into errors. This switch takes a negative
form, to be used to negate -Werror for specific warnings; for
example -Wno-error=switch makes -Wswitch warnings not be errors,
even when -Werror is in effect.
The warning message for each controllable warning includes the
option that controls the warning. That option can then be used
with -Werror= and -Wno-error= as described above. (Printing of the
option in the warning message can be disabled using the
-fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo.
However, -Wno-error=foo does not imply anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the first
error occurred rather than trying to keep going and printing
further error messages.
You can request many specific warnings with options beginning with -W,
for example -Wimplicit to request warnings on implicit declarations.
Each of these specific warning options also has a negative form
beginning -Wno- to turn off warnings; for example, -Wno-implicit. This
manual lists only one of the two forms, whichever is not the default.
For further language-specific options also refer to C++ Dialect Options
and Objective-C and Objective-C++ Dialect Options.
Some options, such as -Wall and -Wextra, turn on other options, such as
-Wunused, which may turn on further options, such as -Wunused-value.
The combined effect of positive and negative forms is that more
specific options have priority over less specific ones, independently
of their position in the command-line. For options of the same
specificity, the last one takes effect. Options enabled or disabled via
pragmas take effect as if they appeared at the end of the command-line.
When an unrecognized warning option is requested (e.g.,
-Wunknown-warning), GCC emits a diagnostic stating that the option is
not recognized. However, if the -Wno- form is used, the behavior is
slightly different: no diagnostic is produced for -Wno-unknown-warning
unless other diagnostics are being produced. This allows the use of
new -Wno- options with old compilers, but if something goes wrong, the
compiler warns that an unrecognized option is present.
-Wpedantic
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject
all programs that use forbidden extensions, and some other programs
that do not follow ISO C and ISO C++. For ISO C, follows the
version of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or
without this option (though a rare few require -ansi or a -std
option specifying the required version of ISO C). However, without
this option, certain GNU extensions and traditional C and C++
features are supported as well. With this option, they are
rejected.
-Wpedantic does not cause warning messages for use of the alternate
keywords whose names begin and end with __. Pedantic warnings are
also disabled in the expression that follows "__extension__".
However, only system header files should use these escape routes;
application programs should avoid them.
Some users try to use -Wpedantic to check programs for strict ISO C
conformance. They soon find that it does not do quite what they
want: it finds some non-ISO practices, but not all---only those for
which ISO C requires a diagnostic, and some others for which
diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful
in some instances, but would require considerable additional work
and would be quite different from -Wpedantic. We don't have plans
to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended
dialect of C, such as gnu90 or gnu99, there is a corresponding base
standard, the version of ISO C on which the GNU extended dialect is
based. Warnings from -Wpedantic are given where they are required
by the base standard. (It does not make sense for such warnings to
be given only for features not in the specified GNU C dialect,
since by definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be nothing
to warn about.)
-pedantic-errors
Give an error whenever the base standard (see -Wpedantic) requires
a diagnostic, in some cases where there is undefined behavior at
compile-time and in some other cases that do not prevent
compilation of programs that are valid according to the standard.
This is not equivalent to -Werror=pedantic, since there are errors
enabled by this option and not enabled by the latter and vice
versa.
-Wall
This enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify to
prevent the warning), even in conjunction with macros. This also
enables some language-specific warnings described in C++ Dialect
Options and Objective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds=1 (only with -O2) -Wc++11-compat
-Wc++14-compat -Wchar-subscripts -Wenum-compare (in C/ObjC; this is
on by default in C++) -Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only) -Wcomment
-Wformat -Wmain (only for C/ObjC and unless -ffreestanding)
-Wmaybe-uninitialized -Wmissing-braces (only for C/ObjC) -Wnonnull
-Wopenmp-simd -Wparentheses -Wpointer-sign -Wreorder -Wreturn-type
-Wsequence-point -Wsign-compare (only in C++) -Wstrict-aliasing
-Wstrict-overflow=1 -Wswitch -Wtrigraphs -Wuninitialized
-Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value
-Wunused-variable -Wvolatile-register-var
Note that some warning flags are not implied by -Wall. Some of
them warn about constructions that users generally do not consider
questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid
in some cases, and there is no simple way to modify the code to
suppress the warning. Some of them are enabled by -Wextra but many
of them must be enabled individually.
-Wextra
This enables some extra warning flags that are not enabled by
-Wall. (This option used to be called -W. The older name is still
supported, but the newer name is more descriptive.)
-Wclobbered -Wempty-body -Wignored-qualifiers
-Wmissing-field-initializers -Wmissing-parameter-type (C only)
-Wold-style-declaration (C only) -Woverride-init -Wsign-compare
-Wtype-limits -Wuninitialized -Wunused-parameter (only with
-Wunused or -Wall) -Wunused-but-set-parameter (only with -Wunused
or -Wall)
The option -Wextra also prints warning messages for the following
cases:
* A pointer is compared against integer zero with "<", "<=", ">",
or ">=".
* (C++ only) An enumerator and a non-enumerator both appear in a
conditional expression.
* (C++ only) Ambiguous virtual bases.
* (C++ only) Subscripting an array that has been declared
"register".
* (C++ only) Taking the address of a variable that has been
declared "register".
* (C++ only) A base class is not initialized in a derived class's
copy constructor.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause
of error, as programmers often forget that this type is signed on
some machines. This warning is enabled by -Wall.
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment,
or whenever a Backslash-Newline appears in a // comment. This
warning is enabled by -Wall.
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the -fprofile-use
option. If a source file is changed between compiling with
-fprofile-gen and with -fprofile-use, the files with the profile
feedback can fail to match the source file and GCC cannot use the
profile feedback information. By default, this warning is enabled
and is treated as an error. -Wno-coverage-mismatch can be used to
disable the warning or -Wno-error=coverage-mismatch can be used to
disable the error. Disabling the error for this warning can result
in poorly optimized code and is useful only in the case of very
minor changes such as bug fixes to an existing code-base.
Completely disabling the warning is not recommended.
-Wno-cpp
(C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Give a warning when a value of type "float" is implicitly promoted
to "double". CPUs with a 32-bit "single-precision" floating-point
unit implement "float" in hardware, but emulate "double" in
software. On such a machine, doing computations using "double"
values is much more expensive because of the overhead required for
software emulation.
It is easy to accidentally do computations with "double" because
floating-point literals are implicitly of type "double". For
example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler performs the entire computation with "double" because
the floating-point literal is a "double".
-Wformat
-Wformat=n
Check calls to "printf" and "scanf", etc., to make sure that the
arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string
make sense. This includes standard functions, and others specified
by format attributes, in the "printf", "scanf", "strftime" and
"strfmon" (an X/Open extension, not in the C standard) families (or
other target-specific families). Which functions are checked
without format attributes having been specified depends on the
standard version selected, and such checks of functions without the
attribute specified are disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by
GNU libc version 2.2. These include all ISO C90 and C99 features,
as well as features from the Single Unix Specification and some BSD
and GNU extensions. Other library implementations may not support
all these features; GCC does not support warning about features
that go beyond a particular library's limitations. However, if
-Wpedantic is used with -Wformat, warnings are given about format
features not in the selected standard version (but not for
"strfmon" formats, since those are not in any version of the C
standard).
-Wformat=1
-Wformat
Option -Wformat is equivalent to -Wformat=1, and -Wno-format is
equivalent to -Wformat=0. Since -Wformat also checks for null
format arguments for several functions, -Wformat also implies
-Wnonnull. Some aspects of this level of format checking can
be disabled by the options: -Wno-format-contains-nul,
-Wno-format-extra-args, and -Wno-format-zero-length. -Wformat
is enabled by -Wall.
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format strings that
contain NUL bytes.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to
a "printf" or "scanf" format function. The C standard
specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with $ operand number specifications, normally
warnings are still given, since the implementation could not
know what type to pass to "va_arg" to skip the unused
arguments. However, in the case of "scanf" formats, this
option suppresses the warning if the unused arguments are all
pointers, since the Single Unix Specification says that such
unused arguments are allowed.
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length
formats. The C standard specifies that zero-length formats are
allowed.
-Wformat=2
Enable -Wformat plus additional format checks. Currently
equivalent to -Wformat -Wformat-nonliteral -Wformat-security
-Wformat-y2k.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not
a string literal and so cannot be checked, unless the format
function takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format
functions that represent possible security problems. At
present, this warns about calls to "printf" and "scanf"
functions where the format string is not a string literal and
there are no format arguments, as in "printf (foo);". This may
be a security hole if the format string came from untrusted
input and contains %n. (This is currently a subset of what
-Wformat-nonliteral warns about, but in future warnings may be
added to -Wformat-security that are not included in
-Wformat-nonliteral.)
-Wformat-signedness
If -Wformat is specified, also warn if the format string
requires an unsigned argument and the argument is signed and
vice versa.
NOTE: In Ubuntu 8.10 and later versions this option is enabled
by default for C, C++, ObjC, ObjC++. To disable, use
-Wno-format-security, or disable all format warnings with
-Wformat=0. To make format security warnings fatal, specify
-Werror=format-security.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats
that may yield only a two-digit year.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring
a non-null value by the "nonnull" function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled
with the -Wno-nonnull option.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables that are initialized with
themselves. Note this option can only be used with the
-Wuninitialized option.
For example, GCC warns about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
This warning is enabled by -Wall in C++.
-Wimplicit-int (C and Objective-C only)
Warn when a declaration does not specify a type. This warning is
enabled by -Wall.
-Wimplicit-function-declaration (C and Objective-C only)
Give a warning whenever a function is used before being declared.
In C99 mode (-std=c99 or -std=gnu99), this warning is enabled by
default and it is made into an error by -pedantic-errors. This
warning is also enabled by -Wall.
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and -Wimplicit-function-declaration. This
warning is enabled by -Wall.
-Wignored-qualifiers (C and C++ only)
Warn if the return type of a function has a type qualifier such as
"const". For ISO C such a type qualifier has no effect, since the
value returned by a function is not an lvalue. For C++, the
warning is only emitted for scalar types or "void". ISO C
prohibits qualified "void" return types on function definitions, so
such return types always receive a warning even without this
option.
This warning is also enabled by -Wextra.
-Wmain
Warn if the type of "main" is suspicious. "main" should be a
function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types. This
warning is enabled by default in C++ and is enabled by either -Wall
or -Wpedantic.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed.
In the following example, the initializer for "a" is not fully
bracketed, but that for "b" is fully bracketed. This warning is
enabled by -Wall in C.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is
expected, or when operators are nested whose precedence people
often get confused about.
Also warn if a comparison like "x<=y<=z" appears; this is
equivalent to "(x<=y ? 1 : 0) <= z", which is a different
interpretation from that of ordinary mathematical notation.
Also warn about constructions where there may be confusion to which
"if" statement an "else" branch belongs. Here is an example of
such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost possible
"if" statement, which in this example is "if (b)". This is often
not what the programmer expected, as illustrated in the above
example by indentation the programmer chose. When there is the
potential for this confusion, GCC issues a warning when this flag
is specified. To eliminate the warning, add explicit braces around
the innermost "if" statement so there is no way the "else" can
belong to the enclosing "if". The resulting code looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
Also warn for dangerous uses of the GNU extension to "?:" with
omitted middle operand. When the condition in the "?": operator is
a boolean expression, the omitted value is always 1. Often
programmers expect it to be a value computed inside the conditional
expression instead.
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of
violations of sequence point rules in the C and C++ standards.
The C and C++ standards define the order in which expressions in a
C/C++ program are evaluated in terms of sequence points, which
represent a partial ordering between the execution of parts of the
program: those executed before the sequence point, and those
executed after it. These occur after the evaluation of a full
expression (one which is not part of a larger expression), after
the evaluation of the first operand of a "&&", "||", "? :" or ","
(comma) operator, before a function is called (but after the
evaluation of its arguments and the expression denoting the called
function), and in certain other places. Other than as expressed by
the sequence point rules, the order of evaluation of subexpressions
of an expression is not specified. All these rules describe only a
partial order rather than a total order, since, for example, if two
functions are called within one expression with no sequence point
between them, the order in which the functions are called is not
specified. However, the standards committee have ruled that
function calls do not overlap.
It is not specified when between sequence points modifications to
the values of objects take effect. Programs whose behavior depends
on this have undefined behavior; the C and C++ standards specify
that "Between the previous and next sequence point an object shall
have its stored value modified at most once by the evaluation of an
expression. Furthermore, the prior value shall be read only to
determine the value to be stored.". If a program breaks these
rules, the results on any particular implementation are entirely
unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] =
b[n++]" and "a[i++] = i;". Some more complicated cases are not
diagnosed by this option, and it may give an occasional false
positive result, but in general it has been found fairly effective
at detecting this sort of problem in programs.
The standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle
cases. Links to discussions of the problem, including proposed
formal definitions, may be found on the GCC readings page, at
.
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr
Do not warn about returning a pointer (or in C++, a reference) to a
variable that goes out of scope after the function returns.
-Wreturn-type
Warn whenever a function is defined with a return type that
defaults to "int". Also warn about any "return" statement with no
return value in a function whose return type is not "void" (falling
off the end of the function body is considered returning without a
value), and about a "return" statement with an expression in a
function whose return type is "void".
For C++, a function without return type always produces a
diagnostic message, even when -Wno-return-type is specified. The
only exceptions are "main" and functions defined in system headers.
This warning is enabled by -Wall.
-Wshift-count-negative
Warn if shift count is negative. This warning is enabled by
default.
-Wshift-count-overflow
Warn if shift count >= width of type. This warning is enabled by
default.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type
and lacks a "case" for one or more of the named codes of that
enumeration. (The presence of a "default" label prevents this
warning.) "case" labels outside the enumeration range also provoke
warnings when this option is used (even if there is a "default"
label). This warning is enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type
and lacks a "case" for one or more of the named codes of that
enumeration. "case" labels outside the enumeration range also
provoke warnings when this option is used. The only difference
between -Wswitch and this option is that this option gives a
warning about an omitted enumeration code even if there is a
"default" label.
-Wswitch-bool
Warn whenever a "switch" statement has an index of boolean type.
It is possible to suppress this warning by casting the controlling
expression to a type other than "bool". For example:
switch ((int) (a == 4))
{
...
}
This warning is enabled by default for C and C++ programs.
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch"
built-in functions are used. These functions changed semantics in
GCC 4.4.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning
of the program (trigraphs within comments are not warned about).
This warning is enabled by -Wall.
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but otherwise
unused (aside from its declaration).
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise unused
(aside from its declaration). This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is enabled by
-Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a
non-inline static function is unused. This warning is enabled by
-Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is
enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
Warn when a typedef locally defined in a function is not used.
This warning is enabled by -Wall.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its
declaration.
To suppress this warning use the "unused" attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with attribute
"warn_unused_result" does not use its return value. The default is
-Wunused-result.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is
unused aside from its declaration. This warning is enabled by
-Wall.
To suppress this warning use the "unused" attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not
used. To suppress this warning cast the unused expression to
"void". This includes an expression-statement or the left-hand side
of a comma expression that contains no side effects. For example,
an expression such as "x[i,j]" causes a warning, while
"x[(void)i,j]" does not.
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you
must either specify -Wextra -Wunused (note that -Wall implies
-Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being
initialized or if a variable may be clobbered by a "setjmp" call.
In C++, warn if a non-static reference or non-static "const" member
appears in a class without constructors.
If you want to warn about code that uses the uninitialized value of
the variable in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered
elements of structure, union or array variables as well as for
variables that are uninitialized or clobbered as a whole. They do
not occur for variables or elements declared "volatile". Because
these warnings depend on optimization, the exact variables or
elements for which there are warnings depends on the precise
optimization options and version of GCC used.
Note that there may be no warning about a variable that is used
only to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the
warnings are printed.
-Wmaybe-uninitialized
For an automatic variable, if there exists a path from the function
entry to a use of the variable that is initialized, but there exist
some other paths for which the variable is not initialized, the
compiler emits a warning if it cannot prove the uninitialized paths
are not executed at run time. These warnings are made optional
because GCC is not smart enough to see all the reasons why the code
might be correct in spite of appearing to have an error. Here is
one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always
initialized, but GCC doesn't know this. To suppress the warning,
you need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might
be changed by a call to "longjmp". These warnings as well are
possible only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know where
"longjmp" will be called; in fact, a signal handler could call it
at any point in the code. As a result, you may get a warning even
when there is in fact no problem because "longjmp" cannot in fact
be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the
functions you use that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas
Warn when a "#pragma" directive is encountered that is not
understood by GCC. If this command-line option is used, warnings
are even issued for unknown pragmas in system header files. This
is not the case if the warnings are only enabled by the -Wall
command-line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters,
invalid syntax, or conflicts between pragmas. See also
-Wunknown-pragmas.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It
warns about code that might break the strict aliasing rules that
the compiler is using for optimization. The warning does not catch
all cases, but does attempt to catch the more common pitfalls. It
is included in -Wall. It is equivalent to -Wstrict-aliasing=3
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active. It
warns about code that might break the strict aliasing rules that
the compiler is using for optimization. Higher levels correspond
to higher accuracy (fewer false positives). Higher levels also
correspond to more effort, similar to the way -O works.
-Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful
when higher levels do not warn but -fstrict-aliasing still breaks
the code, as it has very few false negatives. However, it has many
false positives. Warns for all pointer conversions between
possibly incompatible types, even if never dereferenced. Runs in
the front end only.
Level 2: Aggressive, quick, not too precise. May still have many
false positives (not as many as level 1 though), and few false
negatives (but possibly more than level 1). Unlike level 1, it
only warns when an address is taken. Warns about incomplete types.
Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few false
positives and few false negatives. Slightly slower than levels 1
or 2 when optimization is enabled. Takes care of the common
pun+dereference pattern in the front end: "*(int*)&some_float". If
optimization is enabled, it also runs in the back end, where it
deals with multiple statement cases using flow-sensitive points-to
information. Only warns when the converted pointer is
dereferenced. Does not warn about incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when -fstrict-overflow is active. It
warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it does
not warn about all cases where the code might overflow: it only
warns about cases where the compiler implements some optimization.
Thus this warning depends on the optimization level.
An optimization that assumes that signed overflow does not occur is
perfectly safe if the values of the variables involved are such
that overflow never does, in fact, occur. Therefore this warning
can easily give a false positive: a warning about code that is not
actually a problem. To help focus on important issues, several
warning levels are defined. No warnings are issued for the use of
undefined signed overflow when estimating how many iterations a
loop requires, in particular when determining whether a loop will
be executed at all.
-Wstrict-overflow=1
Warn about cases that are both questionable and easy to avoid.
For example, with -fstrict-overflow, the compiler simplifies
"x + 1 > x" to 1. This level of -Wstrict-overflow is enabled
by -Wall; higher levels are not, and must be explicitly
requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified to
a constant. For example: "abs (x) >= 0". This can only be
simplified when -fstrict-overflow is in effect, because "abs
(INT_MIN)" overflows to "INT_MIN", which is less than zero.
-Wstrict-overflow (with no level) is the same as
-Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified.
For example: "x + 1 > 1" is simplified to "x > 0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above
cases. For example: "(x * 10) / 5" is simplified to "x * 2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude
of a constant involved in a comparison. For example: "x + 2 >
y" is simplified to "x + 1 >= y". This is reported only at the
highest warning level because this simplification applies to
many comparisons, so this warning level gives a very large
number of false positives.
-Wsuggest-attribute=[pure|const|noreturn|format]
Warn for cases where adding an attribute may be beneficial. The
attributes currently supported are listed below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
Warn about functions that might be candidates for attributes
"pure", "const" or "noreturn". The compiler only warns for
functions visible in other compilation units or (in the case of
"pure" and "const") if it cannot prove that the function
returns normally. A function returns normally if it doesn't
contain an infinite loop or return abnormally by throwing,
calling "abort" or trapping. This analysis requires option
-fipa-pure-const, which is enabled by default at -O and higher.
Higher optimization levels improve the accuracy of the
analysis.
-Wsuggest-attribute=format
-Wmissing-format-attribute
Warn about function pointers that might be candidates for
"format" attributes. Note these are only possible candidates,
not absolute ones. GCC guesses that function pointers with
"format" attributes that are used in assignment,
initialization, parameter passing or return statements should
have a corresponding "format" attribute in the resulting type.
I.e. the left-hand side of the assignment or initialization,
the type of the parameter variable, or the return type of the
containing function respectively should also have a "format"
attribute to avoid the warning.
GCC also warns about function definitions that might be
candidates for "format" attributes. Again, these are only
possible candidates. GCC guesses that "format" attributes
might be appropriate for any function that calls a function
like "vprintf" or "vscanf", but this might not always be the
case, and some functions for which "format" attributes are
appropriate may not be detected.
-Wsuggest-final-types
Warn about types with virtual methods where code quality would be
improved if the type were declared with the C++11 "final"
specifier, or, if possible, declared in an anonymous namespace.
This allows GCC to more aggressively devirtualize the polymorphic
calls. This warning is more effective with link time optimization,
where the information about the class hierarchy graph is more
complete.
-Wsuggest-final-methods
Warn about virtual methods where code quality would be improved if
the method were declared with the C++11 "final" specifier, or, if
possible, its type were declared in an anonymous namespace or with
the "final" specifier. This warning is more effective with link
time optimization, where the information about the class hierarchy
graph is more complete. It is recommended to first consider
suggestions of -Wsuggest-final-types and then rebuild with new
annotations.
-Wsuggest-override
Warn about overriding virtual functions that are not marked with
the override keyword.
-Warray-bounds
-Warray-bounds=n
This option is only active when -ftree-vrp is active (default for
-O2 and above). It warns about subscripts to arrays that are always
out of bounds. This warning is enabled by -Wall.
-Warray-bounds=1
This is the warning level of -Warray-bounds and is enabled by
-Wall; higher levels are not, and must be explicitly requested.
-Warray-bounds=2
This warning level also warns about out of bounds access for
arrays at the end of a struct and for arrays accessed through
pointers. This warning level may give a larger number of false
positives and is deactivated by default.
-Wbool-compare
Warn about boolean expression compared with an integer value
different from "true"/"false". For instance, the following
comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }
This warning is enabled by -Wall.
-Wno-discarded-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on pointers are being discarded.
Typically, the compiler warns if a "const char *" variable is
passed to a function that takes a "char *" parameter. This option
can be used to suppress such a warning.
-Wno-discarded-array-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on arrays which are pointer targets
are being discarded. Typically, the compiler warns if a "const int
(*)[]" variable is passed to a function that takes a "int (*)[]"
parameter. This option can be used to suppress such a warning.
-Wno-incompatible-pointer-types (C and Objective-C only)
Do not warn when there is a conversion between pointers that have
incompatible types. This warning is for cases not covered by
-Wno-pointer-sign, which warns for pointer argument passing or
assignment with different signedness.
-Wno-int-conversion (C and Objective-C only)
Do not warn about incompatible integer to pointer and pointer to
integer conversions. This warning is about implicit conversions;
for explicit conversions the warnings -Wno-int-to-pointer-cast and
-Wno-pointer-to-int-cast may be used.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating-
point division by zero is not warned about, as it can be a
legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the
assumption that they usually do not indicate real problems and
would only make the compiler output harder to read. Using this
command-line option tells GCC to emit warnings from system headers
as if they occurred in user code. However, note that using -Wall
in conjunction with this option does not warn about unknown pragmas
in system headers---for that, -Wunknown-pragmas must also be used.
-Wtrampolines
Warn about trampolines generated for pointers to nested functions.
A trampoline is a small piece of data or code that is created at
run time on the stack when the address of a nested function is
taken, and is used to call the nested function indirectly. For
some targets, it is made up of data only and thus requires no
special treatment. But, for most targets, it is made up of code
and thus requires the stack to be made executable in order for the
program to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers. If you are doing this, then you
need to compute (by analyzing the code, or in some other way) the
maximum or likely maximum error that the computation introduces,
and allow for it when performing comparisons (and when producing
output, but that's a different problem). In particular, instead of
testing for equality, you should check to see whether the two
values have ranges that overlap; and this is done with the
relational operators, so equality comparisons are probably
mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that have
no traditional C equivalent, and/or problematic constructs that
should be avoided.
* Macro parameters that appear within string literals in the
macro body. In traditional C macro replacement takes place
within string literals, but in ISO C it does not.
* In traditional C, some preprocessor directives did not exist.
Traditional preprocessors only considered a line to be a
directive if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C
understands but ignores because the # does not appear as the
first character on the line. It also suggests you hide
directives like "#pragma" not understood by traditional C by
indenting them. Some traditional implementations do not
recognize "#elif", so this option suggests avoiding it
altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating-point
constant suffixes. (Traditional C does support the L suffix on
integer constants.) Note, these suffixes appear in macros
defined in the system headers of most modern systems, e.g. the
_MIN/_MAX macros in "". Use of these macros in user
code might normally lead to spurious warnings, however GCC's
integrated preprocessor has enough context to avoid warning in
these cases.
* A function declared external in one block and then used after
the end of the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static" one.
This construct is not accepted by some traditional C compilers.
* The ISO type of an integer constant has a different width or
signedness from its traditional type. This warning is only
issued if the base of the constant is ten. I.e. hexadecimal or
octal values, which typically represent bit patterns, are not
warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a
separate namespace for labels.
* Initialization of unions. If the initializer is zero, the
warning is omitted. This is done under the assumption that the
zero initializer in user code appears conditioned on e.g.
"__STDC__" to avoid missing initializer warnings and relies on
default initialization to zero in the traditional C case.
* Conversions by prototypes between fixed/floating-point values
and vice versa. The absence of these prototypes when compiling
with traditional C causes serious problems. This is a subset
of the possible conversion warnings; for the full set use
-Wtraditional-conversion.
* Use of ISO C style function definitions. This warning
intentionally is not issued for prototype declarations or
variadic functions because these ISO C features appear in your
code when using libiberty's traditional C compatibility macros,
"PARAMS" and "VPARAMS". This warning is also bypassed for
nested functions because that feature is already a GCC
extension and thus not relevant to traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is different from
what would happen to the same argument in the absence of a
prototype. This includes conversions of fixed point to floating
and vice versa, and conversions changing the width or signedness of
a fixed-point argument except when the same as the default
promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a block.
This construct, known from C++, was introduced with ISO C99 and is
by default allowed in GCC. It is not supported by ISO C90.
-Wundef
Warn if an undefined identifier is evaluated in an "#if" directive.
-Wno-endif-labels
Do not warn whenever an "#else" or an "#endif" are followed by
text.
-Wshadow
Warn whenever a local variable or type declaration shadows another
variable, parameter, type, class member (in C++), or instance
variable (in Objective-C) or whenever a built-in function is
shadowed. Note that in C++, the compiler warns if a local variable
shadows an explicit typedef, but not if it shadows a
struct/class/enum.
-Wno-shadow-ivar (Objective-C only)
Do not warn whenever a local variable shadows an instance variable
in an Objective-C method.
-Wlarger-than=len
Warn whenever an object of larger than len bytes is defined.
-Wframe-larger-than=len
Warn if the size of a function frame is larger than len bytes. The
computation done to determine the stack frame size is approximate
and not conservative. The actual requirements may be somewhat
greater than len even if you do not get a warning. In addition,
any space allocated via "alloca", variable-length arrays, or
related constructs is not included by the compiler when determining
whether or not to issue a warning.
-Wno-free-nonheap-object
Do not warn when attempting to free an object that was not
allocated on the heap.
-Wstack-usage=len
Warn if the stack usage of a function might be larger than len
bytes. The computation done to determine the stack usage is
conservative. Any space allocated via "alloca", variable-length
arrays, or related constructs is included by the compiler when
determining whether or not to issue a warning.
The message is in keeping with the output of -fstack-usage.
* If the stack usage is fully static but exceeds the specified
amount, it's:
warning: stack usage is 1120 bytes
* If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
* If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler cannot
assume anything on the bounds of the loop indices. With
-funsafe-loop-optimizations warn if the compiler makes such
assumptions.
-Wno-pedantic-ms-format (MinGW targets only)
When used in combination with -Wformat and -pedantic without GNU
extensions, this option disables the warnings about non-ISO
"printf" / "scanf" format width specifiers "I32", "I64", and "I"
used on Windows targets, which depend on the MS runtime.
-Wpointer-arith
Warn about anything that depends on the "size of" a function type
or of "void". GNU C assigns these types a size of 1, for
convenience in calculations with "void *" pointers and pointers to
functions. In C++, warn also when an arithmetic operation involves
"NULL". This warning is also enabled by -Wpedantic.
-Wtype-limits
Warn if a comparison is always true or always false due to the
limited range of the data type, but do not warn for constant
expressions. For example, warn if an unsigned variable is compared
against zero with "<" or ">=". This warning is also enabled by
-Wextra.
-Wbad-function-cast (C and Objective-C only)
Warn when a function call is cast to a non-matching type. For
example, warn if a call to a function returning an integer type is
cast to a pointer type.
-Wc90-c99-compat (C and Objective-C only)
Warn about features not present in ISO C90, but present in ISO C99.
For instance, warn about use of variable length arrays, "long long"
type, "bool" type, compound literals, designated initializers, and
so on. This option is independent of the standards mode. Warnings
are disabled in the expression that follows "__extension__".
-Wc99-c11-compat (C and Objective-C only)
Warn about features not present in ISO C99, but present in ISO C11.
For instance, warn about use of anonymous structures and unions,
"_Atomic" type qualifier, "_Thread_local" storage-class specifier,
"_Alignas" specifier, "Alignof" operator, "_Generic" keyword, and
so on. This option is independent of the standards mode. Warnings
are disabled in the expression that follows "__extension__".
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset
of ISO C and ISO C++, e.g. request for implicit conversion from
"void *" to a pointer to non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++
1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are
keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is
enabled by -Wall.
-Wc++14-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++
2011 and ISO C++ 2014. This warning is enabled by -Wall.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if a "const char *" is
cast to an ordinary "char *".
Also warn when making a cast that introduces a type qualifier in an
unsafe way. For example, casting "char **" to "const char **" is
unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of
the target is increased. For example, warn if a "char *" is cast
to an "int *" on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const
char[length]" so that copying the address of one into a non-"const"
"char *" pointer produces a warning. These warnings help you find
at compile time code that can try to write into a string constant,
but only if you have been very careful about using "const" in
declarations and prototypes. Otherwise, it is just a nuisance.
This is why we did not make -Wall request these warnings.
When compiling C++, warn about the deprecated conversion from
string literals to "char *". This warning is enabled by default
for C++ programs.
-Wclobbered
Warn for variables that might be changed by "longjmp" or "vfork".
This warning is also enabled by -Wextra.
-Wconditionally-supported (C++ and Objective-C++ only)
Warn for conditionally-supported (C++11 [intro.defs]) constructs.
-Wconversion
Warn for implicit conversions that may alter a value. This includes
conversions between real and integer, like "abs (x)" when "x" is
"double"; conversions between signed and unsigned, like "unsigned
ui = -1"; and conversions to smaller types, like "sqrtf (M_PI)". Do
not warn for explicit casts like "abs ((int) x)" and "ui =
(unsigned) -1", or if the value is not changed by the conversion
like in "abs (2.0)". Warnings about conversions between signed and
unsigned integers can be disabled by using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-
defined conversions; and conversions that never use a type
conversion operator: conversions to "void", the same type, a base
class or a reference to them. Warnings about conversions between
signed and unsigned integers are disabled by default in C++ unless
-Wsign-conversion is explicitly enabled.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-pointer types.
-Wconversion-null is enabled by default.
-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
Warn when a literal '0' is used as null pointer constant. This can
be useful to facilitate the conversion to "nullptr" in C++11.
-Wdate-time
Warn when macros "__TIME__", "__DATE__" or "__TIMESTAMP__" are
encountered as they might prevent bit-wise-identical reproducible
compilations.
-Wdelete-incomplete (C++ and Objective-C++ only)
Warn when deleting a pointer to incomplete type, which may cause
undefined behavior at runtime. This warning is enabled by default.
-Wuseless-cast (C++ and Objective-C++ only)
Warn when an expression is casted to its own type.
-Wempty-body
Warn if an empty body occurs in an "if", "else" or "do while"
statement. This warning is also enabled by -Wextra.
-Wenum-compare
Warn about a comparison between values of different enumerated
types. In C++ enumeral mismatches in conditional expressions are
also diagnosed and the warning is enabled by default. In C this
warning is enabled by -Wall.
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps forward
across the initialization of a variable, or jumps backward to a
label after the variable has been initialized. This only warns
about variables that are initialized when they are declared. This
warning is only supported for C and Objective-C; in C++ this sort
of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be disabled
with the -Wno-jump-misses-init option.
-Wsign-compare
Warn when a comparison between signed and unsigned values could
produce an incorrect result when the signed value is converted to
unsigned. This warning is also enabled by -Wextra; to get the
other warnings of -Wextra without this warning, use -Wextra
-Wno-sign-compare.
-Wsign-conversion
Warn for implicit conversions that may change the sign of an
integer value, like assigning a signed integer expression to an
unsigned integer variable. An explicit cast silences the warning.
In C, this option is enabled also by -Wconversion.
-Wfloat-conversion
Warn for implicit conversions that reduce the precision of a real
value. This includes conversions from real to integer, and from
higher precision real to lower precision real values. This option
is also enabled by -Wconversion.
-Wsized-deallocation (C++ and Objective-C++ only)
Warn about a definition of an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
without a definition of the corresponding sized deallocation
function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by -Wextra along with -fsized-deallocation.
-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and memory
built-in functions if the argument uses "sizeof". This warning
warns e.g. about "memset (ptr, 0, sizeof (ptr));" if "ptr" is not
an array, but a pointer, and suggests a possible fix, or about
"memcpy (&foo, ptr, sizeof (&foo));". This warning is enabled by
-Wall.
-Wsizeof-array-argument
Warn when the "sizeof" operator is applied to a parameter that is
declared as an array in a function definition. This warning is
enabled by default for C and C++ programs.
-Wmemset-transposed-args
Warn for suspicious calls to the "memset" built-in function, if the
second argument is not zero and the third argument is zero. This
warns e.g.@ about "memset (buf, sizeof buf, 0)" where most probably
"memset (buf, 0, sizeof buf)" was meant instead. The diagnostics
is only emitted if the third argument is literal zero. If it is
some expression that is folded to zero, a cast of zero to some
type, etc., it is far less likely that the user has mistakenly
exchanged the arguments and no warning is emitted. This warning is
enabled by -Wall.
-Waddress
Warn about suspicious uses of memory addresses. These include using
the address of a function in a conditional expression, such as
"void func(void); if (func)", and comparisons against the memory
address of a string literal, such as "if (x == "abc")". Such uses
typically indicate a programmer error: the address of a function
always evaluates to true, so their use in a conditional usually
indicate that the programmer forgot the parentheses in a function
call; and comparisons against string literals result in unspecified
behavior and are not portable in C, so they usually indicate that
the programmer intended to use "strcmp". This warning is enabled
by -Wall.
-Wlogical-op
Warn about suspicious uses of logical operators in expressions.
This includes using logical operators in contexts where a bit-wise
operator is likely to be expected.
-Wlogical-not-parentheses
Warn about logical not used on the left hand side operand of a
comparison. This option does not warn if the RHS operand is of a
boolean type. Its purpose is to detect suspicious code like the
following:
int a;
...
if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS into
parentheses:
if ((!a) > 1) { ... }
This warning is enabled by -Wall.
-Waggregate-return
Warn if any functions that return structures or unions are defined
or called. (In languages where you can return an array, this also
elicits a warning.)
-Wno-aggressive-loop-optimizations
Warn if in a loop with constant number of iterations the compiler
detects undefined behavior in some statement during one or more of
the iterations.
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This does not stop errors for incorrect use of supported
attributes.
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This
suppresses warnings for redefinition of "__TIMESTAMP__",
"__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration that specifies the
argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in a
declaration. For example, warn if storage-class specifiers like
"static" are not the first things in a declaration. This warning
is also enabled by -Wextra.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning is
given even if there is a previous prototype.
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in
K&R-style functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. Use this option to detect global functions
that do not have a matching prototype declaration in a header file.
This option is not valid for C++ because all function declarations
provide prototypes and a non-matching declaration declares an
overload rather than conflict with an earlier declaration. Use
-Wmissing-declarations to detect missing declarations in C++.
-Wmissing-declarations
Warn if a global function is defined without a previous
declaration. Do so even if the definition itself provides a
prototype. Use this option to detect global functions that are not
declared in header files. In C, no warnings are issued for
functions with previous non-prototype declarations; use
-Wmissing-prototypes to detect missing prototypes. In C++, no
warnings are issued for function templates, or for inline
functions, or for functions in anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For
example, the following code causes such a warning, because "x.h" is
implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the
following modification does not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
In C++ this option does not warn either about the empty { }
initializer, for example:
struct s { int f, g, h; };
s x = { };
This warning is included in -Wextra. To get other -Wextra warnings
without this one, use -Wextra -Wno-missing-field-initializers.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually
they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in portable
code.
-Wnormalized[=]
In ISO C and ISO C++, two identifiers are different if they are
different sequences of characters. However, sometimes when
characters outside the basic ASCII character set are used, you can
have two different character sequences that look the same. To
avoid confusion, the ISO 10646 standard sets out some normalization
rules which when applied ensure that two sequences that look the
same are turned into the same sequence. GCC can warn you if you
are using identifiers that have not been normalized; this option
controls that warning.
There are four levels of warning supported by GCC. The default is
-Wnormalized=nfc, which warns about any identifier that is not in
the ISO 10646 "C" normalized form, NFC. NFC is the recommended
form for most uses. It is equivalent to -Wnormalized.
Unfortunately, there are some characters allowed in identifiers by
ISO C and ISO C++ that, when turned into NFC, are not allowed in
identifiers. That is, there's no way to use these symbols in
portable ISO C or C++ and have all your identifiers in NFC.
-Wnormalized=id suppresses the warning for these characters. It is
hoped that future versions of the standards involved will correct
this, which is why this option is not the default.
You can switch the warning off for all characters by writing
-Wnormalized=none or -Wno-normalized. You should only do this if
you are using some other normalization scheme (like "D"), because
otherwise you can easily create bugs that are literally impossible
to see.
Some characters in ISO 10646 have distinct meanings but look
identical in some fonts or display methodologies, especially once
formatting has been applied. For instance "\u207F", "SUPERSCRIPT
LATIN SMALL LETTER N", displays just like a regular "n" that has
been placed in a superscript. ISO 10646 defines the NFKC
normalization scheme to convert all these into a standard form as
well, and GCC warns if your code is not in NFKC if you use
-Wnormalized=nfkc. This warning is comparable to warning about
every identifier that contains the letter O because it might be
confused with the digit 0, and so is not the default, but may be
useful as a local coding convention if the programming environment
cannot be fixed to display these characters distinctly.
-Wno-deprecated
Do not warn about usage of deprecated features.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as
deprecated by using the "deprecated" attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant expressions.
-Wno-odr
Warn about One Definition Rule violations during link-time
optimization. Requires -flto-odr-type-merging to be enabled.
Enabled by default.
-Wopenmp-simd
Warn if the vectorizer cost model overrides the OpenMP or the Cilk
Plus simd directive set by user. The -fsimd-cost-model=unlimited
option can be used to relax the cost model.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden
when using designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings
without this one, use -Wextra -Wno-override-init.
-Wpacked
Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable "f.x" in "struct bar" is
misaligned even though "struct bar" does not itself have the packed
attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on
bit-fields of type "char". This has been fixed in GCC 4.4 but the
change can lead to differences in the structure layout. GCC
informs you when the offset of such a field has changed in GCC 4.4.
For example there is no longer a 4-bit padding between field "a"
and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use
-Wno-packed-bitfield-compat to disable this warning.
-Wpadded
Warn if padding is included in a structure, either to align an
element of the structure or to align the whole structure.
Sometimes when this happens it is possible to rearrange the fields
of the structure to reduce the padding and so make the structure
smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even
in cases where multiple declaration is valid and changes nothing.
-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a function.
-Wno-inherited-variadic-ctor
Suppress warnings about use of C++11 inheriting constructors when
the base class inherited from has a C variadic constructor; the
warning is on by default because the ellipsis is not inherited.
-Winline
Warn if a function that is declared as inline cannot be inlined.
Even with this option, the compiler does not warn about failures to
inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or
not to inline a function. For example, the compiler takes into
account the size of the function being inlined and the amount of
inlining that has already been done in the current function.
Therefore, seemingly insignificant changes in the source program
can cause the warnings produced by -Winline to appear or disappear.
-Wno-invalid-offsetof (C++ and Objective-C++ only)
Suppress warnings from applying the "offsetof" macro to a non-POD
type. According to the 2014 ISO C++ standard, applying "offsetof"
to a non-standard-layout type is undefined. In existing C++
implementations, however, "offsetof" typically gives meaningful
results. This flag is for users who are aware that they are
writing nonportable code and who have deliberately chosen to ignore
the warning about it.
The restrictions on "offsetof" may be relaxed in a future version
of the C++ standard.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a
different size. In C++, casting to a pointer type of smaller size
is an error. Wint-to-pointer-cast is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a
different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but can't
be used.
-Wlong-long
Warn if "long long" type is used. This is enabled by either
-Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit
the warning messages, use -Wno-long-long.
-Wvariadic-macros
Warn if variadic macros are used in ISO C90 mode, or if the GNU
alternate syntax is used in ISO C99 mode. This is enabled by
either -Wpedantic or -Wtraditional. To inhibit the warning
messages, use -Wno-variadic-macros.
-Wvarargs
Warn upon questionable usage of the macros used to handle variable
arguments like "va_start". This is default. To inhibit the
warning messages, use -Wno-varargs.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities
of the architecture. Mainly useful for the performance tuning.
Vector operation can be implemented "piecewise", which means that
the scalar operation is performed on every vector element; "in
parallel", which means that the vector operation is implemented
using scalars of wider type, which normally is more performance
efficient; and "as a single scalar", which means that vector fits
into a scalar type.
-Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base with a non-
trivial C++11 move assignment operator. This is dangerous because
if the virtual base is reachable along more than one path, it is
moved multiple times, which can mean both objects end up in the
moved-from state. If the move assignment operator is written to
avoid moving from a moved-from object, this warning can be
disabled.
-Wvla
Warn if variable length array is used in the code. -Wno-vla
prevents the -Wpedantic warning of the variable length array.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may eliminate
reads and/or writes to register variables. This warning is enabled
by -Wall.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning
does not generally indicate that there is anything wrong with your
code; it merely indicates that GCC's optimizers are unable to
handle the code effectively. Often, the problem is that your code
is too big or too complex; GCC refuses to optimize programs when
the optimization itself is likely to take inordinate amounts of
time.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with different
signedness. This option is only supported for C and Objective-C.
It is implied by -Wall and by -Wpedantic, which can be disabled
with -Wno-pointer-sign.
-Wstack-protector
This option is only active when -fstack-protector is active. It
warns about functions that are not protected against stack
smashing.
-Woverlength-strings
Warn about string constants that are longer than the "minimum
maximum" length specified in the C standard. Modern compilers
generally allow string constants that are much longer than the
standard's minimum limit, but very portable programs should avoid
using longer strings.
The limit applies after string constant concatenation, and does not
count the trailing NUL. In C90, the limit was 509 characters; in
C99, it was raised to 4095. C++98 does not specify a normative
minimum maximum, so we do not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with
-Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
Issue a warning for any floating constant that does not have a
suffix. When used together with -Wsystem-headers it warns about
such constants in system header files. This can be useful when
preparing code to use with the "FLOAT_CONST_DECIMAL64" pragma from
the decimal floating-point extension to C99.
-Wno-designated-init (C and Objective-C only)
Suppress warnings when a positional initializer is used to
initialize a structure that has been marked with the
"designated_init" attribute.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your
program or GCC:
-g Produce debugging information in the operating system's native
format (stabs, COFF, XCOFF, or DWARF 2). GDB can work with this
debugging information.
On most systems that use stabs format, -g enables use of extra
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but probably makes other
debuggers crash or refuse to read the program. If you want to
control for certain whether to generate the extra information, use
-gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
GCC allows you to use -g with -O. The shortcuts taken by optimized
code may occasionally produce surprising results: some variables
you declared may not exist at all; flow of control may briefly move
where you did not expect it; some statements may not be executed
because they compute constant results or their values are already
at hand; some statements may execute in different places because
they have been moved out of loops.
Nevertheless it proves possible to debug optimized output. This
makes it reasonable to use the optimizer for programs that might
have bugs.
The following options are useful when GCC is generated with the
capability for more than one debugging format.
-gsplit-dwarf
Separate as much dwarf debugging information as possible into a
separate output file with the extension .dwo. This option allows
the build system to avoid linking files with debug information. To
be useful, this option requires a debugger capable of reading .dwo
files.
-ggdb
Produce debugging information for use by GDB. This means to use
the most expressive format available (DWARF 2, stabs, or the native
format if neither of those are supported), including GDB extensions
if at all possible.
-gpubnames
Generate dwarf .debug_pubnames and .debug_pubtypes sections.
-ggnu-pubnames
Generate .debug_pubnames and .debug_pubtypes sections in a format
suitable for conversion into a GDB index. This option is only
useful with a linker that can produce GDB index version 7.
-gstabs
Produce debugging information in stabs format (if that is
supported), without GDB extensions. This is the format used by DBX
on most BSD systems. On MIPS, Alpha and System V Release 4 systems
this option produces stabs debugging output that is not understood
by DBX or SDB. On System V Release 4 systems this option requires
the GNU assembler.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is
supported), for only symbols that are actually used.
-femit-class-debug-always
Instead of emitting debugging information for a C++ class in only
one object file, emit it in all object files using the class. This
option should be used only with debuggers that are unable to handle
the way GCC normally emits debugging information for classes
because using this option increases the size of debugging
information by as much as a factor of two.
-fdebug-types-section
When using DWARF Version 4 or higher, type DIEs can be put into
their own ".debug_types" section instead of making them part of the
".debug_info" section. It is more efficient to put them in a
separate comdat sections since the linker can then remove
duplicates. But not all DWARF consumers support ".debug_types"
sections yet and on some objects ".debug_types" produces larger
instead of smaller debugging information.
-gstabs+
Produce debugging information in stabs format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program.
-gcoff
Produce debugging information in COFF format (if that is
supported). This is the format used by SDB on most System V
systems prior to System V Release 4.
-gxcoff
Produce debugging information in XCOFF format (if that is
supported). This is the format used by the DBX debugger on IBM
RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program, and may cause
assemblers other than the GNU assembler (GAS) to fail with an
error.
-gdwarf-version
Produce debugging information in DWARF format (if that is
supported). The value of version may be either 2, 3, 4 or 5; the
default version for most targets is 4. DWARF Version 5 is only
experimental.
Note that with DWARF Version 2, some ports require and always use
some non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for
maximum benefit.
-grecord-gcc-switches
This switch causes the command-line options used to invoke the
compiler that may affect code generation to be appended to the
DW_AT_producer attribute in DWARF debugging information. The
options are concatenated with spaces separating them from each
other and from the compiler version. See also
-frecord-gcc-switches for another way of storing compiler options
into the object file. This is the default.
-gno-record-gcc-switches
Disallow appending command-line options to the DW_AT_producer
attribute in DWARF debugging information.
-gstrict-dwarf
Disallow using extensions of later DWARF standard version than
selected with -gdwarf-version. On most targets using non-
conflicting DWARF extensions from later standard versions is
allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than
selected with -gdwarf-version.
-gz[=type]
Produce compressed debug sections in DWARF format, if that is
supported. If type is not given, the default type depends on the
capabilities of the assembler and linker used. type may be one of
none (don't compress debug sections), zlib (use zlib compression in
ELF gABI format), or zlib-gnu (use zlib compression in traditional
GNU format). If the linker doesn't support writing compressed
debug sections, the option is rejected. Otherwise, if the
assembler does not support them, -gz is silently ignored when
producing object files.
-gvms
Produce debugging information in Alpha/VMS debug format (if that is
supported). This is the format used by DEBUG on Alpha/VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how
much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates
-g.
Level 1 produces minimal information, enough for making backtraces
in parts of the program that you don't plan to debug. This
includes descriptions of functions and external variables, and line
number tables, but no information about local variables.
Level 3 includes extra information, such as all the macro
definitions present in the program. Some debuggers support macro
expansion when you use -g3.
-gdwarf-2 does not accept a concatenated debug level, because GCC
used to support an option -gdwarf that meant to generate debug
information in version 1 of the DWARF format (which is very
different from version 2), and it would have been too confusing.
That debug format is long obsolete, but the option cannot be
changed now. Instead use an additional -glevel option to change
the debug level for DWARF.
-gtoggle
Turn off generation of debug info, if leaving out this option
generates it, or turn it on at level 2 otherwise. The position of
this argument in the command line does not matter; it takes effect
after all other options are processed, and it does so only once, no
matter how many times it is given. This is mainly intended to be
used with -fcompare-debug.
-fsanitize=address
Enable AddressSanitizer, a fast memory error detector. Memory
access instructions are instrumented to detect out-of-bounds and
use-after-free bugs. See
for
more details. The run-time behavior can be influenced using the
ASAN_OPTIONS environment variable. When set to "help=1", the
available options are shown at startup of the instrumended program.
See
for a list of supported options.
-fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel. See
for more details.
-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector. Memory access
instructions are instrumented to detect data race bugs. See
for
more details. The run-time behavior can be influenced using the
TSAN_OPTIONS environment variable; see
for a list of supported options.
-fsanitize=leak
Enable LeakSanitizer, a memory leak detector. This option only
matters for linking of executables and if neither
-fsanitize=address nor -fsanitize=thread is used. In that case the
executable is linked against a library that overrides "malloc" and
other allocator functions. See
for more details. The run-time behavior can be influenced using
the LSAN_OPTIONS environment variable.
-fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined behavior
detector. Various computations are instrumented to detect
undefined behavior at runtime. Current suboptions are:
-fsanitize=shift
This option enables checking that the result of a shift
operation is not undefined. Note that what exactly is
considered undefined differs slightly between C and C++, as
well as between ISO C90 and C99, etc.
-fsanitize=integer-divide-by-zero
Detect integer division by zero as well as "INT_MIN / -1"
division.
-fsanitize=unreachable
With this option, the compiler turns the
"__builtin_unreachable" call into a diagnostics message call
instead. When reaching the "__builtin_unreachable" call, the
behavior is undefined.
-fsanitize=vla-bound
This option instructs the compiler to check that the size of a
variable length array is positive.
-fsanitize=null
This option enables pointer checking. Particularly, the
application built with this option turned on will issue an
error message when it tries to dereference a NULL pointer, or
if a reference (possibly an rvalue reference) is bound to a
NULL pointer, or if a method is invoked on an object pointed by
a NULL pointer.
-fsanitize=return
This option enables return statement checking. Programs built
with this option turned on will issue an error message when the
end of a non-void function is reached without actually
returning a value. This option works in C++ only.
-fsanitize=signed-integer-overflow
This option enables signed integer overflow checking. We check
that the result of "+", "*", and both unary and binary "-" does
not overflow in the signed arithmetics. Note, integer
promotion rules must be taken into account. That is, the
following is not an overflow:
signed char a = SCHAR_MAX;
a++;
-fsanitize=bounds
This option enables instrumentation of array bounds. Various
out of bounds accesses are detected. Flexible array members,
flexible array member-like arrays, and initializers of
variables with static storage are not instrumented.
-fsanitize=alignment
This option enables checking of alignment of pointers when they
are dereferenced, or when a reference is bound to
insufficiently aligned target, or when a method or constructor
is invoked on insufficiently aligned object.
-fsanitize=object-size
This option enables instrumentation of memory references using
the "__builtin_object_size" function. Various out of bounds
pointer accesses are detected.
-fsanitize=float-divide-by-zero
Detect floating-point division by zero. Unlike other similar
options, -fsanitize=float-divide-by-zero is not enabled by
-fsanitize=undefined, since floating-point division by zero can
be a legitimate way of obtaining infinities and NaNs.
-fsanitize=float-cast-overflow
This option enables floating-point type to integer conversion
checking. We check that the result of the conversion does not
overflow. Unlike other similar options,
-fsanitize=float-cast-overflow is not enabled by
-fsanitize=undefined. This option does not work well with
"FE_INVALID" exceptions enabled.
-fsanitize=nonnull-attribute
This option enables instrumentation of calls, checking whether
null values are not passed to arguments marked as requiring a
non-null value by the "nonnull" function attribute.
-fsanitize=returns-nonnull-attribute
This option enables instrumentation of return statements in
functions marked with "returns_nonnull" function attribute, to
detect returning of null values from such functions.
-fsanitize=bool
This option enables instrumentation of loads from bool. If a
value other than 0/1 is loaded, a run-time error is issued.
-fsanitize=enum
This option enables instrumentation of loads from an enum type.
If a value outside the range of values for the enum type is
loaded, a run-time error is issued.
-fsanitize=vptr
This option enables instrumentation of C++ member function
calls, member accesses and some conversions between pointers to
base and derived classes, to verify the referenced object has
the correct dynamic type.
While -ftrapv causes traps for signed overflows to be emitted,
-fsanitize=undefined gives a diagnostic message. This currently
works only for the C family of languages.
-fno-sanitize=all
This option disables all previously enabled sanitizers.
-fsanitize=all is not allowed, as some sanitizers cannot be used
together.
-fasan-shadow-offset=number
This option forces GCC to use custom shadow offset in
AddressSanitizer checks. It is useful for experimenting with
different shadow memory layouts in Kernel AddressSanitizer.
-fsanitize-recover[=opts]
-fsanitize-recover= controls error recovery mode for sanitizers
mentioned in comma-separated list of opts. Enabling this option
for a sanitizer component causes it to attempt to continue running
the program as if no error happened. This means multiple runtime
errors can be reported in a single program run, and the exit code
of the program may indicate success even when errors have been
reported. The -fno-sanitize-recover= option can be used to alter
this behavior: only the first detected error is reported and
program then exits with a non-zero exit code.
Currently this feature only works for -fsanitize=undefined (and its
suboptions except for -fsanitize=unreachable and
-fsanitize=return), -fsanitize=float-cast-overflow,
-fsanitize=float-divide-by-zero and -fsanitize=kernel-address. For
these sanitizers error recovery is turned on by default.
-fsanitize-recover=all and -fno-sanitize-recover=all is also
accepted, the former enables recovery for all sanitizers that
support it, the latter disables recovery for all sanitizers that
support it.
Syntax without explicit opts parameter is deprecated. It is
equivalent to
-fsanitize-recover=undefined,float-cast-overflow,float-divide-by-zero
Similarly -fno-sanitize-recover is equivalent to
-fno-sanitize-recover=undefined,float-cast-overflow,float-divide-by-zero
-fsanitize-undefined-trap-on-error
The -fsanitize-undefined-trap-on-error option instructs the
compiler to report undefined behavior using "__builtin_trap" rather
than a "libubsan" library routine. The advantage of this is that
the "libubsan" library is not needed and is not linked in, so this
is usable even in freestanding environments.
-fcheck-pointer-bounds
Enable Pointer Bounds Checker instrumentation. Each memory
reference is instrumented with checks of the pointer used for
memory access against bounds associated with that pointer.
Currently there is only an implementation for Intel MPX available,
thus x86 target and -mmpx are required to enable this feature.
MPX-based instrumentation requires a runtime library to enable MPX
in hardware and handle bounds violation signals. By default when
-fcheck-pointer-bounds and -mmpx options are used to link a
program, the GCC driver links against the libmpx runtime library
and libmpxwrappers library. It also passes '-z bndplt' to a linker
in case it supports this option (which is checked on libmpx
configuration). Note that old versions of linker may ignore
option. Gold linker doesn't support '-z bndplt' option. With no
'-z bndplt' support in linker all calls to dynamic libraries lose
passed bounds reducing overall protection level. It's highly
recommended to use linker with '-z bndplt' support. In case such
linker is not available it is adviced to always use
-static-libmpxwrappers for better protection level or use -static
to completely avoid external calls to dynamic libraries. MPX-based
instrumentation may be used for debugging and also may be included
in production code to increase program security. Depending on
usage, you may have different requirements for the runtime library.
The current version of the MPX runtime library is more oriented for
use as a debugging tool. MPX runtime library usage implies
-lpthread. See also -static-libmpx. The runtime library behavior
can be influenced using various CHKP_RT_* environment variables.
See
for more details.
Generated instrumentation may be controlled by various -fchkp-*
options and by the "bnd_variable_size" structure field attribute
and "bnd_legacy", and "bnd_instrument" function attributes. GCC
also provides a number of built-in functions for controlling the
Pointer Bounds Checker.
-fchkp-check-incomplete-type
Generate pointer bounds checks for variables with incomplete type.
Enabled by default.
-fchkp-narrow-bounds
Controls bounds used by Pointer Bounds Checker for pointers to
object fields. If narrowing is enabled then field bounds are used.
Otherwise object bounds are used. See also
-fchkp-narrow-to-innermost-array and
-fchkp-first-field-has-own-bounds. Enabled by default.
-fchkp-first-field-has-own-bounds
Forces Pointer Bounds Checker to use narrowed bounds for the
address of the first field in the structure. By default a pointer
to the first field has the same bounds as a pointer to the whole
structure.
-fchkp-narrow-to-innermost-array
Forces Pointer Bounds Checker to use bounds of the innermost arrays
in case of nested static array access. By default this option is
disabled and bounds of the outermost array are used.
-fchkp-optimize
Enables Pointer Bounds Checker optimizations. Enabled by default
at optimization levels -O, -O2, -O3.
-fchkp-use-fast-string-functions
Enables use of *_nobnd versions of string functions (not copying
bounds) by Pointer Bounds Checker. Disabled by default.
-fchkp-use-nochk-string-functions
Enables use of *_nochk versions of string functions (not checking
bounds) by Pointer Bounds Checker. Disabled by default.
-fchkp-use-static-bounds
Allow Pointer Bounds Checker to generate static bounds holding
bounds of static variables. Enabled by default.
-fchkp-use-static-const-bounds
Use statically-initialized bounds for constant bounds instead of
generating them each time they are required. By default enabled
when -fchkp-use-static-bounds is enabled.
-fchkp-treat-zero-dynamic-size-as-infinite
With this option, objects with incomplete type whose dynamically-
obtained size is zero are treated as having infinite size instead
by Pointer Bounds Checker. This option may be helpful if a program
is linked with a library missing size information for some symbols.
Disabled by default.
-fchkp-check-read
Instructs Pointer Bounds Checker to generate checks for all read
accesses to memory. Enabled by default.
-fchkp-check-write
Instructs Pointer Bounds Checker to generate checks for all write
accesses to memory. Enabled by default.
-fchkp-store-bounds
Instructs Pointer Bounds Checker to generate bounds stores for
pointer writes. Enabled by default.
-fchkp-instrument-calls
Instructs Pointer Bounds Checker to pass pointer bounds to calls.
Enabled by default.
-fchkp-instrument-marked-only
Instructs Pointer Bounds Checker to instrument only functions
marked with the "bnd_instrument" attribute. Disabled by default.
-fchkp-use-wrappers
Allows Pointer Bounds Checker to replace calls to built-in
functions with calls to wrapper functions. When
-fchkp-use-wrappers is used to link a program, the GCC driver
automatically links against libmpxwrappers. See also
-static-libmpxwrappers. Enabled by default.
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the
optional argument is omitted (or if file is "."), the name of the
dump file is determined by appending ".gkd" to the compilation
output file name.
-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second
time, adding opts and -fcompare-debug-second to the arguments
passed to the second compilation. Dump the final internal
representation in both compilations, and print an error if they
differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty
and nonzero, implicitly enables -fcompare-debug. If
GCC_COMPARE_DEBUG is defined to a string starting with a dash, then
it is used for opts, otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is
equivalent to -fno-compare-debug, which disables the dumping of the
final representation and the second compilation, preventing even
GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set
GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC
rejects as an invalid option in any actual compilation (rather than
preprocessing, assembly or linking). To get just a warning,
setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden
will do.
-fcompare-debug-second
This option is implicitly passed to the compiler for the second
compilation requested by -fcompare-debug, along with options to
silence warnings, and omitting other options that would cause side-
effect compiler outputs to files or to the standard output. Dump
files and preserved temporary files are renamed so as to contain
the ".gk" additional extension during the second compilation, to
avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the
first compilation to be skipped, which makes it useful for little
other than debugging the compiler proper.
-feliminate-dwarf2-dups
Compress DWARF 2 debugging information by eliminating duplicated
information about each symbol. This option only makes sense when
generating DWARF 2 debugging information with -gdwarf-2.
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base
name of the compilation source file matches the base name of file
in which the struct is defined.
This option substantially reduces the size of debugging
information, but at significant potential loss in type information
to the debugger. See -femit-struct-debug-reduced for a less
aggressive option. See -femit-struct-debug-detailed for more
detailed control.
This option works only with DWARF 2.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base
name of the compilation source file matches the base name of file
in which the type is defined, unless the struct is a template or
defined in a system header.
This option significantly reduces the size of debugging
information, with some potential loss in type information to the
debugger. See -femit-struct-debug-baseonly for a more aggressive
option. See -femit-struct-debug-detailed for more detailed
control.
This option works only with DWARF 2.
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler generates
debug information. The intent is to reduce duplicate struct debug
information between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced
and -femit-struct-debug-baseonly, which serves for most needs.
A specification has the
syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to structs that
are used directly (dir:) or used indirectly (ind:). A struct type
is used directly when it is the type of a variable, member.
Indirect uses arise through pointers to structs. That is, when use
of an incomplete struct is valid, the use is indirect. An example
is struct one direct; struct two * indirect;.
The optional second word limits the specification to ordinary
structs (ord:) or generic structs (gen:). Generic structs are a
bit complicated to explain. For C++, these are non-explicit
specializations of template classes, or non-template classes within
the above. Other programming languages have generics, but
-femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for
which the compiler should emit debug information. The values none
and any have the normal meaning. The value base means that the
base of name of the file in which the type declaration appears must
match the base of the name of the main compilation file. In
practice, this means that when compiling foo.c, debug information
is generated for types declared in that file and foo.h, but not
other header files. The value sys means those types satisfying
base or declared in system or compiler headers.
You may need to experiment to determine the best settings for your
application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF 2.
-fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging
information that are identical in different object files. Merging
is not supported by all assemblers or linkers. Merging decreases
the size of the debug information in the output file at the cost of
increasing link processing time. Merging is enabled by default.
-fdebug-prefix-map=old=new
When compiling files in directory old, record debugging information
describing them as in new instead.
-fno-dwarf2-cfi-asm
Emit DWARF 2 unwind info as compiler generated ".eh_frame" section
instead of using GAS ".cfi_*" directives.
-p Generate extra code to write profile information suitable for the
analysis program prof. You must use this option when compiling the
source files you want data about, and you must also use it when
linking.
-pg Generate extra code to write profile information suitable for the
analysis program gprof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-Q Makes the compiler print out each function name as it is compiled,
and print some statistics about each pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by
each pass when it finishes.
-fmem-report
Makes the compiler print some statistics about permanent memory
allocation when it finishes.
-fmem-report-wpa
Makes the compiler print some statistics about permanent memory
allocation for the WPA phase only.
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory
allocation before or after interprocedural optimization.
-fprofile-report
Makes the compiler print some statistics about consistency of the
(estimated) profile and effect of individual passes.
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