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 -dumpbase dumpbase -dumpbase-ext auxdropsuf -dumpdir dumppfx -x language -v -### --help[=class[,...]] --target-help --version -pass-exit-codes -pipe -specs=file -wrapper @file -ffile-prefix-map=old=new -fcanon-prefix-map -fplugin=file -fplugin-arg-name=arg -fdump-ada-spec[-slim] -fada-spec-parent=unit -fdump-go-spec=file

C Language Options

-ansi -std=standard -aux-info filename -fno-asm -fno-builtin -fno-builtin-function -fcond-mismatch -ffreestanding -fgimple -fgnu-tm -fgnu89-inline -fhosted -flax-vector-conversions -fms-extensions -foffload=arg -foffload-options=arg -fopenacc -fopenacc-dim=geom -fopenmp -fopenmp-simd -fopenmp-target-simd-clone[=device-type] -fpermitted-flt-eval-methods=standard -fplan9-extensions -fsigned-bitfields -funsigned-bitfields -fsigned-char -funsigned-char -fstrict-flex-arrays[=n] -fsso-struct=endianness

C++ Language Options

-fabi-version=n -fno-access-control -faligned-new=n -fargs-in-order=n -fchar8_t -fcheck-new -fconstexpr-depth=n -fconstexpr-cache-depth=n -fconstexpr-loop-limit=n -fconstexpr-ops-limit=n -fno-elide-constructors -fno-enforce-eh-specs -fno-gnu-keywords -fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines -fmodule-header[=kind] -fmodule-only -fmodules-ts -fmodule-implicit-inline -fno-module-lazy -fmodule-mapper=specification -fmodule-version-ignore -fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching -fno-nonansi-builtins -fnothrow-opt -fno-operator-names -fno-optional-diags -fpermissive -fno-pretty-templates -fno-rtti -fsized-deallocation -ftemplate-backtrace-limit=n -ftemplate-depth=n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fvisibility-inlines-hidden -fvisibility-ms-compat -fext-numeric-literals -flang-info-include-translate[=header] -flang-info-include-translate-not -flang-info-module-cmi[=module] -stdlib=libstdc++,libc++ -Wabi-tag -Wcatch-value -Wcatch-value=n -Wno-class-conversion -Wclass-memaccess -Wcomma-subscript -Wconditionally-supported -Wno-conversion-null -Wctad-maybe-unsupported -Wctor-dtor-privacy -Wdangling-reference -Wno-delete-incomplete -Wdelete-non-virtual-dtor -Wno-deprecated-array-compare -Wdeprecated-copy -Wdeprecated-copy-dtor -Wno-deprecated-enum-enum-conversion -Wno-deprecated-enum-float-conversion -Weffc++ -Wno-exceptions -Wextra-semi -Wno-inaccessible-base -Wno-inherited-variadic-ctor -Wno-init-list-lifetime -Winvalid-constexpr -Winvalid-imported-macros -Wno-invalid-offsetof -Wno-literal-suffix -Wmismatched-new-delete -Wmismatched-tags -Wmultiple-inheritance -Wnamespaces -Wnarrowing -Wnoexcept -Wnoexcept-type -Wnon-virtual-dtor -Wpessimizing-move -Wno-placement-new -Wplacement-new=n -Wrange-loop-construct -Wredundant-move -Wredundant-tags -Wreorder -Wregister -Wstrict-null-sentinel -Wno-subobject-linkage -Wtemplates -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wself-move -Wsign-promo -Wsized-deallocation -Wsuggest-final-methods -Wsuggest-final-types -Wsuggest-override -Wno-terminate -Wuseless-cast -Wno-vexing-parse -Wvirtual-inheritance -Wno-virtual-move-assign -Wvolatile -Wzero-as-null-pointer-constant

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-property-assign-default -Wno-protocol -Wobjc-root-class -Wselector -Wstrict-selector-match -Wundeclared-selector

Diagnostic Message Formatting Options

-fmessage-length=n -fdiagnostics-plain-output -fdiagnostics-show-location=[once|every-line] -fdiagnostics-color=[auto|never|always] -fdiagnostics-urls=[auto|never|always] -fdiagnostics-format=[text|sarif-stderr|sarif-file|json|json-stderr|json-file] -fno-diagnostics-show-option -fno-diagnostics-show-caret -fno-diagnostics-show-labels -fno-diagnostics-show-line-numbers -fno-diagnostics-show-cwe -fno-diagnostics-show-rule -fdiagnostics-minimum-margin-width=width -fdiagnostics-parseable-fixits -fdiagnostics-generate-patch -fdiagnostics-show-template-tree -fno-elide-type -fdiagnostics-path-format=[none|separate-events|inline-events] -fdiagnostics-show-path-depths -fno-show-column -fdiagnostics-column-unit=[display|byte] -fdiagnostics-column-origin=origin -fdiagnostics-escape-format=[unicode|bytes]

Warning Options

-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w -Wextra -Wall -Wabi=n -Waddress -Wno-address-of-packed-member -Waggregate-return -Walloc-size-larger-than=byte-size -Walloc-zero -Walloca -Walloca-larger-than=byte-size -Wno-aggressive-loop-optimizations -Warith-conversion -Warray-bounds -Warray-bounds=n -Warray-compare -Wno-attributes -Wattribute-alias=n -Wno-attribute-alias -Wno-attribute-warning -Wbidi-chars=[none|unpaired|any|ucn] -Wbool-compare -Wbool-operation -Wno-builtin-declaration-mismatch -Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat -Wc11-c2x-compat -Wc++-compat -Wc++11-compat -Wc++14-compat -Wc++17-compat -Wc++20-compat -Wno-c++11-extensions -Wno-c++14-extensions -Wno-c++17-extensions -Wno-c++20-extensions -Wno-c++23-extensions -Wcast-align -Wcast-align=strict -Wcast-function-type -Wcast-qual -Wchar-subscripts -Wclobbered -Wcomment -Wno-complain-wrong-lang -Wconversion -Wno-coverage-mismatch -Wno-cpp -Wdangling-else -Wdangling-pointer -Wdangling-pointer=n -Wdate-time -Wno-deprecated -Wno-deprecated-declarations -Wno-designated-init -Wdisabled-optimization -Wno-discarded-array-qualifiers -Wno-discarded-qualifiers -Wno-div-by-zero -Wdouble-promotion -Wduplicated-branches -Wduplicated-cond -Wempty-body -Wno-endif-labels -Wenum-compare -Wenum-conversion -Wenum-int-mismatch -Werror -Werror=* -Wexpansion-to-defined -Wfatal-errors -Wfloat-conversion -Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral -Wformat-overflow=n -Wformat-security -Wformat-signedness -Wformat-truncation=n -Wformat-y2k -Wframe-address -Wframe-larger-than=byte-size -Wno-free-nonheap-object -Wno-if-not-aligned -Wno-ignored-attributes -Wignored-qualifiers -Wno-incompatible-pointer-types -Wimplicit -Wimplicit-fallthrough -Wimplicit-fallthrough=n -Wno-implicit-function-declaration -Wno-implicit-int -Winfinite-recursion -Winit-self -Winline -Wno-int-conversion -Wint-in-bool-context -Wno-int-to-pointer-cast -Wno-invalid-memory-model -Winvalid-pch -Winvalid-utf8 -Wno-unicode -Wjump-misses-init -Wlarger-than=byte-size -Wlogical-not-parentheses -Wlogical-op -Wlong-long -Wno-lto-type-mismatch -Wmain -Wmaybe-uninitialized -Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation -Wmissing-attributes -Wmissing-braces -Wmissing-field-initializers -Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-noreturn -Wno-missing-profile -Wno-multichar -Wmultistatement-macros -Wnonnull -Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc] -Wnull-dereference -Wno-odr -Wopenacc-parallelism -Wopenmp-simd -Wno-overflow -Woverlength-strings -Wno-override-init-side-effects -Wpacked -Wno-packed-bitfield-compat -Wpacked-not-aligned -Wpadded -Wparentheses -Wno-pedantic-ms-format -Wpointer-arith -Wno-pointer-compare -Wno-pointer-to-int-cast -Wno-pragmas -Wno-prio-ctor-dtor -Wredundant-decls -Wrestrict -Wno-return-local-addr -Wreturn-type -Wno-scalar-storage-order -Wsequence-point -Wshadow -Wshadow=global -Wshadow=local -Wshadow=compatible-local -Wno-shadow-ivar -Wno-shift-count-negative -Wno-shift-count-overflow -Wshift-negative-value -Wno-shift-overflow -Wshift-overflow=n -Wsign-compare -Wsign-conversion -Wno-sizeof-array-argument -Wsizeof-array-div -Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstack-protector -Wstack-usage=byte-size -Wstrict-aliasing -Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n -Wstring-compare -Wno-stringop-overflow -Wno-stringop-overread -Wno-stringop-truncation -Wstrict-flex-arrays -Wsuggest-attribute=[pure|const|noreturn|format|malloc] -Wswitch -Wno-switch-bool -Wswitch-default -Wswitch-enum -Wno-switch-outside-range -Wno-switch-unreachable -Wsync-nand -Wsystem-headers -Wtautological-compare -Wtrampolines -Wtrigraphs -Wtrivial-auto-var-init -Wtsan -Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas -Wunsuffixed-float-constants -Wunused -Wunused-but-set-parameter -Wunused-but-set-variable -Wunused-const-variable -Wunused-const-variable=n -Wunused-function -Wunused-label -Wunused-local-typedefs -Wunused-macros -Wunused-parameter -Wno-unused-result -Wunused-value -Wunused-variable -Wno-varargs -Wvariadic-macros -Wvector-operation-performance -Wvla -Wvla-larger-than=byte-size -Wno-vla-larger-than -Wvolatile-register-var -Wwrite-strings -Wxor-used-as-pow -Wzero-length-bounds

Static Analyzer Options

-fanalyzer -fanalyzer-call-summaries -fanalyzer-checker=name -fno-analyzer-feasibility -fanalyzer-fine-grained -fno-analyzer-state-merge -fno-analyzer-state-purge -fno-analyzer-suppress-followups -fanalyzer-transitivity -fno-analyzer-undo-inlining -fanalyzer-verbose-edges -fanalyzer-verbose-state-changes -fanalyzer-verbosity=level -fdump-analyzer -fdump-analyzer-callgraph -fdump-analyzer-exploded-graph -fdump-analyzer-exploded-nodes -fdump-analyzer-exploded-nodes-2 -fdump-analyzer-exploded-nodes-3 -fdump-analyzer-exploded-paths -fdump-analyzer-feasibility -fdump-analyzer-json -fdump-analyzer-state-purge -fdump-analyzer-stderr -fdump-analyzer-supergraph -fdump-analyzer-untracked -Wno-analyzer-double-fclose -Wno-analyzer-double-free -Wno-analyzer-exposure-through-output-file -Wno-analyzer-exposure-through-uninit-copy -Wno-analyzer-fd-access-mode-mismatch -Wno-analyzer-fd-double-close -Wno-analyzer-fd-leak -Wno-analyzer-fd-phase-mismatch -Wno-analyzer-fd-type-mismatch -Wno-analyzer-fd-use-after-close -Wno-analyzer-fd-use-without-check -Wno-analyzer-file-leak -Wno-analyzer-free-of-non-heap -Wno-analyzer-imprecise-fp-arithmetic -Wno-analyzer-infinite-recursion -Wno-analyzer-jump-through-null -Wno-analyzer-malloc-leak -Wno-analyzer-mismatching-deallocation -Wno-analyzer-null-argument -Wno-analyzer-null-dereference -Wno-analyzer-out-of-bounds -Wno-analyzer-possible-null-argument -Wno-analyzer-possible-null-dereference -Wno-analyzer-putenv-of-auto-var -Wno-analyzer-shift-count-negative -Wno-analyzer-shift-count-overflow -Wno-analyzer-stale-setjmp-buffer -Wno-analyzer-tainted-allocation-size -Wno-analyzer-tainted-assertion -Wno-analyzer-tainted-array-index -Wno-analyzer-tainted-divisor -Wno-analyzer-tainted-offset -Wno-analyzer-tainted-size -Wanalyzer-too-complex -Wno-analyzer-unsafe-call-within-signal-handler -Wno-analyzer-use-after-free -Wno-analyzer-use-of-pointer-in-stale-stack-frame -Wno-analyzer-use-of-uninitialized-value -Wno-analyzer-va-arg-type-mismatch -Wno-analyzer-va-list-exhausted -Wno-analyzer-va-list-leak -Wno-analyzer-va-list-use-after-va-end -Wno-analyzer-write-to-const -Wno-analyzer-write-to-string-literal

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

-g -glevel -gdwarf -gdwarf-version -gbtf -gctf -gctflevel -ggdb -grecord-gcc-switches -gno-record-gcc-switches -gstrict-dwarf -gno-strict-dwarf -gas-loc-support -gno-as-loc-support -gas-locview-support -gno-as-locview-support -gcolumn-info -gno-column-info -gdwarf32 -gdwarf64 -gstatement-frontiers -gno-statement-frontiers -gvariable-location-views -gno-variable-location-views -ginternal-reset-location-views -gno-internal-reset-location-views -ginline-points -gno-inline-points -gvms -gz[=type] -gsplit-dwarf -gdescribe-dies -gno-describe-dies -fdebug-prefix-map=old=new -fdebug-types-section -fno-eliminate-unused-debug-types -femit-struct-debug-baseonly -femit-struct-debug-reduced -femit-struct-debug-detailed[=spec-list] -fno-eliminate-unused-debug-symbols -femit-class-debug-always -fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking -fvar-tracking-assignments

Optimization Options

-faggressive-loop-optimizations -falign-functions[=n[:m:[n2[:m2]]]] -falign-jumps[=n[:m:[n2[:m2]]]] -falign-labels[=n[:m:[n2[:m2]]]] -falign-loops[=n[:m:[n2[:m2]]]] -fmin-function-alignment=[n] -fno-allocation-dce -fallow-store-data-races -fassociative-math -fauto-profile -fauto-profile[=path] -fauto-inc-dec -fbranch-probabilities -fcaller-saves -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 -ffinite-loops -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-modref -fipa-cp -fipa-cp-clone -fipa-bit-cp -fipa-vrp -fipa-pta -fipa-profile -fipa-pure-const -fipa-reference -fipa-reference-addressable -fipa-stack-alignment -fipa-icf -fira-algorithm=algorithm -flive-patching=level -fira-region=region -fira-hoist-pressure -fira-loop-pressure -fno-ira-share-save-slots -fno-ira-share-spill-slots -fisolate-erroneous-paths-dereference -fisolate-erroneous-paths-attribute -fivopts -fkeep-inline-functions -fkeep-static-functions -fkeep-static-consts -flimit-function-alignment -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 -fmerge-all-constants -fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves -fmove-loop-invariants -fmove-loop-stores -fno-branch-count-reg -fno-defer-pop -fno-fp-int-builtin-inexact -fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -fno-printf-return-value -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-correction -fprofile-use -fprofile-use=path -fprofile-partial-training -fprofile-values -fprofile-reorder-functions -freciprocal-math -free -frename-registers -freorder-blocks -freorder-blocks-algorithm=algorithm -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -freschedule-modulo-scheduled-loops -frounding-math -fsave-optimization-record -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 -fshrink-wrap-separate -fsignaling-nans -fsingle-precision-constant -fsplit-ivs-in-unroller -fsplit-loops -fsplit-paths -fsplit-wide-types -fsplit-wide-types-early -fssa-backprop -fssa-phiopt -fstdarg-opt -fstore-merging -fstrict-aliasing -fipa-strict-aliasing -fthread-jumps -ftracer -ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -fcode-hoisting -ftree-loop-if-convert -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-scev-cprop -ftree-sink -ftree-slsr -ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter -ftree-vectorize -ftree-vrp -ftrivial-auto-var-init -funconstrained-commons -funit-at-a-time -funroll-all-loops -funroll-loops -funsafe-math-optimizations -funswitch-loops -fipa-ra -fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb -fwhole-program -fwpa -fuse-linker-plugin -fzero-call-used-regs --param name=value -O -O0 -O1 -O2 -O3 -Os -Ofast -Og -Oz

Program Instrumentation Options

-p -pg -fprofile-arcs --coverage -ftest-coverage -fprofile-abs-path -fprofile-dir=path -fprofile-generate -fprofile-generate=path -fprofile-info-section -fprofile-info-section=name -fprofile-note=path -fprofile-prefix-path=path -fprofile-update=method -fprofile-filter-files=regex -fprofile-exclude-files=regex -fprofile-reproducible=[multithreaded|parallel-runs|serial] -fsanitize=style -fsanitize-recover -fsanitize-recover=style -fsanitize-trap -fsanitize-trap=style -fasan-shadow-offset=number -fsanitize-sections=s1,s2,... -fsanitize-undefined-trap-on-error -fbounds-check -fcf-protection=[full|branch|return|none|check] -fharden-compares -fharden-conditional-branches -fstack-protector -fstack-protector-all -fstack-protector-strong -fstack-protector-explicit -fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack -fvtable-verify=[std|preinit|none] -fvtv-counts -fvtv-debug -finstrument-functions -finstrument-functions-once -finstrument-functions-exclude-function-list=sym,sym,... -finstrument-functions-exclude-file-list=file,file,... -fprofile-prefix-map=old=new

Preprocessor Options

-Aquestion=answer -A-question[=answer] -C -CC -Dmacro[=defn] -dD -dI -dM -dN -dU -fdebug-cpp -fdirectives-only -fdollars-in-identifiers -fexec-charset=charset -fextended-identifiers -finput-charset=charset -flarge-source-files -fmacro-prefix-map=old=new -fmax-include-depth=depth -fno-canonical-system-headers -fpch-deps -fpch-preprocess -fpreprocessed -ftabstop=width -ftrack-macro-expansion -fwide-exec-charset=charset -fworking-directory -H -imacros file -include file -M -MD -MF -MG -MM -MMD -MP -MQ -MT -Mno-modules -no-integrated-cpp -P -pthread -remap -traditional -traditional-cpp -trigraphs -Umacro -undef -Wp,option -Xpreprocessor option

Assembler Options

-Wa,option -Xassembler option

Linker Options

object-file-name -fuse-ld=linker -llibrary -nostartfiles -nodefaultlibs -nolibc -nostdlib -nostdlib++ -e entry --entry=entry -pie -pthread -r -rdynamic -s -static -static-pie -static-libgcc -static-libstdc++ -static-libasan -static-libtsan -static-liblsan -static-libubsan -shared -shared-libgcc -symbolic -T script -Wl,option -Xlinker option -u symbol -z keyword

Directory Options

-Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir -iplugindir=dir -iprefix file -iquote dir -isysroot dir -isystem dir -iwithprefix dir -iwithprefixbefore dir -Ldir -no-canonical-prefixes --no-sysroot-suffix -nostdinc -nostdinc++ --sysroot=dir

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 -fcommon -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-plt -fno-jump-tables -fno-bit-tests -frecord-gcc-switches -freg-struct-return -fshort-enums -fshort-wchar -fverbose-asm -fpack-struct[=n] -fleading-underscore -ftls-model=model -fstack-reuse=reuse_level -ftrampolines -ftrapv -fwrapv -fvisibility=[default|internal|hidden|protected] -fstrict-volatile-bitfields -fsync-libcalls

Developer Options

-dletters -dumpspecs -dumpmachine -dumpversion -dumpfullversion -fcallgraph-info[=su,da] -fchecking -fchecking=n -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-debug -fdump-earlydebug -fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links -fdump-final-insns[=file] -fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline -fdump-lang-all -fdump-lang-switch -fdump-lang-switch-options -fdump-lang-switch-options=filename -fdump-passes -fdump-rtl-pass -fdump-rtl-pass=filename -fdump-statistics -fdump-tree-all -fdump-tree-switch -fdump-tree-switch-options -fdump-tree-switch-options=filename -fcompare-debug[=opts] -fcompare-debug-second -fenable-kind-pass -fenable-kind-pass=range-list -fira-verbose=n -flto-report -flto-report-wpa -fmem-report-wpa -fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report -fopt-info -fopt-info-options[=file] -fmultiflags -fprofile-report -frandom-seed=string -fsched-verbose=n -fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose -fstats -fstack-usage -ftime-report -ftime-report-details -fvar-tracking-assignments-toggle -gtoggle -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]

Machine-Dependent Options

AArch64 Options -mabi=name -mbig-endian -mlittle-endian -mgeneral-regs-only -mcmodel=tiny -mcmodel=small -mcmodel=large -mstrict-align -mno-strict-align -momit-leaf-frame-pointer -mtls-dialect=desc -mtls-dialect=traditional -mtls-size=size -mfix-cortex-a53-835769 -mfix-cortex-a53-843419 -mlow-precision-recip-sqrt -mlow-precision-sqrt -mlow-precision-div -mpc-relative-literal-loads -msign-return-address=scope -mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti -mharden-sls=opts -march=name -mcpu=name -mtune=name -moverride=string -mverbose-cost-dump -mstack-protector-guard=guard -mstack-protector-guard-reg=sysreg -mstack-protector-guard-offset=offset -mtrack-speculation -moutline-atomics

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

AMD GCN Options -march=gpu -mtune=gpu -mstack-size=bytes

ARC Options -mbarrel-shifter -mjli-always -mcpu=cpu -mA6 -mARC600 -mA7 -mARC700 -mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64 -matomic -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 -mlong-calls -mmedium-calls -msdata -mirq-ctrl-saved -mrgf-banked-regs -mlpc-width=width -G num -mvolatile-cache -mtp-regno=regno -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 -mmillicode -mmixed-code -mq-class -mRcq -mRcw -msize-level=level -mtune=cpu -mmultcost=num -mcode-density-frame -munalign-prob-threshold=probability -mmpy-option=multo -mdiv-rem -mcode-density -mll64 -mfpu=fpu -mrf16 -mbranch-index

ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check -mapcs-reentrant -mno-apcs-reentrant -mgeneral-regs-only -msched-prolog -mno-sched-prolog -mlittle-endian -mbig-endian -mbe8 -mbe32 -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 -mflip-thumb -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking -mtp=name -mtls-dialect=dialect -mword-relocations -mfix-cortex-m3-ldrd -mfix-cortex-a57-aes-1742098 -mfix-cortex-a72-aes-1655431 -munaligned-access -mneon-for-64bits -mslow-flash-data -masm-syntax-unified -mrestrict-it -mverbose-cost-dump -mpure-code -mcmse -mfix-cmse-cve-2021-35465 -mstack-protector-guard=guard -mstack-protector-guard-offset=offset -mfdpic -mbranch-protection=none|standard|pac-ret[+leaf] [+bti]|bti[+pac-ret[+leaf]]

AVR Options -mmcu=mcu -mabsdata -maccumulate-args -mbranch-cost=cost -mcall-prologues -mgas-isr-prologues -mint8 -mdouble=bits -mlong-double=bits -mn_flash=size -mno-interrupts -mmain-is-OS_task -mrelax -mrmw -mstrict-X -mtiny-stack -mfract-convert-truncate -mshort-calls -mskip-bug -nodevicelib -nodevicespecs -Waddr-space-convert -Wmisspelled-isr

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 -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -melf -maout -sim -sim2 -mmul-bug-workaround -mno-mul-bug-workaround

C-SKY Options -march=arch -mcpu=cpu -mbig-endian -EB -mlittle-endian -EL -mhard-float -msoft-float -mfpu=fpu -mdouble-float -mfdivdu -mfloat-abi=name -melrw -mistack -mmp -mcp -mcache -msecurity -mtrust -mdsp -medsp -mvdsp -mdiv -msmart -mhigh-registers -manchor -mpushpop -mmultiple-stld -mconstpool -mstack-size -mccrt -mbranch-cost=n -mcse-cc -msched-prolog -msim

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

eBPF Options -mbig-endian -mlittle-endian -mkernel=version -mframe-limit=bytes -mxbpf -mco-re -mno-co-re -mjmpext -mjmp32 -malu32 -mcpu=version

FR30 Options -msmall-model -mno-lsim

FT32 Options -msim -mlra -mnodiv -mft32b -mcompress -mnopm

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 -mmusl -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 -matomic-libcalls -mbig-switch -mcaller-copies -mdisable-fpregs -mdisable-indexing -mordered -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld -mfixed-range=register-range -mcoherent-ldcw -mjump-in-delay -mlinker-opt -mlong-calls -mlong-load-store -mno-atomic-libcalls -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 -msoft-mult -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

LoongArch Options -march=cpu-type -mtune=cpu-type -mabi=base-abi-type -mfpu=fpu-type -msoft-float -msingle-float -mdouble-float -mbranch-cost=n -mcheck-zero-division -mno-check-zero-division -mcond-move-int -mno-cond-move-int -mcond-move-float -mno-cond-move-float -memcpy -mno-memcpy -mstrict-align -mno-strict-align -mmax-inline-memcpy-size=n -mexplicit-relocs -mno-explicit-relocs -mdirect-extern-access -mno-direct-extern-access -mcmodel=code-model -mrelax -mpass-mrelax-to-as

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 -mlong-jump-table-offsets

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

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 -mpic-data-is-text-relative

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 -mcrc -mno-crc -mginv -mno-ginv -mmicromips -mno-micromips -mmsa -mno-msa -mloongson-mmi -mno-loongson-mmi -mloongson-ext -mno-loongson-ext -mloongson-ext2 -mno-loongson-ext2 -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 -mload-store-pairs -mno-load-store-pairs -munaligned-access -mno-unaligned-access -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-r5900 -mno-fix-r5900 -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 -mcompact-branches=policy -mfp-exceptions -mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci -mno-synci -mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4 -mno-madd4 -mrelax-pic-calls -mno-relax-pic-calls -mmcount-ra-address -mframe-header-opt -mno-frame-header-opt

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 -mwarn-mcu -mcode-region= -mdata-region= -msilicon-errata= -msilicon-errata-warn= -mhwmult= -minrt -mtiny-printf -mmax-inline-shift=

NDS32 Options -mbig-endian -mlittle-endian -mreduced-regs -mfull-regs -mcmov -mno-cmov -mext-perf -mno-ext-perf -mext-perf2 -mno-ext-perf2 -mext-string -mno-ext-string -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 -mgprel-sec=regexp -mr0rel-sec=regexp -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 -march=arch -mbmx -mno-bmx -mcdx -mno-cdx

Nvidia PTX Options -m64 -mmainkernel -moptimize

OpenRISC Options -mboard=name -mnewlib -mhard-mul -mhard-div -msoft-mul -msoft-div -msoft-float -mhard-float -mdouble-float -munordered-float -mcmov -mror -mrori -msext -msfimm -mshftimm -mcmodel=code-model

PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mint32 -mno-int16 -mint16 -mno-int32 -msplit -munix-asm -mdec-asm -mgnu-asm -mlra

PowerPC Options See RS/6000 and PowerPC Options.

PRU Options -mmcu=mcu -minrt -mno-relax -mloop -mabi=variant

RISC-V Options -mbranch-cost=N-instruction -mplt -mno-plt -mabi=ABI-string -mfdiv -mno-fdiv -mdiv -mno-div -misa-spec=ISA-spec-string -march=ISA-string -mtune=processor-string -mpreferred-stack-boundary=num -msmall-data-limit=N-bytes -msave-restore -mno-save-restore -mshorten-memrefs -mno-shorten-memrefs -mstrict-align -mno-strict-align -mcmodel=medlow -mcmodel=medany -mexplicit-relocs -mno-explicit-relocs -mrelax -mno-relax -mriscv-attribute -mno-riscv-attribute -malign-data=type -mbig-endian -mlittle-endian -mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset -mcsr-check -mno-csr-check -minline-atomics -mno-inline-atomics

RL78 Options -msim -mmul=none -mmul=g13 -mmul=g14 -mallregs -mcpu=g10 -mcpu=g13 -mcpu=g14 -mg10 -mg13 -mg14 -m64bit-doubles -m32bit-doubles -msave-mduc-in-interrupts

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 -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 -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 -mswdiv -msingle-pic-base -mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type -minsert-sched-nops=scheme -mcall-aixdesc -mcall-eabi -mcall-freebsd -mcall-linux -mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi -mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return -msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt -mlongcall -mno-longcall -mpltseq -mno-pltseq -mblock-move-inline-limit=num -mblock-compare-inline-limit=num -mblock-compare-inline-loop-limit=num -mno-block-ops-unaligned-vsx -mstring-compare-inline-limit=num -misel -mno-isel -mvrsave -mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mprototype -mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mreadonly-in-sdata -mvxworks -G num -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 -mhtm -mno-htm -mquad-memory -mno-quad-memory -mquad-memory-atomic -mno-quad-memory-atomic -mcompat-align-parm -mno-compat-align-parm -mfloat128 -mno-float128 -mfloat128-hardware -mno-float128-hardware -mgnu-attribute -mno-gnu-attribute -mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset -mprefixed -mno-prefixed -mpcrel -mno-pcrel -mmma -mno-mmma -mrop-protect -mno-rop-protect -mprivileged -mno-privileged

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 -mallow-string-insns -mno-allow-string-insns -mjsr -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 -mhtm -mvx -mzvector -mtpf-trace -mno-tpf-trace -mtpf-trace-skip -mno-tpf-trace-skip -mfused-madd -mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard -mhotpatch=halfwords,halfwords

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 -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mrenesas -mno-renesas -mnomacsave -mieee -mno-ieee -mbitops -misize -minline-ic_invalidate -mpadstruct -mprefergot -musermode -multcost=number -mdiv=strategy -mdivsi3_libfunc=name -mfixed-range=register-range -maccumulate-outgoing-args -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

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 -mstd-struct-return -mno-std-struct-return -munaligned-doubles -mno-unaligned-doubles -muser-mode -mno-user-mode -mv8plus -mno-v8plus -mvis -mno-vis -mvis2 -mno-vis2 -mvis3 -mno-vis3 -mvis4 -mno-vis4 -mvis4b -mno-vis4b -mcbcond -mno-cbcond -mfmaf -mno-fmaf -mfsmuld -mno-fsmuld -mpopc -mno-popc -msubxc -mno-subxc -mfix-at697f -mfix-ut699 -mfix-ut700 -mfix-gr712rc -mlra -mno-lra

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

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 -mlra

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 -m80387 -mhard-float -msoft-float -mno-wide-multiply -mrtd -malign-double -mpreferred-stack-boundary=num -mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe -mcrc32 -mmwait -mrecip -mrecip=opt -mvzeroupper -mprefer-avx128 -mprefer-vector-width=opt -mmove-max=bits -mstore-max=bits -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f -mavx512pf -mavx512er -mavx512cd -mavx512vl -mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi -msha -maes -mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mptwrite -mprefetchwt1 -mclflushopt -mclwb -mxsavec -mxsaves -msse4a -m3dnow -m3dnowa -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -madx -mlzcnt -mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mhle -mlwp -mmwaitx -mclzero -mpku -mthreads -mgfni -mvaes -mwaitpkg -mshstk -mmanual-endbr -mcet-switch -mforce-indirect-call -mavx512vbmi2 -mavx512bf16 -menqcmd -mvpclmulqdq -mavx512bitalg -mmovdiri -mmovdir64b -mavx512vpopcntdq -mavx5124fmaps -mavx512vnni -mavx5124vnniw -mprfchw -mrdpid -mrdseed -msgx -mavx512vp2intersect -mserialize -mtsxldtrk -mamx-tile -mamx-int8 -mamx-bf16 -muintr -mhreset -mavxvnni -mavx512fp16 -mavxifma -mavxvnniint8 -mavxneconvert -mcmpccxadd -mamx-fp16 -mprefetchi -mraoint -mamx-complex -mcldemote -mms-bitfields -mno-align-stringops -minline-all-stringops -minline-stringops-dynamically -mstringop-strategy=alg -mkl -mwidekl -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 -mdaz-ftz -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 -miamcu -mlarge-data-threshold=num -msse2avx -mfentry -mrecord-mcount -mnop-mcount -m8bit-idiv -minstrument-return=type -mfentry-name=name -mfentry-section=name -mavx256-split-unaligned-load -mavx256-split-unaligned-store -malign-data=type -mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset -mstack-protector-guard-symbol=symbol -mgeneral-regs-only -mcall-ms2sysv-xlogues -mrelax-cmpxchg-loop -mindirect-branch=choice -mfunction-return=choice -mindirect-branch-register -mharden-sls=choice -mindirect-branch-cs-prefix -mneeded -mno-direct-extern-access -munroll-only-small-loops -mlam=choice

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 -mauto-litpools -mno-auto-litpools -mtarget-align -mno-target-align -mlongcalls -mno-longcalls -mabi=abi-type -mextra-l32r-costs=cycles

zSeries Options See S/390 and zSeries Options.

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.d

D source code.

file.di

D interface file.

file.dd

D documentation code (Ddoc).

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++-system-header c++-user-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
        d
        f77  f77-cpp-input f95  f95-cpp-input
        go
    

-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).

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 the primary 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.

Though -o names only the primary output, it also affects the naming of auxiliary and dump outputs. See the examples below. Unless overridden, both auxiliary outputs and dump outputs are placed in the same directory as the primary output. In auxiliary outputs, the suffix of the input file is replaced with that of the auxiliary output file type; in dump outputs, the suffix of the dump file is appended to the input file suffix. In compilation commands, the base name of both auxiliary and dump outputs is that of the primary output; in compile and link commands, the primary output name, minus the executable suffix, is combined with the input file name. If both share the same base name, disregarding the suffix, the result of the combination is that base name, otherwise, they are concatenated, separated by a dash.

        gcc -c foo.c ...
    

will use foo.o as the primary output, and place aux outputs and dumps next to it, e.g., aux file foo.dwo for -gsplit-dwarf, and dump file foo.c.???r.final for -fdump-rtl-final.

If a non-linker output file is explicitly specified, aux and dump files by default take the same base name:

        gcc -c foo.c -o dir/foobar.o ...
    

will name aux outputs dir/foobar.* and dump outputs dir/foobar.c.*.

A linker output will instead prefix aux and dump outputs:

        gcc foo.c bar.c -o dir/foobar ...
    

will generally name aux outputs dir/foobar-foo.* and dir/foobar-bar.*, and dump outputs dir/foobar-foo.c.* and dir/foobar-bar.c.*.

The one exception to the above is when the executable shares the base name with the single input:

        gcc foo.c -o dir/foo ...
    

in which case aux outputs are named dir/foo.* and dump outputs named dir/foo.c.*.

The location and the names of auxiliary and dump outputs can be adjusted by the options -dumpbase, -dumpbase-ext, -dumpdir, -save-temps=cwd, and -save-temps=obj.

-dumpbase dumpbase

This option sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file. Intermediate outputs, when preserved, are not regarded as primary outputs, but as auxiliary outputs:

        gcc -save-temps -S foo.c
    

saves the (no longer) temporary preprocessed file in foo.i, and then compiles to the (implied) output file foo.s, whereas:

        gcc -save-temps -dumpbase save-foo -c foo.c
    

preprocesses to in save-foo.i, compiles to save-foo.s (now an intermediate, thus auxiliary output), and then assembles to the (implied) output file foo.o.

Absent this option, dump and aux files take their names from the input file, or from the (non-linker) output file, if one is explicitly specified: dump output files (e.g. those requested by -fdump-* options) with the input name suffix, and aux output files (those requested by other non-dump options, e.g. "-save-temps", "-gsplit-dwarf", "-fcallgraph-info") without it.

Similar suffix differentiation of dump and aux outputs can be attained for explicitly-given -dumpbase basename.suf by also specifying -dumpbase-ext .suf.

If dumpbase is explicitly specified with any directory component, any dumppfx specification (e.g. -dumpdir or -save-temps=*) is ignored, and instead of appending to it, dumpbase fully overrides it:

        gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
          -dumpdir pfx- -save-temps=cwd ...
    

creates auxiliary and dump outputs named alt/foo.*, disregarding dir/ in -o, the ./ prefix implied by -save-temps=cwd, and pfx- in -dumpdir.

When -dumpbase is specified in a command that compiles multiple inputs, or that compiles and then links, it may be combined with dumppfx, as specified under -dumpdir. Then, each input file is compiled using the combined dumppfx, and default values for dumpbase and auxdropsuf are computed for each input file:

        gcc foo.c bar.c -c -dumpbase main ...
    

creates foo.o and bar.o as primary outputs, and avoids overwriting the auxiliary and dump outputs by using the dumpbase as a prefix, creating auxiliary and dump outputs named main-foo.* and main-bar.*.

An empty string specified as dumpbase avoids the influence of the output basename in the naming of auxiliary and dump outputs during compilation, computing default values :

        gcc -c foo.c -o dir/foobar.o -dumpbase " ...
    

will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. Note how their basenames are taken from the input name, but the directory still defaults to that of the output.

The empty-string dumpbase does not prevent the use of the output basename for outputs during linking:

        gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...
    

The compilation of the source files will name auxiliary outputs dir/foo.* and dir/bar.*, and dump outputs dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking will use dir/foobar. as the prefix for dumps and auxiliary files.

-dumpbase-ext auxdropsuf

When forming the name of an auxiliary (but not a dump) output file, drop trailing auxdropsuf from dumpbase before appending any suffixes. If not specified, this option defaults to the suffix of a default dumpbase, i.e., the suffix of the input file when -dumpbase is not present in the command line, or dumpbase is combined with dumppfx.

        gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...
    

creates dir/foo.o as the main output, and generates auxiliary outputs in dir/x-foo.*, taking the location of the primary output, and dropping the .c suffix from the dumpbase. Dump outputs retain the suffix: dir/x-foo.c.*.

This option is disregarded if it does not match the suffix of a specified dumpbase, except as an alternative to the executable suffix when appending the linker output base name to dumppfx, as specified below:

        gcc foo.c bar.c -o main.out -dumpbase-ext .out ...
    

creates main.out as the primary output, and avoids overwriting the auxiliary and dump outputs by using the executable name minus auxdropsuf as a prefix, creating auxiliary outputs named main-foo.* and main-bar.* and dump outputs named main-foo.c.* and main-bar.c.*.

-dumpdir dumppfx

When forming the name of an auxiliary or dump output file, use dumppfx as a prefix:

        gcc -dumpdir pfx- -c foo.c ...
    

creates foo.o as the primary output, and auxiliary outputs named pfx-foo.*, combining the given dumppfx with the default dumpbase derived from the default primary output, derived in turn from the input name. Dump outputs also take the input name suffix: pfx-foo.c.*.

If dumppfx is to be used as a directory name, it must end with a directory separator:

        gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...
    

creates obj/bar.o as the primary output, and auxiliary outputs named dir/bar.*, combining the given dumppfx with the default dumpbase derived from the primary output name. Dump outputs also take the input name suffix: dir/bar.c.*.

It defaults to the location of the output file, unless the output file is a special file like "/dev/null". Options -save-temps=cwd and -save-temps=obj override this default, just like an explicit -dumpdir option. In case multiple such options are given, the last one prevails:

        gcc -dumpdir pfx- -c foo.c -save-temps=obj ...
    

outputs foo.o, with auxiliary outputs named foo.* because -save-temps=* overrides the dumppfx given by the earlier -dumpdir option. It does not matter that =obj is the default for -save-temps, nor that the output directory is implicitly the current directory. Dump outputs are named foo.c.*.

When compiling from multiple input files, if -dumpbase is specified, dumpbase, minus a auxdropsuf suffix, and a dash are appended to (or override, if containing any directory components) an explicit or defaulted dumppfx, so that each of the multiple compilations gets differently-named aux and dump outputs.

        gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...
    

outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. Dump outputs retain the input file suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. Contrast with the single-input compilation:

        gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...
    

that, applying -dumpbase to a single source, does not compute and append a separate dumpbase per input file. Its auxiliary and dump outputs go in dir/pfx-main.*.

When compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also undergoes the dumpbase- transformation above (e.g. the compilation of foo.c and bar.c above, but without -c). If neither -dumpdir nor -dumpbase are given, the linker output base name, minus auxdropsuf, if specified, or the executable suffix otherwise, plus a dash is appended to the default dumppfx instead. Note, however, that unlike earlier cases of linking:

        gcc foo.c bar.c -dumpdir dir/pfx- -o main ...
    

does not append the output name main to dumppfx, because -dumpdir is explicitly specified. The goal is that the explicitly-specified dumppfx may contain the specified output name as part of the prefix, if desired; only an explicitly-specified -dumpbase would be combined with it, in order to avoid simply discarding a meaningful option.

When compiling and then linking from a single input file, the linker output base name will only be appended to the default dumppfx as above if it does not share the base name with the single input file name. This has been covered in single-input linking cases above, but not with an explicit -dumpdir that inhibits the combination, even if overridden by -save-temps=*:

        gcc foo.c -dumpdir alt/pfx- -o dir/main.exe -save-temps=cwd ...
    

Auxiliary outputs are named foo.*, and dump outputs foo.c.*, in the current working directory as ultimately requested by -save-temps=cwd.

Summing it all up for an intuitive though slightly imprecise data flow: the primary output name is broken into a directory part and a basename part; dumppfx is set to the former, unless overridden by -dumpdir or -save-temps=*, and dumpbase is set to the latter, unless overriden by -dumpbase. If there are multiple inputs or linking, this dumpbase may be combined with dumppfx and taken from each input file. Auxiliary output names for each input are formed by combining dumppfx, dumpbase minus suffix, and the auxiliary output suffix; dump output names are only different in that the suffix from dumpbase is retained.

When it comes to auxiliary and dump outputs created during LTO recompilation, a combination of dumppfx and dumpbase, as given or as derived from the linker output name but not from inputs, even in cases in which this combination would not otherwise be used as such, is passed down with a trailing period replacing the compiler-added dash, if any, as a -dumpdir option to lto-wrapper; being involved in linking, this program does not normally get any -dumpbase and -dumpbase-ext, and it ignores them.

When running sub-compilers, lto-wrapper appends LTO stage names to the received dumppfx, ensures it contains a directory component so that it overrides any -dumpdir, and passes that as -dumpbase to sub-compilers.

-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.

--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:

        --help=target,undocumented

        --help=warnings,^joined,^undocumented

        --help=target,optimizers

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

        -Q -O2 --help=optimizers

        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

--version

Display the version number and copyrights of the invoked GCC.

-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.

-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.

-specs=file

Process file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they are processed in order, from left to right.

-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 ....

-ffile-prefix-map=old=new

When compiling files residing in directory old, record any references to them in the result of the compilation as if the files resided in directory new instead. Specifying this option is equivalent to specifying all the individual -f*-prefix-map options. This can be used to make reproducible builds that are location independent. Directories referenced by directives are not affected by these options. See also -fmacro-prefix-map, -fdebug-prefix-map, -fprofile-prefix-map and -fcanon-prefix-map.

-fcanon-prefix-map

For the -f*-prefix-map options normally comparison of old prefix against the filename that would be normally referenced in the result of the compilation is done using textual comparison of the prefixes, or ignoring character case for case insensitive filesystems and considering slashes and backslashes as equal on DOS based filesystems. The -fcanon-prefix-map causes such comparisons to be done on canonicalized paths of old and the referenced filename.

-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

-aux-info filename

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

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

-fno-asm

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

In C++, "inline" is a standard keyword and is not affected by this switch. You may want to use the -fno-gnu-keywords flag instead, which disables "typeof" but not "asm" and "inline". 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. In C2X mode (-std=c2x or -std=gnu2x), this switch only affects the "asm" keyword, since "typeof" is a standard keyword in ISO C2X.

-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))
    

-fcond-mismatch

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

-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.

-fgimple

Enable parsing of function definitions marked with "__GIMPLE". This is an experimental feature that allows unit testing of GIMPLE passes.

-fgnu-tm

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

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

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

-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.

-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.

-flax-vector-conversions

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

-fms-extensions

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

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

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

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

Note that this option is off for all targets except for x86 targets using ms-abi.

-foffload=disable

-foffload=default

-foffload=target-list

Specify for which OpenMP and OpenACC offload targets code should be generated. The default behavior, equivalent to -foffload=default, is to generate code for all supported offload targets. The -foffload=disable form generates code only for the host fallback, while -foffload=target-list generates code only for the specified comma-separated list of offload targets.

Offload targets are specified in GCC's internal target-triplet format. You can run the compiler with -v to show the list of configured offload targets under "OFFLOAD_TARGET_NAMES".

-foffload-options=options

-foffload-options=target-triplet-list=options

With -foffload-options=options, GCC passes the specified options to the compilers for all enabled offloading targets. You can specify options that apply only to a specific target or targets by using the -foffload-options=target-list=options form. The target-list is a comma-separated list in the same format as for the -foffload= option.

Typical command lines are

        -foffload-options=-lgfortran -foffload-options=-lm
        -foffload-options="-lgfortran -lm" -foffload-options=nvptx-none=-latomic
        -foffload-options=amdgcn-amdhsa=-march=gfx906 -foffload-options=-lm
    

-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 v2.6 <https://www.openacc.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread.

-fopenacc-dim=geom

Specify default compute dimensions for parallel offload regions that do not explicitly specify. The geom value is a triple of ':'-separated sizes, in order 'gang', 'worker' and, 'vector'. A size can be omitted, to use a target-specific default value.

-fopenmp

Enable handling of OpenMP directives "#pragma omp" in C/C++, "[[omp::directive(...)]]" and "[[omp::sequence(...)]]" in C++ and "!$omp" in Fortran. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 <https://www.openmp.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd.

-fopenmp-simd

Enable handling of OpenMP's "simd", "declare simd", "declare reduction", "assume", "ordered", "scan", "loop" directives and combined or composite directives with "simd" as constituent with "#pragma omp" in C/C++, "[[omp::directive(...)]]" and "[[omp::sequence(...)]]" in C++ and "!$omp" in Fortran. Other OpenMP directives are ignored.

-fopenmp-target-simd-clone

-fopenmp-target-simd-clone=device-type

In addition to generating SIMD clones for functions marked with the "declare simd" directive, GCC also generates clones for functions marked with the OpenMP "declare target" directive that are suitable for vectorization when this option is in effect. The device-type may be one of "none", "host", "nohost", and "any", which correspond to keywords for the "device_type" clause of the "declare target" directive; clones are generated for the intersection of devices specified. -fopenmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=any and -fno-openmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=none.

At -O2 and higher (but not -Os or -Og) this optimization defaults to -fopenmp-target-simd-clone=nohost; otherwise it is disabled by default.

-fpermitted-flt-eval-methods=style

ISO/IEC TS 18661-3 defines new permissible values for "FLT_EVAL_METHOD" that indicate that operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type. These new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values of "FLT_EVAL_METHOD". As such, code conforming to C11 may not have been written expecting the possibility of the new values.

-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of "FLT_EVAL_METHOD" specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.

style is either "c11" or "ts-18661-3" as appropriate.

The default when in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3.

-fplan9-extensions

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

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

-fsigned-bitfields

-funsigned-bitfields

-fno-signed-bitfields

-fno-unsigned-bitfields

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

-fsigned-char

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

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

-funsigned-char

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

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

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

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

-fstrict-flex-arrays

Control when to treat the trailing array of a structure as a flexible array member for the purpose of accessing the elements of such an array. The positive form is equivalent to -fstrict-flex-arrays=3, which is the strictest. A trailing array is treated as a flexible array member only when it is declared as a flexible array member per C99 standard onwards. The negative form is equivalent to -fstrict-flex-arrays=0, which is the least strict. All trailing arrays of structures are treated as flexible array members.

-fstrict-flex-arrays=level

Control when to treat the trailing array of a structure as a flexible array member for the purpose of accessing the elements of such an array. The value of level controls the level of strictness.

The possible values of level are the same as for the "strict_flex_array" attribute.

You can control this behavior for a specific trailing array field of a structure by using the variable attribute "strict_flex_array" attribute.

-fsso-struct=endianness

Set the default scalar storage order of structures and unions to the specified endianness. The accepted values are big-endian, little-endian and native for the native endianness of the target (the default). This option is not supported for C++.

Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target.

Options Controlling C++ Dialect¶

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

        g++ -g -fstrict-enums -O -c firstClass.C

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

Some options for compiling C programs, such as -std, are also relevant for C++ programs.

Here is a list of options that are only for compiling C++ programs:

-fabi-version=n

Use version n of the C++ ABI. The default is version 0.

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

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

Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.

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

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

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

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

Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.

Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.

Version 9, which first appeared in G++ 5.2, corrects the alignment of "nullptr_t".

Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall).

Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.

Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor.

Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12.

Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.

Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag regression.

Version 16, which first appeared in G++ 11, changes the mangling of "__alignof__" to be distinct from that of "alignof", and dependent operator names.

Version 17, which first appeared in G++ 12, fixes layout of classes that inherit from aggregate classes with default member initializers in C++14 and up.

Version 18, which first appeard in G++ 13, fixes manglings of lambdas that have additional context.

See also -Wabi.

-fabi-compat-version=n

On targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias.

With -fabi-version=0 (the default), this defaults to 13 (GCC 8.2 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2.

If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning.

-fno-access-control

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

-faligned-new

Enable support for C++17 "new" of types that require more alignment than "void* ::operator new(std::size_t)" provides. A numeric argument such as "-faligned-new=32" can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default of alignof(std::max_align_t).

This flag is enabled by default for -std=c++17.

-fchar8_t

-fno-char8_t

Enable support for "char8_t" as adopted for C++20. This includes the addition of a new "char8_t" fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new "__cpp_char8_t" and "__cpp_lib_char8_t" feature test macros.

This option enables functions to be overloaded for ordinary and UTF-8 strings:

        int f(const char *);    // #1
        int f(const char8_t *); // #2
        int v1 = f("text");     // Calls #1
        int v2 = f(u8"text");   // Calls #2
    

and introduces new signatures for user-defined literals:

        int operator""_udl1(char8_t);
        int v3 = u8'x'_udl1;
        int operator""_udl2(const char8_t*, std::size_t);
        int v4 = u8"text"_udl2;
        template<typename T, T...> int operator""_udl3();
        int v5 = u8"text"_udl3;
    

The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards. For example, the following code is well-formed under ISO C++11, but is ill-formed when -fchar8_t is specified.

        const char *cp = u8"xx";// error: invalid conversion from
                                //        `const char8_t*' to `const char*'
        int f(const char*);
        auto v = f(u8"xx");     // error: invalid conversion from
                                //        `const char8_t*' to `const char*'
        std::string s{u8"xx"};  // error: no matching function for call to
                                //        `std::basic_string<char>::basic_string()'
        using namespace std::literals;
        s = u8"xx"s;            // error: conversion from
                                //        `basic_string<char8_t>' to non-scalar
                                //        type `basic_string<char>' requested
    

-fcheck-new

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

-fconcepts

-fconcepts-ts

Enable support for the C++ Concepts feature for constraining template arguments. With -std=c++20 and above, Concepts are part of the language standard, so -fconcepts defaults to on.

Some constructs that were allowed by the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), but didn't make it into the standard, can additionally be enabled by -fconcepts-ts.

-fconstexpr-depth=n

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

-fconstexpr-cache-depth=n

Set the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to n. This is a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows very large from highly recursive evaluations). The default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you.

-fconstexpr-fp-except

Annex F of the C standard specifies that IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers have historically not followed this guidance, instead treating floating point division by zero as non-constant even though it has a well defined value. This flag tells the compiler to give Annex F priority over other rules saying that a particular operation is undefined.

        constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except
    

-fconstexpr-loop-limit=n

Set the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18).

-fconstexpr-ops-limit=n

Set the maximum number of operations during a single constexpr evaluation. Even when number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is 33554432 (1<<25).

-fcontracts

Enable experimental support for the C++ Contracts feature, as briefly added to and then removed from the C++20 working paper (N4820). The implementation also includes proposed enhancements from papers P1290, P1332, and P1429. This functionality is intended mostly for those interested in experimentation towards refining the feature to get it into shape for a future C++ standard.

On violation of a checked contract, the violation handler is called. Users can replace the violation handler by defining

        void
        handle_contract_violation (const std::experimental::contract_violation&);
    

There are different sets of additional flags that can be used together to specify which contracts will be checked and how, for N4820 contracts, P1332 contracts, or P1429 contracts; these sets cannot be used together.

-fcoroutines

Enable support for the C++ coroutines extension (experimental).

-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. This option also causes G++ to call trivial member functions which otherwise would be expanded inline.

In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions.

-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.

-ffold-simple-inlines

-fno-fold-simple-inlines

Permit the C++ frontend to fold calls to "std::move", "std::forward", "std::addressof" and "std::as_const". In contrast to inlining, this means no debug information will be generated for such calls. Since these functions are rarely interesting to debug, this flag is enabled by default unless -fno-inline is active.

-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. This option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.

-fimplicit-constexpr

Make inline functions implicitly constexpr, if they satisfy the requirements for a constexpr function. This option can be used in C++14 mode or later. This can result in initialization changing from dynamic to static and other optimizations.

-fno-implicit-templates

Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. If you use this option, you must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time.

-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.

-fmodules-ts

-fno-modules-ts

Enable support for C++20 modules. The -fno-modules-ts is usually not needed, as that is the default. Even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version.

-fmodule-header

-fmodule-header=user

-fmodule-header=system

Compile a header file to create an importable header unit.

-fmodule-implicit-inline

Member functions defined in their class definitions are not implicitly inline for modular code. This is different to traditional C++ behavior, for good reasons. However, it may result in a difficulty during code porting. This option makes such function definitions implicitly inline. It does however generate an ABI incompatibility, so you must use it everywhere or nowhere. (Such definitions outside of a named module remain implicitly inline, regardless.)

-fno-module-lazy

Disable lazy module importing and module mapper creation.

-fmodule-mapper=[hostname]:port[?ident]

-fmodule-mapper=|program[?ident] args...

-fmodule-mapper==socket[?ident]

-fmodule-mapper=<>[inout][?ident]

-fmodule-mapper=<in>out[?ident]

-fmodule-mapper=file[?ident]

An oracle to query for module name to filename mappings. If unspecified the CXX_MODULE_MAPPER environment variable is used, and if that is unset, an in-process default is provided.

-fmodule-only

Only emit the Compiled Module Interface, inhibiting any object file.

-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.

-fnew-inheriting-ctors

Enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17 but also considered to be a Defect Report against C++11 and C++14. This flag is enabled by default unless -fabi-version=10 or lower is specified.

-fnew-ttp-matching

Enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. This flag is enabled by default for -std=c++17.

-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.

-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.

Mixing code compiled with -frtti with that compiled with -fno-rtti may not work. For example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti.

-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.

-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.

-fstrong-eval-order

Evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. Enabled by default with -std=c++17. -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default without -std=c++17.

-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:

-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++.

-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, ...).

-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.)

-flang-info-include-translate

-flang-info-include-translate-not

-flang-info-include-translate=header

Inform of include translation events. The first will note accepted include translations, the second will note declined include translations. The header form will inform of include translations relating to that specific header. If header is of the form "user" or "<system>" it will be resolved to a specific user or system header using the include path.

-flang-info-module-cmi

-flang-info-module-cmi=module

Inform of Compiled Module Interface pathnames. The first will note all read CMI pathnames. The module form will not reading a specific module's CMI. module may be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in "<>" or "").

-stdlib=libstdc++,libc++

When G++ is configured to support this option, it allows specification of alternate C++ runtime libraries. Two options are available: libstdc++ (the default, native C++ runtime for G++) and libc++ which is the C++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards). The option switches G++ to use the headers from the specified library and to emit "-lstdc++" or "-lc++" respectively, when a C++ runtime is required for linking.

In addition, these warning options have meanings only for C++ programs:

-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.

-Wcomma-subscript (C++ and Objective-C++ only)

Warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++20 and is going to be removed in C++23. However, a comma expression wrapped in "( )" is not deprecated. Example:

        void f(int *a, int b, int c) {
            a[b,c];     // deprecated in C++20, invalid in C++23
            a[(b,c)];   // OK
        }
    

In C++23 it is valid to have comma separated expressions in a subscript when an overloaded subscript operator is found and supports the right number and types of arguments. G++ will accept the formerly valid syntax for code that is not valid in C++23 but used to be valid but deprecated in C++20 with a pedantic warning that can be disabled with -Wno-comma-subscript.

Enabled by default with -std=c++20 unless -Wno-deprecated, and with -std=c++23 regardless of -Wno-deprecated.

-Wctad-maybe-unsupported (C++ and Objective-C++ only)

Warn when performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides. This warning will point out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended. Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types that are designed to support CTAD. This warning can be suppressed with the following pattern:

        struct allow_ctad_t; // any name works
        template <typename T> struct S {
          S(T) { }
        };
        // Guide with incomplete parameter type will never be considered.
        S(allow_ctad_t) -> S<void>;
    

-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.

-Wdangling-reference (C++ and Objective-C++ only)

Warn when a reference is bound to a temporary whose lifetime has ended. For example:

        int n = 1;
        const int& r = std::max(n - 1, n + 1); // r is dangling
    

In the example above, two temporaries are created, one for each argument, and a reference to one of the temporaries is returned. However, both temporaries are destroyed at the end of the full expression, so the reference "r" is dangling. This warning also detects dangling references in member initializer lists:

        const int& f(const int& i) { return i; }
        struct S {
          const int &r; // r is dangling
          S() : r(f(10)) { }
        };
    

Member functions are checked as well, but only their object argument:

        struct S {
           const S& self () { return *this; }
        };
        const S& s = S().self(); // s is dangling
    

Certain functions are safe in this respect, for example "std::use_facet": they take and return a reference, but they don't return one of its arguments, which can fool the warning. Such functions can be excluded from the warning by wrapping them in a "#pragma":

        #pragma GCC diagnostic push
        #pragma GCC diagnostic ignored "-Wdangling-reference"
        const T& foo (const T&) { ... }
        #pragma GCC diagnostic pop
    

-Wdangling-reference also warns about code like

        auto p = std::minmax(1, 2);
    

where "std::minmax" returns "std::pair<const int&, const int&>", and both references dangle after the end of the full expression that contains the call to "std::minmax".

This warning is enabled by -Wextra.

-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.

-Wdeprecated-copy (C++ and Objective-C++ only)

Warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up. This warning is enabled by -Wextra. With -Wdeprecated-copy-dtor, also deprecate if the class has a user-provided destructor.

-Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only)

Disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a different enumeration type. This conversion was deprecated in C++20. For example:

        enum E1 { e };
        enum E2 { f };
        int k = f - e;
    

-Wdeprecated-enum-enum-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion.

-Wno-deprecated-enum-float-conversion (C++ and Objective-C++ only)

Disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a floating-point type. This conversion was deprecated in C++20. For example:

        enum E1 { e };
        enum E2 { f };
        bool b = e <= 3.7;
    

-Wdeprecated-enum-float-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion.

-Wno-init-list-lifetime (C++ and Objective-C++ only)

Do not warn about uses of "std::initializer_list" that are likely to result in dangling pointers. Since the underlying array for an "initializer_list" is handled like a normal C++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array's lifetime. For example:

-Winvalid-constexpr

Warn when a function never produces a constant expression. In C++20 and earlier, for every "constexpr" function and function template, there must be at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed this restriction, so it's possible to have a function or a function template marked "constexpr" for which no invocation satisfies the requirements of a core constant expression.

This warning is enabled as a pedantic warning by default in C++20 and earlier. In C++23, -Winvalid-constexpr can be turned on, in which case it will be an ordinary warning. For example:

        void f (int& i);
        constexpr void
        g (int& i)
        {
          // Warns by default in C++20, in C++23 only with -Winvalid-constexpr.
          f(i);
        }
    

-Winvalid-imported-macros

Verify all imported macro definitions are valid at the end of compilation. This is not enabled by default, as it requires additional processing to determine. It may be useful when preparing sets of header-units to ensure consistent macros.

-Wno-literal-suffix (C++ and Objective-C++ only)

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

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

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

This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization.

These warnings are enabled by default.

-Wno-narrowing (C++ and Objective-C++ only)

For C++11 and later standards, narrowing conversions are diagnosed by default, as required by the standard. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but -Wno-narrowing suppresses the diagnostic. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts.

With -Wnarrowing in C++98, 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.

-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.

-Wnoexcept-type (C++ and Objective-C++ only)

Warn if the C++17 feature making "noexcept" part of a function type changes the mangled name of a symbol relative to C++14. Enabled by -Wabi and -Wc++17-compat.

As an example:

        template <class T> void f(T t) { t(); };
        void g() noexcept;
        void h() { f(g); }
    

In C++14, "f" calls "f<void(*)()>", but in C++17 it calls "f<void(*)()noexcept>".

-Wclass-memaccess (C++ and Objective-C++ only)

Warn when the destination of a call to a raw memory function such as "memset" or "memcpy" is an object of class type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call to "memset" below is undefined because it modifies a non-trivial class object and is, therefore, diagnosed. The safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available.

        std::string str = "abc";
        memset (&str, 0, sizeof str);
    

The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object to "void *" or to a type that can be safely accessed by the raw memory function suppresses the warning.

-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. The -Wdelete-non-virtual-dtor option (enabled by -Wall) should be preferred because it warns about the unsafe cases without false positives.

-Wregister (C++ and Objective-C++ only)

Warn on uses of the "register" storage class specifier, except when it is part of the GNU Explicit Register Variables extension. The use of the "register" keyword as storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17.

-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.

-Wno-pessimizing-move (C++ and Objective-C++ only)

This warning warns when a call to "std::move" prevents copy elision. A typical scenario when copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type.

        struct T {
        ...
        };
        T fn()
        {
          T t;
          ...
          return std::move (t);
        }
    

But in this example, the "std::move" call prevents copy elision.

This warning is enabled by -Wall.

-Wno-redundant-move (C++ and Objective-C++ only)

This warning warns about redundant calls to "std::move"; that is, when a move operation would have been performed even without the "std::move" call. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations such as returning a local variable, where copy elision isn't applicable. Consider:

        struct T {
        ...
        };
        T fn(T t)
        {
          ...
          return std::move (t);
        }
    

Here, the "std::move" call is redundant. Because G++ implements Core Issue 1579, another example is:

        struct T { // convertible to U
        ...
        };
        struct U {
        ...
        };
        U fn()
        {
          T t;
          ...
          return std::move (t);
        }
    

In this example, copy elision isn't applicable because the type of the expression being returned and the function return type differ, yet G++ treats the return value as if it were designated by an rvalue.

This warning is enabled by -Wextra.

-Wrange-loop-construct (C++ and Objective-C++ only)

This warning warns when a C++ range-based for-loop is creating an unnecessary copy. This can happen when the range declaration is not a reference, but probably should be. For example:

        struct S { char arr[128]; };
        void fn () {
          S arr[5];
          for (const auto x : arr) { ... }
        }
    

It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes.

This warning also warns when a loop variable in a range-based for-loop is initialized with a value of a different type resulting in a copy. For example:

        void fn() {
          int arr[10];
          for (const double &x : arr) { ... }
        }
    

In the example above, in every iteration of the loop a temporary value of type "double" is created and destroyed, to which the reference "const double &" is bound.

This warning is enabled by -Wall.

-Wredundant-tags (C++ and Objective-C++ only)

Warn about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity. For example:

        struct foo;
        struct foo *p;   // warn that keyword struct can be eliminated
    

On the other hand, in this example there is no warning:

        struct foo;
        void foo ();   // "hides" struct foo
        void bar (struct foo&);  // no warning, keyword struct is necessary
    

-Wno-subobject-linkage (C++ and Objective-C++ only)

Do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. If a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation because the meaning of B is different in each translation unit. If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main .C file, as those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default.

-Weffc++ (C++ and Objective-C++ only)

Warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of books:

-Wno-exceptions (C++ and Objective-C++ only)

Disable the warning about the case when an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers.

-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-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such as friend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default.

-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)

-Woverloaded-virtual=n

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); // does not override
        };
    

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

        B* b;
        b->f();
    

fails to compile.

In cases where the different signatures are not an accident, the simplest solution is to add a using-declaration to the derived class to un-hide the base function, e.g. add "using A::f;" to "B".

The optional level suffix controls the behavior when all the declarations in the derived class override virtual functions in the base class, even if not all of the base functions are overridden:

        struct C {
          virtual void f();
          virtual void f(int);
        };
        
        struct D: public C {
          void f(int); // does override
        }
    

This pattern is less likely to be a mistake; if D is only used virtually, the user might have decided that the base class semantics for some of the overloads are fine.

At level 1, this case does not warn; at level 2, it does. -Woverloaded-virtual by itself selects level 2. Level 1 is included in -Wall.

-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.

-Wtemplates (C++ and Objective-C++ only)

Warn when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates.

-Wmismatched-new-delete (C++ and Objective-C++ only)

Warn for mismatches between calls to "operator new" or "operator delete" and the corresponding call to the allocation or deallocation function. This includes invocations of C++ "operator delete" with pointers returned from either mismatched forms of "operator new", or from other functions that allocate objects for which the "operator delete" isn't a suitable deallocator, as well as calls to other deallocation functions with pointers returned from "operator new" for which the deallocation function isn't suitable.

For example, the "delete" expression in the function below is diagnosed because it doesn't match the array form of the "new" expression the pointer argument was returned from. Similarly, the call to "free" is also diagnosed.

        void f ()
        {
          int *a = new int[n];
          delete a;   // warning: mismatch in array forms of expressions
        
          char *p = new char[n];
          free (p);   // warning: mismatch between new and free
        }
    

The related option -Wmismatched-dealloc diagnoses mismatches involving allocation and deallocation functions other than "operator new" and "operator delete".

-Wmismatched-new-delete is included in -Wall.

-Wmismatched-tags (C++ and Objective-C++ only)

Warn for declarations of structs, classes, and class templates and their specializations with a class-key that does not match either the definition or the first declaration if no definition is provided.

For example, the declaration of "struct Object" in the argument list of "draw" triggers the warning. To avoid it, either remove the redundant class-key "struct" or replace it with "class" to match its definition.

        class Object {
        public:
          virtual ~Object () = 0;
        };
        void draw (struct Object*);
    

It is not wrong to declare a class with the class-key "struct" as the example above shows. The -Wmismatched-tags option is intended to help achieve a consistent style of class declarations. In code that is intended to be portable to Windows-based compilers the warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys. The option can be used either on its own or in conjunction with -Wredundant-tags.

-Wmultiple-inheritance (C++ and Objective-C++ only)

Warn when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use multiple inheritance.

-Wvirtual-inheritance

Warn when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use virtual inheritance.

-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.

-Wnamespaces

Warn when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also use using directives and qualified names.

-Wno-terminate (C++ and Objective-C++ only)

Disable the warning about a throw-expression that will immediately result in a call to "terminate".

-Wno-vexing-parse (C++ and Objective-C++ only)

Warn about the most vexing parse syntactic ambiguity. This warns about the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration. For instance:

        void f(double a) {
          int i();        // extern int i (void);
          int n(int(a));  // extern int n (int);
        }
    

Another example:

        struct S { S(int); };
        void f(double a) {
          S x(int(a));   // extern struct S x (int);
          S y(int());    // extern struct S y (int (*) (void));
          S z();         // extern struct S z (void);
        }
    

The warning will suggest options how to deal with such an ambiguity; e.g., it can suggest removing the parentheses or using braces instead.

This warning is enabled by default.

-Wno-class-conversion (C++ and Objective-C++ only)

Do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void; such a conversion function will never be called.

-Wvolatile (C++ and Objective-C++ only)

Warn about deprecated uses of the "volatile" qualifier. This includes postfix and prefix "++" and "--" expressions of "volatile"-qualified types, using simple assignments where the left operand is a "volatile"-qualified non-class type for their value, compound assignments where the left operand is a "volatile"-qualified non-class type, "volatile"-qualified function return type, "volatile"-qualified parameter type, and structured bindings of a "volatile"-qualified type. This usage was deprecated in C++20.

Enabled by default with -std=c++20.

-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.

-Waligned-new

Warn about a new-expression of a type that requires greater alignment than the alignof(std::max_align_t) but uses an allocation function without an explicit alignment parameter. This option is enabled by -Wall.

Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions.

-Wno-placement-new

-Wplacement-new=n

Warn about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large.

        char buf [64];
        new (buf) int[64];
    

This warning is enabled by default.

-Wcatch-value

-Wcatch-value=n (C++ and Objective-C++ only)

Warn about catch handlers that do not catch via reference. With -Wcatch-value=1 (or -Wcatch-value for short) warn about polymorphic class types that are caught by value. With -Wcatch-value=2 warn about all class types that are caught by value. With -Wcatch-value=3 warn about all types that are not caught by reference. -Wcatch-value is enabled by -Wall.

-Wconditionally-supported (C++ and Objective-C++ only)

Warn for conditionally-supported (C++11 [intro.defs]) constructs.

-Wno-delete-incomplete (C++ and Objective-C++ only)

Do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default.

-Wextra-semi (C++, Objective-C++ only)

Warn about redundant semicolons after in-class function definitions.

-Wno-inaccessible-base (C++, Objective-C++ only)

This option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the -Wextra option.

        struct A { int a; };
        
        struct B : A { };
        
        struct C : B, A { };
    

-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.

-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.

-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.

-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.

-Wuse-after-free

-Wuse-after-free=n

Warn about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. The warning is enabled at all optimization levels but may yield different results with optimization than without.

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

Warn when an expression is cast to its own type. This warning does not occur when a class object is converted to a non-reference type as that is a way to create a temporary:

        struct S { };
        void g (S&&);
        void f (S&& arg)
        {
          g (S(arg)); // make arg prvalue so that it can bind to S&&
        }
    

-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.

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). On Darwin (macOS, MacOS X) platforms, the -fconstant-cfstrings option, if also present, overrides the -fconstant-string-class setting and cause "@"..."" literals to be laid out as constant CoreFoundation strings. Note that -fconstant-cfstrings is an alias for the target-specific -mconstant-cfstrings equivalent.

-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++. 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-property-assign-default (Objective-C and Objective-C++ only)

Do not warn if a property for an Objective-C object has no assign semantics specified.

-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.

-Wobjc-root-class (Objective-C and Objective-C++ only)

Warn if a class interface lacks a superclass. Most classes will inherit from "NSObject" (or "Object") for example. When declaring classes intended to be root classes, the warning can be suppressed by marking their interfaces with "__attribute__((objc_root_class))".

-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.

Note - this option also affects the display of the #error and #warning pre-processor directives, and the deprecated function/type/variable attribute. It does not however affect the pragma GCC warning and pragma GCC error pragmas.

-fdiagnostics-plain-output

This option requests that diagnostic output look as plain as possible, which may be useful when running dejagnu or other utilities that need to parse diagnostics output and prefer that it remain more stable over time. -fdiagnostics-plain-output is currently equivalent to the following options: -fno-diagnostics-show-caret -fno-diagnostics-show-line-numbers -fdiagnostics-color=never -fdiagnostics-urls=never -fdiagnostics-path-format=separate-events In the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior will be added to this list.

-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 makes GCC use color only when the standard error is a terminal, and when not executing in an emacs shell. 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:range1=32:range2=34:locus=01:\
        quote=01:path=01;36:fixit-insert=32:fixit-delete=31:\
        diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
        type-diff=01;32:fnname=01;32:targs=35
    

where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red. Setting GCC_COLORS to the empty string disables colors. Supported capabilities are as follows.

-fdiagnostics-urls[=WHEN]

Use escape sequences to embed URLs in diagnostics. For example, when -fdiagnostics-show-option emits text showing the command-line option controlling a diagnostic, embed a URL for documentation of that option.

WHEN is never, always, or auto. auto makes GCC use URL escape sequences only when the standard error is a terminal, and when not executing in an emacs shell or any graphical terminal which is known to be incompatible with this feature, see below.

The default depends on how the compiler has been configured. It can be any of the above WHEN options.

GCC can also be configured (via the --with-diagnostics-urls=auto-if-env configure-time option) so that the default is affected by environment variables. Under such a configuration, GCC defaults to using auto if either GCC_URLS or TERM_URLS environment variables are present and non-empty in the environment of the compiler, or never if neither are.

However, even with -fdiagnostics-urls=always the behavior is dependent on those environment variables: If GCC_URLS is set to empty or no, do not embed URLs in diagnostics. If set to st, URLs use ST escape sequences. If set to bel, the default, URLs use BEL escape sequences. Any other non-empty value enables the feature. If GCC_URLS is not set, use TERM_URLS as a fallback. Note: ST is an ANSI escape sequence, string terminator ESC \, BEL is an ASCII character, CTRL-G that usually sounds like a beep.

At this time GCC tries to detect also a few terminals that are known to not implement the URL feature, and have bugs or at least had bugs in some versions that are still in use, where the URL escapes are likely to misbehave, i.e. print garbage on the screen. That list is currently xfce4-terminal, certain known to be buggy gnome-terminal versions, the linux console, and mingw. This check can be skipped with the -fdiagnostics-urls=always.

-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.

-fno-diagnostics-show-labels

By default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, such as the types of expressions:

            printf ("foo %s bar", long_i + long_j);
                         ~^       ~~~~~~~~~~~~~~~
                          |              |
                          char *         long int
    

This option suppresses the printing of these labels (in the example above, the vertical bars and the "char *" and "long int" text).

-fno-diagnostics-show-cwe

Diagnostic messages can optionally have an associated
CWE ("https://cwe.mitre.org/index.html") identifier. GCC itself only provides such metadata for some of the -fanalyzer diagnostics. GCC plugins may also provide diagnostics with such metadata. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata.

-fno-diagnostics-show-rules

Diagnostic messages can optionally have rules associated with them, such as from a coding standard, or a specification. GCC itself does not do this for any of its diagnostics, but plugins may do so. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata.

-fno-diagnostics-show-line-numbers

By default, when printing source code (via -fdiagnostics-show-caret), a left margin is printed, showing line numbers. This option suppresses this left margin.

-fdiagnostics-minimum-margin-width=width

This option controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. It defaults to 6.

-fdiagnostics-parseable-fixits

Emit fix-it hints in a machine-parseable format, suitable for consumption by IDEs. For each fix-it, a line will be printed after the relevant diagnostic, starting with the string "fix-it:". For example:

        fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"
    

The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. In the above example, bytes 3 through 20 of line 45 of "test.c" are to be replaced with the given string:

        00000000011111111112222222222
        12345678901234567890123456789
          gtk_widget_showall (dlg);
          ^^^^^^^^^^^^^^^^^^
          gtk_widget_show_all
    

The filename and replacement string escape backslash as "\\", tab as "\t", newline as "\n", double quotes as "\"", non-printable characters as octal (e.g. vertical tab as "\013").

An empty replacement string indicates that the given range is to be removed. An empty range (e.g. "45:3-45:3") indicates that the string is to be inserted at the given position.

-fdiagnostics-generate-patch

Print fix-it hints to stderr in unified diff format, after any diagnostics are printed. For example:

        --- test.c
        +++ test.c
        @ -42,5 +42,5 @
        
         void show_cb(GtkDialog *dlg)
         {
        -  gtk_widget_showall(dlg);
        +  gtk_widget_show_all(dlg);
         }
    

The diff may or may not be colorized, following the same rules as for diagnostics (see -fdiagnostics-color).

-fdiagnostics-show-template-tree

In the C++ frontend, when printing diagnostics showing mismatching template types, such as:

          could not convert 'std::map<int, std::vector<double> >()'
            from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
    

the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and differing parts of the types, such as:

          map<
            [...],
            vector<
              [double != float]>>
    

The parts that differ are highlighted with color ("double" and "float" in this case).

-fno-elide-type

By default when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types are printed as "[...]" to simplify the error message. For example:

          could not convert 'std::map<int, std::vector<double> >()'
            from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
    

Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag.

-fdiagnostics-path-format=KIND

Specify how to print paths of control-flow events for diagnostics that have such a path associated with them.

KIND is none, separate-events, or inline-events, the default.

none means to not print diagnostic paths.

separate-events means to print a separate "note" diagnostic for each event within the diagnostic. For example:

        test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
        test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
        test.c:27:3: note: (2) when 'i < count'
        test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 1
    

inline-events means to print the events "inline" within the source code. This view attempts to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source.

For example, the same events as above might be printed as:

          'test': events 1-3
            |
            |   25 |   list = PyList_New(0);
            |      |          ^~~~~~~~~~~~~
            |      |          |
            |      |          (1) when 'PyList_New' fails, returning NULL
            |   26 |
            |   27 |   for (i = 0; i < count; i++) {
            |      |   ~~~
            |      |   |
            |      |   (2) when 'i < count'
            |   28 |     item = PyLong_FromLong(random());
            |   29 |     PyList_Append(list, item);
            |      |     ~~~~~~~~~~~~~~~~~~~~~~~~~
            |      |     |
            |      |     (3) when calling 'PyList_Append', passing NULL from (1) as argument 1
            |
    

Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show how stack frames are nested, pushed, and popped.

For example:

          'test': events 1-2
            |
            |  133 | {
            |      | ^
            |      | |
            |      | (1) entering 'test'
            |  134 |   boxed_int *obj = make_boxed_int (i);
            |      |                    ~~~~~~~~~~~~~~~~~~
            |      |                    |
            |      |                    (2) calling 'make_boxed_int'
            |
            +--> 'make_boxed_int': events 3-4
                   |
                   |  120 | {
                   |      | ^
                   |      | |
                   |      | (3) entering 'make_boxed_int'
                   |  121 |   boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
                   |      |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                   |      |                                    |
                   |      |                                    (4) calling 'wrapped_malloc'
                   |
                   +--> 'wrapped_malloc': events 5-6
                          |
                          |    7 | {
                          |      | ^
                          |      | |
                          |      | (5) entering 'wrapped_malloc'
                          |    8 |   return malloc (size);
                          |      |          ~~~~~~~~~~~~~
                          |      |          |
                          |      |          (6) calling 'malloc'
                          |
            <-------------+
            |
         'test': event 7
            |
            |  138 |   free_boxed_int (obj);
            |      |   ^~~~~~~~~~~~~~~~~~~~
            |      |   |
            |      |   (7) calling 'free_boxed_int'
            |
        (etc)
    

-fdiagnostics-show-path-depths

This option provides additional information when printing control-flow paths associated with a diagnostic.

If this is option is provided then the stack depth will be printed for each run of events within -fdiagnostics-path-format=inline-events. If provided with -fdiagnostics-path-format=separate-events, then the stack depth and function declaration will be appended when printing each event.

This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow.

-fno-show-column

Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu.

-fdiagnostics-column-unit=UNIT

Select the units for the column number. This affects traditional diagnostics (in the absence of -fno-show-column), as well as JSON format diagnostics if requested.

The default UNIT, display, considers the number of display columns occupied by each character. This may be larger than the number of bytes required to encode the character, in the case of tab characters, or it may be smaller, in the case of multibyte characters. For example, the character "GREEK SMALL LETTER PI (U+03C0)" occupies one display column, and its UTF-8 encoding requires two bytes; the character "SLIGHTLY SMILING FACE (U+1F642)" occupies two display columns, and its UTF-8 encoding requires four bytes.

Setting UNIT to byte changes the column number to the raw byte count in all cases, as was traditionally output by GCC prior to version 11.1.0.

-fdiagnostics-column-origin=ORIGIN

Select the origin for column numbers, i.e. the column number assigned to the first column. The default value of 1 corresponds to traditional GCC behavior and to the GNU style guide. Some utilities may perform better with an origin of 0; any non-negative value may be specified.

-fdiagnostics-escape-format=FORMAT

When GCC prints pertinent source lines for a diagnostic it normally attempts to print the source bytes directly. However, some diagnostics relate to encoding issues in the source file, such as malformed UTF-8, or issues with Unicode normalization. These diagnostics are flagged so that GCC will escape bytes that are not printable ASCII when printing their pertinent source lines.

This option controls how such bytes should be escaped.

The default FORMAT, unicode displays Unicode characters that are not printable ASCII in the form <U+XXXX>, and bytes that do not correspond to a Unicode character validly-encoded in UTF-8-encoded will be displayed as hexadecimal in the form <XX>.

For example, a source line containing the string before followed by the Unicode character U+03C0 ("GREEK SMALL LETTER PI", with UTF-8 encoding 0xCF 0x80) followed by the byte 0xBF (a stray UTF-8 trailing byte), followed by the string after will be printed for such a diagnostic as:

         before<U+03C0><BF>after
    

Setting FORMAT to bytes will display all non-printable-ASCII bytes in the form <XX>, thus showing the underlying encoding of non-ASCII Unicode characters. For the example above, the following will be printed:

         before<CF><80><BF>after
    

-fdiagnostics-format=FORMAT

Select a different format for printing diagnostics. FORMAT is text, sarif-stderr, sarif-file, json, json-stderr, or json-file.

The default is text.

The sarif-stderr and sarif-file formats both emit diagnostics in SARIF Version 2.1.0 format, either to stderr, or to a file named source.sarif, respectively.

The json format is a synonym for json-stderr. The json-stderr and json-file formats are identical, apart from where the JSON is emitted to - with the former, the JSON is emitted to stderr, whereas with json-file it is written to source.gcc.json.

The emitted JSON consists of a top-level JSON array containing JSON objects representing the diagnostics. The JSON is emitted as one line, without formatting; the examples below have been formatted for clarity.

Diagnostics can have child diagnostics. For example, this error and note:

        misleading-indentation.c:15:3: warning: this 'if' clause does not
          guard... [-Wmisleading-indentation]
           15 |   if (flag)
              |   ^~
        misleading-indentation.c:17:5: note: ...this statement, but the latter
          is misleadingly indented as if it were guarded by the 'if'
           17 |     y = 2;
              |     ^
    

might be printed in JSON form (after formatting) like this:

        [
            {
                "kind": "warning",
                "locations": [
                    {
                        "caret": {
                            "display-column": 3,
                            "byte-column": 3,
                            "column": 3,
                            "file": "misleading-indentation.c",
                            "line": 15
                        },
                        "finish": {
                            "display-column": 4,
                            "byte-column": 4,
                            "column": 4,
                            "file": "misleading-indentation.c",
                            "line": 15
                        }
                    }
                ],
                "message": "this \u2018if\u2019 clause does not guard...",
                "option": "-Wmisleading-indentation",
                "option_url": "https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmisleading-indentation",
                "children": [
                    {
                        "kind": "note",
                        "locations": [
                            {
                                "caret": {
                                    "display-column": 5,
                                    "byte-column": 5,
                                    "column": 5,
                                    "file": "misleading-indentation.c",
                                    "line": 17
                                }
                            }
                        ],
                        "escape-source": false,
                        "message": "...this statement, but the latter is ..."
                    }
                ]
                "escape-source": false,
                "column-origin": 1,
            }
        ]
    

where the "note" is a child of the "warning".

A diagnostic has a "kind". If this is "warning", then there is an "option" key describing the command-line option controlling the warning.

A diagnostic can contain zero or more locations. Each location has an optional "label" string and up to three positions within it: a "caret" position and optional "start" and "finish" positions. A position is described by a "file" name, a "line" number, and three numbers indicating a column position:

        bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka
           'struct s'} and 'T' {aka 'struct t'})
           64 |   return callee_4a () + callee_4b ();
              |          ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
              |          |              |
              |          |              T {aka struct t}
              |          S {aka struct s}

            {
                "children": [],
                "kind": "error",
                "locations": [
                    {
                        "caret": {
                            "column": 23, "file": "bad-binary-ops.c", "line": 64
                        }
                    },
                    {
                        "caret": {
                            "column": 10, "file": "bad-binary-ops.c", "line": 64
                        },
                        "finish": {
                            "column": 21, "file": "bad-binary-ops.c", "line": 64
                        },
                        "label": "S {aka struct s}"
                    },
                    {
                        "caret": {
                            "column": 25, "file": "bad-binary-ops.c", "line": 64
                        },
                        "finish": {
                            "column": 36, "file": "bad-binary-ops.c", "line": 64
                        },
                        "label": "T {aka struct t}"
                    }
                ],
                "escape-source": false,
                "message": "invalid operands to binary + ..."
            }

        demo.c:8:15: error: 'struct s' has no member named 'colour'; did you
          mean 'color'?
            8 |   return ptr->colour;
              |               ^~~~~~
              |               color

            {
                "children": [],
                "fixits": [
                    {
                        "next": {
                            "column": 21,
                            "file": "demo.c",
                            "line": 8
                        },
                        "start": {
                            "column": 15,
                            "file": "demo.c",
                            "line": 8
                        },
                        "string": "color"
                    }
                ],
                "kind": "error",
                "locations": [
                    {
                        "caret": {
                            "column": 15,
                            "file": "demo.c",
                            "line": 8
                        },
                        "finish": {
                            "column": 20,
                            "file": "demo.c",
                            "line": 8
                        }
                    }
                ],
                "escape-source": false,
                "message": "\u2018struct s\u2019 has no member named ..."
            }

            "path": [
                {
                    "depth": 0,
                    "description": "when 'PyList_New' fails, returning NULL",
                    "function": "test",
                    "location": {
                        "column": 10,
                        "file": "test.c",
                        "line": 25
                    }
                },
                {
                    "depth": 0,
                    "description": "when 'i < count'",
                    "function": "test",
                    "location": {
                        "column": 3,
                        "file": "test.c",
                        "line": 27
                    }
                },
                {
                    "depth": 0,
                    "description": "when calling 'PyList_Append', passing NULL from (1) as argument 1",
                    "function": "test",
                    "location": {
                        "column": 5,
                        "file": "test.c",
                        "line": 29
                    }
                }
            ]

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. Additional warnings can be produced by enabling the static analyzer;

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.

The effectiveness of some warnings depends on optimizations also being enabled. For example -Wsuggest-final-types is more effective with link-time optimization and some instances of other warnings may not be issued at all unless optimization is enabled. While optimization in general improves the efficacy of control and data flow sensitive warnings, in some cases it may also cause false positives.

-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 __. This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. 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) -Warray-compare -Warray-parameter=2 (C and Objective-C only) -Wbool-compare -Wbool-operation -Wc++11-compat -Wc++14-compat -Wcatch-value (C++ and Objective-C++ only) -Wchar-subscripts -Wcomment -Wdangling-pointer=2 -Wduplicate-decl-specifier (C and Objective-C only) -Wenum-compare (in C/ObjC; this is on by default in C++) -Wenum-int-mismatch (C and Objective-C only) -Wformat -Wformat-overflow -Wformat-truncation -Wint-in-bool-context -Wimplicit (C and Objective-C only) -Wimplicit-int (C and Objective-C only) -Wimplicit-function-declaration (C and Objective-C only) -Winit-self (only for C++) -Wlogical-not-parentheses -Wmain (only for C/ObjC and unless -ffreestanding) -Wmaybe-uninitialized -Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation (only for C/C++) -Wmismatched-dealloc -Wmismatched-new-delete (only for C/C++) -Wmissing-attributes -Wmissing-braces (only for C/ObjC) -Wmultistatement-macros -Wnarrowing (only for C++) -Wnonnull -Wnonnull-compare -Wopenmp-simd -Wparentheses -Wpessimizing-move (only for C++) -Wpointer-sign -Wrange-loop-construct (only for C++) -Wreorder -Wrestrict -Wreturn-type -Wself-move (only for C++) -Wsequence-point -Wsign-compare (only in C++) -Wsizeof-array-div -Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1 -Wswitch -Wtautological-compare -Wtrigraphs -Wuninitialized -Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value -Wunused-variable -Wuse-after-free=2 -Wvla-parameter (C and Objective-C only) -Wvolatile-register-var -Wzero-length-bounds

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 -Wcast-function-type -Wdangling-reference (C++ only) -Wdeprecated-copy (C++ only) -Wempty-body -Wenum-conversion (C only) -Wignored-qualifiers -Wimplicit-fallthrough=3 -Wmissing-field-initializers -Wmissing-parameter-type (C only) -Wold-style-declaration (C only) -Woverride-init -Wsign-compare (C only) -Wstring-compare -Wredundant-move (only for C++) -Wtype-limits -Wuninitialized -Wshift-negative-value (in C++11 to C++17 and in C99 and newer) -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:

-Wabi (C, Objective-C, C++ and Objective-C++ only)

Warn about code affected by ABI changes. This includes code that may not be compatible with the vendor-neutral C++ ABI as well as the psABI for the particular target.

Since G++ now defaults to updating the ABI with each major release, normally -Wabi warns only about C++ ABI compatibility problems if there is a check added later in a release series for an ABI issue discovered since the initial release. -Wabi warns about more things if an older ABI version is selected (with -fabi-version=n).

-Wabi can also be used with an explicit version number to warn about C++ ABI compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.

If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for C++ ABI warnings.

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.

Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:

-Wno-changes-meaning (C++ and Objective-C++ only)

C++ requires that unqualified uses of a name within a class have the same meaning in the complete scope of the class, so declaring the name after using it is ill-formed:

        struct A;
        struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
        struct B2 { A a; struct A { }; }; // error, 'A' changes meaning
    

By default, the B1 case is only a warning because the two declarations have the same type, while the B2 case is an error. Both diagnostics can be disabled with -Wno-changes-meaning. Alternately, the error case can be reduced to a warning with -Wno-error=changes-meaning or -fpermissive.

Both diagnostics are also suppressed by -fms-extensions.

-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.

-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-generate 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-coverage-invalid-line-number

Warn in case a function ends earlier than it begins due to an invalid linenum macros. The warning is emitted only with --coverage enabled.

By default, this warning is enabled and is treated as an error. -Wno-coverage-invalid-line-number can be used to disable the warning or -Wno-error=coverage-invalid-line-number can be used to disable the error.

-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".

-Wduplicate-decl-specifier (C and Objective-C only)

Warn if a declaration has duplicate "const", "volatile", "restrict" or "_Atomic" specifier. This warning is enabled by -Wall.

-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).

-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.

-Wformat-overflow

-Wformat-overflow=level

Warn about calls to formatted input/output functions such as "sprintf" and "vsprintf" that might overflow the destination buffer. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives.

-Wno-format-zero-length

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

-Wformat-nonliteral

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

-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.

-Wformat-truncation

-Wformat-truncation=level

Warn about calls to formatted input/output functions such as "snprintf" and "vsnprintf" that might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic -Wformat-overflow.

-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.

-Wnonnull-compare

Warn when comparing an argument marked with the "nonnull" function attribute against null inside the function.

-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.

-Wnull-dereference

Warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when -fdelete-null-pointer-checks is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used.

-Winfinite-recursion

Warn about infinitely recursive calls. The warning is effective at all optimization levels but requires optimization in order to detect infinite recursion in calls between two or more functions. -Winfinite-recursion is included in -Wall.

Compare with -Wanalyzer-infinite-recursion which provides a similar diagnostic, but is implemented in a different way (as part of -fanalyzer).

-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++.

-Wno-implicit-int (C and Objective-C only)

This option controls warnings when a declaration does not specify a type. This warning is enabled by default in C99 and later dialects of C, and also by -Wall.

-Wno-implicit-function-declaration (C and Objective-C only)

This option controls warnings when a function is used before being declared. This warning is enabled by default in C99 and later dialects of C, and also by -Wall. The warning is made into an error by -pedantic-errors.

-Wimplicit (C and Objective-C only)

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

-Wimplicit-fallthrough

-Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0.

-Wimplicit-fallthrough=n

Warn when a switch case falls through. For example:

        switch (cond)
          {
          case 1:
            a = 1;
            break;
          case 2:
            a = 2;
          case 3:
            a = 3;
            break;
          }
    

This warning does not warn when the last statement of a case cannot fall through, e.g. when there is a return statement or a call to function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes into account control flow statements, such as ifs, and only warns when appropriate. E.g.

        switch (cond)
          {
          case 1:
            if (i > 3) {
              bar (5);
              break;
            } else if (i < 1) {
              bar (0);
            } else
              return;
          default:
            ...
          }
    

Since there are occasions where a switch case fall through is desirable, GCC provides an attribute, "__attribute__ ((fallthrough))", that is to be used along with a null statement to suppress this warning that would normally occur:

        switch (cond)
          {
          case 1:
            bar (0);
            __attribute__ ((fallthrough));
          default:
            ...
          }
    

C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning using "[[fallthrough]];" instead of the GNU attribute. In C++11 or C++14 users can use "[[gnu::fallthrough]];", which is a GNU extension. Instead of these attributes, it is also possible to add a fallthrough comment to silence the warning. The whole body of the C or C++ style comment should match the given regular expressions listed below. The option argument n specifies what kind of comments are accepted:

        switch (cond)
          {
          case 1:
            bar (0);
            /* FALLTHRU */
          default:
            ...
          }

-Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only)

Control if warnings triggered by the "warn_if_not_aligned" attribute should be issued. These warnings are enabled by default.

-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.

-Wno-ignored-attributes (C and C++ only)

This option controls warnings when an attribute is ignored. This is different from the -Wattributes option in that it warns whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used in a wrong place, etc. This warning is enabled by default.

-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.

-Wmisleading-indentation (C and C++ only)

Warn when the indentation of the code does not reflect the block structure. Specifically, a warning is issued for "if", "else", "while", and "for" clauses with a guarded statement that does not use braces, followed by an unguarded statement with the same indentation.

In the following example, the call to "bar" is misleadingly indented as if it were guarded by the "if" conditional.

          if (some_condition ())
            foo ();
            bar ();  /* Gotcha: this is not guarded by the "if".  */
    

In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line up (defaulting to 8).

The warning is not issued for code involving multiline preprocessor logic such as the following example.

          if (flagA)
            foo (0);
        #if SOME_CONDITION_THAT_DOES_NOT_HOLD
          if (flagB)
        #endif
            foo (1);
    

The warning is not issued after a "#line" directive, since this typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references.

This warning is enabled by -Wall in C and C++.

-Wmissing-attributes

Warn when a declaration of a function is missing one or more attributes that a related function is declared with and whose absence may adversely affect the correctness or efficiency of generated code. For example, the warning is issued for declarations of aliases that use attributes to specify less restrictive requirements than those of their targets. This typically represents a potential optimization opportunity. By contrast, the -Wattribute-alias=2 option controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. Attributes considered include "alloc_align", "alloc_size", "cold", "const", "hot", "leaf", "malloc", "nonnull", "noreturn", "nothrow", "pure", "returns_nonnull", and "returns_twice".

In C++, the warning is issued when an explicit specialization of a primary template declared with attribute "alloc_align", "alloc_size", "assume_aligned", "format", "format_arg", "malloc", or "nonnull" is declared without it. Attributes "deprecated", "error", and "warning" suppress the warning..

You can use the "copy" attribute to apply the same set of attributes to a declaration as that on another declaration without explicitly enumerating the attributes. This attribute can be applied to declarations of functions, variables, or types.

-Wmissing-attributes is enabled by -Wall.

For example, since the declaration of the primary function template below makes use of both attribute "malloc" and "alloc_size" the declaration of the explicit specialization of the template is diagnosed because it is missing one of the attributes.

        template <class T>
        T* __attribute__ ((malloc, alloc_size (1)))
        allocate (size_t);
        
        template <>
        void* __attribute__ ((malloc))   // missing alloc_size
        allocate<void> (size_t);
    

-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.

        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, Objective-C++ and Fortran only)

Warn if a user-supplied include directory does not exist. This option is disabled by default for C, C++, Objective-C and Objective-C++. For Fortran, it is partially enabled by default by warning for -I and -J, only.

-Wno-missing-profile

This option controls warnings if feedback profiles are missing when using the -fprofile-use option. This option diagnoses those cases where a new function or a new file is added between compiling with -fprofile-generate and with -fprofile-use, without regenerating the profiles. In these cases, the profile feedback data files do not contain any profile feedback information for the newly added function or file respectively. Also, in the case when profile count data (.gcda) files are removed, GCC cannot use any profile feedback information. In all these cases, warnings are issued to inform you that a profile generation step is due. Ignoring the warning can result in poorly optimized code. -Wno-missing-profile can be used to disable the warning, but this is not recommended and should be done only when non-existent profile data is justified.

-Wmismatched-dealloc

Warn for calls to deallocation functions with pointer arguments returned from from allocations functions for which the former isn't a suitable deallocator. A pair of functions can be associated as matching allocators and deallocators by use of attribute "malloc". Unless disabled by the -fno-builtin option the standard functions "calloc", "malloc", "realloc", and "free", as well as the corresponding forms of C++ "operator new" and "operator delete" are implicitly associated as matching allocators and deallocators. In the following example "mydealloc" is the deallocator for pointers returned from "myalloc".

        void mydealloc (void*);
        
        __attribute__ ((malloc (mydealloc, 1))) void*
        myalloc (size_t);
        
        void f (void)
        {
          void *p = myalloc (32);
          // ...use p...
          free (p);   // warning: not a matching deallocator for myalloc
          mydealloc (p);   // ok
        }
    

In C++, the related option -Wmismatched-new-delete diagnoses mismatches involving either "operator new" or "operator delete".

Option -Wmismatched-dealloc is included in -Wall.

-Wmultistatement-macros

Warn about unsafe multiple statement macros that appear to be guarded by a clause such as "if", "else", "for", "switch", or "while", in which only the first statement is actually guarded after the macro is expanded.

For example:

        #define DOIT x++; y++
        if (c)
          DOIT;
    

will increment "y" unconditionally, not just when "c" holds. The can usually be fixed by wrapping the macro in a do-while loop:

        #define DOIT do { x++; y++; } while (0)
        if (c)
          DOIT;
    

This warning is enabled by -Wall in C and C++.

-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 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.

For C++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration:

        {
          // Declares a local variable called mymutex.
          std::unique_lock<std::mutex> (mymutex);
          // User meant std::unique_lock<std::mutex> lock (mymutex);
        }
    

This warning is enabled by -Wall.

-Wno-self-move (C++ and Objective-C++ only)

This warning warns when a value is moved to itself with "std::move". Such a "std::move" typically has no effect.

        struct T {
        ...
        };
        void fn()
        {
          T t;
          ...
          t = std::move (t);
        }
    

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 C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this option will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++.

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 <https://gcc.gnu.org/readings.html>.

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

-Wno-return-local-addr

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

-Wreturn-type

Warn whenever a function is defined with a return type that defaults to "int". Also warn about any "return" statement with no return value in a function whose return type is not "void" (falling off the end of the function body is considered returning without a value).

For C only, warn about a "return" statement with an expression in a function whose return type is "void", unless the expression type is also "void". As a GNU extension, the latter case is accepted without a warning unless -Wpedantic is used. Attempting to use the return value of a non-"void" function other than "main" that flows off the end by reaching the closing curly brace that terminates the function is undefined.

Unlike in C, in C++, flowing off the end of a non-"void" function other than "main" results in undefined behavior even when the value of the function is not used.

This warning is enabled by default in C++ and by -Wall otherwise.

-Wno-shift-count-negative

Controls warnings if a shift count is negative. This warning is enabled by default.

-Wno-shift-count-overflow

Controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is enabled by default.

-Wshift-negative-value

Warn if left shifting a negative value. This warning is enabled by -Wextra in C99 (and newer) and C++11 to C++17 modes.

-Wno-shift-overflow

-Wshift-overflow=n

These options control warnings about left shift overflows.

-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.

-Wno-switch-bool

Do not warn when a "switch" statement has an index of boolean type and the case values are outside the range of a 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.

-Wno-switch-outside-range

This option controls warnings when a "switch" case has a value that is outside of its respective type range. This warning is enabled by default for C and C++ programs.

-Wno-switch-unreachable

Do not warn when a "switch" statement contains statements between the controlling expression and the first case label, which will never be executed. For example:

        switch (cond)
          {
           i = 15;
          ...
           case 5:
          ...
          }
    

-Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration:

        switch (cond)
          {
           int i;
          ...
           case 5:
           i = 5;
          ...
          }
    

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.

-Wtrivial-auto-var-init

Warn when "-ftrivial-auto-var-init" cannot initialize the automatic variable. A common situation is an automatic variable that is declared between the controlling expression and the first case label of a "switch" statement.

-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 or static variable is unused aside from its declaration. This option implies -Wunused-const-variable=1 for C, but not for C++. This warning is enabled by -Wall.

To suppress this warning use the "unused" attribute.

-Wunused-const-variable

-Wunused-const-variable=n

Warn whenever a constant static variable is unused aside from its declaration. -Wunused-const-variable=1 is enabled by -Wunused-variable for C, but not for C++. In C this declares variable storage, but in C++ this is not an error since const variables take the place of "#define"s.

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 object with automatic or allocated storage duration is used without having been initialized. In C++, also warn if a non-static reference or non-static "const" member appears in a class without constructors.

In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a "const"-qualified argument of a built-in function known to read the object is also diagnosed by this warning. (-Wmaybe-uninitialized is issued for ordinary functions.)

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 elements of structure, union or array variables as well as for variables that are uninitialized 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 depend 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.

In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC warns about "b" being uninitialized in the following snippet:

        struct A {
          int a;
          int b;
          A() : a(b) { }
        };
    

-Wno-invalid-memory-model

This option controls warnings for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with a memory consistency argument that is either invalid for the operation or outside the range of values of the "memory_order" enumeration. For example, since the "__atomic_store" and "__atomic_store_n" built-ins are only defined for the relaxed, release, and sequentially consistent memory orders the following code is diagnosed:

        void store (int *i)
        {
          __atomic_store_n (i, 0, memory_order_consume);
        }
    

-Winvalid-memory-model is enabled by default.

-Wmaybe-uninitialized

For an object with automatic or allocated storage duration, if there exists a path from the function entry to a use of the object that is initialized, but there exist some other paths for which the object is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time.

In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a "const"-qualified function argument is also diagnosed by this warning. (-Wuninitialized is issued for built-in functions known to read the object.) Annotating the function with attribute "access (none)" indicates that the argument isn't used to access the object and avoids the warning.

These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track of the state of variables.

These warnings are made optional because GCC may not be able to determine when the code is 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". 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.

-Wno-prio-ctor-dtor

Do not warn if a priority from 0 to 100 is used for constructor or destructor. The use of constructor and destructor attributes allow you to assign a priority to the constructor/destructor to control its order of execution before "main" is called or after it returns. The priority values must be greater than 100 as the compiler reserves priority values between 0--100 for the implementation.

-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 signed overflow is undefined. 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.

-Wstring-compare

Warn for calls to "strcmp" and "strncmp" whose result is determined to be either zero or non-zero in tests for such equality owing to the length of one argument being greater than the size of the array the other argument is stored in (or the bound in the case of "strncmp"). Such calls could be mistakes. For example, the call to "strcmp" below is diagnosed because its result is necessarily non-zero irrespective of the contents of the array "a".

        extern char a[4];
        void f (char *d)
        {
          strcpy (d, "string");
          ...
          if (0 == strcmp (a, d))   // cannot be true
            puts ("a and d are the same");
        }
    

-Wstring-compare is enabled by -Wextra.

-Wno-stringop-overflow

-Wstringop-overflow

-Wstringop-overflow=type

Warn for calls to string manipulation functions such as "memcpy" and "strcpy" that are determined to overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to determine the size of the destination. The argument is meaningful only for functions that operate on character arrays but not for raw memory functions like "memcpy" which always make use of Object Size type-0. The option also warns for calls that specify a size in excess of the largest possible object or at most "SIZE_MAX / 2" bytes. The option produces the best results with optimization enabled but can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like "__builtin_memcpy" that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins. For example, the option issues a warning for the "strcpy" call below because it copies at least 5 characters (the string "blue" including the terminating NUL) into the buffer of size 4.

        enum Color { blue, purple, yellow };
        const char* f (enum Color clr)
        {
          static char buf [4];
          const char *str;
          switch (clr)
            {
              case blue: str = "blue"; break;
              case purple: str = "purple"; break;
              case yellow: str = "yellow"; break;
            }
        
          return strcpy (buf, str);   // warning here
        }
    

Option -Wstringop-overflow=2 is enabled by default.

-Wno-stringop-overread

Warn for calls to string manipulation functions such as "memchr", or "strcpy" that are determined to read past the end of the source sequence.

Option -Wstringop-overread is enabled by default.

-Wno-stringop-truncation

Do not warn for calls to bounded string manipulation functions such as "strncat", "strncpy", and "stpncpy" that may either truncate the copied string or leave the destination unchanged.

In the following example, the call to "strncat" specifies a bound that is less than the length of the source string. As a result, the copy of the source will be truncated and so the call is diagnosed. To avoid the warning use "bufsize - strlen (buf) - 1)" as the bound.

        void append (char *buf, size_t bufsize)
        {
          strncat (buf, ".txt", 3);
        }
    

As another example, the following call to "strncpy" results in copying to "d" just the characters preceding the terminating NUL, without appending the NUL to the end. Assuming the result of "strncpy" is necessarily a NUL-terminated string is a common mistake, and so the call is diagnosed. To avoid the warning when the result is not expected to be NUL-terminated, call "memcpy" instead.

        void copy (char *d, const char *s)
        {
          strncpy (d, s, strlen (s));
        }
    

In the following example, the call to "strncpy" specifies the size of the destination buffer as the bound. If the length of the source string is equal to or greater than this size the result of the copy will not be NUL-terminated. Therefore, the call is also diagnosed. To avoid the warning, specify "sizeof buf - 1" as the bound and set the last element of the buffer to "NUL".

        void copy (const char *s)
        {
          char buf[80];
          strncpy (buf, s, sizeof buf);
          ...
        }
    

In situations where a character array is intended to store a sequence of bytes with no terminating "NUL" such an array may be annotated with attribute "nonstring" to avoid this warning. Such arrays, however, are not suitable arguments to functions that expect "NUL"-terminated strings. To help detect accidental misuses of such arrays GCC issues warnings unless it can prove that the use is safe.

-Wstrict-flex-arrays

Warn about inproper usages of flexible array members according to the level of the "strict_flex_array (level)" attribute attached to the trailing array field of a structure if it's available, otherwise according to the level of the option -fstrict-flex-arrays=level.

This option is effective only when level is bigger than 0. Otherwise, it will be ignored with a warning.

when level=1, warnings will be issued for a trailing array reference of a structure that have 2 or more elements if the trailing array is referenced as a flexible array member.

when level=2, in addition to level=1, additional warnings will be issued for a trailing one-element array reference of a structure if the array is referenced as a flexible array member.

when level=3, in addition to level=2, additional warnings will be issued for a trailing zero-length array reference of a structure if the array is referenced as a flexible array member.

-Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]

Warn for cases where adding an attribute may be beneficial. The attributes currently supported are listed below.

-Walloc-zero

Warn about calls to allocation functions decorated with attribute "alloc_size" that specify zero bytes, including those to the built-in forms of the functions "aligned_alloc", "alloca", "calloc", "malloc", and "realloc". Because the behavior of these functions when called with a zero size differs among implementations (and in the case of "realloc" has been deprecated) relying on it may result in subtle portability bugs and should be avoided.

-Walloc-size-larger-than=byte-size

Warn about calls to functions decorated with attribute "alloc_size" that attempt to allocate objects larger than the specified number of bytes, or where the result of the size computation in an integer type with infinite precision would exceed the value of PTRDIFF_MAX on the target. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-alloc-size-larger-than.

-Wno-alloc-size-larger-than

Disable -Walloc-size-larger-than= warnings. The option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger.

-Walloca

This option warns on all uses of "alloca" in the source.

-Walloca-larger-than=byte-size

This option warns on calls to "alloca" with an integer argument whose value is either zero, or that is not bounded by a controlling predicate that limits its value to at most byte-size. It also warns for calls to "alloca" where the bound value is unknown. Arguments of non-integer types are considered unbounded even if they appear to be constrained to the expected range.

For example, a bounded case of "alloca" could be:

        void func (size_t n)
        {
          void *p;
          if (n <= 1000)
            p = alloca (n);
          else
            p = malloc (n);
          f (p);
        }
    

In the above example, passing "-Walloca-larger-than=1000" would not issue a warning because the call to "alloca" is known to be at most 1000 bytes. However, if "-Walloca-larger-than=500" were passed, the compiler would emit a warning.

Unbounded uses, on the other hand, are uses of "alloca" with no controlling predicate constraining its integer argument. For example:

        void func ()
        {
          void *p = alloca (n);
          f (p);
        }
    

If "-Walloca-larger-than=500" were passed, the above would trigger a warning, but this time because of the lack of bounds checking.

Note, that even seemingly correct code involving signed integers could cause a warning:

        void func (signed int n)
        {
          if (n < 500)
            {
              p = alloca (n);
              f (p);
            }
        }
    

In the above example, n could be negative, causing a larger than expected argument to be implicitly cast into the "alloca" call.

This option also warns when "alloca" is used in a loop.

-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is usually only effective when -ftree-vrp is active (default for -O2 and above).

See also -Wvla-larger-than=byte-size.

-Wno-alloca-larger-than

Disable -Walloca-larger-than= warnings. The option is equivalent to -Walloca-larger-than=SIZE_MAX or larger.

-Warith-conversion

Do warn about implicit conversions from arithmetic operations even when conversion of the operands to the same type cannot change their values. This affects warnings from -Wconversion, -Wfloat-conversion, and -Wsign-conversion.

        void f (char c, int i)
        {
          c = c + i; // warns with B<-Wconversion>
          c = c + 1; // only warns with B<-Warith-conversion>
        }
    

-Warray-bounds

-Warray-bounds=n

Warn about out of bounds subscripts or offsets into arrays. This warning is enabled by -Wall. It is more effective when -ftree-vrp is active (the default for -O2 and above) but a subset of instances are issued even without optimization.

By default, the trailing array of a structure will be treated as a flexible array member by -Warray-bounds or -Warray-bounds=n if it is declared as either a flexible array member per C99 standard onwards ([]), a GCC zero-length array extension ([0]), or an one-element array ([1]). As a result, out of bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned by default.

You can add the option -fstrict-flex-arrays or -fstrict-flex-arrays=level to control how this option treat trailing array of a structure as a flexible array member:

when level<=1, no change to the default behavior.

when level=2, additional warnings will be issued for out of bounds subscripts or offsets into one-element arrays;

when level=3, in addition to level=2, additional warnings will be issued for out of bounds subscripts or offsets into zero-length arrays.

-Warray-compare

Warn about equality and relational comparisons between two operands of array type. This comparison was deprecated in C++20. For example:

        int arr1[5];
        int arr2[5];
        bool same = arr1 == arr2;
    

-Warray-compare is enabled by -Wall.

-Warray-parameter

-Warray-parameter=n

Warn about redeclarations of functions involving arguments of array or pointer types of inconsistent kinds or forms, and enable the detection of out-of-bounds accesses to such parameters by warnings such as -Warray-bounds.

If the first function declaration uses the array form the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed by warnings such as -Warray-bounds. At level 1 the warning diagnoses inconsistencies involving array parameters declared using the "T[static N]" form.

For example, the warning triggers for the following redeclarations because the first one allows an array of any size to be passed to "f" while the second one with the keyword "static" specifies that the array argument must have at least four elements.

        void f (int[static 4]);
        void f (int[]);           // warning (inconsistent array form)
        
        void g (void)
        {
          int *p = (int *)malloc (4);
          f (p);                  // warning (array too small)
          ...
        }
    

At level 2 the warning also triggers for redeclarations involving any other inconsistency in array or pointer argument forms denoting array sizes. Pointers and arrays of unspecified bound are considered equivalent and do not trigger a warning.

        void g (int*);
        void g (int[]);     // no warning
        void g (int[8]);    // warning (inconsistent array bound)
    

-Warray-parameter=2 is included in -Wall. The -Wvla-parameter option triggers warnings for similar inconsistencies involving Variable Length Array arguments.

-Wattribute-alias=n

-Wno-attribute-alias

Warn about declarations using the "alias" and similar attributes whose target is incompatible with the type of the alias.

-Wbidi-chars=[none|unpaired|any|ucn]

Warn about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. Such characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. This may be dangerous; for instance, it may seem that a piece of code is not commented out, whereas it in fact is.

There are three levels of warning supported by GCC. The default is -Wbidi-chars=unpaired, which warns about improperly terminated bidi contexts. -Wbidi-chars=none turns the warning off. -Wbidi-chars=any warns about any use of bidirectional control characters.

By default, this warning does not warn about UCNs. It is, however, possible to turn on such checking by using -Wbidi-chars=unpaired,ucn or -Wbidi-chars=any,ucn. Using -Wbidi-chars=ucn is valid, and is equivalent to -Wbidi-chars=unpaired,ucn, if no previous -Wbidi-chars=any was specified.

-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.

-Wbool-operation

Warn about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean is very likely a bug in the program. For C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid. Incrementing a boolean is invalid in C++17, and deprecated otherwise.)

This warning is enabled by -Wall.

-Wduplicated-branches

Warn when an if-else has identical branches. This warning detects cases like

        if (p != NULL)
          return 0;
        else
          return 0;
    

It doesn't warn when both branches contain just a null statement. This warning also warn for conditional operators:

          int i = x ? *p : *p;
    

-Wduplicated-cond

Warn about duplicated conditions in an if-else-if chain. For instance, warn for the following code:

        if (p->q != NULL) { ... }
        else if (p->q != NULL) { ... }
    

-Wframe-address

Warn when the __builtin_frame_address or __builtin_return_address is called with an argument greater than 0. Such calls may return indeterminate values or crash the program. The warning is included in -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.

-Wzero-length-bounds

Warn about accesses to elements of zero-length array members that might overlap other members of the same object. Declaring interior zero-length arrays is discouraged because accesses to them are undefined.

For example, the first two stores in function "bad" are diagnosed because the array elements overlap the subsequent members "b" and "c". The third store is diagnosed by -Warray-bounds because it is beyond the bounds of the enclosing object.

        struct X { int a[0]; int b, c; };
        struct X x;
        
        void bad (void)
        {
          x.a[0] = 0;   // -Wzero-length-bounds
          x.a[1] = 1;   // -Wzero-length-bounds
          x.a[2] = 2;   // -Warray-bounds
        }
    

Option -Wzero-length-bounds is enabled by -Warray-bounds.

-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.

-Wtautological-compare

Warn if a self-comparison always evaluates to true or false. This warning detects various mistakes such as:

        int i = 1;
        ...
        if (i > i) { ... }
    

This warning also warns about bitwise comparisons that always evaluate to true or false, for instance:

        if ((a & 16) == 10) { ... }
    

will always be false.

This warning is enabled by -Wall.

-Wtrampolines

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

-Wfloat-equal

Warn if floating-point values are used in equality comparisons.

The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken.

-Wtraditional (C and Objective-C only)

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

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

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

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

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

-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. If this warning is enabled, it includes also all instances of local shadowing. This means that -Wno-shadow=local and -Wno-shadow=compatible-local are ignored when -Wshadow is used. Same as -Wshadow=global.

-Wno-shadow-ivar (Objective-C only)

Do not warn whenever a local variable shadows an instance variable in an Objective-C method.

-Wshadow=global

Warn for any shadowing. Same as -Wshadow.

-Wshadow=local

Warn when a local variable shadows another local variable or parameter.

-Wshadow=compatible-local

Warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. The creation of this flag (in addition to -Wshadow=local) is based on the idea that when a local variable shadows another one of incompatible type, it is most likely intentional, not a bug or typo, as shown in the following example:

        for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
        {
          for (int i = 0; i < N; ++i)
          {
            ...
          }
          ...
        }
    

Since the two variable "i" in the example above have incompatible types, enabling only -Wshadow=compatible-local does not emit a warning. Because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch that and emit an error or warning. Use of this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. Note that this also means that shadowing "const char *i" by "char *i" does not emit a warning.

This warning is also enabled by -Wshadow=local.

-Wlarger-than=byte-size

Warn whenever an object is defined whose size exceeds byte-size. -Wlarger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-larger-than.

Also warn for calls to bounded functions such as "memchr" or "strnlen" that specify a bound greater than the largest possible object, which is PTRDIFF_MAX bytes by default. These warnings can only be disabled by -Wno-larger-than.

-Wno-larger-than

Disable -Wlarger-than= warnings. The option is equivalent to -Wlarger-than=SIZE_MAX or larger.

-Wframe-larger-than=byte-size

Warn if the size of a function frame exceeds byte-size. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than byte-size 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. -Wframe-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-frame-larger-than.

-Wno-frame-larger-than

Disable -Wframe-larger-than= warnings. The option is equivalent to -Wframe-larger-than=SIZE_MAX or larger.

-Wfree-nonheap-object

Warn when attempting to deallocate an object that was either not allocated on the heap, or by using a pointer that was not returned from a prior call to the corresponding allocation function. For example, because the call to "stpcpy" returns a pointer to the terminating nul character and not to the beginning of the object, the call to "free" below is diagnosed.

        void f (char *p)
        {
          p = stpcpy (p, "abc");
          // ...
          free (p);   // warning
        }
    

-Wfree-nonheap-object is included in -Wall.

-Wstack-usage=byte-size

Warn if the stack usage of a function might exceed byte-size. 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.

-Wno-stack-usage

Disable -Wstack-usage= warnings. The option is equivalent to -Wstack-usage=SIZE_MAX or larger.

-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.

-Wno-pointer-compare

Do not warn if a pointer is compared with a zero character constant. This usually means that the pointer was meant to be dereferenced. For example:

        const char *p = foo ();
        if (p == '\0')
          return 42;
    

Note that the code above is invalid in C++11.

This warning is enabled by default.

-Wtsan

Warn about unsupported features in ThreadSanitizer.

ThreadSanitizer does not support "std::atomic_thread_fence" and can report false positives.

This warning is enabled by default.

-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.

-Wabsolute-value (C and Objective-C only)

Warn for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function is available. For example, calling abs(3.14) triggers the warning because the appropriate function to call to compute the absolute value of a double argument is "fabs". The option also triggers warnings when the argument in a call to such a function has an unsigned type. This warning can be suppressed with an explicit type cast and it is also enabled by -Wextra.

-Wcomment

-Wcomments

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.

-Wtrigraphs

Warn if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments are not warned about, except those that would form escaped newlines.

This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs.

-Wundef

Warn if an undefined identifier is evaluated in an "#if" directive. Such identifiers are replaced with zero.

-Wexpansion-to-defined

Warn whenever defined is encountered in the expansion of a macro (including the case where the macro is expanded by an #if directive). Such usage is not portable. This warning is also enabled by -Wpedantic and -Wextra.

-Wunused-macros

Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested for existence at least once. The preprocessor also warns if the macro has not been used at the time it is redefined or undefined.

Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.

Note: If a macro is actually used, but only used in skipped conditional blocks, then the preprocessor reports it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:

        #if defined the_macro_causing_the_warning
        #endif
    

-Wno-endif-labels

Do not warn whenever an "#else" or an "#endif" are followed by text. This sometimes happens in older programs with code of the form

        #if FOO
        ...
        #else FOO
        ...
        #endif FOO
    

The second and third "FOO" should be in comments. This warning is on by default.

-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__".

-Wc11-c2x-compat (C and Objective-C only)

Warn about features not present in ISO C11, but present in ISO C2X. For instance, warn about omitting the string in "_Static_assert", use of [[]] syntax for attributes, use of decimal floating-point types, 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.

-Wc++17-compat (C++ and Objective-C++ only)

Warn about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. This warning is enabled by -Wall.

-Wc++20-compat (C++ and Objective-C++ only)

Warn about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. This warning is enabled by -Wall.

-Wno-c++11-extensions (C++ and Objective-C++ only)

Do not warn about C++11 constructs in code being compiled using an older C++ standard. Even without this option, some C++11 constructs will only be diagnosed if -Wpedantic is used.

-Wno-c++14-extensions (C++ and Objective-C++ only)

Do not warn about C++14 constructs in code being compiled using an older C++ standard. Even without this option, some C++14 constructs will only be diagnosed if -Wpedantic is used.

-Wno-c++17-extensions (C++ and Objective-C++ only)

Do not warn about C++17 constructs in code being compiled using an older C++ standard. Even without this option, some C++17 constructs will only be diagnosed if -Wpedantic is used.

-Wno-c++20-extensions (C++ and Objective-C++ only)

Do not warn about C++20 constructs in code being compiled using an older C++ standard. Even without this option, some C++20 constructs will only be diagnosed if -Wpedantic is used.

-Wno-c++23-extensions (C++ and Objective-C++ only)

Do not warn about C++23 constructs in code being compiled using an older C++ standard. Even without this option, some C++23 constructs will only be diagnosed if -Wpedantic is used.

-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.

-Wcast-align=strict

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 *" regardless of the target machine.

-Wcast-function-type

Warn when a function pointer is cast to an incompatible function pointer. In a cast involving function types with a variable argument list only the types of initial arguments that are provided are considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types are ignored, like "int" vs. "long" on ILP32 targets. Likewise type qualifiers are ignored. The function type "void (*) (void)" is special and matches everything, which can be used to suppress this warning. In a cast involving pointer to member types this warning warns whenever the type cast is changing the pointer to member type. This warning is enabled by -Wextra.

-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.

-Wno-complain-wrong-lang

By default, language front ends complain when a command-line option is valid, but not applicable to that front end. This may be disabled with -Wno-complain-wrong-lang, which is mostly useful when invoking a single compiler driver for multiple source files written in different languages, for example:

        $ g++ -fno-rtti a.cc b.f90
    

The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. The latter front end diagnoses f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran, which may be disabled with -Wno-complain-wrong-lang.

-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.

Warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion.

-Wdangling-else

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 ();
            }
        }
    

This warning is enabled by -Wparentheses.

-Wdangling-pointer

-Wdangling-pointer=n

Warn about uses of pointers (or C++ references) to objects with automatic storage duration after their lifetime has ended. This includes local variables declared in nested blocks, compound literals and other unnamed temporary objects. In addition, warn about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels but may yield different results with optimization than without.

-Wdate-time

Warn when macros "__TIME__", "__DATE__" or "__TIMESTAMP__" are encountered as they might prevent bit-wise-identical reproducible compilations.

-Wempty-body

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

-Wno-endif-labels

Do not warn about stray tokens after "#else" and "#endif".

-Wenum-compare

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

-Wenum-conversion

Warn when a value of enumerated type is implicitly converted to a different enumerated type. This warning is enabled by -Wextra in C.

-Wenum-int-mismatch (C and Objective-C only)

Warn about mismatches between an enumerated type and an integer type in declarations. For example:

        enum E { l = -1, z = 0, g = 1 };
        int foo(void);
        enum E foo(void);
    

In C, an enumerated type is compatible with "char", a signed integer type, or an unsigned integer type. However, since the choice of the underlying type of an enumerated type is implementation-defined, such mismatches may cause portability issues. In C++, such mismatches are an error. In C, this warning is enabled by -Wall and -Wc++-compat.

-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. In C++, this warning is also enabled by -Wall. In C, it is also enabled by -Wextra.

-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.

-Wno-scalar-storage-order

Do not warn on suspicious constructs involving reverse scalar storage order.

-Wsizeof-array-div

Warn about divisions of two sizeof operators when the first one is applied to an array and the divisor does not equal the size of the array element. In such a case, the computation will not yield the number of elements in the array, which is likely what the user intended. This warning warns e.g. about

        int fn ()
        {
          int arr[10];
          return sizeof (arr) / sizeof (short);
        }
    

This warning is enabled by -Wall.

-Wsizeof-pointer-div

Warn for suspicious divisions of two sizeof expressions that divide the pointer size by the element size, which is the usual way to compute the array size but won't work out correctly with pointers. This warning warns e.g. about "sizeof (ptr) / sizeof (ptr[0])" if "ptr" is not an array, but a pointer. This warning is enabled by -Wall.

-Wsizeof-pointer-memaccess

Warn for suspicious length parameters to certain string and memory built-in functions if the argument uses "sizeof". This warning triggers for example for "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));". -Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like "strncat" or "strncpy" that specify as the bound a "sizeof" expression of the source array. For example, in the following function the call to "strncat" specifies the size of the source string as the bound. That is almost certainly a mistake and so the call is diagnosed.

        void make_file (const char *name)
        {
          char path[PATH_MAX];
          strncpy (path, name, sizeof path - 1);
          strncat (path, ".text", sizeof ".text");
          ...
        }
    

The -Wsizeof-pointer-memaccess option is enabled by -Wall.

-Wno-sizeof-array-argument

Do not 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-elt-size

Warn for suspicious calls to the "memset" built-in function, if the first argument references an array, and the third argument is a number equal to the number of elements, but not equal to the size of the array in memory. This indicates that the user has omitted a multiplication by the element size. This warning is enabled by -Wall.

-Wmemset-transposed-args

Warn for suspicious calls to the "memset" built-in function where the second argument is not zero and the third argument is zero. For example, the call "memset (buf, sizeof buf, 0)" is diagnosed because "memset (buf, 0, sizeof buf)" was meant instead. The diagnostic is only emitted if the third argument is a literal zero. Otherwise, if it is an expression that is folded to zero, or a cast of zero to some type, it is far less likely that the arguments have been mistakenly transposed and no warning is emitted. This warning is enabled by -Wall.

-Waddress

Warn about suspicious uses of address expressions. These include comparing the address of a function or a declared object to the null pointer constant such as in

        void f (void);
        void g (void)
        {
          if (!f)   // warning: expression evaluates to false
            abort ();
        }
    

comparisons of a pointer to a string literal, such as in

        void f (const char *x)
        {
          if (x == "abc")   // warning: expression evaluates to false
            puts ("equal");
        }
    

and tests of the results of pointer addition or subtraction for equality to null, such as in

        void f (const int *p, int i)
        {
          return p + i == NULL;
        }
    

Such uses typically indicate a programmer error: the address of most functions and objects necessarily evaluates to true (the exception are weak symbols), so their use in a conditional might indicate missing parentheses in a function call or a missing dereference in an array expression. The subset of the warning for object pointers can be suppressed by casting the pointer operand to an integer type such as "intptr_t" or "uintptr_t". Comparisons against string literals result in unspecified behavior and are not portable, and suggest the intent was to call "strcmp". The warning is suppressed if the suspicious expression is the result of macro expansion. -Waddress warning is enabled by -Wall.

-Wno-address-of-packed-member

Do not warn when the address of packed member of struct or union is taken, which usually results in an unaligned pointer value. This is enabled by default.

-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. Also warns when the operands of a logical operator are the same:

        extern int a;
        if (a < 0 && a < 0) { ... }
    

-Wlogical-not-parentheses

Warn about logical not used on the left hand side operand of a comparison. This option does not warn if the right operand is considered to be a boolean expression. 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.

Additionally, using -Wno-attributes=, it is possible to suppress warnings about unknown scoped attributes (in C++11 and C2X). For example, -Wno-attributes=vendor::attr disables warning about the following declaration:

        [[vendor::attr]] void f();
    

It is also possible to disable warning about all attributes in a namespace using -Wno-attributes=vendor:: which prevents warning about both of these declarations:

        [[vendor::safe]] void f();
        [[vendor::unsafe]] void f2();
    

Note that -Wno-attributes= does not imply -Wno-attributes.

-Wno-builtin-declaration-mismatch

Warn if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that does not include a prototype is called with arguments whose promoted types do not match those expected by the function. When -Wextra is specified, also warn when a built-in function that takes arguments is declared without a prototype. The -Wbuiltin-declaration-mismatch warning is enabled by default. To avoid the warning include the appropriate header to bring the prototypes of built-in functions into scope.

For example, the call to "memset" below is diagnosed by the warning because the function expects a value of type "size_t" as its argument but the type of 32 is "int". With -Wextra, the declaration of the function is diagnosed as well.

        extern void* memset ();
        void f (void *d)
        {
          memset (d, '\0', 32);
        }
    

-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. A definition using () is not considered an old-style definition in C2X mode, because it is equivalent to (void) in that case, but is considered an old-style definition for older standards.

-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 about the universal zero initializer { 0 }:

        struct s { int f, g, h; };
        struct s x = { 0 };
    

Likewise, in C++ this option does not warn 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-missing-requires

By default, the compiler warns about a concept-id appearing as a C++20 simple-requirement:

        bool satisfied = requires { C<T> };
    

Here satisfied will be true if C<T> is a valid expression, which it is for all T. Presumably the user meant to write

        bool satisfied = requires { requires C<T> };
    

so satisfied is only true if concept C is satisfied for type T.

This warning can be disabled with -Wno-missing-requires.

-Wno-missing-template-keyword

The member access tokens ., -> and :: must be followed by the "template" keyword if the parent object is dependent and the member being named is a template.

        template <class X>
        void DoStuff (X x)
        {
          x.template DoSomeOtherStuff<X>(); // Good.
          x.DoMoreStuff<X>(); // Warning, x is dependent.
        }
    

In rare cases it is possible to get false positives. To silence this, wrap the expression in parentheses. For example, the following is treated as a template, even where m and N are integers:

        void NotATemplate (my_class t)
        {
          int N = 5;
        
          bool test = t.m < N > (0); // Treated as a template.
          test = (t.m < N) > (0); // Same meaning, but not treated as a template.
        }
    

This warning can be disabled with -Wno-missing-template-keyword.

-Wno-multichar

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

-Wnormalized=[none|id|nfc|nfkc]

In ISO C and ISO C++, two identifiers are different if they are different sequences of characters. However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you are using identifiers that have not been normalized; this option controls that warning.

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

Do not warn about usage of functions declared with "warning" attribute. By default, this warning is enabled. -Wno-attribute-warning can be used to disable the warning or -Wno-error=attribute-warning can be used to disable the error when compiled with -Werror flag.

-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. Enabled by default.

-Wopenacc-parallelism

Warn about potentially suboptimal choices related to OpenACC parallelism.

-Wopenmp-simd

Warn if the vectorizer cost model overrides the OpenMP 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.

-Wno-override-init-side-effects (C and Objective-C only)

Do not warn if an initialized field with side effects is overridden when using designated initializers. This warning is enabled by default.

-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;
        };
    

-Wnopacked-bitfield-compat

The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". This was 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.

-Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only)

Warn if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning will be issued on "struct S", like, "warning: alignment 1 of 'struct S' is less than 8", in this code:

        struct __attribute__ ((aligned (8))) S8 { char a[8]; };
        struct __attribute__ ((packed)) S {
          struct S8 s8;
        };
    

This warning is enabled by -Wall.

-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.

-Wrestrict

Warn when an object referenced by a "restrict"-qualified parameter (or, in C++, a "__restrict"-qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to the "strcpy" function below attempts to truncate the string by replacing its initial characters with the last four. However, because the call writes the terminating NUL into "a[4]", the copies overlap and the call is diagnosed.

        void foo (void)
        {
          char a[] = "abcd1234";
          strcpy (a, a + 4);
          ...
        }
    

The -Wrestrict option detects some instances of simple overlap even without optimization but works best at -O2 and above. It is included in -Wall.

-Wnested-externs (C and Objective-C only)

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

-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.

-Winterference-size

Warn about use of C++17 "std::hardware_destructive_interference_size" without specifying its value with --param destructive-interference-size. Also warn about questionable values for that option.

This variable is intended to be used for controlling class layout, to avoid false sharing in concurrent code:

        struct independent_fields {
          alignas(std::hardware_destructive_interference_size)
            std::atomic<int> one;
          alignas(std::hardware_destructive_interference_size)
            std::atomic<int> two;
        };
    

Here one and two are intended to be far enough apart that stores to one won't require accesses to the other to reload the cache line.

By default, --param destructive-interference-size and --param constructive-interference-size are set based on the current -mtune option, typically to the L1 cache line size for the particular target CPU, sometimes to a range if tuning for a generic target. So all translation units that depend on ABI compatibility for the use of these variables must be compiled with the same -mtune (or -mcpu).

If ABI stability is important, such as if the use is in a header for a library, you should probably not use the hardware interference size variables at all. Alternatively, you can force a particular value with --param.

If you are confident that your use of the variable does not affect ABI outside a single build of your project, you can turn off the warning with -Wno-interference-size.

-Wint-in-bool-context

Warn for suspicious use of integer values where boolean values are expected, such as conditional expressions (?:) using non-boolean integer constants in boolean context, like "if (a <= b ? 2 : 3)". Or left shifting of signed integers in boolean context, like "for (a = 0; 1 << a; a++);". Likewise for all kinds of multiplications regardless of the data type. This warning is enabled by -Wall.

-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 cannot be used.

-Winvalid-utf8

Warn if an invalid UTF-8 character is found. This warning is on by default for C++23 if -finput-charset=UTF-8 is used and turned into error with -pedantic-errors.

-Wno-unicode

Don't diagnose invalid forms of delimited or named escape sequences which are treated as separate tokens. Wunicode is enabled by default.

-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.

-Wno-varargs

Do not warn upon questionable usage of the macros used to handle variable arguments like "va_start". These warnings are enabled by default.

-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.

-Wvla

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

-Wvla-larger-than=byte-size

If this option is used, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that allows the array size to exceed byte-size bytes. This is similar to how -Walloca-larger-than=byte-size works, but with variable-length arrays.

Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays.

-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically only effective when -ftree-vrp is active (default for -O2 and above).

See also -Walloca-larger-than=byte-size.

-Wno-vla-larger-than

Disable -Wvla-larger-than= warnings. The option is equivalent to -Wvla-larger-than=SIZE_MAX or larger.

-Wvla-parameter

Warn about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds or forms, and enable the detection of out-of-bounds accesses to such parameters by warnings such as -Warray-bounds.

If the first function declaration uses the VLA form the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed.

For example, the warning triggers for the following redeclarations because the first one allows an array of any size to be passed to "f" while the second one specifies that the array argument must have at least "n" elements. In addition, calling "f" with the associated VLA bound parameter in excess of the actual VLA bound triggers a warning as well.

        void f (int n, int[n]);
        // warning: argument 2 previously declared as a VLA
        void f (int, int[]);
        
        void g (int n)
        {
            if (n > 4)
              return;
            int a[n];
            // warning: access to a by f may be out of bounds
            f (sizeof a, a);
          ...
        }
    

-Wvla-parameter is included in -Wall. The -Warray-parameter option triggers warnings for similar problems involving ordinary array arguments.

-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.

-Wxor-used-as-pow (C, C++, Objective-C and Objective-C++ only)

Warn about uses of "^", the exclusive or operator, where it appears the user meant exponentiation. Specifically, the warning occurs when the left-hand side is the decimal constant 2 or 10 and the right-hand side is also a decimal constant.

In C and C++, "^" means exclusive or, whereas in some other languages (e.g. TeX and some versions of BASIC) it means exponentiation.

This warning is enabled by default. It can be silenced by converting one of the operands to hexadecimal.

-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-lto-type-mismatch

During the link-time optimization, do not warn about type mismatches in global declarations from different compilation units. Requires -flto to be enabled. Enabled by default.

-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.