GCC(1) GNU GCC(1) NAME gcc - GNU project C and C++ compiler SYNOPSIS gcc [-c|-S|-E] [-std=standard] [-g] [-pg] [-Olevel] [-Wwarn...] [-Wpedantic] [-Idir...] [-Ldir...] [-Dmacro[=defn]...] [-Umacro] [-foption...] [-mmachine-option...] [-o outfile] [@file] infile... Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc. DESCRIPTION When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" allow you to stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the output consists of object files output by the assembler. Other options are passed on to one or more stages of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them. Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages. The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or machine-gcc-version to run a specific version of GCC. When you compile C++ programs, you should invoke GCC as g++ instead. The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dv is very different from -d -v. You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant. Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms, whichever one is not the default. Some options take one or more arguments typically separated either by a space or by the equals sign (=) from the option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the 0x prefix. Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a multiple of bytes such as "kB" and "KiB" for kilobyte and kibibyte, respectively, "MB" and "MiB" for megabyte and mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes. OPTIONS Option Summary Here is a summary of all the options, grouped by type. Explanations are in the following sections. Overall Options -c -S -E -o file -dumpbase dumpbase -dumpbase-ext auxdropsuf -dumpdir dumppfx -x language -v -### --help[=class[,...]] --target-help --version -pass-exit-codes -pipe -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 --assemble --compile --dumpbase dumpbase --dumpbase-ext auxdropsuf --dumpdir dumppfx --language=language --output=file --pass-exit-codes --pipe --preprocess --verbose 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 -fpermitted-flt-eval-methods=standard -fplan9-extensions -fsigned-bitfields -funsigned-bitfields -fsigned-char -funsigned-char -fstrict-flex-arrays[=n] -fsso-struct=endianness --ansi C++ Language Options --compile-std-module -fabi-compat-version=n -fabi-version=n -fno-access-control -faligned-new=[n] -fno-assume-sane-operators-new-delete -fchar8_t -fcheck-new -fconcepts -fconcepts-diagnostics-depth=n -fconstexpr-depth=n -fconstexpr-cache-depth=n -fconstexpr-loop-limit=n -fconstexpr-ops-limit=n -fcontracts -fcontract-evaluation-semantic=[ignore|observe|enforce|quick_enforce] -fcontracts-conservative-ipa -fcontract-checks-outlined -fcontract-disable-optimized-checks -fcontracts-client-check=[none|pre|all] -fcontracts-definition-check=[on|off] -fcoroutines -fdiagnostics-all-candidates -fno-elide-constructors -fno-enforce-eh-specs -fext-numeric-literals -fno-gnu-keywords -fno-immediate-escalation -fno-implement-inlines -fimplicit-constexpr -fno-implicit-inline-templates -fno-implicit-templates -fmodule-header[=kind] -fmodule-implicit-inline -fno-module-lazy -fmodule-mapper=specification -fmodule-only -fmodules -fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching -fno-nonansi-builtins -fnothrow-opt -fno-operator-names -fno-optional-diags -fno-pretty-templates -frange-for-ext-temps -freflection -fno-rtti -fsized-deallocation -fstrict-enums -fstrong-eval-order[=kind] -ftemplate-backtrace-limit=n -ftemplate-depth=n -fno-threadsafe-statics -fuse-cxa-atexit -fno-use-cxa-get-exception-ptr -fno-weak -nostdinc++ -fvisibility-inlines-hidden -fvisibility-ms-compat -flang-info-include-translate[=header] -flang-info-include-translate-not -flang-info-module-cmi[=module] -stdlib=libstdc++,libc++ -Wabbreviated-auto-in-template-arg -Wabi-tag -Waligned-new[=kind] -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-defaulted-function-deleted -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 -Wno-deprecated-literal-operator -Wdeprecated-variadic-comma-omission -Weffc++ -Wno-elaborated-enum-base -Wno-exceptions -Wno-expose-global-module-tu-local -Wno-external-tu-local -Wextra-semi -Wno-global-module -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 -Wno-sfinae-incomplete -Wstrict-null-sentinel -Wno-subobject-linkage -Wtemplates -Wno-non-c-typedef-for-linkage -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-template-body -Wno-template-id-cdtor -Wtemplate-names-tu-local -Wno-terminate -Wno-vexing-parse -Wvirtual-inheritance -Wno-virtual-move-assign -Wvolatile 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 OpenMP and OpenACC Options -foffload=arg -foffload-options=arg -fopenacc -fopenacc-dim=geom -fopenmp -fopenmp-simd -fopenmp-target-simd-clone[=device-type] 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] -fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC -fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC -fno-diagnostics-json-formatting -fno-diagnostics-show-option -fno-diagnostics-show-caret -fno-diagnostics-show-event-links -fno-diagnostics-show-labels -fno-diagnostics-show-line-numbers -fno-diagnostics-show-cwe -fno-diagnostics-show-rules -fno-diagnostics-show-highlight-colors -fno-diagnostics-show-nesting -fno-diagnostics-show-nesting-locations -fdiagnostics-show-nesting-levels -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] -fdiagnostics-text-art-charset=[none|ascii|unicode|emoji] -fdiagnostics-show-context[=depth] Warning Options -fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -fpermissive -w -Wextra -Wall -Wabi=n -Waddress -Wno-address-of-packed-member -Waggregate-return -Walloc-size -Walloc-size-larger-than=byte-size -Walloc-zero -Walloca -Walloca-larger-than=byte-size -Wauto-profile -Wno-aggressive-loop-optimizations -Warith-conversion -Warray-bounds -Warray-bounds=n -Warray-compare -Warray-parameter -Warray-parameter=n -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-c23-compat -Wc23-c2y-compat -Wc++-compat -Wc++11-compat -Wc++14-compat -Wc++17-compat -Wc++20-compat -Wc++26-compat -Wno-c++11-extensions -Wno-c++14-extensions -Wno-c++17-extensions -Wno-c++20-extensions -Wno-c++23-extensions -Wcalloc-transposed-args -Wcannot-profile -Wcast-align -Wcast-align=strict -Wcast-function-type -Wcast-qual -Wchar-subscripts -Wclobbered -Wcomment -Wcompare-distinct-pointer-types -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 -Wno-deprecated-openmp -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 -Wflex-array-member-not-at-end -Wfloat-conversion -Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul -Wno-format-diag -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 -Wheader-guard -Wno-if-not-aligned -Wno-ignored-attributes -Wignored-qualifiers -Wno-incompatible-pointer-types -Whardened -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 -Wkeyword-macro -Wlarger-than=byte-size -Wleading-whitespace=kind -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 -Wmusttail-local-addr -Wmaybe-musttail-local-addr -Wno-missing-profile -Wno-multichar -Wmultistatement-macros -Wnonnull -Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc] -Wnull-dereference -Wno-odr -Wopenacc-parallelism -Wopenmp -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-pragma-once-outside-header -Wno-prio-ctor-dtor -Wno-psabi -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=attribute-name -Wswitch -Wno-switch-bool -Wswitch-default -Wswitch-enum -Wno-switch-outside-range -Wno-switch-unreachable -Wsync-nand -Wsystem-headers -Wtautological-compare -Wtrailing-whitespace -Wtrailing-whitespace=kind -Wtrampolines -Wtrigraphs -Wtrivial-auto-var-init -Wno-tsan -Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas -Wunsuffixed-float-constants -Wunterminated-string-initialization -Wunused -Wunused-but-set-parameter -Wunused-but-set-parameter=n -Wunused-but-set-variable -Wunused-but-set-variable=n -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 -Wuse-after-free -Wuse-after-free=n -Wuseless-cast -Wno-varargs -Wvariadic-macros -Wvector-operation-performance -Wvla -Wvla-larger-than=byte-size -Wno-vla-larger-than -Wvolatile-register-var -Wwrite-strings -Wno-xor-used-as-pow -Wzero-as-null-pointer-constant -Wzero-length-bounds --all-warnings --extra-warnings --no-warnings --pedantic --pedantic-errors Static Analyzer Options -fanalyzer -fanalyzer-assume-nothrow -fanalyzer-call-summaries -fanalyzer-checker=name -fno-analyzer-feasibility -fanalyzer-show-events-in-system-headers -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-infinite-loop -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-loop -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-overlapping-buffers -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 -Wno-analyzer-throw-of-unexpected-type -Wanalyzer-symbol-too-complex -Wanalyzer-too-complex -Wno-analyzer-undefined-behavior-ptrdiff -Wno-analyzer-undefined-behavior-strtok -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 -Wdeprecated-non-prototype -Wfree-labels -Wmissing-declarations -Wmissing-parameter-name -Wmissing-parameter-type -Wdeclaration-missing-parameter-type -Wmissing-prototypes -Wmissing-variable-declarations -Wmultiple-parameter-fwd-decl-lists -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 -gno-prune-btf -ggdb -gno-record-gcc-switches -gstrict-dwarf -gas-loc-support -gas-locview-support -gcodeview -gcolumn-info -gdwarf32 -gdwarf64 -gno-statement-frontiers -gno-variable-location-views -gvariable-location-views=incompat5 -ginternal-reset-location-views -ginline-points -gvms -gz[=type] -gsplit-dwarf -gdescribe-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 -fvar-tracking-uninit --debug 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-profile-inlining -fauto-inc-dec -fbranch-probabilities -fcaller-saves -fcombine-stack-adjustments -fconserve-stack -ffold-mem-offsets -fcompare-elim -fcprop-registers -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range -fcx-method -fdata-sections -fdce -fdelayed-branch -fdelete-null-pointer-checks -fdep-fusion -fdevirtualize -fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse -fearly-inlining -fexcess-precision=style -fexpensive-optimizations -fext-dce -ffast-math -ffat-lto-objects -ffinite-loops -ffinite-math-only -ffloat-store -fforward-propagate -ffp-contract=style -ffp-int-builtin-inexact -ffunction-sections -ffuse-ops-with-volatile-access -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity -fgcse-sm -fhoist-adjacent-loads -fif-conversion -fif-conversion2 -findirect-inlining -finline-atomics -finline-functions -finline-functions-called-once -finline-limit=n -finline-small-functions -finline-stringops[=fn] -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-reorder-for-locality -fipa-sra -fipa-stack-alignment -fipa-icf -fipa-icf-functions -fipa-icf-variables -fira-algorithm=algorithm -flate-combine-instructions -flifetime-dse -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=n -flto-toplevel-asm-heuristics -flto-partition=alg -flto-incremental=path -flto-incremental-cache-size=n -fmalloc-dce -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-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-crc -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 -fspeculatively-call-stored-functions -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-cselim -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 -O -O0 -O1 -O2 -O3 -Os -Ofast -Og -Oz --optimize Program Instrumentation Options -p -pg -fprofile-arcs -coverage -ftest-coverage -fcondition-coverage -fpath-coverage 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Directory Options -Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir -imultiarch dir -iplugindir=dir -iprefix file -iquote dir -isysroot dir -isystem dir -iwithprefix dir -iwithprefixbefore dir -Ldir -no-canonical-prefixes --no-sysroot-suffix -nostdinc -nostdinc++ --embed-dir=dir --embed-directory=dir --include-barrier --include-directory=dir --include-directory-after=dir --include-prefix=prefix --include-with-prefix=prefix --include-with-prefix-after=prefix --include-with-prefix-before=prefix --no-canonical-prefixes --no-standard-includes --prefix=prefix --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 -ftrampoline-impl=[stack|heap] -ftrapv -fwrapv -fwrapv-pointer -fvisibility=[default|internal|hidden|protected] -fstrict-volatile-bitfields -fsync-libcalls -fzero-init-padding-bits=value -Qy -Qn 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-internal-locations -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 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--print-multi-directory --print-multi-lib --print-multi-os-directory --print-multiarch --print-prog-name=program --print-search-dirs --print-sysroot --print-sysroot-headers-suffix --save-temps --specs=file --param name=value Target-Specific Options AArch64 Options (AArch64 Options) -mabi=name -mbig-endian -mlittle-endian -menable-sysreg-checking -mgeneral-regs-only -mcmodel=tiny -mcmodel=small -mcmodel=large -mstrict-align -momit-leaf-frame-pointer -mtls-dialect=desc -mtls-dialect=traditional -mtls-size=size -mtp=name -mfix-cortex-a53-835769 -mfix-cortex-a53-843419 -mlow-precision-recip-sqrt -mlow-precision-sqrt -mlow-precision-div -mmax-vectorization -mautovec-preference=name -mpc-relative-literal-loads -msign-return-address=scope -mbranch-protection=features -mharden-sls=opts -march=name -mcpu=name -mtune=name -moverride=string -mstack-protector-guard=guard -mstack-protector-guard-reg=sysreg -mstack-protector-guard-offset=offset -mtrack-speculation -moutline-atomics -mearly-ra -mearly-ldp-fusion -mlate-ldp-fusion -mnarrow-gp-writes -msve-vector-bits=bits Adapteva Epiphany Options (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 -mmay-round-for-trunc -mfp-iarith -mvect-double -max-vect-align=num -msplit-vecmove-early -m1reg-reg AMD GCN Options (AMD GCN Options) -march=gpu -mtune=gpu -mgang-private-size=bytes -msram-ecc=[on|off|any] -mxnack=[on|off|any] -Wopenacc-dims ARC Options (ARC Options) -mbarrel-shifter -mjli-always -mcpu=cpu -mA6 -mARC600 -mA7 -mARC700 -mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr -mea -mmul32x16 -mmul64 -matomic -mnorm -mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap -mlock -mswape -mxy -misize -marclinux -marclinux_prof -mlong-calls -mmedium-calls -msdata -mirq-ctrl-saved -mrgf-banked-regs -mlpc-width=width -G num 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-mfix-cortex-a57-aes-1742098 -mfix-cortex-a72-aes-1655431 -munaligned-access -mslow-flash-data -masm-syntax-unified -mrestrict-it -mpure-code -mcmse -mfix-cmse-cve-2021-35465 -mstack-protector-guard=guard -mstack-protector-guard-offset=offset -mfdpic -mbranch-protection=features -mvectorize-with-neon-quad -mvectorize-with-neon-double AVR Options (AVR Options) -mmcu=mcu -mabsdata -maccumulate-args -mcvt -mbranch-cost=cost -mfuse-add=level -mfuse-move=level -mfuse-move2 -mcall-prologues -mgas-isr-prologues -mint8 -mflmap -mdouble=bits -mlong-double=bits -mno-call-main -mn_flash=size -mfract-convert-truncate -mno-interrupts -mmain-is-OS_task -mrelax -mpmem-wrap-around -mrmw -mstrict-X -mtiny-stack -mrodata-in-ram -msplit-bit-shift -msplit-ldst -mshort-calls -mskip-bug -muse-nonzero-bits -nodevicelib -nodevicespecs -Waddr-space-convert -Wmisspelled-isr Blackfin Options (Blackfin Options) -mcpu=cpu[-sirevision] -msim -momit-leaf-frame-pointer -mspecld-anomaly -mcsync-anomaly -mlow-64k -mstack-check-l1 -mid-shared-library -mleaf-id-shared-library -mshared-library-id=n -msep-data -mlong-calls -mfast-fp -minline-plt -mmulticore -mcorea -mcoreb -msdram -micplb C6X Options (C6X Options) -mbig-endian -mlittle-endian -march=cpu -msim -msdata=sdata-type -mdsbt -mlong-calls CRIS Options (CRIS Options) -mcpu=cpu -march=cpu -mtune=cpu -mmax-stackframe=n -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mbest-lib-options -moverride-best-lib-options -mtrap-using-break8 -mtrap-unaligned-atomic -munaligned-atomic-may-use-library -sim -sim2 -mmul-bug-workaround C-SKY Options (C-SKY Options) -march=arch -mcpu=cpu -mbig-endian -mlittle-endian -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 -msched-prolog -msim Cygwin and MinGW Options (Cygwin and MinGW Options) -mconsole -mcrtdll=library -mdll -mnop-fun-dllimport -mthreads -municode -mwin32 -mwindows -fno-set-stack-executable -fwritable-relocated-rdata -mpe-aligned-commons -muse-libstdc-wrappers Darwin Options (Darwin Options) -all_load -allowable_client -arch name -arch_errors_fatal -asm_macosx_version_min=version -bind_at_load -bundle -bundle_loader -client_name -compatibility_version -current_version -dead_strip -dependency-file -dylib_file -dylinker -dylinker_install_name -dynamic -dynamiclib -exported_symbols_list -fapple-kext -fconstant-cfstrings -ffix-and-continue -filelist -findirect-data -flat_namespace -force_cpusubtype_ALL -force_flat_namespace -framework name -gfull -gused -headerpad_max_install_names -iframework -image_base -init symbol-name -install_name -keep_private_externs -matt-stubs -mconstant-cfstrings -mdynamic-no-pic -mfix-and-continue -mkernel -mmacosx-version-min=version -mone-byte-bool -msymbol-stubs -mtarget-linker[=]version -nodefaultexport -nodefaultrpaths -pagezero_size -preload -read_only_relocs -sectalign -sectcreate -seg_addr_table -seg1addr -segaddr -segprot -segs_read_only_addr -segs_read_write_addr -sub_library -sub_umbrella -twolevel_namespace -twolevel_namespace_hints -umbrella -undefined -unexported_symbols_list -weak_framework name -weak_reference_mismatches -whatsloaded -whyload -F -ObjC -ObjC++ -Wnonportable-cfstrings DEC Alpha Options (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 -msafe-bwa -msafe-partial -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data -mlarge-data -msmall-text -mlarge-text -mmemory-latency=time -mtls-kernel -mtls-size=bitsize -mlong-double-128 -mlong-double-64 eBPF Options (eBPF Options) -mbig-endian -mlittle-endian -mframe-limit=bytes -mxbpf -mco-re -mjmpext -mjmp32 -malu32 -mv3-atomics -mbswap -msdiv -msmov -mcpu=version -masm=dialect -minline-memops-threshold=bytes -Wco-re FR30 Options (FR30 Options) -msmall-model -mno-lsim FRV Options (FRV Options) -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -malloc-cc -mfixed-cc -mdword -mdouble -mmedia -mmuladd -mfdpic -minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp -mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8 -mpack -mno-eflags -mno-cond-move -mno-optimize-membar -mno-scc -mno-cond-exec -mno-vliw-branch -mno-multi-cond-exec -mno-nested-cond-exec -mtomcat-stats -mTLS -mtls -mcpu=cpu FT32 Options (FT32 Options) -msim -mnodiv -mft32b -mcompress -mnopm GNU/Linux Options (GNU/Linux Options) -mglibc -muclibc -mmusl -mbionic -mandroid -tno-android-cc -tno-android-ld H8/300 Options (H8/300 Options) -mrelax -mh -ms -mn -msx -ms2600 -mquickcall -mslowbyte -mexr -mint32 -malign-300 HPPA Options (HPPA Options) -march=architecture-type -mno-atomic-libcalls 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-msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc -msched-spec-control-ldc -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 (LM32 Options) -mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled -msign-extend-enabled -muser-enabled LoongArch Options (LoongArch Options) -march=arch-type -mtune=tune-type -mabi=base-abi-type -mfpu=fpu-type -msimd=simd-type -msoft-float -msingle-float -mdouble-float -mlsx -mlasx -mbranch-cost=n -maddr-reg-reg-cost=n -mcheck-zero-division -mbreak-code=code -mcond-move-int -mcond-move-float -memcpy -mstrict-align -G num -mmax-inline-memcpy-size=n -mexplicit-relocs=style -mexplicit-relocs -mno-explicit-relocs -mdirect-extern-access -mcmodel=code-model -mrelax -mpass-mrelax-to-as -mrecip -mrecip=opt -mfrecipe -mdiv32 -mlam-bh -mlamcas -mld-seq-sa -mscq -mtls-dialect=opt -mannotate-tablejump LynxOS Options (LynxOS Options) -mshared -mthreads -mlegacy-threads M32C Options (M32C Options) -mcpu=cpu -msim -memregs=number M32R/D Options (M32R/D Options) -m32r2 -m32rx -m32r -mdebug -malign-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 M680x0 Options (M680x0 Options) -march=arch -mcpu=cpu -mtune=tune -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060 -m68302 -m68332 -m68851 -mcpu32 -mfidoa -m5200 -m5206e -m528x -m5307 -m5407 -mcfv4e -mbitfield -mc68000 -mc68020 -mrtd -mdiv -mshort -mhard-float -m68881 -msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mshared-library-id=n -mid-shared-library -mxgot -mxtls -mlong-jump-table-offsets MCore Options (MCore Options) -mhardlit -mdiv -mrelax-immediates -mwide-bitfields -m4byte-functions -mcallgraph-data -mslow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment MicroBlaze Options (MicroBlaze Options) -msoft-float -mhard-float -msmall-divides -mcpu=cpu -mmemcpy -mxl-soft-mul -mxl-soft-div -mxl-barrel-shift -mxl-pattern-compare -mxl-gp-opt -mxl-multiply-high -mxl-float-convert -mxl-float-sqrt -mbig-endian -mlittle-endian -mxl-reorder -mxl-mode-app-model -mxl-prefetch -mpic-data-is-text-relative MIPS Options (MIPS Options) -EL -EB -mel -meb -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips32r3 -mips32r5 -mips32r6 -mips64 -mips64r2 -mips64r3 -mips64r5 -mips64r6 -mips16 -mmips16e2 -mflip-mips16 -minterlink-compressed -minterlink-mips16 -mabi=abi -mabicalls -mshared -mplt -mxgot -mgp32 -mgp64 -mfp32 -mfpxx -mfp64 -mhard-float -msoft-float -mno-float -msingle-float -mdouble-float -modd-spreg -mabs=mode -mnan=encoding -mdsp -mdspr2 -mmcu -meva -mvirt -mxpa -mcrc -mginv -mmicromips -mmsa -mloongson-mmi -mloongson-ext -mloongson-ext2 -mfpu=fpu-type -msmartmips -mpaired-single -mdmx -mips3d -mmt -mllsc -mlong64 -mlong32 -msym32 -Gnum -mno-local-sdata -mno-extern-sdata -mno-gopt -membedded-data -muninit-const-in-rodata -mcode-readable=setting -mno-data-in-code -mcode-xonly -msplit-addresses -mexplicit-relocs -mexplicit-relocs=release -mno-check-zero-division -mdivide-traps -mdivide-breaks -mno-load-store-pairs -mstrict-align -mno-unaligned-access -mmemcpy -mlong-calls -mmad -mimadd -mno-fused-madd -nocpp -mfix-24k -mfix-r4000 -mfix-r4400 -mfix-r5900 -mfix-r10000 -mfix-rm7000 -mfix-vr4120 -mfix-vr4130 -mfix-sb1 -mfix4300 -mr10k-cache-barrier=setting -mflush-func=func -mno-flush-func -mbranch-cost=num -mbranch-likely -mcompact-branches=policy -mno-fp-exceptions -mvr4130-align -msynci -mno-lxc1-sxc1 -mno-madd4 -mno-relax-pic-calls -mmcount-ra-address -mframe-header-opt MMIX Options (MMIX Options) -mlibfuncs -mepsilon -mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols -melf -mbranch-predict -mbase-addresses -msingle-exit -mset-data-start=address -mset-program-start=address -mno-set-program-start MN10300 Options (MN10300 Options) -mmult-bug -mno-mult-bug -mam33 -mam33-2 -mam34 -mtune=cpu-type -mno-return-pointer-on-d0 -mno-crt0 -mrelax -mno-liw -mno-setlb Moxie Options (Moxie Options) -meb -mel -mmul.x -mno-crt0 MSP430 Options (MSP430 Options) -msim -masm-hex -mmcu=name -mlarge -msmall -mrelax -mwarn-mcu -mwarn-devices-csv -mcode-region=where -mdata-region=where -muse-lower-region-prefix -msilicon-errata=name[,name...] -msilicon-errata-warn=name[,name...] -mhwmult=type -minrt -mtiny-printf -mmax-inline-shift=n NDS32 Options (NDS32 Options) -mbig-endian -mlittle-endian -EB -EL -mabi=name -mfloat-abi=name -mreduced-regs -mfull-regs -malways-align -malign-functions -mfp-as-gp -mcmov -mhw-abs -mext-perf -mext-perf2 -mext-string -mext-dsp -mext-fpu-fma -mext-fpu-sp -mext-fpu-dp -mv3push -m16-bit -mvh -misr-vector-size=num -misr-secure=num -mcache-block-size=num -march=arch -mcpu=cpu -mconfig-fpu=num -mconfig-mul=type -mconfig-register-ports=kind -mcmodel=code-model -mctor-dtor -mrelax -mrelax-hint -msched-prolog-epilog -mno-ret-in-naked-func -malways-save-lp -munaligned-access -minline-asm-r15 Nvidia PTX Options (Nvidia PTX Options) -m64 -march=arch -misa=arch -march-map=arch -mptx=version -mmainkernel -moptimize -msoft-stack -msoft-stack-reserve-local=size -muniform-simt -mgomp OpenRISC Options (OpenRISC Options) -mboard=name -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 (PDP-11 Options) -mfpu -msoft-float -mac0 -m40 -m45 -m10 -mint32 -mint16 -msplit -munix-asm -mdec-asm -mgnu-asm -mlra Picolibc Options (Picolibc Options) --oslib=library --crt0=[none|minimal|hosted|semihost] --printf=[d|f|l|i|m] --scanf=[d|f|l|i|m] PowerPC Options See RS/6000 and PowerPC Options. PRU Options (PRU Options) -mmcu=mcu -minrt -mno-relax -mloop -mmul -mfillzero -mabi=variant RISC-V Options (RISC-V Options) -mbranch-cost=N-instruction -mabi=ABI-string -mfdiv -mdiv -mno-fence-tso -misa-spec=ISA-spec-string -march=[ISA|Profile|Profile_ISA|processor-string] -mcpu=processor-string -mtune=processor-string -mpreferred-stack-boundary=num -msmall-data-limit=N-bytes -msave-restore -mno-shorten-memrefs -mstrict-align -mscalar-strict-align -mno-vector-strict-align -mcmodel=medlow -mcmodel=medany -mcmodel=large -mexplicit-relocs -mrelax -mriscv-attribute -malign-data=type -mbig-endian -mlittle-endian -mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset -mcsr-check -momit-leaf-frame-pointer -mmovcc -mno-inline-atomics -mno-inline-strlen -mno-inline-strcmp -mno-inline-strncmp -mstringop-strategy=strategy -mtls-dialect=desc -mtls-dialect=trad -mrvv-vector-bits=value -mrvv-max-lmul=value -madjust-lmul-cost -mmax-vectorization -mno-autovec-segment RL78 Options (RL78 Options) -msim -mallregs -mrelax -mes0 -mmul=none -mmul=g13 -mmul=g14 -mmul=rl78 -mcpu=g10 -mcpu=g13 -mcpu=g14 -mcpu=rl78 -mg10 -mg13 -mg14 -mrl78 -msave-mduc-in-interrupts RS/6000 and PowerPC Options (RS/6000 and PowerPC Options) -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mprofile-kernel -mpowerpc64 -maltivec -mpowerpc-gpopt -mpowerpc-gfxopt -mmfcrf -mpopcntb -mpopcntd -mfprnd -mcmpb -mhard-dfp -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -maix64 -maix32 -m64 -m32 -mxl-compat -mpe -malign-power -malign-natural -msoft-float -mhard-float -mmultiple -mupdate -mavoid-indexed-addresses -mfused-madd -mbit-align -mbit-word -mstrict-align -mrelocatable -mrelocatable-lib -mlittle -mlittle-endian -mbig -mbig-endian -mdynamic-no-pic -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 -msplit-patch-nops -mregnames -mlongcall -mpltseq -mtls-markers -mblock-move-inline-limit=num -mblock-compare-inline-limit=num -mblock-compare-inline-loop-limit=num -mblock-ops-unaligned-vsx -mstring-compare-inline-limit=num -misel -mvsx -mvrsave -mmulhw -mdlmzb -mprototype -msim -mmvme -mads -myellowknife -memb -meabi -msdata -msdata=opt -mreadonly-in-sdata -mvxworks -G num -mrecip -mrecip=opt -mno-recip -mrecip-precision -mveclibabi=type -mfriz -mpointers-to-nested-functions -msave-toc-indirect -mpower8-fusion -mcrypto -mhtm -mquad-memory -mquad-memory-atomic -mcompat-align-parm -mfloat128 -mfloat128-hardware -mgnu-attribute -mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset -mprefixed -mpcrel -mmma -mdense-math -mrop-protect -mprivileged -mno-splat-word-constant -mno-splat-float-constant -mno-ieee128-constant -mno-warn-altivec-long RX Options (RX Options) -m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu=name -mbig-endian-data -mlittle-endian-data -msmall-data-limit=N -msim -mas100-syntax -mrelax -mmax-constant-size=N -mint-register=N -mpid -mno-allow-string-insns -mjsr -mno-warn-multiple-fast-interrupts -msave-acc-in-interrupts -mlra S/390 and zSeries Options (S/390 and zSeries Options) -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float -mhard-dfp -mlong-double-64 -mlong-double-128 -mbackchain -mpacked-stack -msmall-exec -mmvcle -m64 -m31 -mdebug -mesa -mzarch -mhtm -mvx -mzvector -mtpf-trace -mtpf-trace-skip -mmain -mfused-madd -mno-fused-madd -mwarn-framesize=framesize -mwarn-dynamicstack -mstack-size=stack-size -mno-stack-size -mstack-guard=stack-guard -mno-stack-guard -mstack-protector-guard=guard -mstack-protector-guard-record -mhotpatch=pre-halfwords,post-halfwords -mno-pic-data-is-text-relative -mindirect-branch=choice -mindirect-branch-jump=choice -mindirect-branch-call=choice -mfunction-return=choice -mfunction-return-mem=choice -mfunction-return-reg=choice -mindirect-branch-table -mfentry -mrecord-mcount -mnop-mcount -mpreserve-args -munaliged-symbols SH Options (SH Options) -m1 -m2 -m2e -m2a -m2a-nofpu -m2a-single -m2a-single-only -m3 -m3e -m4 -m4-nofpu -m4-single -m4-single-only -m4-100 -m4-100-nofpu -m4-100-single -m4-100-single-only -m4-200 -m4-200-nofpu -m4-200-single -m4-200-single-only -m4-300 -m4-300-nofpu -m4-300-single -m4-300-single-only -m4-340 -m4-400 -m4-500 -m4a -m4al -m4a-nofpu -m4a-single -m4a-single-only -mb -ml -mdalign -mrelax -mbigtable -mbitops -mfmovd -mrenesas -mnomacsave -mieee -misize -minline-ic_invalidate -mprefergot -musermode -multcost=number -mdiv=strategy -mdivsi3_libfunc=name -mfixed-range=register-range -maccumulate-outgoing-args -matomic-model=atomic-model -mbranch-cost=num -mzdcbranch -mcbranch-force-delay-slot -mfsca -mfsrra -mpretend-cmove -mfdpic -mtas -mlra Solaris 2 Options (Solaris 2 Options) -mclear-hwcap -mno-clear-hwcap -mimpure-text -mno-impure-text -gsctf SPARC Options (SPARC Options) -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mmemory-model=mem-model -m32 -m64 -mptr32 -mptr64 -mapp-regs -mfaster-structs -mflat -mfpu -mhard-float -msoft-float -mhard-quad-float -msoft-quad-float -mstack-bias -mstd-struct-return -munaligned-doubles -muser-mode -mv8plus -mvis -mvis2 -mvis3 -mvis3b -mvis4 -mvis4b -mcbcond -mfmaf -mfsmuld -mpopc -msubxc -mfix-at697f -mfix-ut699 -mfix-ut700 -mfix-gr712rc System V Options (System V Options) -YP,paths -Ym,dir V850 Options (V850 Options) -mlong-calls -mep -mprolog-function -mspace -mtda=n -msda=n -mzda=n -mv850 -mv850e3v5 -m850e2v4 -mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e -mdisable-callt -mrelax -mlong-jumps -msoft-float -mhard-float -mloop -mrh850-abi -mghs -mgcc-abi -m8byte-align -mbig-switch -mapp-regs -msmall-sld -mno-strict-align -mjump-tables-in-data-section VAX Options (VAX Options) -munix -mgnu -md -md-float -mg -mg-float -mlra -mvaxc-alignment -mqmath Visium Options (Visium Options) -mdebug -msim -mfpu -mhard-float -msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode -muser-mode VMS Options (VMS Options) -mvms-return-codes -mdebug-main=prefix -mmalloc64 -mpointer-size=size VxWorks Options (VxWorks Options) -mrtp -msmp -mvthreads -non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now x86 Options (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 -mieee-fp -mrtd -malign-double -mpreferred-stack-boundary=num -mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe -mcrc32 -mmwait -mrecip -mrecip=opt -mvzeroupper -mstv -mprefer-avx128 -mprefer-vector-width=opt -mpartial-vector-fp-math -mmove-max=bits -mstore-max=bits -mnoreturn-no-callee-saved-registers -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f -mavx512cd -mavx512vl -mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi -msha -maes -mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mptwrite -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 -mgfni -mvaes -mwaitpkg -mshstk -mmanual-endbr -mcet-switch -mforce-indirect-call -mavx512vbmi2 -mavx512bf16 -menqcmd -mvpclmulqdq -mavx512bitalg -mmovdiri -mmovdir64b -mavx512vpopcntdq -mavx512vnni -mprfchw -mrdpid -mrdseed -msgx -mavx512vp2intersect -mserialize -mtsxldtrk -mamx-tile -mamx-int8 -mamx-bf16 -muintr -mhreset -mavxvnni -mamx-fp8 -mavx512fp16 -mavxifma -mavxvnniint8 -mavxneconvert -mcmpccxadd -mamx-fp16 -mprefetchi -mraoint -mamx-complex -mavxvnniint16 -msm3 -msha512 -msm4 -mapxf -musermsr -mavx10.1 -mavx10.2 -mamx-avx512 -mamx-tf32 -mmovrs -mamx-movrs -mavx512bmm -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 -mstack-arg-probe -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 -mrecord-return -mfentry-name=name -mfentry-section=name -mskip-rax-setup -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 -mtls-dialect=type -mrelax-cmpxchg-loop -mindirect-branch=choice -mfunction-return=choice -mindirect-branch-register -mharden-sls=choice -mindirect-branch-cs-prefix -mapx-inline-asm-use-gpr32 -mgather -mscatter -mneeded -mno-direct-extern-access -munroll-only-small-loops -mdispatch-scheduler -mlam=choice x86 Windows Options See Cygwin and MinGW Options. Xstormy16 Options (Xstormy16 Options) -msim Xtensa Options (Xtensa Options) -mconst16 -mforce-no-pic -mno-serialize-volatile -mtext-section-literals -mauto-litpools -mno-target-align -mlongcalls -mabi=abi-type -mextra-l32r-costs=cycles -mstrict-align 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 file.fi 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 file.fii 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.cob file.COB file.cbl file.CBL COBOL source code. 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 --language=language --language 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 c++-system-module objective-c objective-c-header objective-c-cpp-output objc-cpp-output objective-c++ objective-c++-header objective-c++-cpp-output objc++-cpp-output assembler assembler-with-cpp ada adascil adawhy cobol d f77 f77-cpp-input f95 f95-cpp-input go modula-2 modula-2-cpp-output rust algol68 lto Note that -x does not imply a particular language standard. For example -x f77 may also require -std=legacy for some older source codes. -x none Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as 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 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 --assemble 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 --preprocess 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 --output=file --output 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 --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 --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 --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 --verbose 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: optimizers Display all of the optimization options supported by the compiler. warnings Display all of the options controlling warning messages produced by the compiler. target Display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended --help= syntax. params Display the values recognized by the --param option. language Display the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, one needs to select common class. common Display the options that are common to all languages. These are the supported qualifiers: undocumented Display only those options that are undocumented. joined Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as: --help=target. separate Display options taking an argument that appears as a separate word following the original option, such as: -o output-file. Thus for example to display all the undocumented target-specific switches supported by the compiler, use: --help=target,undocumented The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use: --help=warnings,^joined,^undocumented The argument to --help= should not consist solely of inverted qualifiers. Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use: --help=target,optimizers The --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --help is also specified anywhere on the command line then this takes precedence over any --help= option. If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the --help= option is used). Here is a truncated example from the ARM port of gcc: % gcc -Q -mabi=2 --help=target -c The following options are target specific: -mabi= 2 -mabort-on-noreturn [disabled] -mapcs [disabled] The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at -O2 by using: -Q -O2 --help=optimizers Alternatively you can discover which binary optimizations are enabled by -O3 by using: gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts diff /tmp/O2-opts /tmp/O3-opts | grep enabled --version Display the version number and copyrights of the invoked GCC. -pass-exit-codes --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 --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. -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; preprocessed C++ files use the suffix .ii; and C++20 module interface units sometimes use .ixx, .cppm, .cxxm, .c++m, or .ccm. 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 --ansi In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98. -std= Determine the language standard. This option is currently only supported when compiling C or C++. The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the "asm" and "typeof" keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a "?:" expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not. A value for this option must be provided; possible values are c90 c89 iso9899:1990 Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code. iso9899:199409 ISO C90 as modified in amendment 1. c99 c9x iso9899:1999 iso9899:199x ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See for more information. The names c9x and iso9899:199x are deprecated. c11 c1x iso9899:2011 ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated. c17 c18 iso9899:2017 iso9899:2018 ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is same as C11 except for corrections of defects (all of which are also applied with -std=c11) and a new value of "__STDC_VERSION__", and so is supported to the same extent as C11. c23 c2x iso9899:2024 ISO C23, the 2023 revision of the ISO C standard (published in 2024). The name c2x is deprecated. c2y The next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. gnu90 gnu89 GNU dialect of ISO C90 (including some C99 features). gnu99 gnu9x GNU dialect of ISO C99. The name gnu9x is deprecated. gnu11 gnu1x GNU dialect of ISO C11. The name gnu1x is deprecated. gnu17 gnu18 GNU dialect of ISO C17. gnu23 gnu2x GNU dialect of ISO C23. This is the default for C code. The name gnu2x is deprecated. gnu2y The next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. The name gnu2x is deprecated. c++98 c++03 The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code. gnu++98 gnu++03 GNU dialect of -std=c++98. c++11 c++0x The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated. gnu++11 gnu++0x GNU dialect of -std=c++11. The name gnu++0x is deprecated. c++14 c++1y The 2014 ISO C++ standard plus amendments. The name c++1y is deprecated. gnu++14 gnu++1y GNU dialect of -std=c++14. The name gnu++1y is deprecated. c++17 c++1z The 2017 ISO C++ standard plus amendments. The name c++1z is deprecated. gnu++17 gnu++1z GNU dialect of -std=c++17. The name gnu++1z is deprecated. c++20 c++2a The 2020 ISO C++ standard plus amendments. C++20 modules support is still experimental and needs to be enabled with -fmodules option. The name c++2a is deprecated. gnu++20 gnu++2a GNU dialect of -std=c++20. This is the default for C++ code. C++20 modules support is still experimental and needs to be enabled with -fmodules option. The name gnu++2a is deprecated. c++23 c++2b The 2023 ISO C++ standard plus amendments (published in 2024). Support is experimental, and could change in incompatible ways in future releases. The name c++2b is deprecated. gnu++23 gnu++2b GNU dialect of -std=c++23. Support is experimental, and could change in incompatible ways in future releases. The name gnu++2b is deprecated. c++2c c++26 The next revision of the ISO C++ standard, planned for 2026. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. gnu++2c gnu++26 GNU dialect of -std=c++2c. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. -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 C23 mode (-std=c23 or -std=gnu23), this switch only affects the "asm" keyword, since "typeof" is a standard keyword in ISO C23. -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. -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. The -fdeps-* options are used to extract structured dependency information for a source. This involves determining what resources provided by other source files will be required to compile the source as well as what resources are provided by the source. This information can be used to add required dependencies between compilation rules of dependent sources based on their contents rather than requiring such information be reflected within the build tools as well. -fdeps-file=file Where to write structured dependency information. -fdeps-format=format The format to use for structured dependency information. p1689r5 is the only supported format right now. Note that when this argument is specified, the output of -MF is stripped of some information (namely C++ modules) so that it does not use extended makefile syntax not understood by most tools. -fdeps-target=file Analogous to -MT but for structured dependency information. This indicates the target which will ultimately need any required resources and provide any resources extracted from the source that may be required by other sources. -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 -funsigned-char -fsigned-char lets the type "char" be signed, like "signed char", while -funsigned-char lets the type "char" be unsigned, like "unsigned char". Note that -fsigned-char 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. Each target supported by GCC has a default for what "char" should be, which is typically specified in the processor-specific ABI document for that target. 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. These options 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 (C and C++ only) -fstrict-flex-arrays=level (C and C++ only) 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. -fstrict-flex-arrays 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 -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, which is the least strict. In this case all trailing arrays of structures are treated as flexible array members. There are two more levels in between 0 and 3, which are provided to support older code that uses the GCC zero-length array extension ([0]) or one-element array as flexible array members ([1]). When level is 1, the trailing array is treated as a flexible array member when it is declared as either [], [0], or [1]. When level is 2, the trailing array is treated as a flexible array member when it is declared as either [], or [0]. You can control this behavior for a specific trailing array field of a structure by using the variable attribute "strict_flex_array" attribute. The -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. -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: --compile-std-module Build the compiled module interfaces for the header unit and the std and std.compat modules before compiling any source files explicitly specified on the command line. This is intended to be useful for building simple programs that use import std; with a single command. -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 appeared in G++ 13, fixes manglings of lambdas that have additional context. Version 19, which first appeared in G++ 14, fixes manglings of structured bindings to include ABI tags, handling of cv-qualified [[no_unique_address]] members, and adds mangling of C++20 constraints on function templates. Version 20, which first appeared in G++ 15, fixes manglings of lambdas in static data member initializers. Version 21, which first appeared in G++ 16, fixes unnecessary captures in noexcept lambdas (c++/119764), layout of a base class with all explicitly defaulted constructors (c++/120012), and mangling of class and array objects with implicitly zero-initialized non-trailing subobjects (c++/121231). 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 -faligned-new=alignment 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. -fno-assume-sane-operators-new The C++ standard allows replacing the global "new", "new[]", "delete" and "delete[]" operators, though a lot of C++ programs don't replace them and just use the implementation provided version. Furthermore, the C++ standard allows omitting those calls if they are made from new or delete expressions (and by extension the same is assumed if "__builtin_operator_new" or "__builtin_operator_delete" functions are used). This option allows control over some optimizations around calls to those operators. With "-fassume-sane-operators-new-delete" option GCC may assume that calls to the replaceable global operators from new or delete expressions or from "__builtin_operator_new" or "__builtin_operator_delete" calls don't read or modify any global variables or variables whose address could escape to the operators (global state; except for "errno" for the "new" and "new[]" operators). This allows most optimizations across those calls and is something that the implementation provided operators satisfy unless "malloc" implementation details are observable in the code or unless "malloc" hooks are used, but might not be satisfied if a program replaces those operators. This behavior is enabled by default. With "-fno-assume-sane-operators-new-delete" option GCC must assume all these calls (whether from new or delete expressions or called directly) may read and write global state unless proven otherwise (e.g. when GCC compiles their implementation). Use this option if those operators are or may be replaced and code needs to expect such behavior. -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 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::basic_string()' using namespace std::literals; s = u8"xx"s; // error: conversion from // `basic_string' to non-scalar // type `basic_string' 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 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, could additionally be enabled by -fconcepts-ts. The option -fconcepts-ts was deprecated in GCC 14 and removed in GCC 15; users are expected to convert their code to C++20 concepts. -fconcepts-diagnostics-depth=n Specify maximum error replay depth during recursive diagnosis of a constraint satisfaction failure. The default is 1. -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 support for the C++ Contracts feature, as specified in the C++26 working draft (N5003). On violation of an enforced or observed contract (see -fcontract-evaluation-semantic below), a violation handler is called; the standard library provides a default handler that emits information about the contract that failed. Users can replace the default violation handler by defining void handle_contract_violation (const std::contracts::contract_violation&); -fcontract-evaluation-semantic=semantic Set the semantic with which contracts will be evaluated to semantic. Available values for the evaluation mode are: '"ignored"' The contract checks will be elided, with no checking added at either compile or runtime. '"observed"' The contract checks are performed (at runtime) and, if one fails, a handler is called that allows actions such as logging or emitting run-time warnings. When the handler returns, the execution of the code will continue. At compile-time the checks are performed for "constexpr" evaluations, in this case a diagnostic is emitted if the check fails. '"enforce"' The contract checks are performed and the handler is called if one fails. When the handler returns the execution of the code is terminated preventing it from continuing past the failed case. At compile-time the checks are applied to "constexpr" and, if one fails, the program is ill-formed; an error will be emitted. '"quick_enforce"' The contract checks are performed (at runtime) and, if one fails, the execution is terminated immediately (without calling any handler). At compile-time there is no difference in behaviour between this option and the "enforce" case, since no handler is invoked at compile time. -fcontracts-conservative-ipa This prevents inter-procedural analysis from taking action when the body of an inline function is visible in a given TU. This allows for the case that contract evaluation conditions are permitted to differ between TUs which means that such actions would be potentially incorrect. -fcontract-checks-outlined By default, contract checks for "pre" and "post" condition checks are expanded in-line into function bodies. This option causes a small function to be created instead (containing the checks) that is called in the relevant position. This permits different criteria to be applied to the compilation of the checking and the remainder of the function body. -fcontract-disable-optimized-checks When "pre" and "post" condition checks are outlined (using -fcontract-checks-outlined), this option disables the optimisation steps for those functions which can avoid unwanted elision of checking steps. -fcontracts-client-check=mode This option enables insertion of checks at call sites (caller-side checking). The value of mode determines which contract checks are made: '"none"' No caller/client side checking (default) '"pre"' Preconditions are checked at the caller/client side. '"all"' Both pre and post-conditions are checked at the caller/client side. -fcontracts-definition-check=mode This option introduces the ability to disable callee/definition-side checks, which are otherwise enabled by default when the contracts feature is active. The variable mode has the values '"on"' and '"off"'. -fcoroutines Enable support for the C++ coroutines extension. With -std=c++20 and above, coroutines are part of the language standard, so -fcoroutines defaults to on. -fdiagnostics-all-candidates Permit the C++ front end to note all candidates during overload resolution failure, including when a deleted function is selected. -fdump-lang- -fdump-lang-switch -fdump-lang-switch-options -fdump-lang-switch-options=filename Control the dumping of C++-specific information. The options and filename portions behave as described in the -fdump-tree option. The following switch values are accepted: all Enable all of the below. class Dump class hierarchy information. Virtual table information is emitted unless 'slim' is specified. module Dump module information. Options lineno (locations), graph (reachability), blocks (clusters), uid (serialization), alias (mergeable), asmname (Elrond), eh (mapper) & vops (macros) may provide additional information. raw Dump the raw internal tree data. tinst Dump the sequence of template instantiations, indented to show the depth of recursion. The lineno option adds the source location where the instantiation was triggered, and the details option also dumps pre-instantiation substitutions such as those performed during template argument deduction. Lines in the .tinst dump start with I for an instantiation, S for another substitution, and R[IS] for the reopened context of a deferred instantiation. -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", "std::to_underlying" 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. -fno-immediate-escalation Do not enable immediate function escalation whereby certain functions can be promoted to consteval, as specified in P2564R3. For example: consteval int id(int i) { return i; } constexpr int f(auto t) { return t + id(t); // id causes f to be promoted to consteval } void g(int i) { f (3); } compiles in C++20: "f" is an immediate-escalating function (due to the "auto" it is a function template and is declared "constexpr") and id(t) is an immediate-escalating expression, so "f" is promoted to "consteval". Consequently, the call to id(t) is in an immediate context, so doesn't have to produce a constant (that is the mechanism allowing consteval function composition). However, with -fno-immediate-escalation, "f" is not promoted to "consteval", and since the call to consteval function id(t) is not a constant expression, the compiler rejects the code. This option is turned on by default; it is only effective in C++20 mode or later. -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 -fno-modules Enable support for C++20 modules. The -fno-modules 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=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. -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. -frange-for-ext-temps Enable lifetime extension of C++ range based for temporaries. With -std=c++23 and above this is part of the language standard, so lifetime of the temporaries is extended until the end of the loop by default. This option allows enabling that behavior also in earlier versions of the standard. -freflection Enable experimental C++26 Reflection. -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. This option has no effect for an enumeration type with a fixed underlying type. -fstrong-eval-order -fstrong-eval-order=kind 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. -fstrong-eval-order is equivalent to -fstrong-eval-order=all, and is enabled by default with -std=c++17 or later. -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default for C++ dialects prior to C++17. -fstrong-eval-order=none is equivalent to -fno-strong-eval-order. -ftemplate-backtrace-limit=n Set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10. -ftemplate-depth=n Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations. -fno-threadsafe-statics Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe. -fuse-cxa-atexit Register destructors for objects with static storage duration with the "__cxa_atexit" function rather than the "atexit" function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports "__cxa_atexit". -fno-use-cxa-get-exception-ptr Don't use the "__cxa_get_exception_ptr" runtime routine. This causes "std::uncaught_exception" to be incorrect, but is necessary if the runtime routine is not available. -fvisibility-inlines-hidden This switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects. The effect of this is that GCC may, effectively, mark inline methods with "__attribute__ ((visibility ("hidden")))" so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object. You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect. Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. -fvisibility-ms-compat This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio. The flag makes these changes to GCC's linkage model: 1. It sets the default visibility to "hidden", like -fvisibility=hidden. 2. Types, but not their members, are not hidden by default. 3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used. In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior. Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently. -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 "" 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 note 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. -Wno-abbreviated-auto-in-template-arg Disable the error for an "auto" placeholder type used within a template argument list to declare a C++20 abbreviated function template, e.g. void f(S); This feature was proposed in the Concepts TS, but was not adopted into C++20; in the standard, a placeholder in a parameter declaration must appear as a decl-specifier. The error can also be reduced to a warning by -fpermissive or -Wno-error=abbreviated-auto-in-template-arg. -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 after -std=c++23 regardless of -Wno-deprecated. Before -std=c++20, enabled with explicit -Wdeprecated. This warning is upgraded to an error by -pedantic-errors in C++23 mode or later. -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 struct S { S(T) { } }; // Guide with incomplete parameter type will never be considered. S(allow_ctad_t) -> S; -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 The "#pragma" can also surround the class; in that case, the warning will be disabled for all the member functions. -Wdangling-reference also warns about code like auto p = std::minmax(1, 2); where "std::minmax" returns "std::pair", and both references dangle after the end of the full expression that contains the call to "std::minmax". The warning does not warn for "std::span"-like classes. We consider classes of the form: template struct Span { T* data_; std::size len_; }; as "std::span"-like; that is, the class is a non-union class that has a pointer data member and a trivial destructor. The warning can be disabled by using the "gnu::no_dangling" attribute. 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. -Wdeprecated-copy-dtor (C++ and Objective-C++ only) Similar to -Wdeprecated-copy, but 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 or -Wdeprecated. -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 or -Wdeprecated. -Wdeprecated-literal-operator (C++ and Objective-C++ only) Warn that the declaration of a user-defined literal operator with a space before the suffix is deprecated. This warning is enabled by default in C++23, or with explicit -Wdeprecated. string operator "" _i18n(const char*, std::size_t); // deprecated string operator ""_i18n(const char*, std::size_t); // preferred -Wdeprecated-variadic-comma-omission (C++ and Objective-C++ only) Warn that omitting a comma before the varargs "..." at the end of a function parameter list is deprecated. This warning is enabled by default in C++26, or with explicit -Wdeprecated. void f1(int...); // deprecated void f1(int, ...); // preferred template void f2(T...); // ok template void f3(T......); // deprecated -Wno-elaborated-enum-base For C++11 and above, warn if an (invalid) additional enum-base is used in an elaborated-type-specifier. That is, if an enum with given underlying type and no enumerator list is used in a declaration other than just a standalone declaration of the enum. Enabled by default. This warning is upgraded to an error with -pedantic-errors. -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: * If a function returns a temporary "initializer_list", or a local "initializer_list" variable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer. * If a new-expression creates an "initializer_list", the array only lives until the end of the enclosing full-expression, so the "initializer_list" in the heap has a dangling pointer. * When an "initializer_list" variable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so at the next statement the "initializer_list" variable has a dangling pointer. // li's initial underlying array lives as long as li std::initializer_list li = { 1,2,3 }; // assignment changes li to point to a temporary array li = { 4, 5 }; // now the temporary is gone and li has a dangling pointer int i = li.begin()[0] // undefined behavior * When a list constructor stores the "begin" pointer from the "initializer_list" argument, this doesn't extend the lifetime of the array, so if a class variable is constructed from a temporary "initializer_list", the pointer is left dangling by the end of the variable declaration statement. -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 "". For example: #define __STDC_FORMAT_MACROS #include #include int main() { int64_t i64 = 123; printf("My int64: %" PRId64"\n", i64); } In this case, "PRId64" is treated as a separate preprocessing token. This 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 void f(T t) { t(); }; void g() noexcept; void h() { f(g); } In C++14, "f" calls "f", but in C++17 it calls "f". -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: * Define a copy constructor and an assignment operator for classes with dynamically-allocated memory. * Prefer initialization to assignment in constructors. * Have "operator=" return a reference to *this. * Don't try to return a reference when you must return an object. * Distinguish between prefix and postfix forms of increment and decrement operators. * Never overload "&&", "||", or ",". This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too. When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings. -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. -Wsfinae-incomplete (C++ and Objective-C++ only) Warn about a class that is found to be incomplete, or a function with auto return type that has not yet been deduced, in a context where that causes substitution failure rather than an error, and then the class or function is defined later in the translation unit. This is problematic because template instantiations or concept checks could have different results if they first occur either before or after the definition. This warning is enabled by default. -Wsfinae-incomplete=2 adds a warning at the point of substitution failure, to make it easier to track down problems flagged by the default mode. -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-c-typedef-for-linkage (C++ and Objective-C++ only) Disable pedwarn for unnamed classes with a typedef name for linkage purposes containing C++ specific members, base classes, default member initializers or lambda expressions, including those on nested member classes. typedef struct { int a; // non-static data members are ok struct T { int b; }; // member classes too enum E { E1, E2, E3 }; // member enumerations as well int c = 42; // default member initializers are not ok struct U : A { int c; }; // classes with base classes are not ok typedef int V; // typedef is not ok using W = int; // using declaration is not ok decltype([](){}) x; // lambda expressions not ok } S; In all these cases, the tag name S should be added after the struct keyword. -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-template-body (C++ and Objective-C++ only) Disable diagnosing errors when parsing a template, and instead issue an error only upon instantiation of the template. This flag can also be used to downgrade such errors into warnings with Wno-error= or -fpermissive. -Wno-template-id-cdtor (C++ and Objective-C++ only) Disable the warning about the use of simple-template-id as the declarator-id of a constructor or destructor, which became invalid in C++20 via DR 2237. For example: template struct S { S(); // should be S(); ~S(); // should be ~S(); }; -Wtemplate-id-cdtor is enabled by default with -std=c++20; it is also enabled by -Wc++20-compat. -Wtemplate-names-tu-local Warn when a template body hides an exposure of a translation-unit-local entity. In most cases, referring to a translation-unit-local entity (such as an internal linkage declaration) within an entity that is emitted into a module's CMI is an error. However, within the initializer of a variable, or in the body of a non-inline function, this is not an exposure and no error is emitted. This can cause variable or function templates to accidentally become unusable if they reference such an entity, because other translation units that import the template will never be able to instantiate it. This warning attempts to detect cases where this might occur. The presence of an explicit instantiation silences the warning. This flag is enabled by -Wextra. -Wno-expose-global-module-tu-local An exposure of a translation-unit-local entity from a module interface is invalid, as this may cause ODR violations and manifest in link errors or other unexpected behaviour. However, many existing libraries declare TU-local entities in their interface, and avoiding exposures of these entities may be difficult in some cases. As an extension, GCC allows exposures of internal variables and functions that were declared in the global module fragment. This warning indicates when such an invalid exposure has occurred, and can be silenced using diagnostic pragmas either at the site of the exposure, or at the point of declaration of the internal declaration. This also applies to "export using" declarations naming such entities. When combined with -Wtemplate-names-tu-local, GCC will also warn about non-exposure references to TU-local entities in template bodies. Such templates can still be instantiated in other TUs but the above risks regarding exposures of translation-unit-local entities apply. This warning is enabled by default, and is upgraded to an error by -pedantic-errors. -Wno-external-tu-local Warn when naming a TU-local entity outside of the translation unit it was declared in. Such declarations will be ignored during name lookup. This can occur when performing ADL from a template declared in the same TU as the internal function: export module M; template void foo(T t) { bar(t); } struct S {} s; static void bar(S) {} // internal linkage // instantiating foo(s) from outside this TU can see ::bar, // but naming it there is ill-formed. This can be worked around by making "bar" attached to the global module, using "extern "C++"". This warning is enabled by default, and is upgraded to an error by -pedantic-errors. -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. Before -std=c++20, enabled with explicit -Wdeprecated. -Waligned-new -Waligned-new=[none|global|all] 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. -Wplacement-new=1 This is the default warning level of -Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the following "new" expression is not diagnosed at this level even though it has undefined behavior according to the C++ standard because it writes past the end of the one-element array. struct S { int n, a[1]; }; S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]); new (s->a)int [32](); -Wplacement-new=2 At this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2. struct S { int n, a[]; }; S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]); new (s->a)int [32](); -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-defaulted-function-deleted (C++ and Objective-C++ only) Warn when an explicitly defaulted function is deleted by the compiler. That can occur when the function's declared type does not match the type of the function that would have been implicitly declared. This warning is enabled by default. -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. There are various contexts in which an extra semicolon can occur. One is a semicolon after in-class function definitions, which is valid in all C++ dialects (and is never a pedwarn): struct S { void foo () {}; }; Another is an extra semicolon at namespace scope, which has been allowed since C++11 (therefore is a pedwarn in C++98): struct S { }; ; And yet another is an extra semicolon in class definitions, which has been allowed since C++11 (therefore is a pedwarn in C++98): struct S { int a; ; }; -Wno-global-module (C++ and Objective-C++ only) Disable the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor directives. -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. -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 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 / macOS. 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 Controlling OpenMP and OpenACC GCC supports OpenMP extensions to the C, C++, and Fortran languages with the -fopenmp option. Similarly, OpenACC extensions are supported in all three languages with -fopenacc. -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='-fno-math-errno -ffinite-math-only' \ -foffload-options=nvptx-none=-latomic -foffload-options=amdgcn-amdhsa=-march=gfx906 -fopenacc Enable handling of OpenACC directives #pragma acc in C/C++ and !$acc in free-form Fortran and !$acc, c$acc and *$acc in fixed-form Fortran. 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 them. 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, [[omp::decl(...)]], [[omp::directive(...)]], and [[omp::sequence(...)]] in C/C++. In Fortran, it enables !$omp and the conditional compilation sentinel !$. In fixed source form Fortran, the sentinels can also start with c or *. 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" and "loop" directive, and of combined or composite directives with "simd" as constituent with "#pragma omp", "[[omp::directive(...)]]", "[[omp::sequence(...)]]" and "[[omp::decl(...)]]" in C/C++ and "!$omp" in Fortran. It additionally enables the conditional compilation sentinel !$ in Fortran. In fixed source form Fortran, the sentinels can also start with c or *. Other OpenMP directives are ignored. Unless -fopenmp is additionally specified, the "loop" region binds to the current task region, independent of the specified "bind" clause. -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. 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 -fdiagnostics-text-art-charset=none -fno-diagnostics-show-event-links -fno-diagnostics-show-nesting 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:valid=01;31:invalid=01;32\ highlight-a=01;32:highlight-b=01;34 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. "error=" SGR substring for error: markers. "warning=" SGR substring for warning: markers. "note=" SGR substring for note: markers. "path=" SGR substring for colorizing paths of control-flow events as printed via -fdiagnostics-path-format=, such as the identifiers of individual events and lines indicating interprocedural calls and returns. "range1=" SGR substring for first additional range. "range2=" SGR substring for second additional range. "locus=" SGR substring for location information, file:line or file:line:column etc. "quote=" SGR substring for information printed within quotes. "fnname=" SGR substring for names of C++ functions. "targs=" SGR substring for C++ function template parameter bindings. "fixit-insert=" SGR substring for fix-it hints suggesting text to be inserted or replaced. "fixit-delete=" SGR substring for fix-it hints suggesting text to be deleted. "diff-filename=" SGR substring for filename headers within generated patches. "diff-hunk=" SGR substring for the starts of hunks within generated patches. "diff-delete=" SGR substring for deleted lines within generated patches. "diff-insert=" SGR substring for inserted lines within generated patches. "type-diff=" SGR substring for highlighting mismatching types within template arguments in the C++ frontend. "valid=" SGR substring for highlighting valid elements within text art diagrams. "invalid=" SGR substring for highlighting invalid elements within text art diagrams. "highlight-a=" "highlight-b=" SGR substrings for contrasting two different things within diagnostics, such as a pair of mismatching types. See -fdiagnostics-show-highlight-colors. -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-event-links By default, when printing execution paths (via -fdiagnostics-path-format=inline-events), GCC will print lines connecting related events, such as the line connecting events 1 and 2 in: 3 | if (p) | ^ | | | (1) following `false' branch (when `p' is NULL)... ->-+ | | | | |+------------------------------------------------------------+ 4 || return 0; 5 || return *p; || ~ || | |+-------->(2) ...to here | (3) dereference of NULL `p' This option suppresses the printing of such connector lines. -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-highlight-colors GCC can use color for emphasis and contrast when printing diagnostic messages and quoting the user's source. For example, in demo.c: In function `test_bad_format_string_args': ../../src/demo.c:25:18: warning: format `%i' expects argument of type `int', but argument 2 has type `const char *' [-Wformat=] 25 | printf("hello %i", msg); | ~^ ~~~ | | | | int const char * | %s * the %i and "int" in the message and the "int" in the quoted source are colored using "highlight-a" (bold green by default), and * the "const char *" in the message and in the quoted source are both colored using "highlight-b" (bold blue by default). The intent is to draw the reader's eyes to the relationships between the various aspects of the diagnostic message and the source, using color to group related elements and distinguish between mismatching ones. This additional colorization is enabled by default if color printing is enabled (as per -fdiagnostics-color=), but it can be separately disabled via -fno-diagnostics-show-highlight-colors. -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-show-context[=depth] -fno-diagnostics-show-context With this option, the compiler might print the interesting control flow chain that guards the basic block of the statement which has the warning. depth is the maximum depth of the control flow chain. Currently, The list of the impacted warning options includes: -Warray-bounds, -Wstringop-overflow, -Wstringop-overread, -Wstringop-truncation. and -Wrestrict. More warning options might be added to this list in future releases. The forms -fdiagnostics-show-context and -fno-diagnostics-show-context are aliases for -fdiagnostics-show-context=1 and -fdiagnostics-show-context=0, respectively. -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 >()' from 'map<[...],vector>' to 'map<[...],vector> 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 >()' from 'map<[...],vector>' to 'map<[...],vector> 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). 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 , and bytes that do not correspond to a Unicode character validly-encoded in UTF-8-encoded will be displayed as hexadecimal in the form . 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: beforeafter Setting FORMAT to bytes will display all non-printable-ASCII bytes in the form , thus showing the underlying encoding of non-ASCII Unicode characters. For the example above, the following will be printed: before<80>after -fdiagnostics-text-art-charset=CHARSET Some diagnostics can contain "text art" diagrams: visualizations created from text, intended to be viewed in a monospaced font. This option selects which characters should be used for printing such diagrams, if any. CHARSET is none, ascii, unicode, or emoji. The none value suppresses the printing of such diagrams. The ascii value will ensure that such diagrams are pure ASCII ("ASCII art"). The unicode value will allow for conservative use of unicode drawing characters (such as box-drawing characters). The emoji value further adds the possibility of emoji in the output (such as emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select the emoji variant of the character). The default is emoji, except when the environment variable LANG is set to C, in which case the default is ascii. -fno-diagnostics-show-nesting Some GCC diagnostics have an internal tree-like structure of nested sub-diagnostics, such as for problems when instantiating C++ templates. By default GCC uses indentation and bullet points in its text output to show the nesting structure of these diagnostics, moves location information to separate lines to make the structure clearer, and eliminates redundant repeated information. Selecting -fno-diagnostics-show-nesting suppresses this indentation, reformatting, and elision, restoring an older `look" for the diagnostics. -fno-diagnostics-show-nesting-locations When fdiagnostics-show-nesting is enabled, file names and line- and column- numbers are displayed on separate lines from the messages. This location information can be disabled altogether with -fno-diagnostics-show-nesting-locations. This option exists for use by GCC developers, for writing DejaGnu test cases. -fdiagnostics-show-nesting-levels When fdiagnostics-show-nesting is enabled, use fdiagnostics-show-nesting-levels to also display numbers showing the depth of the nesting. This option exists for use by GCC developers for debugging nested diagnostics, but may be of use to plugin authors. -fdiagnostics-format=FORMAT Select a different format for printing diagnostics. FORMAT is text, sarif-stderr or sarif-file. Using this option replaces any additional "output sinks" added by -fdiagnostics-add-output=, or that set by -fdiagnostics-set-output=. 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. -fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC Add an additional "output sink" for emitting diagnostics. DIAGNOSTICS-OUTPUT-SPEC should specify a scheme, optionally followed by ":" and one or more KEY=VALUE pairs, in this form: := :=,= etc. Schemes, keys, or values with a name prefixed "experimental" may change or be removed without notice. Keys can be per-scheme, or related to GCC as a whole. SCHEME can be text Emit diagnostics to stderr using GCC's classic text output format. Supported keys for the "text" scheme are: color=[yes|no] Override colorization settings from -fdiagnostics-color for this text output. show-nesting=[yes|no] Enable a mode that emphasizes hierarchical relationships within diagnostics messages, as per -fdiagnostics-show-nesting. Defaults to "yes". show-nesting-locations=[yes|no] If "show-nesting=yes", then by default locations are shown; set this key to "no" to disable printing such locations. This exists for use by GCC developers, for writing DejaGnu test cases. show-nesting-levels=[yes|no] This is a debugging option for use with "show-nesting=yes". Set this key to "yes" to print explicit nesting levels in the output. This exists for use by GCC developers. sarif Emit diagnostics to a file in SARIF format. Supported keys for the "sarif" scheme are: file=FILENAME Specify the filename to write the SARIF output to, potentially with a leading absolute or relative path. If not specified, it defaults to source.sarif. serialization=[json] Specify the serialization format to use when writing out the SARIF. Currently this can only be "json", but is present as an extension point for experimenting with other serializations. version=[2.1|2.2-prerelease] Specify the version of SARIF to use for the output. If not specified, defaults to 2.1. "2.2-prerelease" uses an unofficial draft of the future SARIF 2.2 specification and should only be used for experimentation in this release. There is also this key intended for use by GCC developers, rather than end-users, and subject to change or removal without notice: state-graphs=[yes|no] This is a debugging feature and defaults to "no". If "state-graphs=yes", then attempt to capture detailed state information from -fanalyzer in the generated SARIF. experimental-html Emit diagnostics to a file in HTML format. This scheme is experimental, and may go away in future GCC releases. The keys and details of the output are also subject to change. Supported keys for the "experimental-html" scheme are: css=[yes|no] Add an embedded