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ADC Home > Reference Library > Reference > Mac OS X > Mac OS X Man Pages
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This document is a Mac OS X manual page. Manual pages are a command-line technology for providing documentation. You can view these manual pages locally using the man(1) command. These manual pages come from many different sources, and thus, have a variety of writing styles. This manual page is associated with the Mac OS X developer tools. The software or headers described may not be present on your Mac OS X installation until you install the developer tools package. This package is available on your Mac OS X installation DVD, and the latest versions can be downloaded from developer.apple.com. For more information about the manual page format, see the manual page for manpages(5). |
GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the
same options as gcc.
In Apple's version of GCC, both cc and gcc are actually symbolic links to a compiler named like
gcc-version; which compiler is linked to may be changed using the command gcc_select. Similarly, c++
and g++ are links to a compiler named like g++-version.
Note that Apple's GCC includes a number of extensions to standard GCC (flagged below with ``APPLE
ONLY''), and that not all generic GCC options are available or supported on Darwin / Mac OS X. In
particular, Apple does not currently support the compilation of Fortran, Ada, or Java, although there
are third parties who have made these work.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The
``overall options'' allow you to stop this process at an intermediate stage. For example, the -c
option says not to run the linker. Then the output consists of object files output by the assembler.
Other options are passed on to one stage of processing. Some options control the preprocessor and
others the compiler itself. Yet other options control the assembler and linker; most of these are
not documented here, since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful for C programs; when an option
is only useful with another language (usually C++), the explanation says so explicitly. If the
description for a particular option does not mention a source language, you can use that option with
all supported languages.
The gcc program accepts options and file names as operands. Many options have multi-letter names;
therefore multiple single-letter options may not be grouped: -dr is very different from -d -r.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order
does matter when you use several options of the same kind; for example, if you specify -L more than
once, the directories are searched in the order specified.
Many options have long names starting with -f or with -W---for example, -fforce-mem,
-fstrength-reduce, -Wformat and so on. Most of these have both positive and negative forms; the
negative form of -ffoo would be -fno-foo. This manual documents only one of these two forms,
whichever one is not the default.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
Overall Options
-c -S -E -o file -combine -pipe -pass-exit-codes -ObjC (APPLE ONLY) -ObjC++ (APPLE ONLY)
-arch arch (APPLE ONLY) -fsave-repository=file -x language -v -### --help --target-help
--version
C Language Options
-ansi -std=standard -aux-info filename -faltivec (APPLE ONLY) -fasm-blocks (APPLE ONLY)
-fno-asm -fno-builtin -fno-builtin-function -fhosted -ffreestanding -fms-extensions
-trigraphs -no-integrated-cpp -traditional -traditional-cpp -fallow-single-precision
-fcond-mismatch -fconstant-cfstrings (APPLE ONLY) -fnon-lvalue-assign (APPLE ONLY)
-fno-nested-functions -fpch-preprocess (APPLE ONLY) -fsigned-bitfields -fsigned-char
-fpascal-strings (APPLE ONLY) -Wno-#warnings (APPLE ONLY) -Wextra-tokens (APPLE ONLY)
-Wnewline-eof (APPLE ONLY) -Wno-altivec-long-deprecated (APPLE ONLY) -funsigned-bitfields
-funsigned-char -fwritable-strings
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space -fno-const-strings
-fno-elide-constructors -fno-enforce-eh-specs -ffor-scope -fno-for-scope -fno-gnu-keywords
-fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines -fms-extensions
-fno-nonansi-builtins -fno-operator-names -fno-optional-diags -fpermissive -frepo -fno-rtti
-fstats -ftemplate-depth-n -fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++
-fno-default-inline -fvisibility-inlines-hidden -fvisibility-ms-compat -Wabi
-Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Weffc++ -Wno-deprecated
-Wstrict-null-sentinel -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wsign-promo
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers
-fobjc-call-cxx-cdtors (APPLE ONLY) -fobjc-sjlj-exceptions -fobjc-gc -freplace-objc-classes
-fzero-link -gen-decls -Wno-protocol -Wselector -Wstrict-selector-match -Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall -Waggregate-return -Wcast-align
-Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Wno-deprecated-declarations
-Wdisabled-optimization -Wno-div-by-zero -Wno-endif-labels -Werror
-Werror-implicit-function-declaration -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2
-Wno-format-extra-args -Wformat-nonliteral -Wformat-security -Wformat-y2k -Wimplicit
-Wimplicit-function-declaration -Wimplicit-int -Wimport -Wno-import -Winit-self -Winline
-Wno-int-to-pointer-cast -Wno-invalid-offsetof -Winvalid-pch -Wlarger-than-len -Wlong-long
-Wmain -Wmissing-braces -Wmissing-field-initializers -Wmissing-format-attribute
-Wmissing-include-dirs -Wmissing-noreturn -Wmost (APPLE ONLY) -Wno-multichar -Wnonnull -Wpacked
-Wpadded -Wparentheses -Wpointer-arith -Wno-pointer-to-int-cast -Wredundant-decls -Wreturn-type
-Wsequence-point -Wshadow -Wstack-protector -Wsign-compare -Wstrict-aliasing
-Wstrict-aliasing=2 -Wswitch -Wswitch-default -Wswitch-enum -Wsystem-headers -Wtrigraphs
-Wundef -Wuninitialized -Wunknown-pragmas -Wunreachable-code -Wunused -Wunused-function
-Wunused-label -Wunused-parameter -Wunused-value -Wunused-variable -Wwrite-strings
-Wvariadic-macros
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes -Wnested-externs
-Wold-style-definition -Wstrict-prototypes -Wtraditional -Wdeclaration-after-statement
-Wno-discard-qual -Wno-pointer-sign
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered -fdump-translation-unit[-n]
-fdump-class-hierarchy[-n] -fdump-ipa-all -fdump-ipa-cgraph -fdump-tree-all
-fdump-tree-original[-n] -fdump-tree-optimized[-n] -fdump-tree-inlined[-n] -fdump-tree-cfg
-fdump-tree-vcg -fdump-tree-alias -fdump-tree-ch -fdump-tree-ssa[-n] -fdump-tree-pre[-n]
-fdump-tree-ccp[-n] -fdump-tree-dce[-n] -fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n]
-fdump-tree-scev [-n] -fdump-tree-ddall [-n] -fdump-tree-elck [-n] -fdump-tree-dom[-n]
-fdump-tree-dse[-n] -fdump-tree-phiopt[-n] -fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n]
-fdump-tree-nrv -fdump-tree-vect -fdump-tree-sra[-n] -fdump-tree-fre[-n] -fdump-tree-loop[-n]
-fdump-tree-vect[-n] -ftree-vectorizer-verbose=n -flimit-debug-info -feliminate-dwarf2-dups
-feliminate-unused-debug-types -feliminate-unused-debug-symbols -fmem-report -fopt-diary
-fprofile-arcs -ftree-based-profiling -frandom-seed=string -fsched-verbose=n -ftest-coverage
-ftime-report -fvar-tracking -g -glevel -gcoff -gdwarf-2 -ggdb -gstabs -gstabs+ -gvms
-gxcoff -gxcoff+ -p -pg -print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib -print-prog-name=program -print-search-dirs -Q
-save-temps -time
Optimization Options
-falign-functions=n -falign-jumps=n -falign-labels=n -falign-loops=n -falign-loops-max-skip=n
-falign-jumps-max-skip=n -fbounds-check -fmudflap -fmudflapth -fmudflapir -fbranch-probabilities
-fprofile-values -fvpt -fbranch-target-load-optimize -fbranch-target-load-optimize2
-fbtr-bb-exclusive -fcaller-saves -fcprop-registers -fcreate-profile -fcse-follow-jumps
-fcse-skip-blocks -fcx-limited-range -fdata-sections -fdelayed-branch
-fdelete-null-pointer-checks -fexpensive-optimizations -ffast-math -ffloat-store -fforce-addr
-fforce-mem -ffunction-sections -fgcse -fgcse-lm -fgcse-sm -fgcse-las -fgcse-after-reload
-floop-optimize -fcrossjumping -fif-conversion -fif-conversion2 -finline-functions
-finline-limit=n -fkeep-inline-functions -fkeep-static-consts -fmerge-constants
-fmerge-all-constants -fmodulo-sched -fno-branch-count-reg -fno-default-inline -fno-defer-pop
-floop-optimize2 -fmove-loop-invariants -fno-function-cse -fno-guess-branch-probability
-fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -funsafe-math-optimizations
-ffinite-math-only -fno-trapping-math -fno-zero-initialized-in-bss -mstackrealign
-fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls -fprefetch-loop-arrays
-fprofile-generate -fprofile-use -fregmove -frename-registers -freorder-blocks
-freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -frerun-loop-opt
-frounding-math -fschedule-insns -fschedule-insns2 -fno-sched-interblock -fno-sched-spec
-fsched-spec-load -fsched-spec-load-dangerous -fsched-stalled-insns=n -sched-stalled-insns-dep=n
-fsched2-use-superblocks -fsched2-use-traces -freschedule-modulo-scheduled-loops -fsignaling-nans
-fsingle-precision-constant -fspeculative-prefetching -fstack-protector -fstack-protector-all
-fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps -funroll-all-loops -funroll-loops
-fpeel-loops -fsplit-ivs-in-unroller -funswitch-loops -fvariable-expansion-in-unroller -ftree-pre
-ftree-ccp -ftree-dce -ftree-loop-optimize -ftree-loop-linear -ftree-loop-im -ftree-loop-ivcanon
-fivopts -ftree-dominator-opts -ftree-dse -ftree-copyrename -ftree-ch -ftree-sra -ftree-ter
-ftree-lrs -ftree-fre -ftree-vectorize -fuse-profile -fweb -fscalar-evolutions -fall-data-deps
--param name=value -O -O0 -O1 -O2 -O3 -Os -Oz (APPLE ONLY) -fast (APPLE ONLY)
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn] -E -H -idirafter
dir -include file -imacros file -iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem
dir -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -fworking-directory -remap -trigraphs
-undef -Umacro -Wp,option -Xpreprocessor option
Assembler Option
-Wa,option -Xassembler option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -pie -s -static
-static-libgcc -shared -shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -iquotedir -Ldir -specs=file -I-Target -ITarget
Target Options
-V version -b machine
Machine Dependent Options
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check
-mapcs-float -mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant -msched-prolog
-mno-sched-prolog -mlittle-endian -mbig-endian -mwords-little-endian -mfloat-abi=name
-msoft-float -mhard-float -mfpe -mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name
-mfpu=name -mstructure-size-boundary=n -mabort-on-noreturn -mlong-calls -mno-long-calls
-msingle-pic-base -mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport
-mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns -mpoke-function-name -mthumb -marm
-mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking
Darwin Options -all_load -allowable_client -arch -arch_errors_fatal -arch_only -bind_at_load
-bundle -bundle_loader -client_name -compatibility_version -current_version -dead_strip
-dependency-file -dylib_file -dylinker_install_name -dynamic -dynamiclib
-exported_symbols_list -filelist -flat_namespace -force_cpusubtype_ALL -force_flat_namespace
-headerpad_max_install_names -iframework -image_base -init -install_name -keep_private_externs
-multi_module -multiply_defined -multiply_defined_unused -noall_load
-no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs -noprebind -noseglinkedit
-pagezero_size -prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs
-sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder
-segaddr -segs_read_only_addr -segs_read_write_addr -seg_addr_table -seg_addr_table_filename
-seglinkedit -segprot -segs_read_only_addr -segs_read_write_addr -single_module -static
-sub_library -sub_umbrella -twolevel_namespace -umbrella -undefined -unexported_symbols_list
-weak_reference_mismatches -whatsloaded -F -gused -gfull -mmacosx-version-min=version -mkernel
-mone-byte-bool
i386 and x86-64 Options -mtune=cpu-type -march=cpu-type -mfpmath=unit -masm=dialect
-mno-fancy-math-387 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib -mno-wide-multiply -mrtd
-malign-double -mpreferred-stack-boundary=num -mmmx -msse -msse2 -msse3 -mssse3 -m3dnow
-mthreads -mno-align-stringops -minline-all-stringops -mpush-args -maccumulate-outgoing-args
-m128bit-long-double -m96bit-long-double -mregparm=num -momit-leaf-frame-pointer -mno-red-zone
-mno-tls-direct-seg-refs -mcmodel=code-model -m32 -m64
PowerPC Options See RS/6000 and PowerPC Options.
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mpower -mno-power -mpower2
-mno-power2 -mpowerpc -mpowerpc64 -mno-powerpc -maltivec -mno-altivec -mpim-altivec
-mno-pim-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt
-mnew-mnemonics -mold-mnemonics -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m64
-m32 -mxl-compat -mno-xl-compat -mpe -malign-power -malign-natural -msoft-float -mhard-float
-mmultiple -mno-multiple -mstring -mno-string -mupdate -mno-update -mfused-madd
-mno-fused-madd -mbit-align -mno-bit-align -mstrict-align -mno-strict-align -mrelocatable
-mno-relocatable -mrelocatable-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle
-mlittle-endian -mbig -mbig-endian -mdynamic-no-pic -mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme -mcall-sysv -mcall-netbsd
-maix-struct-return -msvr4-struct-return -mabi=altivec -mabi=no-altivec -mabi=spe -mabi=no-spe
-misel=yes -misel=no -mspe=yes -mspe=no -mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single
-mfloat-gprs=double -mprototype -mno-prototype -msim -mmvme -mads -myellowknife -memb
-msdata -msdata=opt -mvxworks -mwindiss -G num -pthread
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions
-funwind-tables -fasynchronous-unwind-tables -finhibit-size-directive -finstrument-functions
-fno-common -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE -freg-struct-return
-fshared-data -fshort-enums -fshort-double -fshort-wchar -fverbose-asm -fpack-struct[=n]
-fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym -fargument-alias
-fargument-noalias -fargument-noalias-global -fleading-underscore -ftls-model=model -ftrapv
-fwrapv -fbounds-check -fvisibility
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking,
always in that order. GCC is capable of preprocessing and compiling several files either into
several assembler input files, or into one assembler input file; then each assembler input file
produces an object file, and linking combines all the object files (those newly compiled, and those
specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
file.c
C source code which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc library to make an Objective-C
program work.
file.mi
Objective-C source code which should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the libobjc library to make an
Objective-C++ program work. Note that .M refers to a literal capital M.
file.mii
Objective-C++ source code which should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header.
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code which must be preprocessed. Note that in .cxx, the last two letters must both be
literally x. Likewise, .C refers to a literal capital C.
file.mm
file.M
Objective-C++ source code which must be preprocessed. (APPLE ONLY)
file.mii
Objective-C++ source code which should not be preprocessed. (APPLE ONLY)
file.hh
file.H
C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR
Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fortran source code which must be preprocessed (with the traditional preprocessor).
file.r
Fortran source code which must be preprocessed with a RATFOR preprocessor (not included with
GCC).
file.f90
file.f95
Fortran 90/95 source code which should not be preprocessed.
file.ads
Ada source code file which contains a library unit declaration (a declaration of a package,
subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a
package, generic, or subprogram renaming declaration). Such files are also called specs.
file.adb
Ada source code file containing a library unit body (a subprogram or package body). Such files
are also called bodies.
file.s
Assembler code. Apple's version of GCC runs the preprocessor on these files as well as those
ending in .S.
file.S
Assembler code which must be preprocessed.
other
An object file to be fed straight into linking. Any file name with no recognized suffix is
treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files (rather than letting the compiler
choose a default based on the file name suffix). This option applies to all following input
files until the next -x option. Possible values for language are:
c c-header c-cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
f95
java
treelang
-x none
Turn off any specification of a language, so that subsequent files are handled according to their
file name suffixes (as they are if -x has not been used at all).
-ObjC
-ObjC++
These are similar in effect to -x objective-c and -x objective-c++, but affect only the choice of
compiler for files already identified as source files. (APPLE ONLY)
-arch arch
Compile for the specified target architecture arch. The allowable values are i386, ppc and
ppc64. Multiple options work, and direct the compiler to produce ``universal'' binaries
including object code for each architecture specified with -arch. This option only works if
assembler and libraries are available for each architecture specified. (APPLE ONLY)
-fsave-repository=file
Save debug info in separate object file. This is available only while building PCH in -gfull
mode.
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase of the compiler returns a non-success nonsuccess
success return code. If you specify -pass-exit-codes, the gcc program will instead return with
numerically highest error produced by any phase that returned an error indication.
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc
where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some
combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking stage simply is not done.
The ultimate output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix .c, .i, .s,
etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S Stop after the stage of compilation proper; do not assemble. The output is in the form of an
assembler code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i,
etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of
preprocessed source code, which is sent to the standard output.
Input files which don't require preprocessing are ignored.
-o file
Place output in file file. This applies regardless to whatever sort of output is being produced,
whether it be an executable file, an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the object file for
source.suffix in source.o, its assembler file in source.s, a precompiled header file in
source.suffix.gch, and all preprocessed C source on standard output.
-v Print (on standard error output) the commands executed to run the stages of compilation. Also
print the version number of the compiler driver program and of the preprocessor and the compiler
proper.
-###
Like -v except the commands are not executed and all command arguments are quoted. This is
useful for shell scripts to capture the driver-generated command lines.
-pipe
Use pipes rather than temporary files for communication between the various stages of
compilation. This fails to work on some systems where the assembler is unable to read from a
pipe; but the GNU assembler has no trouble.
-combine
If you are compiling multiple source files, this option tells the driver to pass all the source
files to the compiler at once (for those languages for which the compiler can handle this). This
will allow intermodule analysis (IMA) to be performed by the compiler. Currently the only
language for which this is supported is C. If you pass source files for multiple languages to
the driver, using this option, the driver will invoke the compiler(s) that support IMA once each,
passing each compiler all the source files appropriate for it. For those languages that do not
support IMA this option will be ignored, and the compiler will be invoked once for each source
file in that language. If you use this option in conjunction with -save-temps, the compiler will
generate multiple pre-processed files (one for each source file), but only one (combined) .o or
.s file.
--help
Print (on the standard output) a description of the command line options understood by gcc. If
the -v option is also specified then --help will also be passed on to the various processes
invoked by gcc, so that they can display the command line options they accept. If the -Wextra
option is also specified then command line options which have no documentation associated with
them will also be displayed.
--target-help
Print (on the standard output) a description of target specific command line options for each
tool.
--version
Display the version number and copyrights of the invoked GCC.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++
header files often use .hh or .H; and preprocessed C++ files use the suffix .ii. GCC recognizes
files with these names and compiles them as C++ programs even if you call the compiler the same way
as for compiling C programs (usually with the name gcc).
However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and treats .c,
.h and .i files as C++ source files instead of C source files unless -x is used, and automatically
specifies linking against the C++ library. This is also useful when precompiling a C header file
with a .h extension for use in C++ compilations. On many systems, g++ is also installed with the
name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for
compiling programs in any language; or command-line options meaningful for C and related languages;
or options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C
and Objective-C++) that the compiler accepts:
-ansi
In C mode, support all ISO C90 programs. In C++ mode, remove GNU extensions that conflict with
ISO C++.
This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C
code), or of standard C++ (when compiling C++ code), such as the "asm" and "typeof" keywords, and
predefined macros such as "unix" and "vax" that identify the type of system you are using. It
also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it
disables recognition of C++ style // comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" continue to work
despite -ansi. You would not want to use them in an ISO C program, of course, but it is useful
to put them in header files that might be included in compilations done with -ansi. Alternate
predefined macros such as "__unix__" and "__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that,
-pedantic is required in addition to -ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is used. Some header files may
notice this macro and refrain from declaring certain functions or defining certain macros that
the ISO standard doesn't call for; this is to avoid interfering with any programs that might use
these names for other things.
Functions which would normally be built in but do not have semantics defined by ISO C (such as
"alloca" and "ffs") are not built-in functions with -ansi is used.
-std=
Determine the language standard. This option is currently only supported when compiling C or
C++. A value for this option must be provided; possible values are
c89
iso9899:1990
ISO C90 (same as -ansi).
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported; see
<http://gcc.gnu.org/gcc-4.0/c99status.html for more information. The names c9x and
iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including some C99 features).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will become the
default. The name gnu9x is deprecated.
c++98
The 1998 ISO C++ standard plus amendments.
gnu++98
The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
Even when this option is not specified, you can still use some of the features of newer standards
in so far as they do not conflict with previous C standards. For example, you may use
"__restrict__" even when -std=c99 is not specified.
The -std options specifying some version of ISO C have the same effects as -ansi, except that
features that were not in ISO C90 but are in the specified version (for example, // comments and
the "inline" keyword in ISO C99) are not disabled.
-aux-info filename
Output to the given filename prototyped declarations for all functions declared and/or defined in
a translation unit, including those in header files. This option is silently ignored in any
language other than C.
Besides declarations, the file indicates, in comments, the origin of each declaration (source
file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or
O for old, respectively, in the first character after the line number and the colon), and whether
it came from a declaration or a definition (C or F, respectively, in the following character).
In the case of function definitions, a K&R-style list of arguments followed by their declarations
is also provided, inside comments, after the declaration.
-faltivec
This flag is provided for compatibility with Metrowerks CodeWarrior and MrC compilers as well as
previous Apple versions of GCC. It causes the -mpim-altivec option to be turned on.
-fasm-blocks
Enable the use of blocks and entire functions of assembly code within a C or C++ file. The
syntax follows that used in CodeWarrior. (APPLE ONLY)
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these words as
identifiers. You can use the keywords "__asm__", "__inline__" and "__typeof__" instead. -ansi
implies -fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm" and "inline" are standard
keywords. You may want to use the -fno-gnu-keywords flag instead, which has the same effect. In
C99 mode (-std=c99 or -std=gnu99), this switch only affects the "asm" and "typeof" keywords,
since "inline" is a standard keyword in ISO C99.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with __builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more efficiently; for
instance, calls to "alloca" may become single instructions that adjust the stack directly, and
calls to "memcpy" may become inline copy loops. The resulting code is often both smaller and
faster, but since the function calls no longer appear as such, you cannot set a breakpoint on
those calls, nor can you change the behavior of the functions by linking with a different
library. In addition, when a function is recognized as a built-in function, GCC may use
information about that function to warn about problems with calls to that function, or to
generate more efficient code, even if the resulting code still contains calls to that function.
For example, warnings are given with -Wformat for bad calls to "printf", when "printf" is built
in, and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function
must not begin with __builtin_. If a function is named this is not built-in in this version of
GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to
enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define
macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation takes place in a hosted environment. This implies -fbuiltin. A hosted
environment is one in which the entire standard library is available, and in which "main" has a
return type of "int". Examples are nearly everything except a kernel. This is equivalent to
-fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding environment. This implies -fno-builtin. A
freestanding environment is one in which the standard library may not exist, and program startup
may not necessarily be at "main". The most obvious example is an OS kernel. This is equivalent
to -fno-hosted.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
Some cases of unnamed fields in structures and unions are only accepted with this option.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance)
implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling. This option allows a user
supplied "cc1", "cc1plus", or "cc1obj" via the -B option. The user supplied compilation step can
then add in an additional preprocessing step after normal preprocessing but before compiling.
The default is to use the integrated cpp (internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are now
only supported with the -E switch. The preprocessor continues to support a pre-standard mode.
See the GNU CPP manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments. The value
of such an expression is void. This option is not supported for C++.
-fno-nested-functions
Disable nested functions. This option is not supported for C++ or Objective-C++. On Darwin,
nested functions are disabled by default.
-fpch-preprocess
Enable PCH processing even when -E or -save-temps is used.
-fnon-lvalue-assign
C and C++ forbid the use of casts and conditional expressions as lvalues, e.g.:
float *p, q, r;
((int *)p)++;
(cond ? q : r) = 3.0;
As a transitional measure, the Apple version of GCC 4.0 allows casts and conditional expressions
to be used as lvalues in certain situations. This is accomplished via the -fnon-lvalue-assign
switch, which is on by default. Whenever an lvalue cast or an lvalue conditional expression is
encountered, the compiler will issue a deprecation warning and then rewrite the expression as
follows:
(type)expr ---becomes---> *(type *)&expr
cond ? expr1 : expr2 ---becomes---> *(cond ? &expr1 : &expr2)
To disallow lvalue casts and lvalue conditional expressions altogether, specify
-fno-non-lvalue-assign; lvalue casts and lvalue conditional expressions will be disallowed in
future versions of Apple's GCC.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It is either like "unsigned char"
by default or like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on
the signedness of an object. But many programs have been written to use plain "char" and expect
it to be signed, or expect it to be unsigned, depending on the machines they were written for.
This option, and its inverse, let you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed char" or "unsigned char", even
though its behavior is always just like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the negative form of
-funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the declaration does not
use either "signed" or "unsigned". By default, such a bit-field is signed, because this is
consistent: the basic integer types such as "int" are signed types.
-fconstant-cfstrings
Enable the automatic creation of a CoreFoundation-type constant string whenever a special builtin
"__builtin__CFStringMakeConstantString" is called on a literal string. (APPLE ONLY)
-fpascal-strings
Allow Pascal-style string literals to be constructed. (APPLE ONLY)
-fwritable-strings
Store string constants in the writable data segment and don't uniquize them. This is for
compatibility with old programs which assume they can write into string constants.
Writing into string constants is a very bad idea; ``constants'' should be constant.
This option is deprecated.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++ programs; but you
can also use most of the GNU compiler options regardless of what language your program is in. For
example, you might compile a file "firstClass.C" like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use the other options
with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++
3.4. Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will
always be the version that conforms most closely to the C++ ABI specification. Therefore, the
ABI obtained using version 0 will change as ABI bugs are fixed.
The default is version 2.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in the access
control code.
-fcheck-new
Check that the pointer returned by "operator new" is non-null before attempting to modify the
storage allocated. This check is normally unnecessary because the C++ standard specifies that
"operator new" will only return 0 if it is declared trw), in which case the compiler will
always check the return value even without this option. In all other cases, when "operator new"
has a non-empty exception specification, memory exhaustion is signalled by throwing
"std::bad_alloc". See also new (nothrow).
-fconserve-space
Put uninitialized or runtime-initialized global variables into the common segment, as C does.
This saves space in the executable at the cost of not diagnosing duplicate definitions. If you
compile with this flag and your program mysteriously crashes after "main()" has completed, you
may have an object that is being destroyed twice because two definitions were merged.
This option is no longer useful on most targets, now that support has been added for putting
variables into BSS without making them common.
-fno-const-strings
Give string constants type "char *" instead of type "const char *". By default, G++ uses type
"const char *" as required by the standard. Even if you use -fno-const-strings, you cannot
actually modify the value of a string constant, unless you also use -fwritable-strings.
This option might be removed in a future release of G++. For maximum portability, you should
structure your code so that it works with string constants that have type "const char *".
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary which is only used to
initialize another object of the same type. Specifying this option disables that optimization,
and forces G++ to call the copy constructor in all cases.
-fno-enforce-eh-specs
Don't check for violation of exception specifications at runtime. This option violates the C++
standard, but may be useful for reducing code size in production builds, much like defining
NDEBUG. The compiler will still optimize based on the exception specifications.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited
to the for loop itself, as specified by the C++ standard. If -fno-for-scope is specified, the
scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as
was the case in old versions of G++, and other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to allow and give a warning for
old-style code that would otherwise be invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this word as an identifier. You can
use the keyword "__typeof__" instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only
emit code for explicit instantiations.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates, either. The default is to
handle inlines differently so that compiles with and without optimization will need the same set
of explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions controlled by #pragma
implementation. This will cause linker errors if these functions are not inlined everywhere they
are called.
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a
pointer to member function via non-standard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include
"ffs", "alloca", "_exit", "index", "bzero", "conjf", and other related functions.
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor"
as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to issue. Currently, the
only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using
-fpermissive will allow some nonconforming code to compile.
-frepo
Enable automatic template instantiation at link time. This option also implies
-fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual functions for use by the C++
runtime type identification features (dynamic_cast and typeid). If you don't use those parts of
the language, you can save some space by using this flag. Note that exception handling uses the
same information, but it will generate it as needed.
-fstats
Emit statistics about front-end processing at the end of the compilation. This information is
generally only useful to the G++ development team.
-ftemplate-depth-n
Set the maximum instantiation depth for template classes to n. A limit on the template
instantiation depth is needed to detect endless recursions during template class instantiation.
ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe
initialization of local statics. You can use this option to reduce code size slightly in code
that doesn't need to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the "__cxa_atexit" function
rather than the "atexit" function. This option is required for fully standards-compliant
handling of static destructors, but will only work if your C library supports "__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This will cause
"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 methods where
the addresses of the two functions were 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 behaviour of this switch is not quite the same as marking the methods as hidden directly.
Normally if there is a class with default visibility which has a hidden method, the effect of
this is that the method must be defined in only one shared object. This switch does not have
this restriction.
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.
-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 which are defined in more than one different shared object:
those declarations are permitted if they would have been permitted when this option was not used.
This option is discouraged, rather, it is preferable for types to be explicitly exported as
desired on a per-class basis. Unfortunately because Visual Studio can't compare two different
hidden types as unequal for the purposes of type_info and exception handling, users are able to
write code that relies upon this 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 will be different, so changing one will not
change the other; and that pointers to function members defined in different shared objects will
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++ will use
weak symbols if they are available. This option exists only for testing, and should not be used
by end-users; it will result in inferior code and has no benefits. This option may be removed in
a future release of G++.
-nostdinc++
Do not search for header files in the standard directories specific to C++, but do still search
the other standard directories. (This option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options have meanings only for C++
programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline functions; they just won't be inlined
by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI.
Although an effort has been made to warn about all such cases, there are probably some cases that
are not warned about, even though G++ is generating incompatible code. There may also be cases
where warnings are emitted even though the code that is generated will be compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that
code generated by G++ may not be binary compatible with code generated by other compilers.
The known incompatibilities at this point include:
Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the
same byte as a base class. For example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte as"A::f1"; other compilers will not.
You can avoid this problem by explicitly padding "A" so that its size is a multiple of the
byte size on your platform; that will cause G++ and other compilers to layout "B"
identically.
Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when
laying out virtual bases. For example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for "A"; other compilers will.
You can avoid this problem by explicitly padding "A" so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++ and other compilers to layout
"C" identically.
Incorrect handling of bit-fields with declared widths greater than that of their underlying
types, when the bit-fields appear in a union. For example:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make the union too small by the
number of bits in an "int".
Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset; it should be placed at offset
zero. G++ mistakenly believes that the "A" data member of "B" is already at offset zero.
Names of template functions whose types involve "typename" or template template parameters
can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable because all the constructors or destructors in that class are
private, and it has neither friends nor public static member functions.
-Wnon-virtual-dtor (C++ only)
Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it
declares a non-virtual one. This warning is enabled by -Wall.
-Wreorder (C++ only)
Warn when the order of member initializers given in the code does not match the order in which
they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler will rearrange the member initializers for i and j to match the declaration order of
the members, emitting a warning to that effect. This warning is enabled by -Wall.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)
Warn about violations of the following style guidelines from Scott Meyers' Effective C++ book:
Item 11: Define a copy constructor and an assignment operator for classes with dynamically
allocated memory.
Item 12: Prefer initialization to assignment in constructors.
Item 14: Make destructors virtual in base classes.
Item 15: Have "operator=" return a reference to *this.
Item 23: Don't try to return a reference when you must return an object.
Also warn about violations of the following style guidelines from Scott Meyers' More Effective
C++ book:
Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.
Item 7: Never overload "&&", "||", or ",".
When selecting this option, be aware that the standard library headers do not obey all of these
guidelines; use grep -v to filter out those warnings.
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features.
-Wstrict-null-sentinel (C++ only)
Warn also about the use of an uncasted "NULL" as sentinel. When compiling only with GCC this is
a valid sentinel, as "NULL" is defined to "__null". Although it is a null pointer constant not a
null pointer, it is guaranteed to of the same size as a pointer. But this use is not portable
across different compilers.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared within a template. Since the
advent of explicit template specification support in G++, if the name of the friend is an
unqualified-id (i.e., friend foo(int)), the C++ language specification demands that the friend
declare or define an ordinary, nontemplate function. (Section 14.5.3). Before G++ implemented
explicit specification, unqualified-ids could be interpreted as a particular specialization of a
templatized function. Because this non-conforming behavior is no longer the default behavior for
G++, -Wnon-template-friend allows the compiler to check existing code for potential trouble spots
and is on by default. This new compiler behavior can be turned off with -Wno-non-template-friend
which keeps the conformant compiler code but disables the helpful warning.
-Wold-style-cast (C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style newstyle
style casts (static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended
effects and much easier to search for.
-Woverloaded-virtual (C++ only)
Warn when a function declaration hides virtual functions from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
-Wsign-promo (C++ only)
Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed
type, over a conversion to an unsigned type of the same size. Previous versions of G++ would try
to preserve unsignedness, but the standard mandates the current behavior.
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator = (const A&);, while cfront will use
the user-defined operator =.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. See
This section describes the command-line options that are only meaningful for Objective-C and
Objective-C++ programs, but you can also use most of the language-independent GNU compiler options.
For example, you might compile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs;
you can use the other options with any language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C compilations may also use
options specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may
use C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each literal string specified with the
syntax "@"..."". The default class name is "NXConstantString" if the GNU runtime is being used,
and "NSConstantString" if the NeXT runtime is being used (see below). The -fconstant-cfstrings
option, if also present, will override the -fconstant-string-class setting and cause "@"...""
literals to be laid out as constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime. This is the default
for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems,
including Darwin and Mac OS X. The macro "__NEXT_RUNTIME__" is predefined if (and only if) this
option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches (e.g., "[receiver message:arg]") in this
translation unit ensure that the receiver is not "nil". This allows for more efficient entry
points in the runtime to be used. Currently, this option is only available in conjunction with
the NeXT runtime on Mac OS X 10.3 and later.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial nontrivial
trivial default constructor. If so, synthesize a special "- (id) .cxx_construct" instance method
that will run non-trivial default constructors on any such instance variables, in order, and then
return "self". Similarly, check if any instance variable is a C++ object with a non-trivial
destructor, and if so, synthesize a special "- (void) .cxx_destruct" method that will run all
such default destructors, in reverse order.
The "- (id) .cxx_construct" and/or "- (void) .cxx_destruct" methods thusly generated will only
operate on instance variables declared in the current Objective-C class, and not those inherited
from superclasses. It is the responsibility of the Objective-C runtime to invoke all such
methods in an object's inheritance hierarchy. The "- (id) .cxx_construct" methods will be
invoked by the runtime immediately after a new object instance is allocated; the "- (void)
.cxx_destruct" methods will be invoked immediately before the runtime deallocates an object
instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the
"- (id) .cxx_construct" and "- (void) .cxx_destruct" methods.
-fobjc-sjlj-exceptions
Enable syntactic support for structured exception handling in Objective-C, similar to what is
offered by C++ and Java. This option is available in conjunction with the NeXT setjmp base
exceptions on Mac OS X 10.3 and later. This option is on by default with the NeXT runtime.
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
The @throw statement may appear anywhere in an Objective-C or Objective-C++ program; when used
inside of a @catch block, the @throw may appear without an argument (as shown above), in which
case the object caught by the @catch will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and caught using this scheme.
When an object is thrown, it will be caught by the nearest @catch clause capable of handling
objects of that type, analogously to how "catch" blocks work in C++ and Java. A "@catch(id ...)"
clause (as shown above) may also be provided to catch any and all Objective-C exceptions not
caught by previous @catch clauses (if any).
The @finally clause, if present, will be executed upon exit from the immediately preceding "@try
... @catch" section. This will happen regardless of whether any exceptions are thrown, caught or
rethrown inside the "@try ... @catch" section, analogously to the behavior of the "finally"
clause in Java.
There are several caveats to using the new exception mechanism:
Although currently designed to be binary compatible with "NS_HANDLER"-style idioms provided
by the "NSException" class, the new exceptions can only be used on Mac OS X 10.3 (Panther)
and later systems, due to additional functionality needed in the (NeXT) Objective-C runtime.
As mentioned above, the new exceptions do not support handling types other than Objective-C
objects. Furthermore, when used from Objective-C++, the Objective-C exception model does
not interoperate with C++ exceptions at this time. This means you cannot @throw an exception
from Objective-C and "catch" it in C++, or vice versa (i.e., "throw ... @catch").
The -fobjc-sjlj-exceptions switch also enables the use of synchronization blocks for thread-safe
execution:
@synchronized (ObjCClass *guard) {
...
}
Upon entering the @synchronized block, a thread of execution shall first check whether a lock has
been placed on the corresponding "guard" object by another thread. If it has, the current thread
shall wait until the other thread relinquishes its lock. Once "guard" becomes available, the
current thread will place its own lock on it, execute the code contained in the @synchronized
block, and finally relinquish the lock (thereby making "guard" available to other threads).
Unlike Java, Objective-C does not allow for entire methods to be marked @synchronized. Note that
throwing exceptions out of @synchronized blocks is allowed, and will cause the guarding object to
be unlocked properly.
-fobjc-gc
Enable garbage collection (GC) for Objective-C objects. The resulting binary requires additional
runtime support which is not present in any released version of Mac OS X.
When the -fobjc-gc switch is specified, the compiler will replace assignments to instance
variables (ivars) and to certain kinds of pointers to Objective-C object instances with calls to
interceptor functions provided by the runtime garbage collector. Two type qualifiers, "__strong"
and "__weak", also become available. The "__strong" qualifier may be used to indicate that
assignments to variables of this type should generate a GC interceptor call, e.g.:
__strong void *p; // assignments to 'p' will have interceptor calls
int *q; // assignments to 'q' ordinarly will not
...
(__strong int *)q = 0; // this assignment will call an interceptor
Conversely, the "__weak" type qualifier may be used to suppress interceptor call generation:
__weak id q; // assignments to 'q' will not have interceptor calls
id p; // assignments to 'p' will have interceptor calls
...
(__weak id)p = 0; // suppress interceptor call for this assignment
-fobjc-gc-only
Use this option to indicate that the Objective-C program supports garbage collection (GC) only -that onlythat
that is, it does not contain retain/release logic. This flag implies -fobjc-gc as well. With
this flag, framework is marked as not honoring retain/release.
-freplace-objc-classes
Emit a special marker instructing l(1) not to statically link in the resulting object file, and
allow dl(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue Fix-andContinue
Continue debugging mode, where the object file in question may be recompiled and dynamically
reloaded in the course of program execution, without the need to restart the program itself.
Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime
on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls to
"objc_getClass("...")" (when the name of the class is known at compile time) with static class
references that get initialized at load time, which improves run-time performance. Specifying
the -fzero-link flag suppresses this behavior and causes calls to "objc_getClass("...")" to be
retained. This is useful in Zero-Link debugging mode, since it allows for individual class
implementations to be modified during program execution.
-gen-decls
Dump interface declarations for all classes seen in the source file to a file named
sourcename.decl.
-Wno-protocol
If a class is declared to implement a protocol, a warning is issued for every method in the
protocol that is not implemented by the class. The default behavior is to issue a warning for
every method not explicitly implemented in the class, even if a method implementation is
inherited from the superclass. If you use the -Wno-protocol option, then methods inherited from
the superclass are considered to be implemented, and no warning is issued for them.
-Wselector
Warn if multiple methods of different types for the same selector are found during compilation.
The check is performed on the list of methods in the final stage of compilation. Additionally, a
check is performed for each selector appearing in a "@selector(...)" expression, and a
corresponding method for that selector has been found during compilation. Because these checks
scan the method table only at the end of compilation, these warnings are not produced if the
final stage of compilation is not reached, for example because an error is found during
compilation, or because the -fsyntax-only option is being used.
-Wstrict-selector-match
Warn if multiple methods with differing argument and/or return types are found for a given
selector when attempting to send a message using this selector to a receiver of type "id" or
"Class". When this flag is off (which is the default behavior), the compiler will omit such
warnings if any differences found are confined to types which share the same size and alignment.
-Wundeclared-selector
Warn if a "@selector(...)" expression referring to an undeclared selector is found. A selector
is considered undeclared if no method with that name has been declared before the
"@selector(...)" expression, either explicitly in an @interface or @protocol declaration, or
implicitly in an @implementation section. This option always performs its checks as soon as a
"@selector(...)" expression is found, while -Wselector only performs its checks in the final
stage of compilation. This also enforces the coding style convention that methods and selectors
must be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect
(e.g. its width, ...). The options described below can be used to control the diagnostic messages
formatting algorithm, e.g. how many characters per line, how often source location information should
be reported. Right now, only the C++ front end can honor these options. However it is expected, in
the near future, that the remaining front ends would be able to digest them correctly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n characters. The default is 72
characters for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once
source location information; that is, in case the message is too long to fit on a single physical
line and has to be wrapped, the source location won't be emitted (as prefix) again, over and
over, in subsequent continuation lines. This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the
same source location information (as prefix) for physical lines that result from the process of
breaking a message which is too long to fit on a single line.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are not inherently erroneous but
which are risky or suggest there may have been an error.
You can request many specific warnings with options beginning -W, for example -Wimplicit to request
warnings on implicit declarations. Each of these specific warning options also has a negative form
beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the
two forms, whichever is not the default.
The following options control the amount and kinds of warnings produced by GCC; for further,
language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect
Options.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use
forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C,
follows the version of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a
rare few will require -ansi or a -std option specifying the required version of ISO C). However,
without this option, certain GNU extensions and traditional C and C++ features are supported as
well. With this option, they are rejected.
-pedantic does not cause warning messages for use of the alternate keywords whose names begin and
end with __. Pedantic warnings are also disabled in the expression that follows "__extension__".
However, only system header files should use these escape routes; application programs should
avoid them.
Some users try to use -pedantic to check programs for strict ISO C conformance. They soon find
that it does not do quite what they want: it finds some non-ISO practices, but not all---only
those for which ISO C requires a diagnostic, and some others for which diagnostics have been
added.
A feature to report any failure to conform to ISO C might be useful in some instances, but would
require considerable additional work and would be quite different from -pedantic. We don't have
plans to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended dialect of C, such as gnu89 or
gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended
dialect is based. Warnings from -pedantic are given where they are required by the base
standard. (It would not make sense for such warnings to be given only for features not in the
specified GNU C dialect, since by definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be nothing to warn about.)
-pedantic-errors
Like -pedantic, except that errors are produced rather than warnings.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-Wno-#warnings
Inhibit warning messages issued by #warning.
-Wextra-tokens
Warn about extra tokens at the end of prepreprocessor directives. (APPLE ONLY)
-Wnewline-eof
Warn about files missing a newline at the end of the file. (APPLE ONLY)
-Wno-altivec-long-deprecated
Do not warn about the use of the deprecated 'long' keyword in AltiVec data types. (APPLE ONLY)
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause of error, as programmers
often forget that this type is signed on some machines. This warning is enabled by -Wall.
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline BackslashNewline
Newline appears in a // comment. This warning is enabled by -Wall.
-Wfatal-errors
This option causes the compiler to abort compilation on the first error occurred rather than
trying to keep going and printing further error messages.
-Wformat
Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types
appropriate to the format string specified, and that the conversions specified in the format
string make sense. This includes standard functions, and others specified by format attributes,
in the "printf", "scanf", "strftime" and "strfmon" (an X/Open extension, not in the C standard)
families (or other target-specific families). Which functions are checked without format
attributes having been specified depends on the standard version selected, and such checks of
functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These
include all ISO C90 and C99 features, as well as features from the Single Unix Specification and
some BSD and GNU extensions. Other library implementations may not support all these features;
GCC does not support warning about features that go beyond a particular library's limitations.
However, if -pedantic is used with -Wformat, warnings will be given about format features not in
the selected standard version (but not for "strfmon" formats, since those are not in any version
of the C standard).
Since -Wformat also checks for null format arguments for several functions, -Wformat also implies
-Wnonnull.
-Wformat is included in -Wall. For more control over some aspects of format checking, the
options -Wformat-y2k, -Wno-format-extra-args, -Wno-format-zero-length, -Wformat-nonliteral,
-Wformat-security, and -Wformat=2 are available, but are not included in -Wall.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats which may yield only a two-digit
year.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a "printf" or "scanf" format
function. The C standard specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are specified with $ operand number
specifications, normally warnings are still given, since the implementation could not know what
type to pass to "va_arg" to skip the unused arguments. However, in the case of "scanf" formats,
this option will suppress the warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that
zero-length formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string literal and so cannot be
checked, unless the format function takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions that represent possible
security problems. At present, this warns about calls to "printf" and "scanf" functions where
the format string is not a string literal and there are no format arguments, as in "printf
(foo);". This may be a security hole if the format string came from untrusted input and contains
%n. (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings
may be added to -Wformat-security that are not included in -Wformat-nonliteral.)
-Wformat=2
Enable -Wformat plus format checks not included in -Wformat. Currently equivalent to -Wformat
-Wformat-nonliteral -Wformat-security -Wformat-y2k.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a non-null value by the
"nonnull" function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables which are initialized with themselves. Note this option can
only be used with the -Wuninitialized option, which in turn only works with -O1 and above.
For example, GCC will warn about "i" being uninitialized in the following snippet only when
-Winit-self has been specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int
Warn when a declaration does not specify a type. This warning is enabled by -Wall.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being declared. The form
-Wno-error-implicit-function-declaration is not supported. This warning is enabled by -Wall (as
a warning, not an error).
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
-Wmain
Warn if the type of main is suspicious. main should be a function with external linkage,
returning int, taking either zero arguments, two, or three arguments of appropriate types. This
warning is enabled by -Wall.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the following example, the
initializer for a is not fully bracketed, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a
context where a truth value is expected, or when operators are nested whose precedence people
often get confused about. Only the warning for an assignment used as a truth value is supported
when compiling C++; the other warnings are only supported when compiling C.
Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which
is a different interpretation from that of ordinary mathematical notation.
Also warn about constructions where there may be confusion to which "if" statement an "else"
branch belongs. Here is an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every "else" branch belongs to the innermost possible "if" statement, which in this example
is "if (b)". This is often not what the programmer expected, as illustrated in the above example
by indentation the programmer chose. When there is the potential for this confusion, GCC will
issue a warning when this flag is specified. To eliminate the warning, add explicit braces
around the innermost "if" statement so there is no way the "else" could belong to the enclosing
"if". The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of violations of sequence point rules
in the C standard.
The C standard defines the order in which expressions in a C program are evaluated in terms of
sequence points, which represent a partial ordering between the execution of parts of the
program: those executed before the sequence point, and those executed after it. These occur
after the evaluation of a full expression (one which is not part of a larger expression), after
the evaluation of the first operand of a "&&", "||", "? :" or "," (comma) operator, before a
function is called (but after the evaluation of its arguments and the expression denoting the
called function), and in certain other places. Other than as expressed by the sequence point
rules, the order of evaluation of subexpressions of an expression is not specified. All these
rules describe only a partial order rather than a total order, since, for example, if two
functions are called within one expression with no sequence point between them, the order in
which the functions are called is not specified. However, the standards committee have ruled
that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take
effect. Programs whose behavior depends on this have undefined behavior; the C standard
specifies that ``Between the previous and next sequence point an object shall have its stored
value modified at most once by the evaluation of an expression. Furthermore, the prior value
shall be read only to determine the value to be stored.''. If a program breaks these rules, the
results on any particular implementation are entirely unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and "a[i++] = i;". Some
more complicated cases are not diagnosed by this option, and it may give an occasional false
positive result, but in general it has been found fairly effective at detecting this sort of
problem in programs.
The present implementation of this option only works for C programs. A future implementation may
also work for C++ programs.
The C standard is worded confusingly, therefore there is some debate over the precise meaning of
the sequence point rules in subtle cases. Links to discussions of the problem, including
proposed formal definitions, may be found on the GCC readings page, at
<http://gcc.gnu.org/readings.html.
This warning is enabled by -Wall.
-Wreturn-type
Warn whenever a function is defined with a return-type that defaults to "int". Also warn about
any "return" statement with no return-value in a function whose return-type is not "void".
For C, also warn if the return type of a function has a type qualifier such as "const". Such a
type qualifier has no effect, since the value returned by a function is not an lvalue. ISO C
prohibits qualified "void" return types on function definitions, so such return types always
receive a warning even without this option.
For C++, a function without return type always produces a diagnostic message, even when
-Wno-return-type is specified. The only exceptions are main and functions defined in system
headers.
This warning is enabled by -Wall.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or
more of the named codes of that enumeration. (The presence of a "default" label prevents this
warning.) "case" labels outside the enumeration range also provoke warnings when this option is
used. This warning is enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or
more of the named codes of that enumeration. "case" labels outside the enumeration range also
provoke warnings when this option is used.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs
within comments are not warned about). This warning is enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a non\-inline static function is
unused. This warning is enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused aside from its
declaration This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used. This warning is enabled
by -Wall.
To suppress this warning cast the expression to void.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must either specify -Wextra
-Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being initialized or if a variable may be
clobbered by a "setjmp" call.
These warnings are possible only in optimizing compilation, because they require data flow
information that is computed only when optimizing. If you don't specify -O, you simply won't get
these warnings.
If you want to warn about code which uses the uninitialized value of the variable in its own
initializer, use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered elements of structure, union or
array variables as well as for variables which are uninitialized or clobbered as a whole. They
do not occur for variables or elements declared "volatile". Because these warnings depend on
optimization, the exact variables or elements for which there are warnings will depend on the
precise optimization options and version of GCC used.
Note that there may be no warning about a variable that is used only to compute a value that
itself is never used, because such computations may be deleted by data flow analysis before the
warnings are printed.
These warnings are made optional because GCC is not smart enough to see all the reasons why the
code might be correct despite appearing to have an error. Here is one example of how this can
happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn't know
this. Here is another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because "save_y" is used only if it is set.
This option also warns when a non-volatile automatic variable might be changed by a call to
"longjmp". These warnings as well are possible only in optimizing compilation.
The compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be called; in
fact, a signal handler could call it at any point in the code. As a result, you may get a
warning even when there is in fact no problem because "longjmp" cannot in fact be called at the
place which would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return
as "noreturn".
This warning is enabled by -Wall.
-Wunknown-pragmas
Warn when a #pragma directive is encountered which is not understood by GCC. If this command
line option is used, warnings will even be issued for unknown pragmas in system header files.
This is not the case if the warnings were only enabled by the -Wall command line option.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It warns about code which might
break the strict aliasing rules that the compiler is using for optimization. The warning does
not catch all cases, but does attempt to catch the more common pitfalls. It is included in
-Wall.
-Wstrict-aliasing=2
This option is only active when -fstrict-aliasing is active. It warns about all code which might
break the strict aliasing rules that the compiler is using for optimization. This warning
catches all cases, but it will also give a warning for some ambiguous cases that are safe.
-Wall
All of the above -W options combined. This enables all the warnings about constructions that
some users consider questionable, and that are easy to avoid (or modify to prevent the warning),
even in conjunction with macros. This also enables some language-specific warnings described in
C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
-Wmost
This is equivalent to -Wall -Wno-parentheses. (APPLE ONLY)
The following -W... options are not implied by -Wall. Some of them warn about constructions that
users generally do not consider questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid in some cases, and there is no
simple way to modify the code to suppress the warning.
-Wextra
(This option used to be called -W. The older name is still supported, but the newer name is more
descriptive.) Print extra warning messages for these events:
A function can return either with or without a value. (Falling off the end of the function
body is considered returning without a value.) For example, this function would evoke such a
warning:
foo (a)
{
if (a > 0)
return a;
}
An expression-statement or the left-hand side of a comma expression contains no side effects.
To suppress the warning, cast the unused expression to void. For example, an expression such
as x[i,j] will cause a warning, but x[(void)i,j] will not.
An unsigned value is compared against zero with < or >=.
Storage-class specifiers like "static" are not the first things in a declaration. According
to the C Standard, this usage is obsolescent.
If -Wall or -Wunused is also specified, warn about unused arguments.
A comparison between signed and unsigned values could produce an incorrect result when the
signed value is converted to unsigned. (But don't warn if -Wno-sign-compare is also
specified.)
An aggregate has an initializer which does not initialize all members. This warning can be
independently controlled by -Wmissing-field-initializers.
A function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }
An empty body occurs in an if or else statement.
A pointer is compared against integer zero with <, <=, >, or >=.
A variable might be changed by longjmp or vfork.
Any of several floating-point events that often indicate errors, such as overflow, underflow,
loss of precision, etc.
*<(C++ only)>
An enumerator and a non-enumerator both appear in a conditional expression.
*<(C++ only)>
A non-static reference or non-static const member appears in a class without constructors.
*<(C++ only)>
Ambiguous virtual bases.
*<(C++ only)>
Subscripting an array which has been declared register.
*<(C++ only)>
Taking the address of a variable which has been declared register.
*<(C++ only)>
A base class is not initialized in a derived class' copy constructor.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating point division by zero is not
warned about, as it can be a legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from system headers
are normally suppressed, on the assumption that they usually do not indicate real problems and
would only make the compiler output harder to read. Using this command line option tells GCC to
emit warnings from system headers as if they occurred in user code. However, note that using
-Wall in conjunction with this option will not warn about unknown pragmas in system headers---for
that, -Wunknown-pragmas must also be used.
-Wfloat-equal
Warn if floating point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to consider
floating-point values as approximations to infinitely precise real numbers. If you are doing
this, then you need to compute (by analyzing the code, or in some other way) the maximum or
likely maximum error that the computation introduces, and allow for it when performing
comparisons (and when producing output, but that's a different problem). In particular, instead
of testing for equality, you would check to see whether the two values have ranges that overlap;
and this is done with the relational operators, so equality comparisons are probably mistaken.
-Wfour-char-constants
Warn about four char constants, e.g. OSType 'APPL'. This warning is disabled by default.
-Wtraditional (C only)
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about
ISO C constructs that have no traditional C equivalent, and/or problematic constructs which
should be avoided.
Macro parameters that appear within string literals in the macro body. In traditional C
macro replacement takes place within string literals, but does not in ISO C.
In traditional C, some preprocessor directives did not exist. Traditional preprocessors
would only consider a line to be a directive if the # appeared in column 1 on the line.
Therefore -Wtraditional warns about directives that traditional C understands but would
ignore because the # does not appear as the first character on the line. It also suggests
you hide directives like #pragma not understood by traditional C by indenting them. Some
traditional implementations would not recognize #elif, so it suggests avoiding it altogether.
A function-like macro that appears without arguments.
The unary plus operator.
The U integer constant suffix, or the F or L floating point constant suffixes. (Traditional
C does support the L suffix on integer constants.) Note, these suffixes appear in macros
defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in
"<limits.h>". Use of these macros in user code might normally lead to spurious warnings,
however GCC's integrated preprocessor has enough context to avoid warning in these cases.
A function declared external in one block and then used after the end of the block.
A "switch" statement has an operand of type "long".
A non-"static" function declaration follows a "static" one. This construct is not accepted
by some traditional C compilers.
The ISO type of an integer constant has a different width or signedness from its traditional
type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or
octal values, which typically represent bit patterns, are not warned about.
Usage of ISO string concatenation is detected.
Initialization of automatic aggregates.
Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
Initialization of unions. If the initializer is zero, the warning is omitted. This is done
under the assumption that the zero initializer in user code appears conditioned on e.g.
"__STDC__" to avoid missing initializer warnings and relies on default initialization to zero
in the traditional C case.
Conversions by prototypes between fixed/floating point values and vice versa. The absence of
these prototypes when compiling with traditional C would cause serious problems. This is a
subset of the possible conversion warnings, for the full set use -Wconversion.
Use of ISO C style function definitions. This warning intentionally is not issued for
prototype declarations or variadic functions because these ISO C features will appear in your
code when using libiberty's traditional C compatibility macros, "PARAMS" and "VPARAMS". This
warning is also bypassed for nested functions because that feature is already a GCC extension
and thus not relevant to traditional C compatibility.
-Wdeclaration-after-statement (C only)
Warn when a declaration is found after a statement in a block. This construct, known from C++,
was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90 and
was not supported by GCC versions before GCC 3.0.
-Wno-discard-qual
This flag allows user to suppress warning that is issued when qualification is discarded in
situations like, initialization, assignment and argument passing.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable shadows another local variable, parameter or global variable or
whenever a built-in function is shadowed.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
-Wpointer-arith
Warn about anything that depends on the ``size of'' a function type or of "void". GNU C assigns
these types a size of 1, for convenience in calculations with "void *" pointers and pointers to
functions.
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type. For example, warn if "int
malloc()" is cast to "anything *".
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For
example, warn if a "const char *" is cast to an ordinary "char *".
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target is increased. For
example, warn if a "char *" is cast to an "int *" on machines where integers can only be accessed
at two- or four-byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type "const char[length]" so that copying the address
of one into a non-"const" "char *" pointer will get a warning; when compiling C++, warn about the
deprecated conversion from string constants to "char *". These warnings will help you find at
compile time code that can try to write into a string constant, but only if you have been very
careful about using "const" in declarations and prototypes. Otherwise, it will just be a
nuisance; this is why we did not make -Wall request these warnings.
-Wconversion
Warn if a prototype causes a |