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 remain-
der. g++ accepts mostly the same options as gcc.
Note: In Apple's version of GCC, both cc and gcc are actually symbolic
links to gcc-3.3, while c++ and g++ are links to g++-3.3, unless the
ggcccc_sseelleecctt(1) command has been used to point them at another compiler
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 particu-
lar, 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 docu-
mented 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 -pipe -pass-exit-codes -x language -ObjC
(APPLE ONLY) -ObjC++ (APPLE ONLY) -arch arch (APPLE ONLY) -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-func-
tion -fhosted -ffreestanding -fms-extensions -trigraphs
-no-integrated-cpp -traditional -traditional-cpp -fallow-sin-
gle-precision -fcond-mismatch -fconstant-cfstrings (APPLE ONLY)
-fpch-preprocess (APPLE ONLY) -fsigned-bitfields -fsigned-char
-funsigned-bitfields -funsigned-char -fpascal-strings (APPLE ONLY)
-fcoalesce (APPLE ONLY) -fweak-coalesced (APPLE ONLY) -Wno-#warn-
ings (APPLE ONLY) -Wextra-tokens (APPLE ONLY) -Wpragma-once (APPLE
ONLY) -Wnewline-eof (APPLE ONLY) -Wno-altivec-long-deprecated
(APPLE ONLY) -fwritable-strings
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space
-fno-const-strings -fdollars-in-identifiers -fno-elide-construc-
tors -fno-enforce-eh-specs -fexternal-templates -falt-exter-
nal-templates -ffor-scope -fno-for-scope -fno-gnu-keywords
-fno-implicit-templates -fno-implicit-inline-templates -fno-imple-
ment-inlines -findirect-virtual-calls (APPLE ONLY) -fapple-kext
(APPLE ONLY) -fcoalesce-templates (APPLE ONLY) -fms-extensions
-fno-nonansi-builtins -fno-operator-names -fno-optional-diags
-fpermissive -frepo -fno-rtti -fstats -ftemplate-depth-n
-fuse-cxa-atexit -fvtable-gc -fno-weak -nostdinc++
-fno-default-inline -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor
-Wreorder -Weffc++ -Wno-deprecated -Wno-non-template-friend
-Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions
-Wsign-promo -Wsynth
Objective-C Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers (APPLE ONLY) -fobjc-direct-dispatch (APPLE ONLY)
-fobjc-exceptions (APPLE ONLY) -freplace-objc-classes (APPLE ONLY)
-fzero-link (APPLE ONLY) -gen-decls -Wno-protocol -Wselector -Wun-
declared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -W -Wall -Waggre-
gate-return -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment
-Wconversion -Wno-deprecated-declarations -Wdisabled-optimization
-Wno-div-by-zero -Werror -Wfloat-equal -Wformat -Wformat=2
-Wformat-nonliteral -Wformat-security -Wimplicit -Wimplicit-int
-Wimplicit-function-declaration -Werror-implicit-function-declara-
tion -Wimport -Winline -Winvalid-pch -Wno-endif-labels
-Wno-invalid-offsetof (APPLE ONLY) -Wlarger-than-len -Wlong-long
-Wno-long-double (APPLE ONLY) -Wmain -Wmissing-braces -Wmiss-
ing-format-attribute -Wmissing-noreturn -Wmost (APPLE ONLY)
-Wno-multichar -Wno-format-extra-args -Wno-format-y2k -Wno-import
-Wnonnull -Wpacked -Wpadded -Wparentheses -Wpointer-arith -Wre-
dundant-decls -Wreturn-type -Wsequence-point -Wshadow -Wsign-com-
pare -Wstrict-aliasing -Wswitch -Wswitch-default -Wswitch-enum
-Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized -Wun-
known-pragmas -Wunreachable-code -Wunused -Wunused-function
-Wunused-label -Wunused-parameter -Wunused-value -Wunused-vari-
able -Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes
-Wnested-externs -Wstrict-prototypes -Wtraditional
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered
-fdump-translation-unit[-n] -fdump-class-hierarchy[-n]
-fdump-tree-original[-n] -fdump-tree-optimized[-n]
-fdump-tree-inlined[-n] -feliminate-dwarf2-dups -fmem-report
-fprofile-arcs -fsched-verbose=n -ftest-coverage -ftime-report -g
-glevel -gcoff -gdwarf -gdwarf-1 -gdwarf-1+ -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 -fsave-repository=loca-
tion -grepository
Optimization Options
-falign-functions=n -falign-jumps=n -falign-labels=n
-falign-loops=n -falign-loops-max-skip=n -falign-jumps-max-skip=n
-fbranch-probabilities -fcaller-saves -fcprop-registers -fcse-fol-
low-jumps -fcse-skip-blocks -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 -floop-optimize -fcrossjumping -fif-conversion
-fif-conversion2 -finline-functions -finline-limit=n
-fkeep-inline-functions -fkeep-static-consts -fmerge-constants
-fmerge-all-constants -fmove-all-movables -fnew-ra
-fno-branch-count-reg -fno-default-inline -fno-defer-pop
-fno-function-cse -fno-guess-branch-probability -fno-inline
-fno-math-errno -fno-peephole -fno-peephole2 -funsafe-math-opti-
mizations -ffinite-math-only -fno-trapping-math -fno-zero-initial-
ized-in-bss -fomit-frame-pointer -foptimize-register-move -fopti-
mize-sibling-calls -fprefetch-loop-arrays -freduce-all-givs -freg-
move -frename-registers -freorder-blocks -freorder-functions -fre-
run-cse-after-loop -frerun-loop-opt -fschedule-insns -fsched-
ule-insns2 -fno-sched-interblock -fno-sched-spec
-fsched-spec-load -fsched-spec-load-dangerous -fsignaling-nans
-fsingle-precision-constant -fssa -fssa-ccp -fssa-dce
-fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps -fun-
roll-all-loops -funroll-loops --param name=value -O -O0 -O1 -O2
-O3 -Os -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 -remap -depen-
dency-file (APPLE ONLY) -trigraphs -undef -Umacro -Wp,option
Assembler Option
-Wa,option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs -nost-
dlib -no-c++filt (APPLE ONLY) -s -static -static-libgcc -shared
-shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -I- -Fdir (APPLE ONLY) -Ldir -specs=file
Target Options
-V version -b machine
Machine Dependent 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 -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-call -mno-xl-call -mpe -malign-mac68k (APPLE ONLY)
-malign-power (APPLE ONLY) -malign-natural (APPLE ONLY)
-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-relocat-
able-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig
-mbig-endian -mdynamic-no-pic (APPLE ONLY) -mlong-branch (APPLE
ONLY) -mcall-aix -mcall-sysv -mcall-netbsd -maix-struct-return
-msvr4-struct-return -mabi=altivec -mabi=no-altivec -mabi=spe
-mabi=no-spe -misel=yes -misel=no -mprototype -mno-prototype -msim
-mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks
-mwindiss -G num -pthread
Darwin Options
-all_load -allowable_client -arch -arch_errors_fatal -arch_only
-bind_at_load -bundle -bundle_loader -client_name -compatibil-
ity_version -current_version -dead_strip
-no_dead_strip_inits_and_terms -dependency-file -dylib_file
-dylinker_install_name -dynamic -dynamiclib -exported_symbols_list
-filelist -flat_namespace -force_cpusubtype_ALL -force_flat_names-
pace -headerpad_max_install_names -image_base -init -install_name
-keep_private_externs -multi_module -multiply_defined -multi-
ply_defined_unused -noall_load -nomultidefs -noprebind
-noseglinkedit -pagezero_size -prebind -prebind_all_twolevel_mod-
ules -private_bundle -read_only_relocs -sectalign -sectobjectsym-
bols -whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder
-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
-mone-byte-bool
i386 and x86-64 Options
-mcpu=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-bound-
ary=num -mmmx -msse -msse2 -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 -mcmodel=code-model -m32
-m64
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 -fno-gnu-linker -fpcc-struct-return -fpic -fPIC
-freg-struct-return -fshared-data -fshort-enums -fshort-double
-fshort-wchar -fvolatile -fvolatile-global -fvolatile-static
-fverbose-asm -fpack-struct -fstack-check -fstack-limit-regis-
ter=reg -fstack-limit-symbol=sym -fargument-alias -fargu-
ment-noalias -fargument-noalias-global -fleading-underscore
-ftls-model=model -ftrapv -fbounds-check
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order. The first three
stages apply to an individual source file, and end by producing an
object file; linking combines all the object files (those newly com-
piled, 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 library
libobjc.a to make an Objective-C program work.
file.mi
Objective-C source code which should not be preprocessed.
file.h
C or C++ header file to be turned into a precompiled header.
file.cc
file.cp
file.cxx
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 tradi-
tional preprocessor).
file.r
Fortran source code which must be preprocessed with a RATFOR pre-
processor (not included with GCC).
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 cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objc-cpp-output
objective-c++ (APPLE ONLY) objective-c++-header (APPLE ONLY)
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
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 also cause the -ObjC flag to be passed to lldd(1), causing static
archive libraries that define an Objective-C class or category to
be linked in in their entirety. (APPLE ONLY)
-arch arch
Compile for the specified target architecture arch. The allowable
values are i386 and ppc. Multiple options work, and direct the
compiler to produce ``fat'' binaries including object code for each
architecture specified with -arch. This option only works if
assembler and libraries are available for each architecture speci-
fied. (APPLE ONLY)
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase
of the compiler returns a non-success return code. If you specify
-pass-exit-codes, the gcc program will instead return with numeri-
cally 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-assem-
bler 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.
Since only one output file can be specified, it does not make sense
to use -o when compiling more than one input file, unless you are
producing an executable file as output.
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, 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 com-
piler driver program and of the preprocessor and the compiler
proper.
-###
Like -v except the commands are not executed and all command argu-
ments 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.
--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 -W 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; prepro-
cessed 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, C++ programs often require class libraries as well as a com-
piler that understands the C++ language---and under some circumstances,
you might want to compile programs or header files from standard input,
or otherwise without a suffix that flags them as C++ programs. You
might also like to precompile a C header file with a .h extension to be
used in C++ compilations. g++ is a program that calls GCC with the
default language set to C++, and automatically specifies linking
against the C++ library. On many systems, g++ is also installed with
the name c++.
When you compile C++ programs, you may specify many of the same com-
mand-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++ 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 seman-
tics 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-3.3/c99status/> for more infor-
mation. The names c9x and iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including some C99 fea-
tures).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented
in GCC, this will become the default. The name gnu9x is depre-
cated.
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 func-
tions declared and/or defined in a translation unit, including
those in header files. This option is silently ignored in any lan-
guage 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
Enable the AltiVec language extensions, as defined in Motorola's
AltiVec PIM. This includes the recognition of "vector" and "pixel"
as (context-dependent) keywords, the definition of built-in func-
tions such as "vec_add", and other extensions. Note that unlike
the option -maltivec, the extensions do not require the inclusion
of any special header files. (APPLE ONLY)
-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 CodeWar-
rior. (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.
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-func-
tion 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.
-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 (inter-
nal 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++.
-fpch-preprocess
Enable PCH processing even when -E or -save-temps is used.
-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.
-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.
-fconstant-cfstrings
Enable the automatic creation of a CoreFoundation-type constant
string whenever a special builtin "__builtin__CFStringMakeCon-
stantString" is called on a literal string, and for "@"..."" liter-
als in Objective-C (thus overriding the -fconstant-string-class
setting). This option will also be enabled if the environment
variable "MACOSX_DEPLOYMENT_TARGET" exists and is set to "10.2" or
greater. (APPLE ONLY)
-fpascal-strings
Allow Pascal-style string literals to be constructed. (APPLE ONLY)
-fcoalesce
Coalesce duplicated functions and data. The linker will discard all
but one, saving space. Enabled by default. (APPLE ONLY)
-fweak-coalesced
Use the new OS X "weak_definitions" section attribute for coalesced
items. A single "normal" definition will be chosen by the linker
over any number of weakly-coalesced ones. (APPLE ONLY)
Options Controlling C++ Dialect
This section describes the command-line options that are only meaning-
ful 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 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 1.
-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. The current
Working Paper requires that "operator new" never return a null
pointer, so this check is normally unnecessary.
An alternative to using this option is to specify that your "opera-
tor new" does not throw any exceptions; if you declare it tthhrrooww(()),
G++ will check the return value. 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 stan-
dard. Even if you use -fno-const-strings, you cannot actually mod-
ify the value of a string constant, unless you also use
-fwritable-strings.
This option might be removed in a future release of G++. For maxi-
mum portability, you should structure your code so that it works
with string constants that have type "const char *".
-fdollars-in-identifiers
Accept $ in identifiers. You can also explicitly prohibit use of $
with the option -fno-dollars-in-identifiers. (GNU C allows $ by
default on most target systems, but there are a few exceptions.)
Traditional C allowed the character $ to form part of identifiers.
However, ISO C and C++ forbid $ in identifiers.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a tempo-
rary 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 reduc-
ing code size in production builds, much like defining NDEBUG. The
compiler will still optimize based on the exception specifications.
-fexternal-templates
instantiation; template instances are emitted or not according to
the location of the template definition.
This option is deprecated.
-falt-external-templates
Similar to -fexternal-templates, but template instances are emitted
or not according to the place where they are first instantiated.
This option is deprecated.
-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 instantia-
tions.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates,
either. The default is to handle inlines differently so that com-
piles 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
errors if these functions are not inlined everywhere they are
called.
-findirect-virtual-calls
Do not make direct calls to virtual functions; instead, always go
through the vtable. (APPLE ONLY)
-fapple-kext
Alter vtables, destructors, and other implementation details to
more closely resemble the GCC 2.95 ABI. This is to make kernel
extensions loadable by Darwin kernels, and is required to build any
Darwin kernel extension. -fno-exceptions and -static must also be
used with this flag. (APPLE ONLY)
-fcoalesce-templates
Mark instantiated templates as "coalesced": the linker will discard
all but one, thus saving space. (APPLE ONLY)
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as
implicit int and getting a pointer to member function via non-stan-
dard 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 messages about nonconformant code from errors to warn-
ings. By default, G++ effectively sets -pedantic-errors without
-pedantic; this option reverses that. This behavior and this
option are superseded by -pedantic, which works as it does for GNU
C.
-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.
-fskip-unused-source
Skip declarations in the input files that aren't actually needed.
This flag doesn't change the meaning of correct source code, but it
can suppress diagnostics about incorrect source code and suppress
output of debugging information for the parts of the input that are
skipped.
-fstats
Emit statistics about front-end processing at the end of the compi-
lation. 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 end-
less recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than
17.
-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". This option is not supported on Mac OS X.
-fvtable-gc
Emit special relocations for vtables and virtual function refer-
ences so that the linker can identify unused virtual functions and
zero out vtable slots that refer to them. This is most useful with
-ffunction-sections and -Wl,--gc-sections, in order to also discard
the functions themselves.
This optimization requires GNU as and GNU ld. Not all systems sup-
port this option. -Wl,--gc-sections is ignored without -static.
-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 avail-
able. 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 func-
tions; 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 prob-
lem 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 exam-
ple:
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 a class are private and the class has no friends or
public static member functions. This warning is enabled by
default.
-Wnon-virtual-dtor (C++ only)
Warn when a class declares a non-virtual destructor that should
probably be virtual, because it looks like the class will be used
polymorphically. 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) { }
};
Here the compiler will warn that the member initializers for i and
j will be rearranged to match the declaration order of the members.
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.
and about violations of the following style guidelines from Scott
Meyers' More Effective C++ book:
* Item 6: Distinguish between prefix and postfix forms of incre-
ment and decrement operators.
* Item 7: Never overload "&&", "||", or ",".
If you use this option, you should be aware that the standard
library headers do not obey all of these guidelines; you can use
grep -v to filter out those warnings.
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared
within a template. With the advent of explicit template specifica-
tion 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 func-
tion. (Section 14.5.3). Before G++ implemented explicit specifi-
cation, unqualified-ids could be interpreted as a particular spe-
cialization of a templatized function. Because this non-conforming
behavior is no longer the default behavior for G++, -Wnon-tem-
plate-friend allows the compiler to check existing code for poten-
tial trouble spots, and is on by default. This new compiler behav-
ior 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 casts (static_cast, reinter-
pret_cast, and const_cast) are less vulnerable to unintended
effects, and much easier to grep 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 this:
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
enumeral type to a signed type over a conversion to an unsigned
type of the same size. Previous versions of G++ would try to pre-
serve unsignedness, but the standard mandates the current behavior.
-Wsynth (C++ only)
Warn when G++'s synthesis behavior does not match that of cfront.
For instance:
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 Dialect
This section describes the command-line options that are only meaning-
ful for Objective-C programs; but you can also use most of the GNU com-
piler options regardless of what language your program is in. For
example, you might compile a file "some_class.m" like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, only -fgnu-runtime is an option meant only for Objec-
tive-C programs; you can use the other options with any language sup-
ported by GCC.
Here is a list of options that are only for compiling Objective-C pro-
grams:
-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 (APPLE ONLY)
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 run-
time to be used. Currently, this option is only available in con-
junction with the NeXT runtime.
-fobjc-direct-dispatch (APPLE ONLY)
For some functions (such as "objc_msgSend") called very frequently
by Objective-C programs, special entry points exist in high memory
that may be jumped to directly (e.g., via the "bla" instruction on
the PowerPC) for improved performance. The -fobjc-direct-dispatch
option will cause such jumps to be generated. This option is only
available in conjunction with the NeXT runtime; furthermore, pro-
grams built with the -fobjc-direct-dispatch option will only run on
Mac OS X 10.4 (Tiger) or later systems.
-fobjc-exceptions (APPLE ONLY)
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++ and Java. This
option will also be enabled if the environment variable
"MACOSX_DEPLOYMENT_TARGET" exists and is set to "10.3" or greater.
@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-exceptions switch also enables the use of synchroniza-
tion blocks for thread-safe execution:
ObjCClass *lockObject = ...;
...
@synchronized (lockObject) {
...
@throw expr;
...
}
Unlike Java, Objective-C does not allow for entire methods to be
marked @synchronized. Note that throwing exceptions out of @syn-
chronized blocks is allowed, and will cause the guarding object to
be unlocked properly.
-freplace-objc-classes (APPLE ONLY)
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 -freplace-objc-classes flag suppresses this behavior
and causes calls to "objc_getClass("...")" to be retained. This is
useful in Fix-and-Continue debugging mode, since it allows for
individual class implementations to be modified during program exe-
cution.
-fzero-link (APPLE ONLY)
Emit a special marker instructing lldd(1) not to statically link in
the resulting object file, and allow ddyylldd(1) to load it in at run
time instead. This is used in conjunction with the Zero Link
debugging mode.
-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 implemen-
tation is inherited from the superclass. If you use the "-Wno-pro-
tocol" option, then methods inherited from the superclass are con-
sidered 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 that for each selector appearing in a "@selec-
tor(...)" expression, a corresponding method with 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.
-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 (explicitly, in an @inter-
face or @protocol declaration, or implicitly, in an @implementation
section) before the "@selector(...)" expression. 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), and so additionally enforces the coding
style convention that methods and selectors must be declared before
being used.
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 format-
ting 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 cor-
rectly.
-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 @ref{C++
Dialect Options} and @ref{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 ver-
sion 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++ fea-
tures 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__". How-
ever, 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 diag-
nostics 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 warn-
ings.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-Wno-#warnings
Inhibit warning messages issued by #warning.
-Wpragma-once
-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.
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment,
or whenever a Backslash-Newline appears in a // comment.
-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.
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 for-
mat 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 -Wno-format-y2k, -Wno-for-
mat-extra-args, -Wno-format-zero-length, -Wformat-nonliteral,
-Wformat-security, and -Wformat=2 are available, but are not
included in -Wall.
-Wno-format-y2k
If -Wformat is specified, do not 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 Speci-
fication 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 cur-
rently 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. Cur-
rently equivalent to -Wformat -Wformat-nonliteral -Wformat-secu-
rity.
-Wnonnull
Enable warning 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.
-Wimplicit-int
Warn when a declaration does not specify a type.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being
declared.
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration.
-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.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed.
In the following example, the initializer for a is not fully brack-
eted, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is
expected, or when operators are nested whose precedence people
often get confused about.
Also warn 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 ();
}
}
-Wsequence-point
Warn about code that may have undefined semantics because of viola-
tions of sequence point rules in the C standard.
The C standard defines the order in which expressions in a C pro-
gram 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 argu-
ments and the expression denoting the called function), and in cer-
tain 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. How-
ever, 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 posi-
tive 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 pro-
grams. 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 sub-
tle cases. Links to discussions of the problem, including proposed
formal definitions, may be found on our readings page, at
<http://gcc.gnu.org/readings/>.
-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++, a function without return type always produces a diagnos-
tic message, even when -Wno-return-type is specified. The only
exceptions are main and functions defined in system headers.
-Wswitch
Warn whenever a "switch" statement has an index of enumeral type
and lacks a "case" for one or more of the named codes of that enu-
meration. (The presence of a "default" label prevents this warn-
ing.) "case" labels outside the enumeration range also provoke
warnings when this option is used.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumeral type
and lacks a "case" for one or more of the named codes of that enu-
meration. "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).
-Wunused-function
Warn whenever a static function is declared but not defined or a
non\-inline static function is unused.
-Wunused-label
Warn whenever a label is declared but not used.
To suppress this warning use the unused attribute.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its decla-
ration.
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
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not
used.
To suppress this warning cast the 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 -W -Wunused or separately specify
-Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being initial-
ized 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 opti-
mizing. If you don't specify -O, you simply won't get these warn-
ings.
These warnings occur only for variables that are candidates for
register allocation. Therefore, they do not occur for a variable
that is declared "volatile", or whose address is taken, or whose
size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
structures, unions or arrays, even when they are in registers.
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 warn-
ings 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 ini-
tialized, 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 pos-
sible 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 func-
tions you use that never return as "noreturn".
-Wunknown-pragmas
stood 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.
-Wall
All of the above -W options combined. This enables all the warn-
ings 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 @ref{C++ Dialect Options} and @ref{Objec-
tive-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 question-
able, 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.
-W 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 with-
out 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 <=.
* 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.
* Storage-class specifiers like "static" are not the first things
in a declaration. According to the C Standard, this usage is
obsolescent.
* 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. (But don't warn about
the GNU extension of "volatile void" return types. That exten-
sion will be warned about if -pedantic is specified.)
* If -Wall or -Wunused is also specified, warn about unused argu-
ments.
* 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 speci-
fied.)
* An aggregate has a partly bracketed initializer. For example,
the following code would evoke such a warning, because braces
are missing around the initializer for "x.h":
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
* An aggregate has an initializer which does not initialize all
members. For example, the following code would cause such a
warning, because "x.h" would be implicitly initialized to zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
-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 legiti-
mate 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.
-Wtraditional (C only)
Warn about certain constructs that behave differently in tradi-
tional 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 under-
stands but would ignore because the # does not appear as the
first character on the line. It also suggests you hide direc-
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 sepa-
rate 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 inten-
tionally 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 exten-
sion and thus not relevant to traditional C compatibility.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wendif-labels
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 conve-
nience in calculations with "void *" pointers and pointers to func-
tions.
-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 care-
ful 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 type conversion that is different from
what would happen to the same argument in the absence of a proto-
type. This includes conversions of fixed point to floating and
vice versa, and conversions changing the width or signedness of a
fixed point argument except when the same as the default promotion.
Also, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the assign-
ment "x = -1" if "x" is unsigned. But do not warn about explicit
casts like "(unsigned) -1".
-Wsign-compare
Warn when a comparison between signed and unsigned values could
produce an incorrect result when the signed value is converted to
unsigned. This warning is enabled by -W, and by -Wall in C++ only.
-Waggregate-return
Warn if any functions that return structures or unions are defined
or called. (In languages where you can return an array, this also
elicits a warning.)
-Wstrict-prototypes (C only)
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration which specifies the
argument types.)
-Wmissing-prototypes (C only)
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. The aim is to detect global functions that
fail to be declared in header files.
-Wmissing-declarations
Warn if a global function is defined without a previous declara-
tion. Do so even if the definition itself provides a prototype.
Use this option to detect global functions that are not declared in
header files.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute "nore-
turn". Note these are only possible candidates, not absolute ones.
Care should be taken to manually verify functions actually do not
ever return before adding the "noreturn" attribute, otherwise sub-
tle code generation bugs could be introduced. You will not get a
warning for "main" in hosted C environments.
-Wmissing-format-attribute
If -Wformat is enabled, also warn about functions which might be
candidates for "format" attributes. Note these are only possible
candidates, not absolute ones. GCC will guess that "format"
attributes might be appropriate for any function that calls a func-
tion like "vprintf" or "vscanf", but this might not always be the
case, and some functions for which "format" attributes are appro-
priate may not be detected. This option has no effect unless
-Wformat is enabled (possibly by -Wall).
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually
they indicate a typo in the user's code, as they have implementa-
tion-defined values, and should not be used in portable code.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as
deprecated by using the "deprecated" attribute. (@pxref{Function
Attributes}, @pxref{Variable Attributes}, @pxref{Type Attributes}.)
-Wpacked
Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable "f.x" in "struct bar" will be
misaligned even though "struct bar" does not itself have the packed
attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpadded
Warn if padding is included in a structure, either to align an ele-
ment of the structure or to align the whole structure. Sometimes
when this happens it is possible to rearrange the fields of the
structure to reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even
in cases where multiple declaration is valid and changes nothing.
-Wnested-externs (C only)
Warn if an "extern" declaration is encountered within a function.
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at
least a whole line of source code will never be executed, because
some condition is never satisfied or because it is after a proce-
dure that never returns.
It is possible for this option to produce a warning even though
there are circumstances under which part of the affected line can
be executed, so care should be taken when removing apparently-
unreachable code.
For instance, when a function is inlined, a warning may mean that
the line is unreachable in only one inlined copy of the function.
This option is not made part of -Wall because in a debugging ver-
sion of a program there is often substantial code which checks cor-
rect functioning of the program and is, hopefully, unreachable
because the program does work. Another common use of unreachable
code is to provide behavior which is selectable at compile-time.
-Winline
Warn if a function can not be inlined and it was declared as
inline.
-Wno-invalid-offsetof (C++ only)
Suppress warnings from applying the offsetof macro to a non-POD
type.
-Wno-long-double
Inhibit warning if the long double type is used. (APPLE ONLY)
-Winvalid-pch
Warn if a precompiled header is found in the search path but can't
be used.
-Wlong-long
Warn if long long type is used. This is default. To inhibit the
warning messages, use -Wno-long-long. Flags -Wlong-long and
-Wno-long-long are taken into account only when -pedantic flag is
used.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning
does not generally indicate that there is anything wrong with your
code; it merely indicates that GCC's optimizers were unable to han-
dle the code effectively. Often, the problem is that your code is
too big or too complex; GCC will refuse to optimize programs when
the optimization itself is likely to take inordinate amounts of
time.
-Werror
Make all warnings into errors.
Options for Debugging Your Program or GCC</