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.
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 gc_slc(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
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 -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-function -fhosted -ffreestanding -fms-extensions -trigraphs
-no-integrated-cpp -traditional -traditional-cpp -fallow-single-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-#warnings (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-constructors -fno-enforce-eh-specs -fexternal-templates
-falt-external-templates -ffor-scope -fno-for-scope -fno-gnu-keywords -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-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 -Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -W -Wall -Waggregate-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-declaration -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 -Wmissing-format-attribute -Wmissing-noreturn -Wmost (APPLE ONLY)
-Wno-multichar -Wno-format-extra-args -Wno-format-y2k -Wno-import -Wnonnull -Wpacked
-Wpadded -Wparentheses -Wpointer-arith -Wredundant-decls -Wreturn-type -Wsequence-point
-Wshadow -Wsign-compare -Wstrict-aliasing -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
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=location -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-follow-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-optimizations -ffinite-math-only -fno-trapping-math -fno-zero-initialized-in-bss
-fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls -fprefetch-loop-arrays
-freduce-all-givs -fregmove -frename-registers -freorder-blocks -freorder-functions
-frerun-cse-after-loop -frerun-loop-opt -fschedule-insns -fschedule-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 -funroll-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 -dependency-file (APPLE
ONLY) -trigraphs -undef -Umacro -Wp,option
Assembler Option
-Wa,option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs -nostdlib -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-relocatable-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 -compatibility_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_namespace -headerpad_max_install_names -image_base -init -install_name
-keep_private_externs -multi_module -multiply_defined -multiply_defined_unused -noall_load
-nomultidefs -noprebind -noseglinkedit -pagezero_size -prebind -prebind_all_twolevel_modules
-private_bundle -read_only_relocs -sectalign -sectobjectsymbols -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-boundary=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-register=reg -fstack-limit-symbol=sym
-fargument-alias -fargument-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 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 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 traditional preprocessor).
file.r
Fortran source code which must be preprocessed with a RATFOR preprocessor (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 l(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 specified. (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 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.
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 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.
--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; 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, C++ programs often require class libraries as well as a compiler 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 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++ 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-3.3/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
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
functions 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 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.
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.
-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++.
-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__CFStringMakeConstantString" is called on a literal string, and for "@"..."" literals
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 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 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 "operator new" does not throw any
exceptions; if you declare it trw), 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 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 *".
-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 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.
-fexternal-templates
Cause #pragma interface and implementation to apply to template 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 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.
-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-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 messages about nonconformant code from errors to warnings. 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 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.
-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 references 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 support 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 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 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 increment 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 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 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
preserve 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 meaningful for Objective-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 "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 Objective-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 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 (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 runtime to be used. Currently, this option is only available in conjunction 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, programs 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 synchronization 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 @synchronized 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 execution.
-fzero-link (APPLE ONLY)
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 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 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 that for each selector appearing in a "@selector(...)" 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
@interface 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
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 @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 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.
-Wpragma-once
Warn about the use of #pragma once. (APPLE ONLY)
-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 BackslashNewline
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 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 -Wno-format-y2k, -Wno-format-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 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.
-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 bracketed, 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 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 our readings page, at
<http://gcc.gnu.org/readings.html.
-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 diagnostic 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 enumeration. (The presence of a "default" label prevents this
warning.) "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 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).
-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 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
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 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.
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
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".
-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.
-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
@ref{C++ Dialect Options} and @ref{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.
-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 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 <=.
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 extension will be warned about
if -pedantic is specified.)
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 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 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.
-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.
-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 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 type conversion that is different from what would happen to the same
argument in the absence of a prototype. This includes conversions of fixed point to floating and
vice versa, and conversions changing the width or signedness of a fixed point argument except
when the same as the default promotion.
Also, warn if a negative integer constant expression is implicitly converted to an unsigned type.
For example, warn about the assignment "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 declaration. Do so even if the
definition itself provides a prototype. Use this option to detect global functions that are not
declared in header files.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute "noreturn". 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 subtle 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 function like "vprintf" or
"vscanf", but this might not always be the case, and some functions for which "format" attributes
are appropriate may not be detected. 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 implementation-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 element of the structure or to
align the whole structure. Sometimes when this happens it is possible to rearrange the fields of
the structure to reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases where multiple
declaration is valid and changes nothing.
-Wnested-externs (C 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
procedure 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 version of a program there is often
substantial code which checks correct 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
handle 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
GCC has various special options that are used for debugging either your program or GCC:
-g Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or
DWARF). GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only
GDB can use; this extra information makes debugging work better in GDB but will probably make
other debuggers crash or refuse to read the program. If you want to control for certain whether
to generate the extra information, use -gstabs+ or -gstabs (see below).
Unlike most other C compilers, GCC allows you to use -g with -O. The shortcuts taken by
optimized code may occasionally produce surprising results: some variables you declared may not
exist at all; flow of control may briefly move where you did not expect it; some statements may
not be executed because they compute constant results or their values were already at hand; some
statements may execute in different places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the
optimizer for programs that might have bugs.
The following options are useful when GCC is generated with the capability for more than one
debugging format.
-ggdb
Produce debugging information for use by GDB. This means to use the most expressive format
available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB
extensions if at all possible.
-gstabs
Produce debugging information in stabs format (if that is supported), without GDB extensions.
This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4
systems this option produces stabs debugging output which is not understood by DBX or SDB. On
System V Release 4 systems this option requires the GNU assembler.
-gstabs+
Produce debugging information in stabs format (if that is supported), using GNU extensions
understood only by the GNU debugger (GDB). The use of these extensions is likely to make other
debuggers crash or refuse to read the program.
(Other debug formats, such as -gcoff, are not supported in Darwin or Mac OS X.)
-glevel
-ggdblevel
-gstabslevel
Request debugging information and also use level to specify how much information. The default
level is 2.
Level 1 produces minimal information, enough for making backtraces in parts of the program that
you don't plan to debug. This includes descriptions of functions and external variables, but no
information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the program.
Some debuggers support macro expansion when you use -g3.
Note that in order to avoid confusion between DWARF1 debug level 2, and DWARF2, neither -gdwarf
nor -gdwarf-2 accept a concatenated debug level. Instead use an additional -glevel option to
change the debug level for DWARF1 or DWARF2.
-feliminate-dwarf2-dups
Compress DWARF2 debugging information by eliminating duplicated information about each symbol.
This option only makes sense when generating DWARF2 debugging information with -gdwarf-2.
-p Generate extra code to write profile information suitable for the analysis program prof. You
must use this option when compiling the source files you want data about, and you must also use
it when linking.
-pg Generate extra code to write profile information suitable for the analysis program gprof. You
must use this option when compiling the source files you want data about, and you must also use
it when linking.
-Q Makes the compiler print out each function name as it is compiled, and print some statistics
about each pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it finishes.
-fmem-report
Makes the compiler print some statistics about permanent memory allocation when it finishes.
-fprofile-arcs
Instrument arcs during compilation to generate coverage data or for profile-directed block
ordering. During execution the program records how many times each branch is executed and how
many times it is taken. When the compiled program exits it saves this data to a file called
auxname.da for each source file. auxname is generated from the name of the output file, if
explicitly specified and it is not the final executable, otherwise it is the basename of the
source file. In both cases any suffix is removed (e.g. foo.da for input file dir/foo.c, or
dir/foo.da for output file specified as -o dir/foo.o).
For profile-directed block ordering, compile the program with -fprofile-arcs plus optimization
and code generation options, generate the arc profile information by running the program on a
selected workload, and then compile the program again with the same optimization and code
generation options plus -fbranch-probabilities.
The other use of -fprofile-arcs is for use with gcov, when it is used with the -ftest-coverage
option.
With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then
finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be
instrumented: the compiler adds code to count the number of times that these arcs are executed.
When an arc is the only exit or only entrance to a block, the instrumentation code can be added
to the block; otherwise, a new basic block must be created to hold the instrumentation code.
-ftest-coverage
Create data files for the gcov code-coverage utility. See -fprofile-arcs option above for a
description of auxname.
auxname.bb
A mapping from basic blocks to line numbers, which gcov uses to associate basic block
execution counts with line numbers.
auxname.bbg
A list of all arcs in the program flow graph. This allows gcov to reconstruct the program
flow graph, so that it can compute all basic block and arc execution counts from the
information in the auxname.da file.
Use -ftest-coverage with -fprofile-arcs; the latter option adds instrumentation to the program,
which then writes execution counts to another data file:
auxname.da
Runtime arc execution counts, used in conjunction with the arc information in the file
auxname.bbg.
Coverage data will map better to the source files if -ftest-coverage is used without
optimization.
-dletters
Says to make debugging dumps during compilation at times specified by letters. This is used for
debugging the compiler. The file names for most of the dumps are made by appending a pass number
and a word to the dumpname. dumpname is generated from the name of the output file, if explicitly
specified and it is not an executable, otherwise it is the basename of the source file. In both
cases any suffix is removed (e.g. foo.00.rtl or foo.01.sibling). Here are the possible letters
for use in letters, and their meanings:
A Annotate the assembler output with miscellaneous debugging information.
b Dump after computing branch probabilities, to file.14.bp.
B Dump after block reordering, to file.32.bbro.
c Dump after instruction combination, to the file file.19.combine.
C Dump after the first if conversion, to the file file.15.ce1.
d Dump after delayed branch scheduling, to file.34.dbr.
D Dump all macro definitions, at the end of preprocessing, in addition to normal output.
e Dump after SSA optimizations, to file.04.ssa and file.07.ussa.
E Dump after the second if conversion, to file.29.ce3.
f Dump after life analysis, to file.18.life.
F Dump after purging "ADDRESSOF" codes, to file.10.addressof.
g Dump after global register allocation, to file.24.greg.
h Dump after finalization of EH handling code, to file.02.eh.
k Dump after reg-to-stack conversion, to file.31.stack.
o Dump after post-reload optimizations, to file.25.postreload.
G Dump after GCSE, to file.11.gcse.
i Dump after sibling call optimizations, to file.01.sibling.
j Dump after the first jump optimization, to file.03.jump.
k Dump after conversion from registers to stack, to file.31.stack.
l Dump after local register allocation, to file.23.lreg.
L Dump after loop optimization, to file.12.loop.
M Dump after performing the machine dependent reorganization pass, to file.33.mach.
n Dump after register renumbering, to file.28.rnreg.
N Dump after the register move pass, to file.21.regmove.
r Dump after RTL generation, to file.00.rtl.
R Dump after the second scheduling pass, to file.30.sched2.
s Dump after CSE (including the jump optimization that sometimes follows CSE), to file.09.cse.
S Dump after the first scheduling pass, to file.22.sched.
t Dump after the second CSE pass (including the jump optimization that sometimes follows CSE),
to file.17.cse2.
u Dump after null pointer elimination pass to file.08.null.
w Dump after the second flow pass, to file.26.flow2.
X Dump after SSA dead code elimination, to file.06.ssadce.
z Dump after the peephole pass, to file.27.peephole2.
a Produce all the dumps listed above.
m Print statistics on memory usage, at the end of the run, to standard error.
p Annotate the assembler output with a comment indicating which pattern and alternative was
used. The length of each instruction is also printed.
P Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp
annotation.
v For each of the other indicated dump files (except for file.00.rtl), dump a representation of
the control flow graph suitable for viewing with VCG to file.pass.vcg.
x Just generate RTL for a function instead of compiling it. Usually used with r.
y Dump debugging information during parsing, to standard error.
-fdump-unnumbered
When doing debugging dumps (see -d option above), suppress instruction numbers and line number
note output. This makes it more feasible to use diff on debugging dumps for compiler invocations
with different options, in particular with and without -g.
-fdump-translation-unit (C and C++ only)
-fdump-translation-unit-options (C and C++ only)
Dump a representation of the tree structure for the entire translation unit to a file. The file
name is made by appending .tu to the source file name. If the -options form is used, options
controls the details of the dump as described for the -fdump-tree options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class's hierarchy and virtual function table layout to a file. The
file name is made by appending .class to the source file name. If the -options form is used,
options controls the details of the dump as described for the -fdump-tree options.
-fdump-tree-switch (C++ only)
-fdump-tree-switch-options (C++ only)
Control the dumping at various stages of processing the intermediate language tree to a file.
The file name is generated by appending a switch specific suffix to the source file name. If the
-options form is used, options is a list of - separated options that control the details of the
dump. Not all options are applicable to all dumps, those which are not meaningful will be
ignored. The following options are available
address
Print the address of each node. Usually this is not meaningful as it changes according to
the environment and source file. Its primary use is for tying up a dump file with a debug
environment.
slim
Inhibit dumping of members of a scope or body of a function merely because that scope has
been reached. Only dump such items when they are directly reachable by some other path.
all Turn on all options.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
inlined
Dump after function inlining, to file.inlined.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of debugging output
the scheduler prints. This information is written to standard error, unless -dS or -dR is
specified, in which case it is output to the usual dump listing file, .sched or .sched2
respectively. However for n greater than nine, the output is always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same information as -dRS. For n greater
than one, it also output basic block probabilities, detailed ready list information and unit/insn
info. For n greater than two, it includes RTL at abort point, control-flow and regions info.
And for n over four, -fsched-verbose also includes dependence info.
-save-temps
Store the usual ``temporary'' intermediate files permanently; place them in the current directory
and name them based on the source file. Thus, compiling foo.c with -c -save-temps would produce
files foo.i and foo.s, as well as foo.o. This creates a preprocessed foo.i output file even
though the compiler now normally uses an integrated preprocessor.
-time
Report the CPU time taken by each subprocess in the compilation sequence. For C source files,
this is the compiler proper and assembler (plus the linker if linking is done). The output looks
like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the ``user time,'' that is time spent executing the program
itself. The second number is ``system time,'' time spent executing operating system routines on
behalf of the program. Both numbers are in seconds.
-fsave-repository=location
Creates separate symbol repository at location for given input header file. Separate repository
contains only debugging symbols in stabs format.
-grepository
Instructs compiler to use separate symbol repository with debugging symbols. Compiler searches
for such repositories in include paths.
-print-file-name=library
Print the full absolute name of the library file library that would be used when linking---and
don't do anything else. With this option, GCC does not compile or link anything; it just prints
the file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by any other switches present in
the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler switches that enable them. The
directory name is separated from the switches by ;, and each switch starts with an @} instead of
the @samp{-, without spaces between multiple switches. This is supposed to ease shell-processing. shellprocessing.
processing.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a.
You can do
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a list of program and library
directories gcc will search---and don't do anything else.
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such
file or directory. To resolve this you either need to put cpp0 and the other compiler components
where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the
directory where you installed them. Don't forget the trailing '/'.
-dumpmachine
Print the compiler's target machine (for example, i686-pc-linux-gnu)---and don't do anything
else.
-dumpversion
Print the compiler version (for example, 3.0)---and don't do anything else.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else. (This is used when GCC itself
is being built.)
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make
debugging produce the expected results. Statements are independent: if you stop the program with a
breakpoint between statements, you can then assign a new value to any variable or change the program
counter to any other statement in the function and get exactly the results you would expect from the
source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size
at the expense of compilation time and possibly the ability to debug the program.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are
listed.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large
function.
With -O, the compiler tries to reduce code size and execution time, without performing any
optimizations that take a great deal of compilation time.
In Apple's version of GCC, -fstrict-aliasing, -freorder-blocks, and -fsched-interblock are
disabled by default when optimizing.
-O2 Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed spacespeed
speed tradeoff. The compiler does not perform loop unrolling or function inlining when you
specify -O2. As compared to -O, this option increases both compilation time and the performance
of the generated code.
-O2 turns on all optimization flags specified by -O. It also turns on the following optimization
flags: -fforce-mem -foptimize-sibling-calls -fstrength-reduce -fcse-follow-jumps
-fcse-skip-blocks -frerun-cse-after-loop -frerun-loop-opt -fgcse -fgcse-lm -fgcse-sm
-fdelete-null-pointer-checks -fexpensive-optimizations -fregmove -fschedule-insns
-fschedule-insns2 -fsched-interblock -fsched-spec -fcaller-saves -fpeephole2 -freorder-blocks
-freorder-functions -fstrict-aliasing -falign-functions -falign-jumps -falign-loops
-falign-labels
Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the
-finline-functions and -frename-registers options.
-O0 Do not optimize. This is the default.
-fast
Optimize for maximum performance. -fast changes the overall optimization strategy of GCC in order
to produce the fastest possible running code for PPC7450 and G5 architectures. By default, -fast
optimizes for G5. Programs optimized for G5 will not run on PPC7450. To optimize for PPC7450, add
-mcpu=7450 on command line.
-fast currently enables the following optimization flags (for G5 and PPC7450). These flags may
change in the future. You cannot override any of these options if you use -fast except by setting
-mcpu=7450. Note that -ffast-math, -fstrict-aliasing and -malign-natural are unsafe in some
situations. To build shared libraries with -fast, specify -fPIC on command line.
-O3 -funroll-loops -fstrict-aliasing -fsched-interblock -falign-loops=16 -falign-jumps=16
-falign-functions=16 -falign-jumps-max-skip=15 -falign-loops-max-skip=15 -malign-natural
-ffast-math -mdynamic-no-pic -mpowerpc-gpopt -force_cpusubtype_ALL -fstrict-aliasing -mtune=G5
-mcpu=G5 -mpowerpc64
-Os Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size.
It also performs further optimizations designed to reduce code size.
-Os disables the following optimization flags: -falign-functions -falign-jumps -falign-loops
-falign-labels -freorder-blocks -fprefetch-loop-arrays
If you use multiple -O options, with or without level numbers, the last such option is the one
that is effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and
negative forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the
forms is listed---the one you typically will use. You can figure out the other form by either
removing no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are
related to ones that are. You can use the following flags in the rare cases when ``fine-tuning'' of
optimizations to be performed is desired.
-fno-default-inline
Do not make member functions inline by default merely because they are defined inside the class
scope (C++ only). Otherwise, when you specify -O, member functions defined inside class scope
are compiled inline by default; i.e., you don't need to add inline in front of the member
function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that function returns. For machines
which must pop arguments after a function call, the compiler normally lets arguments accumulate
on the stack for several function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
-fforce-mem
Force memory operands to be copied into registers before doing arithmetic on them. This produces
better code by making all memory references potential common subexpressions. When they are not
common subexpressions, instruction combination should eliminate the separate register-load.
Enabled at levels -O2, -O3, -Os.
-fforce-addr
Force memory address constants to be copied into registers before doing arithmetic on them. This
may produce better code just as -fforce-mem may.
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that don't need one. This avoids the
instructions to save, set up and restore frame pointers; it also makes an extra register
available in many functions. It also makes debugging impossible on some machines.
On some machines, such as the VAX, this flag has no effect, because the standard calling sequence
automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The
machine-description macro "FRAME_POINTER_REQUIRED" controls whether a target machine supports
this flag.
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-fno-inline
Don't pay attention to the "inline" keyword. Normally this option is used to keep the compiler
from expanding any functions inline. Note that if you are not optimizing, no functions can be
expanded inline.
-finline-functions
Integrate all simple functions into their callers. The compiler heuristically decides which
functions are simple enough to be worth integrating in this way.
If all calls to a given function are integrated, and the function is declared "static", then the
function is normally not output as assembler code in its own right.
Enabled at level -O3.
-finline-limit=n
By default, gcc limits the size of functions that can be inlined. This flag allows the control
of this limit for functions that are explicitly marked as inline (i.e., marked with the inline
keyword or defined within the class definition in c++). n is the size of functions that can be
inlined in number of pseudo instructions (not counting parameter handling). The default value of
n is 600. Increasing this value can result in more inlined code at the cost of compilation time
and memory consumption. Decreasing usually makes the compilation faster and less code will be
inlined (which presumably means slower programs). This option is particularly useful for
programs that use inlining heavily such as those based on recursive templates with C++.
Inlining is actually controlled by a number of parameters, which may be specified individually by
using --param name=value. The -finline-limit=n option sets some of these parameters as follows:
@item max-inline-insns
is set to I<n>.
@item max-inline-insns-single
is set to I<n>/2.
@item max-inline-insns-single-auto
is set to I<n>/2.
@item min-inline-insns
is set to 130 or I<n>/4, whichever is smaller.
@item max-inline-insns-rtl
is set to I<n>.
Using -finline-limit=600 thus results in the default settings for these parameters. See below
for a documentation of the individual parameters controlling inlining.
Note: pseudo instruction represents, in this particular context, an abstract measurement of
function's size. In no way, it represents a count of assembly instructions and as such its exact
meaning might change from one release to an another.
-fkeep-inline-functions
Even if all calls to a given function are integrated, and the function is declared "static",
nevertheless output a separate run-time callable version of the function. This switch does not
affect "extern inline" functions.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't turned on, even if the variables
aren't referenced.
GCC enables this option by default. If you want to force the compiler to check if the variable
was referenced, regardless of whether or not optimization is turned on, use the
-fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating point constants) across
compilation units.
This option is the default for optimized compilation if the assembler and linker support it. Use
-fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even
constant initialized arrays or initialized constant variables with integral or floating point
types. Languages like C or C++ require each non-automatic variable to have distinct location, so
using this option will result in non-conforming behavior.
-fno-branch-count-reg
Do not use ``decrement and branch'' instructions on a count register, but instead generate a
sequence of instructions that decrement a register, compare it against zero, then branch based
upon the result. This option is only meaningful on architectures that support such instructions,
which include x86, PowerPC, IA-64 and S/390.
The default is -fbranch-count-reg, enabled when -fstrength-reduce is enabled.
-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a constant function
contain the function's address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler
output may be confused by the optimizations performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are initialized to zero
into BSS. This can save space in the resulting code.
This option turns off this behavior because some programs explicitly rely on variables going to
the data section. E.g., so that the resulting executable can find the beginning of that section
and/or make assumptions based on that.
The default is -fzero-initialized-in-bss.
-fstrength-reduce
Perform the optimizations of loop strength reduction and elimination of iteration variables.
Enabled at levels -O2, -O3, -Os.
-fthread-jumps
Perform optimizations where we check to see if a jump branches to a location where another
comparison subsumed by the first is found. If so, the first branch is redirected to either the
destination of the second branch or a point immediately following it, depending on whether the
condition is known to be true or false.
Enabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination, scan through jump instructions when the target of the jump
is not reached by any other path. For example, when CSE encounters an "if" statement with an
"else" clause, CSE will follow the jump when the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps which conditionally skip
over blocks. When CSE encounters a simple "if" statement with no else clause, -fcse-skip-blocks
causes CSE to follow the jump around the body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations has been performed.
Enabled at levels -O2, -O3, -Os.
-frerun-loop-opt
Run the loop optimizer twice.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass also performs global constant
and copy propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may get better runtime
performance if you disable the global common subexpression elimination pass by adding -fno-gcse
to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination will attempt to move loads
which are only killed by stores into themselves. This allows a loop containing a load/store
sequence to be changed to a load outside the loop, and a copy/store within the loop.
Enabled by default when gcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, A store motion pass is run after global common subexpression
elimination. This pass will attempt to move stores out of loops. When used in conjunction with
-fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a
store after the loop.
Enabled by default when gcse is enabled.
-floop-optimize
Perform loop optimizations: move constant expressions out of loops, simplify exit test conditions
and optionally do strength-reduction and loop unrolling as well.
Enabled at levels -O, -O2, -O3, -Os.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent code and save code
size. The resulting code may or may not perform better than without cross-jumping.
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents. This include use of
conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard
arithmetics. The use of conditional execution on chips where it is available is controlled by
"if-conversion2".
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Use conditional execution (where available) to transform conditional jumps into branch-less
equivalents.
Enabled at levels -O, -O2, -O3, -Os.
-fdelete-null-pointer-checks
Use global dataflow analysis to identify and eliminate useless checks for null pointers. The
compiler assumes that dereferencing a null pointer would have halted the program. If a pointer
is checked after it has already been dereferenced, it cannot be null.
In some environments, this assumption is not true, and programs can safely dereference null
pointers. Use -fno-delete-null-pointer-checks to disable this optimization for programs which
depend on that behavior.
Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instructions and as operands of other simple
instructions in order to maximize the amount of register tying. This is especially helpful on
machines with two-operand instructions.
Note -fregmove and -foptimize-register-move are the same optimization.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions to exploit instruction slots
available after delayed branch instructions.
Enabled at levels -O, -O2, -O3, -Os.
-fschedule-insns
If supported for the target machine, attempt to reorder instructions to eliminate execution
stalls due to required data being unavailable. This helps machines that have slow floating point
or memory load instructions by allowing other instructions to be issued until the result of the
load or floating point instruction is required.
Enabled at levels -O2, -O3, -Os.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after
register allocation has been done. This is especially useful on machines with a relatively small
number of registers and where memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Don't schedule instructions across basic blocks. This is normally enabled by default when
scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Don't allow speculative motion of non-load instructions. This is normally enabled by default
when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load
Allow speculative motion of some load instructions. This only makes sense when scheduling before
register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes sense when scheduling before
register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fcaller-saves
Enable values to be allocated in registers that will be clobbered by function calls, by emitting
extra instructions to save and restore the registers around such calls. Such allocation is done
only when it seems to result in better code than would otherwise be produced.
This option is always enabled by default on certain machines, usually those which have no call-preserved callpreserved
preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
-fmove-all-movables
Forces all invariant computations in loops to be moved outside the loop.
-freduce-all-givs
Forces all general-induction variables in loops to be strength-reduced.
Note: When compiling programs written in Fortran, -fmove-all-movables and -freduce-all-givs are
enabled by default when you use the optimizer.
These options may generate better or worse code; results are highly dependent on the structure of
loops within the source code.
These two options are intended to be removed someday, once they have helped determine the
efficacy of various approaches to improving loop optimizations.
Please let us (<gcc@gcc.gnu.org> and <fortran@gnu.org>) know how use of these options affects the
performance of your production code. We're very interested in code that runs slower when these
options are enabled.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference between -fno-peephole and
-fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the
other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
-fbranch-probabilities
-fno-guess-branch-probability
Do not guess branch probabilities using a randomized model.
Sometimes gcc will opt to use a randomized model to guess branch probabilities, when none are
available from either profiling feedback (-fprofile-arcs) or __builtin_expect. This means that
different runs of the compiler on the same program may produce different object code.
In a hard real-time system, people don't want different runs of the compiler to produce code that
has different behavior; minimizing non-determinism is of paramount import. This switch allows
users to reduce non-determinism, possibly at the expense of inferior optimization.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of taken branches and
improve code locality.
Enabled at levels -O2, -O3, -Os.
-freorder-functions
Reorder basic blocks in the compiled function in order to reduce number of taken branches and
improve code locality. This is implemented by using special subsections "text.hot" for most
frequently executed functions and "text.unlikely" for unlikely executed functions. Reordering is
done by the linker so object file format must support named sections and linker must place them
in a reasonable way.
Also profile feedback must be available in to make this option effective. See -fprofile-arcs for
details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allows the compiler to assume the strictest aliasing rules applicable to the language being
compiled. For C (and C++), this activates optimizations based on the type of expressions. In
particular, an object of one type is assumed never to reside at the same address as an object of
a different type, unless the types are almost the same. For example, an "unsigned int" can alias
an "int", but not a "void*" or a "double". A character type may alias any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most recently written to
(called ``type-punning'') is common. Even with -fstrict-aliasing, type-punning is allowed,
provided the memory is accessed through the union type. So, the code above will work as
expected. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Every language that wishes to perform language-specific alias analysis should define a function
that computes, given an "tree" node, an alias set for the node. Nodes in different alias sets
are not allowed to alias. For an example, see the C front-end function "c_get_alias_set".
Enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two greater than n, skipping up to n bytes.
For instance, -falign-functions=32 aligns functions to the next 32-byte boundary, but
-falign-functions=24 would align to the next 32-byte boundary only if this can be done by
skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be
aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
If n is not specified, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up to n bytes like
-falign-functions. This option can easily make code slower, because it must insert dummy
operations for when the branch target is reached in the usual flow of the code.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their
values are used instead.
If n is not specified, use a machine-dependent default which is very likely to be 1, meaning no
alignment.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions. The hope
is that the loop will be executed many times, which will make up for any execution of the dummy
operations.
-falign-loops-max-skip
-falign-loops-max-skip=n
When aligning loops to a power-of-two boundary, only do so if can skip by up to n bytes.
If n is not specified, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets where the targets can only be
reached by jumping, skipping up to n bytes like -falign-functions. In this case, no dummy
operations need be executed.
-falign-jumps-max-skip
-falign-jumps-max-skip=n
When aligning branch targets to a power-of-two boundary, only do so if can skip by up to n bytes.
If n is not specified, use a machine-dependent default.
Enabled at levels -O2, -O3.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left over after
register allocation. This optimization will most benefit processors with lots of registers. It
can, however, make debugging impossible, since variables will no longer stay in a ``home
register''.
Enabled at levels -O3.
-fno-cprop-registers
After register allocation and post-register allocation instruction splitting, we perform a copy-propagation copypropagation
propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.
Disabled at levels -O, -O2, -O3, -Os.
The following options control compiler behavior regarding floating point arithmetic. These options
trade off between speed and correctness. All must be specifically enabled.
-ffloat-store
Do not store floating point variables in registers, and inhibit other options that might change
whether a floating point value is taken from a register or memory.
This option prevents undesirable excess precision on machines such as the 68000 where the
floating registers (of the 68881) keep more precision than a "double" is supposed to have.
Similarly for the x86 architecture. For most programs, the excess precision does only good, but
a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for such
programs, after modifying them to store all pertinent intermediate computations into variables.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trapping-math, -ffinite-math-only and
-fno-signaling-nans.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option should never be turned on by any -O option since it can result in incorrect output
for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math
functions.
-fno-math-errno
Do not set ERRNO after calling math functions that are executed with a single instruction, e.g.,
sqrt. A program that relies on IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
This option should never be turned on by any -O option since it can result in incorrect output
for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math
functions.
The default is -fmath-errno.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are
valid and (b) may violate IEEE or ANSI standards. When used at link-time, it may include
libraries or startup files that change the default FPU control word or other similar
optimizations.
This option should never be turned on by any -O option since it can result in incorrect output
for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math
functions.
The default is -fno-unsafe-math-optimizations.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments and results are not
NaNs or +-Infs.
This option should never be turned on by any -O option since it can result in incorrect output
for programs which depend on an exact implementation of IEEE or ISO rules/specifications.
The default is -fno-finite-math-only.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-visible traps. These
traps include division by zero, overflow, underflow, inexact result and invalid operation. This
option implies -fno-signaling-nans. Setting this option may allow faster code if one relies on
``non-stop'' IEEE arithmetic, for example.
This option should never be turned on by any -O option since it can result in incorrect output
for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math
functions.
The default is -ftrapping-math.
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point floatingpoint
point operations. Setting this option disables optimizations that may change the number of
exceptions visible with signaling NaNs. This option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations
that affect signaling NaN behavior.
-fsingle-precision-constant
Treat floating point constant as single precision constant instead of implicitly converting it to
double precision constant.
The following options control optimizations that may improve performance, but are not enabled by any
-O options. This section includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on the number of times each branch was
taken. When the program compiled with -fprofile-arcs exits it saves arc execution counts to a
file called sourcename.da for each source file The information in this data file is very
dependent on the structure of the generated code, so you must use the same source code and the
same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These
can be used to improve optimization. Currently, they are only used in one place: in reorg.c,
instead of guessing which path a branch is mostly to take, the REG_BR_PROB values are used to
exactly determine which path is taken more often.
-fnew-ra
Use a graph coloring register allocator. Currently this option is meant for testing, so we are
interested to hear about miscompilations with -fnew-ra.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control
flow of the function allowing other optimizations to do better job.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the
loop. -funroll-loops implies both -fstrength-reduce and -frerun-cse-after-loop. This option
makes code larger, and may or may not make it run faster.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This
usually makes programs run more slowly. -funroll-all-loops implies the same options as
-funroll-loops,
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the
performance of loops that access large arrays.
Disabled at level -Os.
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the target supports
arbitrary sections. The name of the function or the name of the data item determines the
section's name in the output file.
Use these options on systems where the linker can perform optimizations to improve locality of
reference in the instruction space. HPPA processors running HP-UX and SPARC processors running
Solaris 2 have linkers with such optimizations. Other systems using the ELF object format as
well as AIX may have these optimizations in the future.
Only use these options when there are significant benefits from doing so. When you specify these
options, the assembler and linker will create larger object and executable files and will also be
slower. You will not be able to use "gprof" on all systems if you specify this option and you
may have problems with debugging if you specify both this option and -g.
-fssa
Perform optimizations in static single assignment form. Each function's flow graph is translated
into SSA form, optimizations are performed, and the flow graph is translated back from SSA form.
Users should not specify this option, since it is not yet ready for production use.
-fssa-ccp
Perform Sparse Conditional Constant Propagation in SSA form. Requires -fssa. Like -fssa, this
is an experimental feature.
-fssa-dce
Perform aggressive dead-code elimination in SSA form. Requires -fssa. Like -fssa, this is an
experimental feature.
--param name=value
In some places, GCC uses various constants to control the amount of optimization that is done.
For example, GCC will not inline functions that contain more that a certain number of
instructions. You can control some of these constants on the command-line using the --param
option.
In each case, the value is an integer. The allowable choices for name are given in the following
table:
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for an instruction to fill a
delay slot. If more than this arbitrary number of instructions is searched, the time savings
from filling the delay slot will be minimal so stop searching. Increasing values mean more
aggressive optimization, making the compile time increase with probably small improvement in
executable run time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to consider when
searching for a block with valid live register information. Increasing this arbitrarily
chosen value means more aggressive optimization, increasing the compile time. This parameter
should be removed when the delay slot code is rewritten to maintain the control-flow graph.
max-gcse-memory
The approximate maximum amount of memory that will be allocated in order to perform the
global common subexpression elimination optimization. If more memory than specified is
required, the optimization will not be done.
max-gcse-passes
The maximum number of passes of GCSE to run.
max-pending-list-length
The maximum number of pending dependencies scheduling will allow before flushing the current
state and starting over. Large functions with few branches or calls can create excessively
large lists which needlessly consume memory and resources.
max-inline-insns-single
Several parameters control the tree inliner used in gcc. This number sets the maximum number
of instructions (counted in gcc's internal representation) in a single function that the tree
inliner will consider for inlining. This only affects functions declared inline and methods
implemented in a class declaration (C++). The default value is 300.
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of functions that would otherwise
not be considered for inlining by the compiler will be investigated. To those functions, a
different (more restrictive) limit compared to functions declared inline can be applied. The
default value is 300.
max-inline-insns
The tree inliner does decrease the allowable size for single functions to be inlined after we
already inlined the number of instructions given here by repeated inlining. This number
should be a factor of two or more larger than the single function limit. Higher numbers
result in better runtime performance, but incur higher compile-time resource (CPU time,
memory) requirements and result in larger binaries. Very high values are not advisable, as
too large binaries may adversely affect runtime performance. The default value is 600.
max-inline-slope
After exceeding the maximum number of inlined instructions by repeated inlining, a linear
function is used to decrease the allowable size for single functions. The slope of that
function is the negative reciprocal of the number specified here. The default value is 32.
min-inline-insns
The repeated inlining is throttled more and more by the linear function after exceeding the
limit. To avoid too much throttling, a minimum for this function is specified here to allow
repeated inlining for very small functions even when a lot of repeated inlining already has
been done. The default value is 130.
max-inline-insns-rtl
For languages that use the RTL inliner (this happens at a later stage than tree inlining),
you can set the maximum allowable size (counted in RTL instructions) for the RTL inliner with
this parameter. The default value is 600.
max-unrolled-insns
The maximum number of instructions that a loop should have if that loop is unrolled, and if
the loop is unrolled, it determines how many times the loop code is unrolled.
hot-bb-count-fraction
Select fraction of the maximal count of repetitions of basic block in program given basic
block needs to have to be considered hot.
hot-bb-frequency-fraction
Select fraction of the maximal frequency of executions of basic block in function given basic
block needs to have to be considered hot
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given percentage of executed
instructions is covered. This limits unnecessary code size expansion.
The tracer-dynamic-coverage-feedback is used only when profile feedback is available. The
real profiles (as opposed to statically estimated ones) are much less balanced allowing the
threshold to be larger value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given percentage. This is rather hokey
argument, as most of the duplicates will be eliminated later in cross jumping, so it may be
set to much higher values than is the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than this threshold (in
percent).
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge do have probability lower than this threshold.
Similarly to tracer-dynamic-coverage two values are present, one for compilation for profile
feedback and one for compilation without. The value for compilation with profile feedback
needs to be more conservative (higher) in order to make tracer effective.
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This parameter specifies
the minimum percentage by which the garbage collector's heap should be allowed to expand
between collections. Tuning this may improve compilation speed; it has no effect on code
generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If
"getrlimit" is available, the notion of "RAM" is the smallest of actual RAM, RLIMIT_RSS,
RLIMIT_DATA and RLIMIT_AS. If GCC is not able to calculate RAM on a particular platform, the
lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes a
full collection to occur at every opportunity. This is extremely slow, but can be useful for
debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins bothering to collect garbage.
The first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. ggc-minheapsize.
heapsize. Again, tuning this may improve compilation speed, and has no effect on code
generation.
The default is RAM/8, with a lower bound of 4096 (four megabytes) and an upper bound of
131072 (128 megabytes). If "getrlimit" is available, the notion of "RAM" is the smallest of
actual RAM, RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If GCC is not able to calculate RAM on a
particular platform, the lower bound is used. Setting this parameter very large effectively
disables garbage collection. Setting this parameter and ggc-min-expand to zero causes a full
collection to occur at every opportunity.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before actual
compilation.
If you use the -E option, nothing is done except preprocessing. Some of these options make sense
only together with -E because they cause the preprocessor output to be unsuitable for actual
compilation.
You can use -Wp,option to bypass the compiler driver and pass option directly through to the
preprocessor. If option contains commas, it is split into multiple options at the commas. However,
many options are modified, translated or interpreted by the compiler driver before being passed to
the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is
undocumented and subject to change, so whenever possible you should avoid using -Wp and let the
driver handle the options instead.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
Predefine name as a macro, with definition definition. There are no restrictions on the contents
of definition, but if you are invoking the preprocessor from a shell or shell-like program you
may need to use the shell's quoting syntax to protect characters such as spaces that have a
meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with
surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most
shells, so you will need to quote the option. With sh and csh, -D'name(args...)=definition'
works.
-D and -U options are processed in the order they are given on the command line. All -imacros
file and -include file options are processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or provided with a -D option.
-undef
Do not predefine any system-specific macros. The common predefined macros remain defined.
-I dir
Add the directory dir to the list of directories to be searched for header files. Directories
named by -I are searched before the standard system include directories. If the directory dir is
a standard system include directory, the option is ignored to ensure that the default search
order for system directories and the special treatment of system headers are not defeated .
-o file
Write output to file. This is the same as specifying file as the second non-option argument to
cpp. gcc has a different interpretation of a second non-option argument, so you must use -o to
specify the output file.
-Wall
Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment
and -Wtrigraphs. Note that many of the preprocessor's warnings are on by default and have no
options to control them.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline backslashnewline
newline appears in a // comment. (Both forms have the same effect.)
-Wtrigraphs
Warn if any trigraphs are encountered. This option used to take effect only if -trigraphs was
also specified, but now works independently. Warnings are not given for trigraphs within
comments, as they do not affect the meaning of the program.
-Wtraditional
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 problematic constructs which should
be avoided.
-Wimport
Warn the first time #import is used.
-Wundef
Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of
defined. Such identifiers are replaced with zero.
-Wunused-macros
Warn about macros defined in the main file that are unused. A macro is used if it is expanded or
tested for existence at least once. The preprocessor will also warn if the macro has not been
used at the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are not
warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will
report it as unused. To avoid the warning in such a case, you might improve the scope of the
macro's definition by, for example, moving it into the first skipped block. Alternatively, you
could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This usually happens in code of the
form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are not in older programs. This
warning is on by default.
-Werror
Make all warnings into hard errors. Source code which triggers warnings will be rejected.
-Wsystem-headers
Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your
own code, therefore suppressed. If you are responsible for the system library, you may want to
see them.
-w Suppress all warnings, including those which GNU CPP issues by default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by
default, since they trigger frequently on harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This
includes mandatory diagnostics that GCC issues without -pedantic but treats as warnings.
-M Instead of outputting the result of preprocessing, output a rule suitable for make describing the
dependencies of the main source file. The preprocessor outputs one make rule containing the
object file name for that source file, a colon, and the names of all the included files,
including those coming from -include or -imacros command line options.
Unless specified explicitly (with -MT or -MQ), the object file name consists of the basename of
the source file with any suffix replaced with object file suffix. If there are many included
files then the rule is split into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such
debug output with the dependency rules you should explicitly specify the dependency output file
with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output will still be
sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.
-MM Like -M but do not mention header files that are found in system header directories, nor header
files that are included, directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an #include directive does not
in itself determine whether that header will appear in -MM dependency output. This is a slight
change in semantics from GCC versions 3.0 and earlier.
-MF file
@anchor{-MF} When used with -M or -MM, specifies a file to write the dependencies to. If no -MF
switch is given the preprocessor sends the rules to the same place it would have sent
preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
-dependency-file
Like -MF. (APPLE ONLY)
-MG In conjunction with an option such as -M requesting dependency generation, -MG assumes missing
header files are generated files and adds them to the dependency list without raising an error.
The dependency filename is taken directly from the "#include" directive without prepending any
path. -MG also suppresses preprocessed output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each dependency other than the main file,
causing each to depend on nothing. These dummy rules work around errors make gives if you remove
header files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By default CPP takes the name of
the main input file, including any path, deletes any file suffix such as .c, and appends the
platform's usual object suffix. The result is the target.
An -MT option will set the target to be exactly the string you specify. If you want multiple
targets, you can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file
based on whether an -o option is given. If it is, the driver uses its argument but with a suffix
of .d, otherwise it take the basename of the input file and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency
output file (but @pxref{-MF}), but if used without -E, each -o is understood to specify a target
object file.
Since -E is not implied, -MD can be used to generate a dependency output file as a side-effect of
the compilation process.
-MMD
Like -MD except mention only user header files, not system -header files.
-fpch-deps
When using precompiled headers, this flag will cause the dependency-output flags to also list the
files from the precompiled header's dependencies. If not specified only the precompiled header
would be listed and not the files that were used to create it because those files are not
consulted when a precompiled header is used.
-x c
-x c++
-x objective-c
-x objective-c++
-x assembler-with-cpp
Specify the source language: C, C++, Objective-C, Objective-C++, or assembly. This has nothing
to do with standards conformance or extensions; it merely selects which base syntax to expect.
If you give none of these options, cpp will deduce the language from the extension of the source
file: .c, .cc, .m, .mm, or .S. Some other common extensions for C++ and assembly are also
recognized. If cpp does not recognize the extension, it will treat the file as C; this is the
most generic mode.
Note: Previous versions of cpp accepted a -lang option which selected both the language and the
standards conformance level. This option has been removed, because it conflicts with the -l
option.
-std=standard
-ansi
Specify the standard to which the code should conform. Currently CPP knows about C and C++
standards; others may be added in the future.
standard may be one of:
"iso9899:1990"
"c89"
The ISO C standard from 1990. c89 is the customary shorthand for this version of the
standard.
The -ansi option is equivalent to -std=c89.
"iso9899:199409"
The 1990 C standard, as amended in 1994.
"iso9899:1999"
"c99"
"iso9899:199x"
"c9x"
The revised ISO C standard, published in December 1999. Before publication, this was known
as C9X.
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"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.
-I- Split the include path. Any directories specified with -I options before -I- are searched only
for headers requested with "#include "file""; they are not searched for "#include <file>". If
additional directories are specified with -I options after the -I-, those directories are
searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current file directory as the first
search directory for "#include "file"".
-nostdinc
Do not search the standard system directories for header files. Only the directories you have
specified with -I options (and the directory of the current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard directories, but do still search the
other standard directories. (This option is used when building the C++ library.)
-include file
Process file as if "#include "file"" appeared as the first line of the primary source file.
However, the first directory searched for file is the preprocessor's working directory instead of
the directory containing the main source file. If not found there, it is searched for in the
remainder of the "#include "..."" search chain as normal.
If multiple -include options are given, the files are included in the order they appear on the
command line.
-imacros file
Exactly like -include, except that any output produced by scanning file is thrown away. Macros
it defines remain defined. This allows you to acquire all the macros from a header without also
processing its declarations.
All files specified by -imacros are processed before all files specified by -include.
-idirafter dir
Search dir for header files, but do it after all directories specified with -I and the standard
system directories have been exhausted. dir is treated as a system include directory.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a
directory, you should include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add the resulting directory to
the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix
puts it where -idirafter would.
Use of these options is discouraged.
-isystem dir
Search dir for header files, after all directories specified by -I but before the standard system
directories. Mark it as a system directory, so that it gets the same special treatment as is
applied to the standard system directories.
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed. This suppresses
things like macro expansion, trigraph conversion, escaped newline splicing, and processing of
most directives. The preprocessor still recognizes and removes comments, so that you can pass a
file preprocessed with -C to the compiler without problems. In this mode the integrated
preprocessor is little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are
the extensions that GCC uses for preprocessed files created by -save-temps.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor report correct column numbers in
warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than
100, the option is ignored. The default is 8.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if diagnostics are being
scanned by a program that does not understand the column numbers, such as dejagnu.
-A predicate=answer
Make an assertion with the predicate predicate and answer answer. This form is preferred to the
older form -A predicate(answer), which is still supported, because it does not use shell special
characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-A- Cancel all predefined assertions and all assertions preceding it on the command line. Also,
undefine all predefined macros and all macros preceding it on the command line. (This is a
historical wart and may change in the future.)
-dCHARS
CHARS is a sequence of one or more of the following characters, and must not be preceded by a
space. Other characters are interpreted by the compiler proper, or reserved for future versions
of GCC, and so are silently ignored. If you specify characters whose behavior conflicts, the
result is undefined.
M Instead of the normal output, generate a list of #define directives for all the macros
defined during the execution of the preprocessor, including predefined macros. This gives
you a way of finding out what is predefined in your version of the preprocessor. Assuming
you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
will show all the predefined macros.
D Like M except in two respects: it does not include the predefined macros, and it outputs both
the #define directives and the result of preprocessing. Both kinds of output go to the
standard output file.
N Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of preprocessing.
-P Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when
running the preprocessor on something that is not C code, and will be sent to a program which
might be confused by the linemarkers.
-C Do not discard comments. All comments are passed through to the output file, except for comments
in processed directives, which are deleted along with the directive.
You should be prepared for side effects when using -C; it causes the preprocessor to treat
comments as tokens in their own right. For example, comments appearing at the start of what
would be a directive line have the effect of turning that line into an ordinary source line,
since the first token on the line is no longer a #.
-CC Do not discard comments, including during macro expansion. This is like -C, except that comments
contained within macros are also passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the -CC option causes all C++-style comments
inside a macro to be converted to C-style comments. This is to prevent later use of that macro
from inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
-gcc
Define the macros __GNUC__, __GNUC_MINOR__ and __GNUC_PATCHLEVEL__. These are defined
automatically when you use gcc -E; you can turn them off in that case with -no-gcc.
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.
-trigraphs
Process trigraph sequences. These are three-character sequences, all starting with ??, that are
defined by ISO C to stand for single characters. For example, ??/ stands for \, so '??/n' is a
character constant for a newline. By default, GCC ignores trigraphs, but in standard-conforming
modes it converts them. See the -std and -ansi options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-Replacement: ??Replacement:
Replacement: [ ] { } # \ ^ | ~
-remap
Enable special code to work around file systems which only permit very short file names, such as
MS-DOS.
--help
--target-help
Print text describing all the command line options instead of preprocessing anything.
-v Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the
final form of the include path.
-H Print the name of each header file used, in addition to other normal activities. Each name is
indented to show how deep in the #include stack it is. Precompiled header files are also
printed, even if they are found to be invalid; an invalid precompiled header file is printed with
...x and a valid one with ...! .
-version
--version
Print out GNU CPP's version number. With one dash, proceed to preprocess as normal. With two
dashes, exit immediately.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains commas, it is split into multiple
options at the commas.
Options for Linking
These options come into play when the compiler links object files into an executable output file.
They are meaningless if the compiler is not doing a link step.
In addition to the options listed below, Apple's GCC also accepts and passes nearly all of the
options defined by the linker ld and by the library tool libtool. Common options include -framework,
-dynamic, -bundle, -flat_namespace, and so forth. See the ld and libtool man pages for further
details.
object-file-name
A file name that does not end in a special recognized suffix is considered to name an object file
or library. (Object files are distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as input to the linker.
-c
-S
-E If any of these options is used, then the linker is not run, and object file names should not be
used as arguments.
-llibrary
-l library
Search the library named library when linking. (The second alternative with the library as a
separate argument is only for POSIX compliance and is not recommended.)
It makes a difference where in the command you write this option; the linker searches and
processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o
searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those
functions may not be loaded.
The linker searches a standard list of directories for the library, which is actually a file
named liblibrary.a. The linker then uses this file as if it had been specified precisely by
name.
The directories searched include several standard system directories plus any that you specify
with -L.
Normally the files found this way are library files---archive files whose members are object
files. The linker handles an archive file by scanning through it for members which define
symbols that have so far been referenced but not defined. But if the file that is found is an
ordinary object file, it is linked in the usual fashion. The only difference between using an -l
option and specifying a file name is that -l surrounds library with lib and .a and searches
several directories.
-lobjc
You need this special case of the -l option in order to link an Objective-C program.
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are
used normally, unless -nostdlib or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you specify will be
passed to the linker. The standard startup files are used normally, unless -nostartfiles is
used. The compiler may generate calls to memcmp, memset, and memcpy for System V (and ISO C)
environments or to bcopy and bzero for BSD environments. These entries are usually resolved by
entries in libc. These entry points should be supplied through some other mechanism when this
option is specified.
-nostdlib
Do not use the standard system startup files or libraries when linking. No startup files and
only the libraries you specify will be passed to the linker. The compiler may generate calls to
memcmp, memset, and memcpy for System V (and ISO C) environments or to bcopy and bzero for BSD
environments. These entries are usually resolved by entries in libc. These entry points should
be supplied through some other mechanism when this option is specified.
-no-c++filt
By default all linker diagnostic output is piped through c++filt. This option suppresses that
behavior. (APPLE ONLY)
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of
internal subroutines that GCC uses to overcome shortcomings of particular machines, or special
needs for some languages.
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other
words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well.
This ensures that you have no unresolved references to internal GCC library subroutines. (For
example, __main, used to ensure C++ constructors will be called.)
-s Remove all symbol table and relocation information from the executable.
-static
On systems that support dynamic linking, this prevents linking with the shared libraries. On
other systems, this option has no effect.
This option will not work on Mac OS X unless all of your libraries (including libgcc.a) have also
been compiled with -static.
-shared
Produce a shared object which can then be linked with other objects to form an executable. Not
all systems support this option. For predictable results, you must also specify the same set of
options that were used to generate code (-fpic, -fPIC, or model suboptions) when you specify this
option.[1]
This option is not supported on Mac OS X.
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options force the use of either the
shared or static version respectively. If no shared version of libgcc was built when the
compiler was configured, these options have no effect.
There are several situations in which an application should use the shared libgcc instead of the
static version. The most common of these is when the application wishes to throw and catch
exceptions across different shared libraries. In that case, each of the libraries as well as the
application itself should use the shared libgcc.
Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared
library or a main executable, because C++ and Java programs typically use exceptions, so this is
the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find that they will not
always be linked with the shared libgcc. If GCC finds, at its configuration time, that you have
a GNU linker that does not support option --eh-frame-hdr, it will link the shared version of
libgcc into shared libraries by default. Otherwise, it will take advantage of the linker and
optimize away the linking with the shared version of libgcc, linking with the static version of
libgcc by default. This allows exceptions to propagate through such shared libraries, without
incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link
it using the G++ or GCJ driver, as appropriate for the languages used in the program, or using
the option -shared-libgcc, such that it is linked with the shared libgcc.
-symbolic
Bind references to global symbols when building a shared object. Warn about any unresolved
references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few
systems support this option.
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-specific linker
options which GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xlinker twice, once for the
option and once for the argument. For example, to pass -assert definitions, you must write
-Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions",
because this passes the entire string as a single argument, which is not what the linker expects.
-Wl,option
Pass option as an option to the linker. If option contains commas, it is split into multiple
options at the commas.
-u symbol
Pretend the symbol symbol is undefined, to force linking of library modules to define it. You
can use -u multiple times with different symbols to force loading of additional library modules.
Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of the
compiler:
-Idir
Add the directory dir to the head of the list of directories to be searched for header files.
This can be used to override a system header file, substituting your own version, since these
directories are searched before the system header file directories. However, you should not use
this option to add directories that contain vendor-supplied system header files (use -isystem for
that). If you use more than one -I option, the directories are scanned in left-to-right order;
the standard system directories come after.
If a standard system include directory, or a directory specified with -isystem, is also specified
with -I, the -I option will be ignored. The directory will still be searched but as a system
directory at its normal position in the system include chain. This is to ensure that GCC's
procedure to fix buggy system headers and the ordering for the include_next directive are not
inadvertently changed. If you really need to change the search order for system directories, use
the -nostdinc and/or -isystem options.
-I- Any directories you specify with -I options before the -I- option are searched only for the case
of #include "file"; they are not searched for #include <file>.
If additional directories are specified with -I options after the -I-, these directories are
searched for all #include directives. (Ordinarily all -I directories are used this way.)
In addition, the -I- option inhibits the use of the current directory (where the current input
file came from) as the first search directory for #include "file". There is no way to override
this effect of -I-. With -I. you can specify searching the directory which was current when the
compiler was invoked. That is not exactly the same as what the preprocessor does by default, but
it is often satisfactory.
-I- does not inhibit the use of the standard system directories for header files. Thus, -I- and
-nostdinc are independent.
-Ldir
Add directory dir to the list of directories to be searched for -l.
-Fdir
In Apple's version of GCC only, add the directory dir to the head of the list of directories to
be searched for frameworks.
The framework search algorithm is, for an inclusion of <Fmwk/Header.h>, to look for files named
path/Fmwk.framework/Headers/Header.h or path/Fmwk.framework/PrivateHeaders/Header.h where path
includes /System/Library/Frameworks/ /Library/Frameworks/, and /Local/Library/Frameworks/, plus
any additional paths specified by -F.
All the -F options are also passed to the linker.
-Bprefix
This option specifies where to find the executables, libraries, include files, and data files of
the compiler itself.
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries
prefix as a prefix for each program it tries to run, both with and without machine/version/.
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that
name is not found, or if -B was not specified, the driver tries two standard prefixes, which are
/usr/lib/gcc/ and /usr/local/lib/gcc-lib/. If neither of those results in a file name that is
found, the unmodified program name is searched for using the directories specified in your PATH
environment variable.
The compiler will check to see if the path provided by the -B refers to a directory, and if
necessary it will add a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker,
because the compiler translates these options into -L options for the linker. They also apply to
includes files in the preprocessor, because the compiler translates these options into -isystem
options for the preprocessor. In this case, the compiler appends include to the prefix.
The run-time support file libgcc.a can also be searched for using the -B prefix, if needed. If
it is not found there, the two standard prefixes above are tried, and that is all. The file is
left out of the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use the environment variable
GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the
range 0 to 9, then it will be replaced by [dir/]include. This is to help with boot-strapping the
compiler.
-specs=file
Process file after the compiler reads in the standard specs file, in order to override the
defaults that the gcc driver program uses when determining what switches to pass to cc1, cc1plus,
as, ld, etc. More than one -specs=file can be specified on the command line, and they are
processed in order, from left to right.
Specifying Target Machine and Compiler Version
The usual way to run GCC is to run the executable called gcc, or <machine>-gcc when cross-compiling,
or <machine>-gcc-<version> to run a version other than the one that was installed last. Sometimes
this is inconvenient, so GCC provides options that will switch to another cross-compiler or version.
-b machine
The argument machine specifies the target machine for compilation.
The value to use for machine is the same as was specified as the machine type when configuring
GCC as a cross-compiler. For example, if a cross-compiler was configured with configure i386v,
meaning to compile for an 80386 running System V, then you would specify -b i386v to run that
cross compiler.
-V version
The argument version specifies which version of GCC to run. This is useful when multiple
versions are installed. For example, version might be 2.0, meaning to run GCC version 2.0.
The -V and -b options work by running the <machine>-gcc-<version> executable, so there's no real
reason to use them if you can just run that directly.
Hardware Models and Configurations
Earlier we discussed the standard option -b which chooses among different installed compilers for
completely different target machines, such as VAX vs. 68000 vs. 80386.
In addition, each of these target machine types can have its own special options, starting with -m,
to choose among various hardware models or configurations---for example, 68010 vs 68020, floating
coprocessor or none. A single installed version of the compiler can compile for any model or
configuration, according to the options specified.
Some configurations of the compiler also support additional special options, usually for
compatibility with other compilers on the same platform.
These options are defined by the macro "TARGET_SWITCHES" in the machine description. The default for
the options is also defined by that macro, which enables you to change the defaults.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
GCC supports two related instruction set architectures for the RS/6000 and PowerPC. The POWER
instruction set are those instructions supported by the rios chip set used in the original
RS/6000 systems and the PowerPC instruction set is the architecture of the Motorola MPC5xx,
MPC6xx, MPC8xx microprocessors, and the IBM 4xx microprocessors.
Neither architecture is a subset of the other. However there is a large common subset of
instructions supported by both. An MQ register is included in processors supporting the POWER
architecture.
You use these options to specify which instructions are available on the processor you are using.
The default value of these options is determined when configuring GCC. Specifying the
-mcpu=cpu_type overrides the specification of these options. We recommend you use the
-mcpu=cpu_type option rather than the options listed above.
The -mpower option allows GCC to generate instructions that are found only in the POWER
architecture and to use the MQ register. Specifying -mpower2 implies -power and also allows GCC
to generate instructions that are present in the POWER2 architecture but not the original POWER
architecture.
The -mpowerpc option allows GCC to generate instructions that are found only in the 32-bit subset
of the PowerPC architecture. Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to
use the optional PowerPC architecture instructions in the General Purpose group, including
floating-point square root. Specifying -mpowerpc-gfxopt implies -mpowerpc and also allows GCC to
use the optional PowerPC architecture instructions in the Graphics group, including floating-point floatingpoint
point select.
The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found
in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC
defaults to -mno-powerpc64.
If you specify both -mno-power and -mno-powerpc, GCC will use only the instructions in the common
subset of both architectures plus some special AIX common-mode calls, and will not use the MQ
register. Specifying both -mpower and -mpowerpc permits GCC to use any instruction from either
architecture and to allow use of the MQ register; specify this for the Motorola MPC601.
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code. With -mnew-mnemonics, GCC uses
the assembler mnemonics defined for the PowerPC architecture. With -mold-mnemonics it uses the
assembler mnemonics defined for the POWER architecture. Instructions defined in only one
architecture have only one mnemonic; GCC uses that mnemonic irrespective of which of these
options is specified.
GCC defaults to the mnemonics appropriate for the architecture in use. Specifying -mcpu=cpu_type
sometimes overrides the value of these option. Unless you are building a cross-compiler, you
should normally not specify either -mnew-mnemonics or -mold-mnemonics, but should instead accept
the default.
-mcpu=cpu_type
Set architecture type, register usage, choice of mnemonics, and instruction scheduling parameters
for machine type cpu_type. Supported values for cpu_type are rios, rios1, rsc, rios2, rs64a,
601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, power, power2, powerpc, 403, 505,
801, 821, 823, and 860 and common.
-mcpu=common selects a completely generic processor. Code generated under this option will run
on any POWER or PowerPC processor. GCC will use only the instructions in the common subset of
both architectures, and will not use the MQ register. GCC assumes a generic processor model for
scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 specify generic POWER, POWER2, pure
32-bit PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine types, with an
appropriate, generic processor model assumed for scheduling purposes.
The other options specify a specific processor. Code generated under those options will run best
on that processor, and may not run at all on others.
The -mcpu options automatically enable or disable other -m options as follows:
common
-mno-power, -mno-powerpc
power
power2
rios1
rios2
rsc -mpower, -mno-powerpc, -mno-new-mnemonics
powerpc
rs64a
602
603
603e
604
620
630
740
7400
7450
750
505 -mno-power, -mpowerpc, -mnew-mnemonics
601 -mpower, -mpowerpc, -mnew-mnemonics
403
821
860 -mno-power, -mpowerpc, -mnew-mnemonics, -msoft-float
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the
architecture type, register usage, or choice of mnemonics, as -mcpu=cpu_type would. The same
values for cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated
will use the architecture, registers, and mnemonics set by -mcpu, but the scheduling parameters
set by -mtune.
-maltivec
-mno-altivec
These switches enable or disable the use of built-in functions that allow access to the AltiVec
instruction set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec
ABI enhancements.
This option is not supported on Mac OS X; use -faltivec instead.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not change the default ABI, instead it
adds the SPE ABI extensions to the current ABI.
-mabi=no-spe
Disable Booke SPE ABI extensions for the current ABI.
-misel=yes/no
-misel
This switch enables or disables the generation of ISEL instructions.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every executable file.
The -mfull-toc option is selected by default. In that case, GCC will allocate at least one TOC
entry for each unique non-automatic variable reference in your program. GCC will also place
floating-point constants in the TOC. However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the available TOC space,
you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options.
-mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc
forces GCC to generate code to calculate the sum of an address and a constant at run-time instead
of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC
to produce very slightly slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify
-mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When
you specify this option, GCC will produce code that is slower and larger but which uses extremely
little TOC space. You may wish to use this option only on files that contain less frequently
executed code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the
infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64 and -mpowerpc,
while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.
-mxl-call
-mno-xl-call
On AIX, pass floating-point arguments to prototyped functions beyond the register save area (RSA)
on the stack in addition to argument FPRs. The AIX calling convention was extended but not
initially documented to handle an obscure K&R C case of calling a function that takes the address
of its arguments with fewer arguments than declared. AIX XL compilers access floating point
arguments which do not fit in the RSA from the stack when a subroutine is compiled without
optimization. Because always storing floating-point arguments on the stack is inefficient and
rarely needed, this option is not enabled by default and only is necessary when calling
subroutines compiled by AIX XL compilers without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message
passing with special startup code to enable the application to run. The system must have PE
installed in the standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with
the -specs= option to specify the appropriate directory location. The Parallel Environment does
not support threads, so the -mpe option and the -pthread option are incompatible.
-malign-mac68k
-malign-power
-malign-natural
The option -malign-mac68k causes structure fields to be aligned on 2-byte boundaries, in order to
be compatible with m68k compiler output. The option -malign-power is the standard alignment mode
for the PowerPC. The option -malign-natural is an extension of PowerPC alignment that aligns
larger data types such as doubles on their natural boundaries. (APPLE ONLY)
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set. Software floating point
emulation is provided if you use the -msoft-float option, and pass the option to GCC when
linking.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word instructions and the store multiple
word instructions. These instructions are generated by default on POWER systems, and not
generated on PowerPC systems. Do not use -mmultiple on little endian PowerPC systems, since
those instructions do not work when the processor is in little endian mode. The exceptions are
PPC740 and PPC750 which permit the instructions usage in little endian mode.
-mstring
-mno-string
Generate code that uses (does not use) the load string instructions and the store string word
instructions to save multiple registers and do small block moves. These instructions are
generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mstring
on little endian PowerPC systems, since those instructions do not work when the processor is in
little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage in
little endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that update the base
register to the address of the calculated memory location. These instructions are generated by
default. If you use -mno-update, there is a small window between the time that the stack pointer
is updated and the address of the previous frame is stored, which means code that walks the stack
frame across interrupts or signals may get corrupted data.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply and accumulate instructions.
These instructions are generated by default if hardware floating is used.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain
bit-fields to be aligned to the base type of the bit-field.
For example, by default a structure containing nothing but 8 "unsigned" bit-fields of length 1
would be aligned to a 4 byte boundary and have a size of 4 bytes. By using -mno-bit-align, the
structure would be aligned to a 1 byte boundary and be one byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references
will be handled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows (does not allow) the program to be
relocated to a different address at runtime. If you use -mrelocatable on any module, all objects
linked together must be compiled with -mrelocatable or -mrelocatable-lib.
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows (does not allow) the program to be
relocated to a different address at runtime. Modules compiled with -mrelocatable-lib can be
linked with either modules compiled without -mrelocatable and -mrelocatable-lib or with modules
compiled with the -mrelocatable options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer
to a global area pointing to the addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the processor in little endian mode.
The -mlittle-endian option is the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the processor in big endian mode.
The -mbig-endian option is the same as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external
references are relocatable. The resulting code is suitable for applications, but not shared
libraries. (APPLE ONLY)
-mlong-branch
On Darwin and Mac OS X systems, compile calls to use a 32-bit destination address. This is to
support kernel extensions, which may load anywhere within the kernel address space. (APPLE ONLY)
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling conventions that adheres to
the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement.
This is the default unless you configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aix
On System V.4 and embedded PowerPC systems compile code using calling conventions that are
similar to those used on AIX. This is the default if you configured GCC using powerpc-*-eabiaix.
-mcall-solaris
On System V.4 and embedded PowerPC systems compile code for the Solaris operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.
-mcall-gnu
On System V.4 and embedded PowerPC systems compile code for the Hurd-based GNU system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).
-mabi=altivec
Extend the current ABI with AltiVec ABI extensions. This does not change the default ABI,
instead it adds the AltiVec ABI extensions to the current ABI.
This option is effectively permanently enabled on Mac OS X.
-mabi=no-altivec
Disable AltiVec ABI extensions for the current ABI.
This option will not work on Mac OS X.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions
are properly prototyped. Otherwise, the compiler must insert an instruction before every non
prototyped call to set or clear bit 6 of the condition code register (CR) to indicate whether
floating point values were passed in the floating point registers in case the function takes a
variable arguments. With -mprototype, only calls to prototyped variable argument functions will
set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the
standard C libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim.
configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C
libraries are libmvme.a and libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C
libraries are libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C
libraries are libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.
-mwindiss
Specify that you are compiling for the WindISS simulation environment.
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that eabi
extended relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary
Interface (eabi) which is a set of modifications to the System V.4 specifications. Selecting
-meabi means that the stack is aligned to an 8 byte boundary, a function "__eabi" is called to
from "main" to set up the eabi environment, and the -msdata option can use both "r2" and "r13" to
point to two separate small data areas. Selecting -mno-eabi means that the stack is aligned to a
16 byte boundary, do not call an initialization function from "main", and the -msdata option will
only use "r13" to point to a single small data area. The -meabi option is on by default if you
configured GCC using one of the powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized "const" global and static data
in the .sdata2 section, which is pointed to by register "r2". Put small initialized non-"const"
global and static data in the .sdata section, which is pointed to by register "r13". Put small
uninitialized global and static data in the .sbss section, which is adjacent to the .sdata
section. The -msdata=eabi option is incompatible with the -mrelocatable option. The
-msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata
section, which is pointed to by register "r13". Put small uninitialized global and static data
in the .sbss section, which is adjacent to the .sdata section. The -msdata=sysv option is
incompatible with the -mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as
-msdata=eabi, otherwise compile code the same as -msdata=sysv.
-msdata-data
On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata
section. Put small uninitialized global and static data in the .sbss section. Do not use
register "r13" to address small data however. This is the default behavior unless other -msdata
options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data in the .data section, and
all uninitialized data in the .bss section.
-G num
On embedded PowerPC systems, put global and static items less than or equal to num bytes into the
small data or bss sections instead of the normal data or bss section. By default, num is 8. The
-G num switch is also passed to the linker. All modules should be compiled with the same -G num
value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly
language output using symbolic forms.
-mlongcall
-mno-longcall
Default to making all function calls via pointers, so that functions which reside further than 64
megabytes (67,108,864 bytes) from the current location can be called. This setting can be
overridden by the "shortcall" function attribute, or by "#pragma longcall(0)".
Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On
these systems, long calls are unnecessary and generate slower code. As of this writing, the AIX
linker can do this, as can the GNU linker for PowerPC/64. It is planned to add this feature to
the GNU linker for 32-bit PowerPC systems as well.
In the future, we may cause GCC to ignore all longcall specifications when the linker is known to
generate glue.
-pthread
Adds support for multithreading with the pthreads library. This option sets flags for both the
preprocessor and linker.
Darwin Options
-all_load
Loads all members of static archive libraries. See man ld(1) for more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all undefined
references when the file is loaded or launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more information.
-bundle_loader executable
This specifies the executable that will be loading the build output file being linked. See man
ld(1) for more information.
-mone-byte-bool
Override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when
compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on
x86.
Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible
with code generated without that switch. Using this switch may require recompiling all other
modules in a program, including system libraries. Use this switch to conform to a non-default
data model.
-allowable_client client_name
-arch_only
-client_name
-compatibility_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_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-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
This options are available for Darwin linker. Darwin linker man page describes them in detail.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of computers:
-mcpu=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set
of available instructions. The choices for cpu-type are i386, i486, i586, i686, pentium,
pentium-mmx, pentiumpro, pentium2, pentium3, pentium4, k6, k6-2, k6-3, athlon, athlon-tbird,
athlon-4, athlon-xp, athlon-mp, winchip-c6, winchip2 and c3.
While picking a specific cpu-type will schedule things appropriately for that particular chip,
the compiler will not generate any code that does not run on the i386 without the -march=cpu-type
option being used. i586 is equivalent to pentium and i686 is equivalent to pentiumpro. k6 and
athlon are the AMD chips as opposed to the Intel ones.
-march=cpu-type
Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as
for -mcpu. Moreover, specifying -march=cpu-type implies -mcpu=cpu-type.
-m386
-m486
-mpentium
-mpentiumpro
These options are synonyms for -mcpu=i386, -mcpu=i486, -mcpu=pentium, and -mcpu=pentiumpro
respectively. These synonyms are deprecated.
-mfpmath=unit
generate floating point arithmetics for selected unit unit. the choices for unit are:
387 Use the standard 387 floating point coprocessor present majority of chips and emulated
otherwise. Code compiled with this option will run almost everywhere. The temporary results
are computed in 80bit precision instead of precision specified by the type resulting in
slightly different results compared to most of other chips. See -ffloat-store for more
detailed description.
This is the default choice for i386 compiler.
sse Use scalar floating point instructions present in the SSE instruction set. This instruction
set is supported by Pentium3 and newer chips, in the AMD line by Athlon-4, Athlon-xp and
Athlon-mp chips. The earlier version of SSE instruction set supports only single precision
arithmetics, thus the double and extended precision arithmetics is still done using 387.
Later version, present only in Pentium4 and the future AMD x86-64 chips supports double
precision arithmetics too.
For i387 you need to use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions
and make this option effective. For x86-64 compiler, these extensions are enabled by
default.
The resulting code should be considerably faster in majority of cases and avoid the numerical
instability problems of 387 code, but may break some existing code that expects temporaries
to be 80bit.
This is the default choice for x86-64 compiler.
sse,387
Attempt to utilize both instruction sets at once. This effectively double the amount of
available registers and on chips with separate execution units for 387 and SSE the execution
resources too. Use this option with care, as it is still experimental, because gcc register
allocator does not model separate functional units well resulting in instable performance.
-masm=dialect
Output asm instructions using selected dialect. Supported choices are intel or att (the default
one).
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating point comparisons. These handle correctly
the case where the result of a comparison is unordered.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries
are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but
this can't be done directly in cross-compilation. You must make your own arrangements to provide
suitable library functions for cross-compilation.
On machines where a function returns floating point results in the 80387 register stack, some
floating point opcodes may be emitted even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types "float" and "double" in an FPU
register, even if there is no FPU. The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers
instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify
this option to avoid generating those instructions. This option is the default on FreeBSD,
OpenBSD and NetBSD. This option is overridden when -march indicates that the target cpu will
always have an FPU and so the instruction will not need emulation. As of revision 2.6.1, these
instructions are not generated unless you also use the -funsafe-math-optimizations switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long" variables on a two word
boundary or a one word boundary. Aligning "double" variables on a two word boundary will produce
code that runs somewhat faster on a Pentium at the expense of more memory.
Warning: if you use the -malign-double switch, structures containing the above types will be
aligned differently than the published application binary interface specifications for the 386
and will not be binary compatible with structures in code compiled without that switch.
-m128bit-long-double
Control the size of "long double" type. i386 application binary interface specify the size to be
12 bytes, while modern architectures (Pentium and newer) prefer "long double" aligned to 8 or 16
byte boundary. This is impossible to reach with 12 byte long doubles in the array accesses.
Warning: if you use the -m128bit-long-double switch, the structures and arrays containing "long
double" will change their size as well as function calling convention for function taking "long
double" will be modified.
-m96bit-long-double
Set the size of "long double" to 96 bits as required by the i386 application binary interface.
This is the default.
-msvr3-shlib
-mno-svr3-shlib
Control whether GCC places uninitialized local variables into the "bss" or "data" segments.
-msvr3-shlib places them into "bss". These options are meaningful only on System V Release 3.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of
arguments return with the "ret" num instruction, which pops their arguments while returning.
This saves one instruction in the caller since there is no need to pop the arguments there.
You can specify that an individual function is called with this calling sequence with the
function attribute stdcall. You can also override the -mrtd option by using the function
attribute cdecl.
Warning: this calling convention is incompatible with the one normally used on Unix, so you
cannot use it if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of
arguments (including "printf"); otherwise incorrect code will be generated for calls to those
functions.
In addition, seriously incorrect code will result if you call a function with too many arguments.
(Normally, extra arguments are harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments. By default, no registers are used
to pass arguments, and at most 3 registers can be used. You can control this behavior for a
specific function by using the function attribute regparm.
Warning: if you use this switch, and num is nonzero, then you must build all modules with the
same value, including any libraries. This includes the system libraries and startup modules.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If
-mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits), except when
optimizing for code size (-Os), in which case the default is the minimum correct alignment (4
bytes for x86, and 8 bytes for x86-64).
On Pentium and PentiumPro, "double" and "long double" values should be aligned to an 8 byte
boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium
III, the Streaming SIMD Extension (SSE) data type "__m128" suffers similar penalties if it is not
16 byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as
that required by any value stored on the stack. Further, every function must be generated such
that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack
boundary from a function compiled with a lower preferred stack boundary will most likely misalign
the stack. It is recommended that libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space, and generally increases code size. Code
that is sensitive to stack space usage, such as embedded systems and operating system kernels,
may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.
-mmmx
-mno-mmx
-msse
-mno-sse
-msse2
-mno-sse2
-m3dnow
-mno-3dnow
These switches enable or disable the use of built-in functions that allow direct access to the
MMX, SSE and 3Dnow extensions of the instruction set.
To have SSE/SSE2 instructions generated automatically from floating-point code, see -mfpmath=sse.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is shorter and usually equally
fast as method using SUB/MOV operations and is enabled by default. In some cases disabling it
may improve performance because of improved scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing arguments will be computed in the
function prologue. This is faster on most modern CPUs because of reduced dependencies, improved
scheduling and reduced stack usage when preferred stack boundary is not equal to 2. The drawback
is a notable increase in code size. This switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on Mingw32. Code that relies on thread-safe exception
handling must compile and link all code with the -mthreads option. When compiling, -mthreads
defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans
up per thread exception handling data.
-mno-align-stringops
Do not align destination of inlined string operations. This switch reduces code size and
improves performance in case the destination is already aligned, but gcc don't know about it.
-minline-all-stringops
By default GCC inlines string operations only when destination is known to be aligned at least to
4 byte boundary. This enables more inlining, increase code size, but may improve performance of
code that depends on fast memcpy, strlen and memset for short lengths.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to
save, set up and restore frame pointers and makes an extra register available in leaf functions.
The option -fomit-frame-pointer removes the frame pointer for all functions which might make
debugging harder.
These -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit
environments.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and
pointer to 32 bits and generates code that runs on any i386 system. The 64-bit environment sets
int to 32 bits and long and pointer to 64 bits and generates code for AMD's x86-64 architecture.
-mno-red-zone
Do not use a so called red zone for x86-64 code. The red zone is mandated by the x86-64 ABI, it
is a 128-byte area beyond the location of the stack pointer that will not be modified by signal
or interrupt handlers and therefore can be used for temporary data without adjusting the stack
pointer. The flag -mno-red-zone disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols must be linked in the lower 2
GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically
linked. This is the default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address
space. This model has to be used for Linux kernel code.
-mcmodel=medium
Generate code for the medium model: The program is linked in the lower 2 GB of the address space
but symbols can be located anywhere in the address space. Programs can be statically or
dynamically linked, but building of shared libraries are not supported with the medium model.
-mcmodel=large
Generate code for the large model: This model makes no assumptions about addresses and sizes of
sections. Currently GCC does not implement this model.
Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In
the table below, only one of the forms is listed---the one which is not the default. You can figure
out the other form by either removing no- or adding it.
-fbounds-check
For front-ends that support it, generate additional code to check that indices used to access
arrays are within the declared range. This is currently only supported by the Java and Fortran
77 front-ends, where this option defaults to true and false respectively.
-ftrapv
This option generates traps for signed overflow on addition, subtraction, multiplication
operations.
-fexceptions
Enable exception handling. Generates extra code needed to propagate exceptions. For some
targets, this implies GCC will generate frame unwind information for all functions, which can
produce significant data size overhead, although it does not affect execution. If you do not
specify this option, GCC will enable it by default for languages like C++ which normally require
exception handling, and disable it for languages like C that do not normally require it.
However, you may need to enable this option when compiling C code that needs to interoperate
properly with exception handlers written in C++. You may also wish to disable this option if you
are compiling older C++ programs that don't use exception handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw exceptions. Note that this requires
platform-specific runtime support that does not exist everywhere. Moreover, it only allows
trapping instructions to throw exceptions, i.e. memory references or floating point instructions.
It does not allow exceptions to be thrown from arbitrary signal handlers such as "SIGALRM".
-funwind-tables
Similar to -fexceptions, except that it will just generate any needed static data, but will not
affect the generated code in any other way. You will normally not enable this option; instead, a
language processor that needs this handling would enable it on your behalf.
-fasynchronous-unwind-tables
Generate unwind table in dwarf2 format, if supported by target machine. The table is exact at
each instruction boundary, so it can be used for stack unwinding from asynchronous events (such
as debugger or garbage collector).
-fpcc-struct-return
Return ``short'' "struct" and "union" values in memory like longer ones, rather than in
registers. This convention is less efficient, but it has the advantage of allowing
intercallability between GCC-compiled files and files compiled with other compilers, particularly
the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration
macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code
compiled with the -freg-struct-return switch. Use it to conform to a non-default application
binary interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible. This is more efficient for small
structures than -fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever
convention is standard for the target. If there is no standard convention, GCC defaults to
-fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we
can choose the standard, and we chose the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not binary compatible with code
compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application
binary interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the declared range of possible
values. Specifically, the "enum" type will be equivalent to the smallest integer type which has
enough room.
Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with
code generated without that switch. Use it to conform to a non-default application binary
interface.
-fshort-double
Use the same size for "double" as for "float".
Warning: the -fshort-double switch causes GCC to generate code that is not binary compatible with
code generated without that switch. Use it to conform to a non-default application binary
interface.
-fshort-wchar
Override the underlying type for wchar_t to be short unsigned int instead of the default for the
target. This option is useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with
code generated without that switch. Use it to conform to a non-default application binary
interface.
-fshared-data
Requests that the data and non-"const" variables of this compilation be shared data rather than
private data. The distinction makes sense only on certain operating systems, where shared data
is shared between processes running the same program, while private data exists in one copy per
process.
-fno-common
In C, allocate even uninitialized global variables in the data section of the object file, rather
than generating them as common blocks. This has the effect that if the same variable is declared
(without "extern") in two different compilations, you will get an error when you link them. The
only reason this might be useful is if you wish to verify that the program will work on other
systems which always work this way.
-fno-ident
Ignore the #ident directive.
-fno-gnu-linker
Do not output global initializations (such as C++ constructors and destructors) in the form used
by the GNU linker (on systems where the GNU linker is the standard method of handling them). Use
this option when you want to use a non-GNU linker, which also requires using the collect2 program
to make sure the system linker includes constructors and destructors. (collect2 is included in
the GCC distribution.) For systems which must use collect2, the compiler driver gcc is
configured to do this automatically.
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that would cause trouble if the
function is split in the middle, and the two halves are placed at locations far apart in memory.
This option is used when compiling crtstuff.c; you should not need to use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to make it more readable. This
option is generally only of use to those who actually need to read the generated assembly code
(perhaps while debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when
comparing two assembler files.
-fvolatile
Consider all memory references through pointers to be volatile.
-fvolatile-global
Consider all memory references to extern and global data items to be volatile. GCC does not
consider static data items to be volatile because of this switch.
-fvolatile-static
Consider all memory references to static data to be volatile.
-fpic
Generate position-independent code (PIC) suitable for use in a shared library, if supported for
the target machine. Such code accesses all constant addresses through a global offset table
(GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader
is not part of GCC; it is part of the operating system). If the GOT size for the linked
executable exceeds a machine-specific maximum size, you get an error message from the linker
indicating that -fpic does not work; in that case, recompile with -fPIC instead. (These maximums
are 16k on the m88k, 8k on the SPARC, and 32k on the m68k and RS/6000. The 386 has no such
limit.)
Position-independent code requires special support, and therefore works only on certain machines.
For the 386, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM
RS/6000 is always position-independent.
-fpic is not supported on Mac OS X.
-fPIC
If supported for the target machine, emit position-independent code, suitable for dynamic linking
and avoiding any limit on the size of the global offset table. This option makes a difference on
the m68k, m88k, and the SPARC.
Position-independent code requires special support, and therefore works only on certain machines.
-fPIC is the default on Darwin and Mac OS X.
-ffixed-reg
Treat the register named reg as a fixed register; generated code should never refer to it (except
perhaps as a stack pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are machine-specific and are
defined in the "REGISTER_NAMES" macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is clobbered by function calls. It
may be allocated for temporaries or variables that do not live across a call. Functions compiled
this way will not save and restore the register reg.
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for
other registers that have fixed pervasive roles in the machine's execution model will produce
disastrous results.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by functions. It may be allocated
even for temporaries or variables that live across a call. Functions compiled this way will save
and restore the register reg if they use it.
It is an error to used this flag with the frame pointer or stack pointer. Use of this flag for
other registers that have fixed pervasive roles in the machine's execution model will produce
disastrous results.
A different sort of disaster will result from the use of this flag for a register in which
function values may be returned.
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct
Pack all structure members together without holes.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with
code generated without that switch. Additionally, it makes the code suboptimal. Use it to
conform to a non-default application binary interface.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just after function entry and
just before function exit, the following profiling functions will be called with the address of
the current function and its call site. (On some platforms, "__builtin_return_address" does not
work beyond the current function, so the call site information may not be available to the
profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function, which may be looked up
exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other functions. The
profiling calls will indicate where, conceptually, the inline function is entered and exited.
This means that addressable versions of such functions must be available. If all your uses of a
function are expanded inline, this may mean an additional expansion of code size. If you use
extern inline in your C code, an addressable version of such functions must be provided. (This
is normally the case anyways, but if you get lucky and the optimizer always expands the functions
inline, you might have gotten away without providing static copies.)
A function may be given the attribute "no_instrument_function", in which case this
instrumentation will not be done. This can be used, for example, for the profiling functions
listed above, high-priority interrupt routines, and any functions from which the profiling
functions cannot safely be called (perhaps signal handlers, if the profiling routines generate
output or allocate memory).
-fstack-check
Generate code to verify that you do not go beyond the boundary of the stack. You should specify
this flag if you are running in an environment with multiple threads, but only rarely need to
specify it in a single-threaded environment since stack overflow is automatically detected on
nearly all systems if there is only one stack.
Note that this switch does not actually cause checking to be done; the operating system must do
that. The switch causes generation of code to ensure that the operating system sees the stack
being extended.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain value, either the value of
a register or the address of a symbol. If the stack would grow beyond the value, a signal is
raised. For most targets, the signal is raised before the stack overruns the boundary, so it is
possible to catch the signal without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use
the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce
a stack limit of 128KB. Note that this may only work with the GNU linker.
-fargument-alias
-fargument-noalias
-fargument-noalias-global
Specify the possible relationships among parameters and between parameters and global data.
-fargument-alias specifies that arguments (parameters) may alias each other and may alias global
storage.-fargument-noalias specifies that arguments do not alias each other, but may alias global
storage.-fargument-noalias-global specifies that arguments do not alias each other and do not
alias global storage.
Each language will automatically use whatever option is required by the language standard. You
should not need to use these options yourself.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are
represented in the object file. One use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code that is not binary
compatible with code generated without that switch. Use it to conform to a non-default
application binary interface. Not all targets provide complete support for this switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model argument should be one of
"global-dynamic", "local-dynamic", "initial-exec" or "local-exec".
The default without -fpic is "initial-exec"; with -fpic the default is "global-dynamic".
ENVIRONMENT
This section describes several environment variables that affect how GCC operates. Some of them work
by specifying directories or prefixes to use when searching for various kinds of files. Some are
used to specify other aspects of the compilation environment.
Note that you can also specify places to search using options such as -B, -I and -L. These take
precedence over places specified using environment variables, which in turn take precedence over
those specified by the configuration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses localization information that allow GCC
to work with different national conventions. GCC inspects the locale categories LC_CTYPE and
LC_MESSAGES if it has been configured to do so. These locale categories can be set to any value
supported by your installation. A typical value is en_UK for English in the United Kingdom.
The LC_CTYPE environment variable specifies character classification. GCC uses it to determine
the character boundaries in a string; this is needed for some multibyte encodings that contain
quote and escape characters that would otherwise be interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES;
otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If
none of these variables are set, GCC defaults to traditional C English behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary
files to hold the output of one stage of compilation which is to be used as input to the next
stage: for example, the output of the preprocessor, which is the input to the compiler proper.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed
by the compiler. No slash is added when this prefix is combined with the name of a subprogram,
but you can specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an appropriate prefix to use based
on the pathname it was invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual
places for the subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc-lib/ where prefix is the value of "prefix"
when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are used for linking.
In addition, the prefix is used in an unusual way in finding the directories to search for header
files. For each of the standard directories whose name normally begins with
/usr/local/lib/gcc-lib (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing
that beginning with the specified prefix to produce an alternate directory name. Thus, with
-Bfoo/, GCC will search foo/bar where it would normally search /usr/local/lib/bar. These
alternate directories are searched first; the standard directories come next.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries
the directories thus specified when searching for subprograms, if it can't find the subprograms
using GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When
configured as a native compiler, GCC tries the directories thus specified when searching for
special linker files, if it can't find them using GCC_EXEC_PREFIX. Linking using GCC also uses
these directories when searching for ordinary libraries for the -l option (but directories
specified with -L come first).
LANG
This variable is used to pass locale information to the compiler. One way in which this
information is used is to determine the character set to be used when character literals, string
literals and comments are parsed in C and C++. When the compiler is configured to allow
multibyte characters, the following values for LANG are recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the compiler will use mblen and
mbtowc as defined by the default locale to recognize and translate multibyte characters.
Some additional environments variables affect the behavior of the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special character, much like PATH,
in which to look for header files. The special character, "PATH_SEPARATOR", is target-dependent
and determined at GCC build time. For Windows-based targets it is a semicolon, and for almost
all other targets it is a colon.
CPATH specifies a list of directories to be searched as if specified with -I, but after any paths
given with -I options on the command line. This environment variable is used regardless of which
language is being preprocessed.
The remaining environment variables apply only when preprocessing the particular language
indicated. Each specifies a list of directories to be searched as if specified with -isystem,
but after any paths given with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to search its current working
directory. Empty elements can appear at the beginning or end of a path. For instance, if the
value of CPATH is ":/special/include", that has the same effect as -I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies for Make based on the
non-system header files processed by the compiler. System header files are ignored in the
dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are
written to that file, guessing the target name from the source file name. Or the value can have
the form file target, in which case the rules are written to file file using target as the target
name.
In other words, this environment variable is equivalent to combining the options -MM and -MF,
with an optional -MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are
not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is
omitted.
BUGS
To report bugs to Apple, see <http://developer.apple.com/bugreporter.
FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On
multi-libbed systems, gcc -shared must select the correct support libraries to link against.
Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where
they are not necessary is innocuous.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), g77(1), as(1), ld(1), gdb(1), adb(1), dbx(1),
sdb(1), gcc_select(1) and the Info entries for gcc, cpp, g77, as, ld, binutils and gdb.
AUTHOR
See the Info entry for gcc, or <http://gcc.gnu.org/onlinedocs/gcc/Contributors.html, for
contributors to GCC.
COPYRIGHT
Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003 Free
Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free
Documentation License, Version 1.2 or any later version published by the Free Software Foundation;
with the Invariant Sections being ``GNU General Public License'' and ``Funding Free Software'', the
Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy
of the license is included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
gcc-3.3 2007-09-23 GCC(1)
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