GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below
for the remainder. g++ accepts mostly the same options as
gcc.
In Apple's version of GCC, both cc and gcc are actually
symbolic links to gcc3, while c++ and g++ are links to
g++3.
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 cur-
rently 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, com-
pilation, 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 com-
piler 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 exam-
ple, 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 lan-
guage -ObjC (APPLE ONLY) -ObjC++ (APPLE ONLY) -arch
arch (APPLE ONLY) -v -### --target-help --help
C Language Options
-ansi -std=standard -aux-info filename -faltivec
(APPLE ONLY) -fno-asm -fno-builtin -fno-builtin-func-
tion -fhosted -ffreestanding -trigraphs -no-inte-
grated-cpp -traditional -traditional-cpp -fallow-
single-precision -fcond-mismatch -fconstant-cfstrings
(APPLE ONLY) -fsigned-bitfields -fsigned-char -fun-
signed-bitfields -funsigned-char -fwritable-strings
-fshort-wchar -fpascal-strings (APPLE ONLY) -fcoalesce
(APPLE ONLY) -fweak-coalesced (APPLE ONLY) -Wno-#warn-
ings (APPLE ONLY) -Wextra-tokens (APPLE ONLY)
-Wpragma-once (APPLE ONLY) -Wnewline-eof (APPLE ONLY)
-Wno-altivec-long-deprecated (APPLE ONLY)
C++ Language Options
-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 -Wctor-dtor-privacy -Wnon-virtual-dtor -Wre-
order -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 -gen-decls -Wno-protocol -Wselector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-loca-
tion=[once|every-line]
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -W
-Wall -Waggregate-return -Wcast-align -Wcast-qual
-Wchar-subscripts -Wcomment -Wconversion -Wno-depre-
cated-declarations -Wdisabled-optimization -Wdiv-by-
zero -Werror -Wfloat-equal -Wformat -Wformat=2
-Wformat-nonliteral -Wformat-security -Wimplicit
-Wimplicit-int -Wimplicit-function-declaration -Wer-
ror-implicit-function-declaration -Wimport -Winline
-Wlarger-than-len -Wno-long-double (APPLE ONLY)
-Wlong-long -Wmain -Wmissing-braces -Wmissing-
declarations -Wmissing-format-attribute -Wmissing-
noreturn -Wmost (APPLE ONLY) -Wmultichar -Wno-format-
extra-args -Wno-format-y2k -Wno-import -Wpacked
-Wpadded -Wparentheses -Wpointer-arith -Wredundant-
decls -Wreturn-type -Wsequence-point -Wshadow
-Wsign-compare -Wswitch -Wsystem-headers -Wtrigraphs
-Wundef -Wuninitialized -Wunknown-pragmas -Wunreach-
able-code -Wunused -Wunused-function -Wunused-label
-Wunused-parameter -Wunused-value -Wunused-variable
-Wwrite-strings
C-only Warning Options
-Wbad-function-cast -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] -fmem-
report -fpretend-float -fprofile-arcs -ftest-cover-
age -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
Optimization Options
-falign-functions=n -falign-jumps=n -falign-labels=n
-falign-loops=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 -ffunc-
tion-sections -fgcse -fgcse-lm -fgcse-sm -finline-
functions -finline-limit=n -fkeep-inline-functions
-fkeep-static-consts -fmerge-constants -fmerge-all-
constants -fmove-all-movables -fno-default-inline
-fno-defer-pop -fno-function-cse -fno-guess-branch-
probability -fno-inline -fno-math-errno -fno-peep-
hole -fno-peephole2 -funsafe-math-optimizations -fno-
trapping-math -fomit-frame-pointer -foptimize-regis-
ter-move -foptimize-sibling-calls -fprefetch-loop-
arrays -freduce-all-givs -fregmove -frename-registers
-frerun-cse-after-loop -frerun-loop-opt -fschedule-
insns -fschedule-insns2 -fsingle-precision-constant
-fssa -fssa-ccp -fssa-dce -fstrength-reduce -fstrict-
aliasing -fthread-jumps -ftrapv -funroll-all-loops
-funroll-loops --param name=value -O -O0 -O1 -O2
-O3 -Os
Preprocessor Options
-$ -Aquestion=answer -A-question[=answer] -C -dD
-dI -dM -dN -Dmacro[=defn] -E -H -idirafter dir
-include file -imacros file -iprefix file -iwithpre-
fix dir -iwithprefixbefore dir -isystem dir -M -MM
-MF -MG -MP -MQ -MT -nostdinc -P -remap -depen-
dency-file (APPLE ONLY) -no-cpp-precomp (APPLE ONLY)
--dump-pch name (APPLE ONLY) --load-pch name (APPLE
ONLY) -trigraphs -undef -Umacro -Wp,option
Assembler Option
-Wa,option
Linker Options
object-file-name -llibrary -nostartfiles -node-
faultlibs -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
-b machine -V version
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 -mmini-
mal-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 -mlit-
tle-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
-mprototype -mno-prototype -msim -mmvme -mads
-myellowknife -memb -msdata -msdata=opt -mvxworks -G
num -pthread
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 -msse-math -m3dnow -mthreads -mno-align-
stringops -minline-all-stringops -mpush-args -maccu-
mulate-outgoing-args -m128bit-long-double -m96bit-
long-double -mregparm=num -momit-leaf-frame-pointer
-mno-red-zone -m32 -m64
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexcep-
tions -fnon-call-exceptions -funwind-tables -fasyn-
chronous-unwind-tables -finhibit-size-directive -fin-
strument-functions -fno-common -fno-ident -fno-gnu-
linker -fpcc-struct-return -fpic -fPIC -freg-struct-
return -fshared-data -fshort-enums -fshort-double
-fvolatile -fvolatile-global -fvolatile-static -fver-
bose-asm -fpack-struct -fstack-check -fstack-limit-
register=reg -fstack-limit-symbol=sym -fargument-
alias -fargument-noalias -fargument-noalias-global
-fleading-underscore
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 prepro-
cessed.
file.h
C header file (not to be compiled or linked).
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 prepro-
cessed. (APPLE ONLY)
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 sub-
program 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 pre-
processor 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++-cpp-output
objective-c objc-cpp-output
objective-c++ (APPLE ONLY)
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
java
-x none
Turn off any specification of a language, so that sub-
sequent files are handled according to their file name
suffixes (as they are if -x has not been used at all).
-ObjC
-ObjC++
These are similar in effect to -x objective-c and -x
objective-c++, but affect only the choice of compiler
for files already identified as source files. (APPLE
ONLY)
-arch arch
Compile for the specified target architecture arch.
The allowable values are i386 and ppc. Multiple
options work, and direct the compiler to produce
``fat'' binaries including object code for each archi-
tecture 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
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 prepro-
cessed source code, which is sent to the standard out-
put.
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 exe-
cutable file in a.out, the object file for source.suf-
fix 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 ver-
sion 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 communica-
tion 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 com-
mand line options which have no documentation associ-
ated with them will also be displayed.
--target-help
Print (on the standard output) a description of target
specific command line options for each tool.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes
.C, .cc, .cpp, .c++, .cp, or .cxx; 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 pro-
grams from standard input, or otherwise without a suffix
that flags them as C++ programs. g++ is a program that
calls GCC with the default language set to C++, and auto-
matically 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 pro-
grams in any language; or command-line options meaningful
for C and related languages; or options that are meaning-
ful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or lan-
guages derived from C, such as C++ and Objective-C) that
the compiler accepts:
-ansi
In C mode, support all ISO C89 programs. In C++ mode,
remove GNU extensions that conflict with ISO C++.
This turns off certain features of GCC that are incom-
patible with ISO C89 (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 com-
piler, it disables recognition of C++ style // com-
ments 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 func-
tions 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 cur-
rently only supported when compiling C. A value for
this option must be provided; possible values are
c89
iso9899:1990
ISO C89 (same as -ansi).
iso9899:199409
ISO C89 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-3.1/c99sta-
tus.html> for more information. The names c9x and
iso9899:199x are deprecated.
gnu89
Default, ISO C89 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.
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 C89 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 trans-
lation 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 declara-
tions 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) key-
words, 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)
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a key-
word, 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 key-
word in ISO C99.
-fno-builtin
-fno-builtin-function (C and Objective-C only)
Don't recognize built-in functions that do not begin
with __builtin_ as prefix.
GCC normally generates special code to handle certain
built-in functions more efficiently; for instance,
calls to "alloca" may become single instructions that
adjust the stack directly, and calls to "memcpy" may
become inline copy loops. The resulting code is often
both smaller and faster, but since the function calls
no longer appear as such, you cannot set a breakpoint
on those calls, nor can you change the behavior of the
functions by linking with a different library.
In C++, -fno-builtin is always in effect. The
-fbuiltin option has no effect. Therefore, in C++,
the only way to get the optimization benefits of
built-in functions is to call the function using the
__builtin_ prefix. The GNU C++ Standard Library uses
built-in functions to implement many functions (like
"std::strchr"), so that you automatically get effi-
cient code.
With the -fno-builtin-function option, not available
when compiling C++, only the built-in function func-
tion 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 envi-
ronment. This implies -fbuiltin. A hosted environ-
ment 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 ker-
nel. This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation takes place in a freestanding
environment. This implies -fno-builtin. A freestand-
ing 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 ker-
nel. This is equivalent to -fno-hosted.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std
options for strict ISO C conformance) implies -tri-
graphs.
-no-integrated-cpp
Invoke the external cpp during compilation. The
default is to use the integrated cpp (internal cpp).
This option also allows a user-supplied cpp via the -B
option. This flag is applicable in both C and C++
modes.
We do not guarantee to retain this option in future,
and we may change its semantics.
-no-cpp-precomp
By default, Apple's GCC preprocesses C and Objective-C
with a special preprocessor called cpp-precomp that
supports precompiled headers. This preprocessor can-
not always handle every construct that GCC supports;
use -no-cpp-precomp to switch to using GNU cpp
instead. (APPLE ONLY)
--dump-pch name
Dump the state of the compiler into a directory named
name, after processing all the other arguments. This
is useful for creating precompiled headers. (APPLE
ONLY)
--load-pch name
Restore the state of the compiler from the directory
name before processing the other arguments. The net
effect is similar to -include, but it happens much
more quickly. (APPLE ONLY)
So for instance if the file myprefix.c #includes vari-
ous headers that are useful to all files in your pro-
gram, you can do
gcc --dump-pch foo -c myprefix.c
gcc --load-pch foo myfile1.c
gcc --load-pch foo myfile2.c
gcc --load-pch foo myfile2.c
...
-traditional
Attempt to support some aspects of traditional C com-
pilers. Specifically:
o All "extern" declarations take effect globally
even if they are written inside of a function def-
inition. This includes implicit declarations of
functions.
o The newer keywords "typeof", "inline", "signed",
"const" and "volatile" are not recognized. (You
can still use the alternative keywords such as
"__typeof__", "__inline__", and so on.)
o Comparisons between pointers and integers are
always allowed.
o Integer types "unsigned short" and "unsigned char"
promote to "unsigned int".
o Out-of-range floating point literals are not an
error.
o Certain constructs which ISO regards as a single
invalid preprocessing number, such as 0xe-0xd, are
treated as expressions instead.
o String ``constants'' are not necessarily constant;
they are stored in writable space, and identical
looking constants are allocated separately. (This
is the same as the effect of -fwritable-strings.)
o All automatic variables not declared "register"
are preserved by "longjmp". Ordinarily, GNU C
follows ISO C: automatic variables not declared
"volatile" may be clobbered.
o The character escape sequences \x and \a evaluate
as the literal characters x and a respectively.
Without -traditional, \x is a prefix for the hex-
adecimal representation of a character, and \a
produces a bell.
This option is deprecated and may be removed.
You may wish to use -fno-builtin as well as -tradi-
tional if your program uses names that are normally
GNU C built-in functions for other purposes of its
own.
You cannot use -traditional if you include any header
files that rely on ISO C features. Some vendors are
starting to ship systems with ISO C header files and
you cannot use -traditional on such systems to compile
files that include any system headers.
The -traditional option also enables -traditional-cpp.
-traditional-cpp
In Apple's version of GCC, this means to use GNU cpp
instead of cpp-precomp to preprocess. This meaning of
the option is deprecated, and will eventually revert
to its standard meaning.
-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++.
-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. Like-
wise, 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 inte-
ger types such as "int" are signed types.
However, when -traditional is used, bit-fields are all
unsigned no matter what.
-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. The option -traditional also has
this effect.
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. (APPLE ONLY)
-fallow-single-precision
Do not promote single precision math operations to
double precision, even when compiling with -tradi-
tional.
Traditional K&R C promotes all floating point opera-
tions to double precision, regardless of the sizes of
the operands. On the architecture for which you are
compiling, single precision may be faster than double
precision. If you must use -traditional, but want to
use single precision operations when the operands are
single precision, use this option. This option has
no effect when compiling with ISO or GNU C conventions
(the default).
-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.
-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 lan-
guage 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 lan-
guage supported by GCC.
Here is a list of options that are only for compiling C++
programs:
-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 allo-
cated. The current Working Paper requires that "oper-
ator 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 throw(), G++ will check the return
value. See also new (nothrow).
-fconserve-space
Put uninitialized or runtime-initialized global vari-
ables 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 pro-
hibit use of $ with the option -fno-dollars-in-identi-
fiers. (GNU C allows $ by default on most target sys-
tems, 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 cre-
ating 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 defi-
nition.
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 opti-
mization will need the same set of explicit instantia-
tions.
-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 built using older compilers, and is required
to build any Darwin kernel extension. (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 excep-
tion handling uses the same information, but it will
generate it as needed.
-fstats
Emit statistics about front-end processing at the end
of the compilation. This information is generally
only useful to the G++ development team.
-ftemplate-depth-n
Set the maximum instantiation depth for template
classes to n. A limit on the template instantiation
depth is needed to detect endless recursions during
template class instantiation. ANSI/ISO C++ conforming
programs must not rely on a maximum depth greater than
17.
-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 destruc-
tors, 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 func-
tion 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 direc-
tories 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 genera-
tion 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.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable, because all the con-
structors or destructors in a class are private and
the class has no friends or public static member func-
tions.
-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.
-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 initializ-
ers for i and j will be rearranged to match the decla-
ration order of the members.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)
Warn about violations of the following style guide-
lines from Scott Meyers' Effective C++ book:
o Item 11: Define a copy constructor and an assign-
ment operator for classes with dynamically allo-
cated memory.
o Item 12: Prefer initialization to assignment in
constructors.
o Item 14: Make destructors virtual in base
classes.
o Item 15: Have "operator=" return a reference to
"*this".
o 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:
o Item 6: Distinguish between prefix and postfix
forms of increment and decrement operators.
o 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 tem-
platized function. Because this non-conforming behav-
ior 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 confor-
mant compiler code but disables the helpful warning.
-Wold-style-cast (C++ only)
Warn if an old-style (C-style) cast to a non-void type
is used within a C++ program. The new-style casts
(static_cast, 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 con-
version to an unsigned type of the same size. Previ-
ous versions of G++ would try to preserve unsigned-
ness, 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 com-
pile 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 "NXCon-
stantString".
-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.
-gen-decls
Dump interface declarations for all classes seen in
the source file to a file named sourcename.decl.
-Wno-protocol
Do not warn if methods required by a protocol are not
implemented in the class adopting it.
-Wselector
Warn if a selector has multiple methods of different
types defined.
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 loca-
tion 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 pre-
fix) 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 begin-
ning -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 any-
thing beyond that.
-pedantic
Issue all the warnings demanded by strict ISO C and
ISO C++; reject all programs that use forbidden exten-
sions, 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 prop-
erly 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 diag-
nostics have been added.
A feature to report any failure to conform to ISO C
might be useful in some instances, but would require
considerable additional work and would be quite dif-
ferent 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 defini-
tion 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 appears in
a // comment.
-Wformat
Check calls to "printf" and "scanf", etc., to make
sure that the arguments supplied have types appropri-
ate to the format string specified, and that the con-
versions specified in the format string make sense.
This includes standard functions, and others specified
by format attributes, in the "printf", "scanf", "strf-
time" 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 C89 and C99 features, as well as features from the
Single Unix Specification and some BSD and GNU exten-
sions. Other library implementations may not support
all these features; GCC does not support warning about
features that go beyond a particular library's limita-
tions. 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).
-Wformat is included in -Wall. For more control over
some aspects of format checking, the options -Wno-for-
mat-y2k, -Wno-format-extra-args, -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 specifica-
tions, 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.
-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 for-
mat functions that represent possible security prob-
lems. 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 con-
tains %n. (This is currently a subset of what -Wfor-
mat-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.
-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-decla-
ration.
-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 con-
fusion 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 enclos-
ing "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 expres-
sion), 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 func-
tion), 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 modi-
fications to the values of objects take effect. Pro-
grams 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 evalua-
tion 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 dis-
cussions 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 pro-
duces a diagnostic message, even when -Wno-return-type
is specified. The only exceptions are main and func-
tions 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.
-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 all the above -Wunused options combined.
In order to get a warning about an unused function
parameter, you must either specify -W -Wunused or sep-
arately 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 compi-
lation, 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 can-
didates 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 can-
not 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 "nore-
turn".
-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:
-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.
-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.
-Wmost
This is equivalent to -Wall -Wno-parentheses. (APPLE
ONLY)
-Wdiv-by-zero
Warn about compile-time integer division by zero.
This is default. To inhibit the warning messages, use
-Wno-div-by-zero. Floating point division by zero is
not warned about, as it can be a legitimate way of
obtaining infinities and NaNs.
-Wmultichar
Warn if a multicharacter constant ('FOOF') is used.
This is default. To inhibit the warning messages, use
-Wno-multichar. Usually they indicate a typo in the
user's code, as they have implementation-defined val-
ues, and should not be used in portable code.
-Wsystem-headers
Print warning messages for constructs found in system
header files. Warnings from system headers are nor-
mally 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 head-
ers---for that, -Wunknown-pragmas must also be used.
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:
o 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;
}
o 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.
o An unsigned value is compared against zero with <
or <=.
o A comparison like x<=y<=z appears; this is equiva-
lent to (x<=y ? 1 : 0) <= z, which is a different
interpretation from that of ordinary mathematical
notation.
o Storage-class specifiers like "static" are not the
first things in a declaration. According to the C
Standard, this usage is obsolescent.
o 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.)
o If -Wall or -Wunused is also specified, warn about
unused arguments.
o 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.)
o 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 };
o An aggregate has an initializer which does not
initialize all members. For example, the follow-
ing 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 };
-Wfloat-equal
Warn if floating point values are used in equality
comparisons.
The idea behind this is that sometimes it is conve-
nient (for the programmer) to consider floating-point
values as approximations to infinitely precise real
numbers. If you are doing this, then you need to com-
pute (by analysing the code, or in some other way) the
maximum or likely maximum error that the computation
introduces, and allow for it when performing compar-
isons (and when producing output, but that's a differ-
ent problem). In particular, instead of testing for
equality, you would check to see whether the two val-
ues 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 con-
structs that have no traditional C equivalent, and/or
problematic constructs which should be avoided.
o Macro parameters that appear within string liter-
als in the macro body. In traditional C macro
replacement takes place within string literals,
but does not in ISO C.
o 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 tradi-
tional 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 indent-
ing them. Some traditional implementations would
not recognize #elif, so it suggests avoiding it
altogether.
o A function-like macro that appears without argu-
ments.
o The unary plus operator.
o 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 pre-
processor has enough context to avoid warning in
these cases.
o A function declared external in one block and then
used after the end of the block.
o A "switch" statement has an operand of type
"long".
o A non-"static" function declaration follows a
"static" one. This construct is not accepted by
some traditional C compilers.
o The ISO type of an integer constant has a differ-
ent 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 val-
ues, which typically represent bit patterns, are
not warned about.
o Usage of ISO string concatenation is detected.
o Initialization of automatic aggregates.
o Identifier conflicts with labels. Traditional C
lacks a separate namespace for labels.
o 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.
o 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.
-Wundef
Warn if an undefined identifier is evaluated in an #if
directive.
-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-match-
ing 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 dec-
larations 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 conver-
sions 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 also enabled by -W; to get the other warnings of -W
without this warning, use -W -Wno-sign-compare.
-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 previ-
ous 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 previ-
ous 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, other-
wise 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 (pos-
sibly by -Wall).
-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{Vari-
able 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 mis-
aligned 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 substan-
tial code which checks correct functioning of the pro-
gram and is, hopefully, unreachable because the pro-
gram 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-long-double
Inhibit warning if the long double type is used.
(APPLE ONLY)
-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 debug-
ging either your program or GCC:
-g Produce debugging information in the operating sys-
tem'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 vari-
ables you declared may not exist at all; flow of con-
trol may briefly move where you did not expect it;
some statements may not be executed because they com-
pute 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 for-
mat.
-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 pro-
duces 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 sup-
ported 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 mak-
ing backtraces in parts of the program that you don't
plan to debug. This includes descriptions of func-
tions 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 debug-
gers support macro expansion when you use -g3.
-p Generate extra code to write profile information suit-
able 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 suit-
able 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.
-a Generate extra code to write profile information for
basic blocks, which will record the number of times
each basic block is executed, the basic block start
address, and the function name containing the basic
block. If -g is used, the line number and filename of
the start of the basic block will also be recorded.
If not overridden by the machine description, the
default action is to append to the text file bb.out.
This data could be analyzed by a program like "tcov".
Note, however, that the format of the data is not what
"tcov" expects. Eventually GNU "gprof" should be
extended to process this data.
-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 perma-
nent memory allocation when it finishes.
-fprofile-arcs
Instrument arcs during compilation to generate cover-
age data or for profile-directed block ordering. Dur-
ing 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 sourcename.da for each source file.
For profile-directed block ordering, compile the pro-
gram with -fprofile-arcs plus optimization and code
generation options, generate the arc profile informa-
tion by running the program on a selected workload,
and then compile the program again with the same opti-
mization 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. GCC supports two methods of determining code
coverage: the options that support "gcov", and options
-a and -ax, which write information to text files.
The options that support "gcov" do not need to instru-
ment every arc in the program, so a program compiled
with them runs faster than a program compiled with -a,
which adds instrumentation code to every basic block
in the program. The tradeoff: since "gcov" does not
have execution counts for all branches, it must start
with the execution counts for the instrumented
branches, and then iterate over the program flow graph
until the entire graph has been solved. Hence, "gcov"
runs a little more slowly than a program which uses
information from -a and -ax.
With -fprofile-arcs, for each function of your program
GCC creates a program flow graph, then finds a span-
ning tree for the graph. Only arcs that are not on
the spanning tree have to be instrumented: the com-
piler 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.
This option makes it possible to estimate branch prob-
abilities and to calculate basic block execution
counts. In general, basic block execution counts as
provided by -a do not give enough information to esti-
mate all branch probabilities.
-ftest-coverage
Create data files for the "gcov" code-coverage util-
ity. The data file names begin with the name of your
source file:
sourcename.bb
A mapping from basic blocks to line numbers, which
"gcov" uses to associate basic block execution
counts with line numbers.
sourcename.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
"sourcename.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:
sourcename.da
Runtime arc execution counts, used in conjunction
with the arc information in the file "source-
name.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 debug-
ging the compiler. The file names for most of the
dumps are made by appending a pass number and a word
to the source file name (e.g. foo.c.00.rtl or
foo.c.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.29.bbro.
c Dump after instruction combination, to the file
file.16.combine.
C Dump after the first if conversion, to the file
file.17.ce.
d Dump after delayed branch scheduling, to
file.31.dbr.
D Dump all macro definitions, at the end of prepro-
cessing, 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.26.ce2.
f Dump after life analysis, to file.15.life.
F Dump after purging "ADDRESSOF" codes, to
file.09.addressof.
g Dump after global register allocation, to
file.21.greg.
h Dump after finalization of EH handling code, to
file.02.eh.
k Dump after reg-to-stack conversion, to
file.28.stack.
o Dump after post-reload optimizations, to
file.22.postreload.
G Dump after GCSE, to file.10.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.32.stack.
l Dump after local register allocation, to
file.20.lreg.
L Dump after loop optimization, to file.11.loop.
M Dump after performing the machine dependent reor-
ganisation pass, to file.30.mach.
n Dump after register renumbering, to file.25.rnreg.
N Dump after the register move pass, to file.18.reg-
move.
r Dump after RTL generation, to file.00.rtl.
R Dump after the second scheduling pass, to
file.27.sched2.
s Dump after CSE (including the jump optimization
that sometimes follows CSE), to file.08.cse.
S Dump after the first scheduling pass, to
file.19.sched.
t Dump after the second CSE pass (including the jump
optimization that sometimes follows CSE), to
file.12.cse2.
w Dump after the second flow pass, to file.23.flow2.
X Dump after SSA dead code elimination, to
file.06.ssadce.
z Dump after the peephole pass, to file.24.peep-
hole2.
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 indi-
cating 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 anno-
tation.
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 com-
piling it. Usually used with r.
y Dump debugging information during parsing, to
standard error.
-fdump-unnumbered
When doing debugging dumps (see -d option above), sup-
press 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 suf-
fix 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 meaning-
ful will be ignored. The following options are avail-
able
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.
-fpretend-float
When running a cross-compiler, pretend that the target
machine uses the same floating point format as the
host machine. This causes incorrect output of the
actual floating constants, but the actual instruction
sequence will probably be the same as GCC would make
when running on the target machine.
-save-temps
Store the usual ``temporary'' intermediate files per-
manently; 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 nor-
mally 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.
-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{-, with-
out spaces between multiple switches. This is sup-
posed to ease shell-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 -nodefault-
libs 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 direc-
tory 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:
-O
-O1 Optimize. Optimizing compilation takes somewhat more
time, and a lot more memory for a large function.
Without -O, 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.
With -O, the compiler tries to reduce code size and
execution time, without performing any optimizations
that take a great deal of compilation time.
When you specify -O, the compiler turns on -fthread-
jumps and -fdefer-pop on all machines. The compiler
turns on -fdelayed-branch on machines that have delay
slots, and -fomit-frame-pointer on machines that can
support debugging even without a frame pointer. On
some machines the compiler also turns on other flags.
In Apple's version of GCC, -fstrict-aliasing, -fre-
order-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 trade-
off. 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 optional optimizations except for
loop unrolling, function inlining, and register renam-
ing. It also turns on the -fforce-mem option on all
machines and frame pointer elimination on machines
where doing so does not interfere with debugging.
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-func-
tions and -frename-registers options.
-O0 Do not optimize.
-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.
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
which is not the default. You can figure out the other
form by either removing no- or adding it.
-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 mem-
ory.
This option prevents undesirable excess precision on
machines such as the 68000 where the floating regis-
ters (of the 68881) keep more precision than a "dou-
ble" 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.
-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 func-
tions 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.
-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 subexpres-
sions, instruction combination should eliminate the
separate register-load. The -O2 option turns on this
option.
-fforce-addr
Force memory address constants to be copied into reg-
isters before doing arithmetic on them. This may pro-
duce better code just as -fforce-mem may.
-fomit-frame-pointer
Don't keep the frame pointer in a register for func-
tions that don't need one. This avoids the instruc-
tions to save, set up and restore frame pointers; it
also makes an extra register available in many func-
tions. 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 automat-
ically handles the frame pointer and nothing is saved
by pretending it doesn't exist. The machine-descrip-
tion macro "FRAME_POINTER_REQUIRED" controls whether a
target machine supports this flag.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
-ftrapv
This option generates traps for signed overflow on
addition, subtraction, multiplication operations.
-fno-inline
Don't pay attention to the "inline" keyword. Normally
this option is used to keep the compiler from expand-
ing 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.
-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 (ie 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++.
Note: pseudo instruction represents, in this particu-
lar 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 optimiza-
tion 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 refer-
enced, 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) accross compilation
units.
This option is default for optimized compilation if
assembler and linker support it. Use -fno-merge-con-
stants to inhibit this behavior.
-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-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.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, and
-fno-trapping-math.
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 sim-
ilar 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.
-fno-trapping-math
Compile code assuming that floating-point operations
cannot generate user-visible traps. 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.
The following options control specific optimizations. The
-O2 option turns on all of these optimizations except
-funroll-loops and -funroll-all-loops. On most machines,
the -O option turns on the -fthread-jumps and -fdelayed-
branch options, but specific machines may handle it dif-
ferently.
You can use the following flags in the rare cases when
``fine-tuning'' of optimizations to be performed is
desired.
Not all of the optimizations performed by GCC have -f
options to control them.
-fstrength-reduce
Perform the optimizations of loop strength reduction
and elimination of iteration variables.
-fthread-jumps
Perform optimizations where we check to see if a jump
branches to a location where another comparison sub-
sumed 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.
-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.
-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".
-frerun-cse-after-loop
Re-run common subexpression elimination after loop
optimizations has been performed.
-frerun-loop-opt
Run the loop optimizer twice.
-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 elmina-
tion pass by adding -fno-gcse to the command line.
-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.
-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.
-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 opti-
mization for programs which depend on that behavior.
-fexpensive-optimizations
Perform a number of minor optimizations that are rela-
tively expensive.
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instruc-
tions 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. GCC enables this optimization by
default with -O2 or higher.
Note -fregmove and -foptimize-register-move are the
same optimization.
-fdelayed-branch
If supported for the target machine, attempt to
reorder instructions to exploit instruction slots
available after delayed branch instructions.
-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.
-fschedule-insns2
Similar to -fschedule-insns, but requests an addi-
tional pass of instruction scheduling after register
allocation has been done. This is especially useful
on machines with a relatively small number of regis-
ters and where memory load instructions take more than
one cycle.
-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 per-
form 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 optimiza-
tions in the future.
Only use these options when there are significant ben-
efits 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 spec-
ify this option and you may have problems with debug-
ging if you specify both this option and -g.
-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
registers to use instead.
For all machines, optimization level 2 and higher
enables this flag by default.
-funroll-loops
Unroll loops whose number of iterations can be deter-
mined 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 instruc-
tions to prefetch memory to improve the performance of
loops that access large arrays.
-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 vari-
ous approaches to improving loop optimizations.
Please let us (<gcc@gcc.gnu.org> and <for-
tran@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-peep-
hole2 is in how they are implemented in the compiler;
some targets use one, some use the other, a few use
both.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs,
you can compile it a second time using -fbranch-proba-
bilities, to improve optimizations based on the number
of times each branch was taken. When the program com-
piled 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 depen-
dent on the structure of the generated code, so you
must use the same source code and the same optimiza-
tion options for both compilations.
With -fbranch-probabilities, GCC puts a REG_EXEC_COUNT
note on the first instruction of each basic block, and
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.
-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 differ-
ent runs of the compiler to produce code that has dif-
ferent behavior; minimizing non-determinism is of
paramount import. This switch allows users to reduce
non-determinism, possibly at the expense of inferior
optimization.
-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-spe-
cific 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 func-
tion "c_get_alias_set".
-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.
-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.
This option does not work on Mac OS X.
-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.
If n is not specified, use a machine-dependent
default.
This option does not work on Mac OS X.
-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-func-
tions. In this case, no dummy operations need be exe-
cuted.
If n is not specified, use a machine-dependent
default.
This option does not work on Mac OS X.
-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 experi-
mental feature.
-fssa-dce
Perform aggressive dead-code elimination in SSA form.
Requires -fssa. Like -fssa, this is an experimental
feature.
-fsingle-precision-constant
Treat floating point constant as single precision con-
stant instead of implicitly converting it to double
precision constant.
-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 pro-
cessors with lots of registers. It can, however, make
debugging impossible, since variables will no longer
stay in a ``home register''.
-fno-cprop-registers
After register allocation and post-register allocation
instruction splitting, we perform a copy-propagation
pass to try to reduce scheduling dependencies and
occasionally eliminate the copy.
--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 num-
ber 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 com-
pile 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 optimiza-
tion 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
If an function contains more than this many
instructions, it will not be inlined. This option
is precisely equivalent to -finline-limit.
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 prepro-
cessing. 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, trans-
lated or interpreted by the compiler driver before being
passed to the preprocessor, and -Wp forcibly bypasses this
phase. The preprocessor's direct interface is undocu-
mented 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 defini-
tion, 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). Paren-
theses 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 com-
mon 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 direc-
tories.
It is dangerous to specify a standard system include
directory in an -I option. This defeats the special
treatment of system headers . It can also defeat the
repairs to buggy system headers which GCC makes when
it is installed.
-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 -Wtri-
graphs. 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 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 speci-
fied, 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 con-
structs 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.
-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 -pedan-
tic but treats as warnings.
-M Instead of outputting the result of preprocessing,
output a rule suitable for make describing the depen-
dencies 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 out-
put 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.
-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 dou-
ble 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 seman-
tics 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 When used with -M or -MM, -MG says to treat missing
header files as generated files and assume they live
in the same directory as the source file. It sup-
presses preprocessed output, as a missing header file
is ordinarily an error.
This feature is used in automatic updating of make-
files.
-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 suf-
fix 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 compi-
lation process.
-MMD
Like -MD except mention only user header files, not
system -header files.
-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 only knows about the standards for C;
other language standards will be added in the future.
standard may be one of:
""iso9899:1990""
""c89""
The ISO C standard from 1990. c89 is the custom-
ary 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.
-I- Split the include path. Any directories specified
with -I options before -I- are searched only for head-
ers requested with "#include "file""; they are not
searched for "#include <file>". If additional direc-
tories are specified with -I options after the -I-,
those directories are searched for all #include direc-
tives.
In addition, -I- inhibits the use of the directory of
the current file directory as the first search direc-
tory 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 preproces-
sor's working directory instead of the directory con-
taining 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 dec-
larations.
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 -iwithpre-
fix 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 direc-
tories. 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 com-
ments, 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 tok-
enizer 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 cre-
ated 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 dur-
ing the execution of the preprocessor, including
predefined macros. This gives you a way of find-
ing 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 preprocess-
ing. Both kinds of output go to the standard out-
put 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 #.
-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
Try to imitate the behavior of old-fashioned C, as
opposed to ISO C.
-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: [ ] { } # \ ^ | ~
-remap
Enable special code to work around file systems which
only permit very short file names, such as MS-DOS.
-$ Forbid the use of $ in identifiers. The C standard
allows implementations to define extra characters that
can appear in identifiers. By default GNU CPP permits
$, a common extension.
-h
--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.
-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 con-
tents.) 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 rec-
ommended.)
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 spec-
ified. Thus, foo.o -lz bar.o searches library z after
file foo.o but before bar.o. If bar.o refers to func-
tions 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 libli-
brary.a. The linker then uses this file as if it had
been specified precisely by name.
The directories searched include several standard sys-
tem 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 nor-
mally, 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 gener-
ate 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 sup-
plied 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 behav-
ior. (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 lan-
guages.
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 subop-
tions) 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 appli-
cation 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 propa-
gate 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 lan-
guages 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 link-
ing 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, it
will be searched only in the position requested by -I.
Also, it will not be considered a system include
directory. If that directory really does contain sys-
tem headers, there is a good chance that they will
break. For instance, if GCC's installation procedure
edited the headers in /usr/include to fix bugs,
-I/usr/include will cause the original, buggy headers
to be found instead of the corrected ones. GCC will
issue a warning when a system include directory is
hidden in this way.
-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/Frame-
works/, and /Local/Library/Frameworks/, plus any addi-
tional 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 com-
piler 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 pre-
fix.
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 pre-
fix is to use the environment variable GCC_EXEC_PRE-
FIX.
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
By default, GCC compiles code for the same type of machine
that you are using. However, it can also be installed as
a cross-compiler, to compile for some other type of
machine. In fact, several different configurations of
GCC, for different target machines, can be installed side
by side. Then you specify which one to use with the -b
option.
In addition, older and newer versions of GCC can be
installed side by side. One of them (probably the newest)
will be the default, but you may sometimes wish to use
another.
-b machine
The argument machine specifies the target machine for
compilation. This is useful when you have installed
GCC as a cross-compiler.
The value to use for machine is the same as was speci-
fied 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.
When you do not specify -b, it normally means to com-
pile for the same type of machine that you are using.
-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 default version, when you do not specify -V, is
the last version of GCC that you installed.
The -b and -V options actually work by controlling part of
the file name used for the executable files and libraries
used for compilation. A given version of GCC, for a given
target machine, is normally kept in the directory
/usr/local/lib/gcc-lib/machine/version.
Thus, sites can customize the effect of -b or -V either by
changing the names of these directories or adding alter-
nate names (or symbolic links). If in directory
/usr/local/lib/gcc-lib/ the file 80386 is a link to the
file i386v, then -b 80386 becomes an alias for -b i386v.
In one respect, the -b or -V do not completely change to a
different compiler: the top-level driver program gcc that
you originally invoked continues to run and invoke the
other executables (preprocessor, compiler per se, assem-
bler and linker) that do the real work. However, since no
real work is done in the driver program, it usually does
not matter that the driver program in use is not the one
for the specified target. It is common for the interface
to the other executables to change incompatibly between
compiler versions, so unless the version specified is very
close to that of the driver (for example, -V 3.0 with a
driver program from GCC version 3.0.1), use of -V may not
work; for example, using -V 2.95.2 will not work with a
driver program from GCC 3.0.
The only way that the driver program depends on the target
machine is in the parsing and handling of special machine-
specific options. However, this is controlled by a file
which is found, along with the other executables, in the
directory for the specified version and target machine.
As a result, a single installed driver program adapts to
any specified target machine, and sufficiently similar
compiler versions.
The driver program executable does control one significant
thing, however: the default version and target machine.
Therefore, you can install different instances of the
driver program, compiled for different targets or ver-
sions, under different names.
For example, if the driver for version 2.0 is installed as
ogcc and that for version 2.1 is installed as gcc, then
the command gcc will use version 2.1 by default, while
ogcc will use 2.0 by default. However, you can choose
either version with either command with the -V option.
Hardware Models and Configurations
Earlier we discussed the standard option -b which chooses
among different installed compilers for completely differ-
ent 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 speci-
fied.
Some configurations of the compiler also support addi-
tional 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 Pow-
erPC:
-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 Pow-
erPC 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. How-
ever there is a large common subset of instructions
supported by both. An MQ register is included in pro-
cessors 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 con-
figuring 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 instruc-
tions 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 Gen-
eral Purpose group, including floating-point square
root. Specifying -mpowerpc-gfxopt implies -mpowerpc
and also allows GCC to use the optional PowerPC archi-
tecture instructions in the Graphics group, including
floating-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 assem-
bler code. With -mnew-mnemonics, GCC uses the assem-
bler 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 some-
times 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 instruc-
tions 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 pur-
poses.
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-powerc
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 speci-
fied, 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 -fal-
tivec instead.
-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 avail-
able 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 Par-
allel 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 com-
patible 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 multi-
ple 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 Pow-
erPC 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 han-
dled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows
(does not allow) the program to be relocated to a dif-
ferent 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 dif-
ferent 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 refer-
ences 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 pow-
erpc-*-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 config-
ured 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.
-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 incom-
patible 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, other-
wise 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 com-
piled 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.
-pthread
Adds support for multithreading with the pthreads
library. This option sets flags for both the prepro-
cessor and linker.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 fam-
ily of computers:
-mcpu=cpu-type
Tune to cpu-type everything applicable about the gen-
erated code, except for the ABI and the set of avail-
able 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 and athlon-mp.
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 precesion 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 sup-
ports 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 effectivly 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. Sup-
ported 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 val-
ues of types "float" and "double" in an FPU register,
even if there is no FPU. The idea is that the operat-
ing 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 instruc-
tion 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.
-m128bit-long-double
Control the size of "long double" type. i386 applica-
tion 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 vari-
ables 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 argu-
ments 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 com-
piler.
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. (Nor-
mally, 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 cor-
rect 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 perfor-
mance 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 sen-
sitive 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.
-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 per-
formance 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 han-
dling 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 reg-
ister available in leaf functions. The option -fomit-
frame-pointer removes the frame pointer for all func-
tions 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 han-
dlers and therefore can be used for temporary data
without adjusting the stack pointer. The flag -mno-
red-zone disables this red zone.
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.
-fexceptions
Enable exception handling. Generates extra code
needed to propagate exceptions. For some targets,
this implies GCC will generate frame unwind informa-
tion for all functions, which can produce significant
data size overhead, although it does not affect execu-
tion. If you do not specify this option, GCC will
enable it by default for languages like C++ which nor-
mally 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 com-
piling 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 every-
where. Moreover, it only allows trapping instructions
to throw exceptions, i.e. memory references or float-
ing 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 gen-
erate 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 con-
vention is less efficient, but it has the advantage of
allowing intercallability between GCC-compiled files
and files compiled with other compilers.
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.
-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 con-
vention, 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.
-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.
-fshort-double
Use the same size for "double" as for "float".
-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 gen-
erating 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 pro-
gram 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++ con-
structors and destructors) in the form used by the GNU
linker (on systems where the GNU linker is the stan-
dard 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. (col-
lect2 is included in the GCC distribution.) For sys-
tems 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 any-
thing 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 debug-
ging the compiler itself).
-fno-verbose-asm, the default, causes the extra infor-
mation 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 maxi-
mum 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; gen-
erated 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 allo-
cated 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 tem-
poraries or variables that live across a call. Func-
tions 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.
Usually you would not want to use this option, since
it makes the code suboptimal, and the offsets of
structure members won't agree with system libraries.
-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 avail-
able. 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 pro-
vided. (This is normally the case anyways, but if you
get lucky and the optimizer always expands the func-
tions inline, you might have gotten away without pro-
viding static copies.)
A function may be given the attribute "no_instru-
ment_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 sin-
gle-threaded environment since stack overflow is auto-
matically 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 tar-
gets, 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,--def-
sym,__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 stor-
age.-fargument-noalias specifies that arguments do not
alias each other, but may alias global storage.-fargu-
ment-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-under-
score, forcibly change the way C symbols are repre-
sented in the object file. One use is to help link
with legacy assembly code.
Be warned that you should know what you are doing when
invoking this option, and that not all targets provide
complete support for it.
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 configu-
ration 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 King-
dom.
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the charac-
ter 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 over-
rides the value of LC_CTYPE and LC_MESSAGES; other-
wise, LC_CTYPE and LC_MESSAGES default to the value of
the LANG environment variable. If none of these vari-
ables 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 out-
put 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 com-
bined with the name of a subprogram, but you can spec-
ify 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 pre-
fix/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 nor-
mally begins with /usr/local/lib/gcc-lib (more pre-
cisely, 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 nor-
mally search /usr/local/lib/bar. These alternate
directories are searched first; the standard directo-
ries come next.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list
of directories, much like PATH. GCC tries the direc-
tories thus specified when searching for subprograms,
if it can't find the subprograms using GCC_EXEC_PRE-
FIX.
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 speci-
fied 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 sepa-
rated 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. The 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.
DEPENDENCIES_OUTPUT
@anchor{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 equiva-
lent to combining the options -MM and -MF, with an
optional -MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as the environment variable
DEPENDENCIES_OUTPUT, except that system header files
are not ignored, so it implies -M rather than -MM.
BUGS
To report bugs to Apple, see <http://devel-
oper.apple.com/bugreporter>.
FOOTNOTES
1. On some systems, gcc -shared needs to build supplemen-
tary 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. Supply-
ing 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) 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/online-
docs/gcc/Contributors.html>, for contributors to GCC.
COPYRIGHT
Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996,
1997, 1998, 1999, 2000, 2001, 2002 Free Software Founda-
tion, Inc.
Permission is granted to copy, distribute and/or modify
this document under the terms of the GNU Free Documenta-
tion License, Version 1.1 or any later version published
by the Free Software Foundation; with the Invariant Sec-
tions 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.
2002-06-19 gcc-3.1 GCC(1)