GCC(1) GNU Tools GCC(1)
NAME
gcc, g++ - GNU project C and C++ Compiler (gcc-2.95)
SYNOPSIS
gcc [ option | filename ]...
g++ [ option | filename ]...
WARNING
The information in(1,8) this man(1,5,7) page is an extract from the full documenta-
tion of the GNU C compiler, and is limited to the meaning of the
options.
This man(1,5,7) page is not kept up to date except when volunteers want to
maintain it. If you find a discrepancy between the man(1,5,7) page and the
software, please check the Info file(1,n), which is the authoritative docu-
mentation.
If we find that the things in(1,8) this man(1,5,7) page that are out of date cause
significant confusion or complaints, we will stop distributing the man(1,5,7)
page. The alternative, updating the man(1,5,7) page when we update(7,n) the Info
file(1,n), is impossible because the rest of the work of maintaining GNU CC
leaves us no time(1,2,n) for that. The GNU project regards man(1,5,7) pages as obso-
lete and should not let them take time(1,2,n) away from other things.
For complete and current documentation, refer to the Info file(1,n) `gcc' or
the manual Using and Porting GNU CC (for version(1,3,5) 2.0). Both are made
from the Texinfo source file(1,n) gcc.texinfo.
DESCRIPTION
The C and C++ compilers are integrated. Both process input files
through one or more of four stages: preprocessing, compilation, assem-
bly, and linking. Source filename suffixes identify the source lan-
guage, but which name you use for the compiler governs default assump-
tions:
gcc assumes preprocessed (.i) files are C and assumes C style link-
ing.
g++ assumes preprocessed (.i) files are C++ and assumes C++ style
linking.
Suffixes of source file(1,n) names indicate the language and kind of pro-
cessing to be done:
.c C source; preprocess, compile, assemble
.C C++ source; preprocess, compile, assemble
.cc C++ source; preprocess, compile, assemble
.cxx C++ source; preprocess, compile, assemble
.m Objective-C source; preprocess, compile, assemble
.i preprocessed C; compile, assemble
.ii preprocessed C++; compile, assemble
.s Assembler source; assemble
.S Assembler source; preprocess, assemble
.h Preprocessor file(1,n); not usually named(5,8) on command line
Files with other suffixes are passed to the linker. Common cases
include:
.o Object file(1,n)
.a Archive file(1,n)
Linking is always the last stage unless you use one of the -c, -S, or
-E options to avoid it (or unless compilation errors stop the whole
process). For the link(1,2) stage, all .o files corresponding to source
files, -l libraries, unrecognized filenames (including named(5,8) .o object
files and .a archives) are passed to the linker in(1,8) command-line order.
OPTIONS
Options must be separate: `-dr' is quite different from `-d -r '.
Most `-f' and `-W' options have two contrary forms: -fname and
-fno-name (or -Wname and -Wno-name). Only the non-default forms are
shown here.
Here is a summary of all the options, grouped by type. Explanations
are in(1,8) the following sections.
Overall Options
-c -S -E -o file(1,n) -pipe -v -x language
Language Options
-ansi -fall-virtual -fcond-mismatch -fdollars-in-identifiers
-fenum-int-equiv -fexternal-templates -fno-asm -fno-builtin
-fhosted -fno-hosted -ffreestanding -fno-freestanding
-fno-strict-prototype -fsigned-bitfields -fsigned-char
-fthis-is-variable -funsigned-bitfields -funsigned-char
-fwritable-strings -traditional -traditional-cpp -trigraphs
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -W -Wall
-Waggregate-return -Wcast-align -Wcast-qual -Wchar-subscript
-Wcomment -Wconversion -Wenum-clash -Werror -Wformat
-Wid-clash-len -Wimplicit -Wimplicit-int
-Wimplicit-function-declaration -Winline -Wlong-long -Wmain
-Wmissing-prototypes -Wmissing-declarations -Wnested-externs
-Wno-import -Wparentheses -Wpointer-arith -Wredundant-decls
-Wreturn-type -Wshadow -Wstrict-prototypes -Wswitch
-Wtemplate-debugging -Wtraditional -Wtrigraphs -Wuninitialized
-Wunused -Wwrite-strings
Debugging Options
-a -dletters -fpretend-float -g -glevel -gcoff -gxcoff -gxcoff+
-gdwarf -gdwarf+ -gstabs -gstabs+ -ggdb -p -pg -save-temps
-print-file-name=library -print-libgcc-file-name
-print-prog-name=program
Optimization Options
-fcaller-saves -fcse-follow-jumps -fcse-skip-blocks
-fdelayed-branch -felide-constructors -fexpensive-optimizations
-ffast-math -ffloat-store -fforce-addr -fforce-mem
-finline-functions -fkeep-inline-functions -fmemoize-lookups
-fno-default-inline -fno-defer-pop -fno-function-cse -fno-inline
-fno-peephole -fomit-frame-pointer -frerun-cse-after-loop
-fschedule-insns -fschedule-insns2 -fstrength-reduce
-fthread-jumps -funroll-all-loops -funroll-loops -O -O2 -O3
Preprocessor Options
-Aassertion -C -dD -dM -dN -Dmacro[=defn] -E -H -idirafter dir
-include file(1,n) -imacros file(1,n) -iprefix file(1,n) -iwithprefix dir -M
-MD -MM -MMD -nostdinc -P -Umacro -undef
Assembler Option
-Wa,option
Linker Options
-llibrary -nostartfiles -nostdlib -static -shared -symbolic
-Xlinker option -Wl,option -u symbol
Directory Options
-Bprefix -Idir -I- -Ldir
Target Options
-b machine -V version(1,3,5)
Configuration Dependent Options
M680x0 Options
-m68000 -m68020 -m68020-40 -m68030 -m68040 -m68881 -mbitfield
-mc68000 -mc68020 -mfpa -mnobitfield -mrtd -mshort -msoft-float
VAX Options
-mg -mgnu -munix
SPARC Options
-mepilogue -mfpu -mhard-float -mno-fpu -mno-epilogue
-msoft-float -msparclite -mv8 -msupersparc -mcypress
Convex Options
-margcount -mc1 -mc2 -mnoargcount
AMD29K Options
-m29000 -m29050 -mbw -mdw -mkernel-registers -mlarge -mnbw
-mnodw -msmall -mstack-check -muser-registers
M88K Options
-m88000 -m88100 -m88110 -mbig-pic -mcheck-zero-division
-mhandle-large-shift -midentify-revision
-mno-check-zero-division -mno-ocs-debug-info
-mno-ocs-frame-position -mno-optimize-arg-area
-mno-serialize-volatile -mno-underscores -mocs-debug-info
-mocs-frame-position -moptimize-arg-area -mserialize-volatile
-mshort-data-num -msvr3 -msvr4 -mtrap-large-shift
-muse-div-instruction -mversion-03.00 -mwarn-passed-structs
RS6000 Options
-mfp-in-toc -mno-fop-in-toc
RT Options
-mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs
-mfull-fp-blocks -mhc-struct-return -min-line-mul
-mminimum-fp-blocks -mnohc-struct-return
MIPS Options
-mcpu=cpu(5,8,8 cpu-ldap) type -mips2 -mips3 -mint64 -mlong64 -mlonglong128
-mmips-as -mgas -mrnames -mno-rnames -mgpopt -mno-gpopt -mstats
-mno-stats -mmemcpy -mno-memcpy -mno-mips-tfile -mmips-tfile
-msoft-float -mhard-float -mabicalls -mno-abicalls -mhalf-pic
-mno-half-pic -G num -nocpp
i386 Options
-m486 -mno-486 -msoft-float -mno-fp-ret-in-387
HPPA Options
-mpa-risc-1-0 -mpa-risc-1-1 -mkernel -mshared-libs
-mno-shared-libs -mlong-calls -mdisable-fpregs
-mdisable-indexing -mtrailing-colon
i960 Options
-mcpu-type -mnumerics -msoft-float -mleaf-procedures
-mno-leaf-procedures -mtail-call -mno-tail-call -mcomplex-addr
-mno-complex-addr -mcode-align -mno-code-align -mic-compat
-mic2.0-compat -mic3.0-compat -masm-compat -mintel-asm
-mstrict-align -mno-strict-align -mold-align -mno-old-align
DEC Alpha Options
-mfp-regs -mno-fp-regs -mno-soft-float -msoft-float
System V Options
-G -Qy -Qn -YP,paths -Ym,dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg
-finhibit-size-directive -fnonnull-objects -fno-common
-fno-ident -fno-gnu-linker -fpcc-struct-return -fpic -fPIC
-freg-struct-return -fshared-data -fshort-enums -fshort-double
-fvolatile -fvolatile-global -fverbose-asm
OVERALL OPTIONS
-x language
Specify explicitly the language for the following input files
(rather than choosing a default based on the file(1,n) name suffix) .
This option applies to all following input files until the next
`-x' option. Possible values of language are `c', `objec-
tive-c', `c-header', `c++', `cpp-output', `assembler', and `as-
sembler-with-cpp'.
-x none
Turn off any specification of a language, so that subsequent
files are handled according to their file(1,n) name suffixes (as they
are if(3,n) `-x' has not been used at all).
If you want only some of the four stages (preprocess, compile, assem-
ble, link(1,2)), 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 com-
piler output is an object file(1,n) corresponding to each source
file.
By default, GCC makes the object file(1,n) name for a source file(1,n) by
replacing the suffix `.c', `.i', `.s', etc., with `.o'. Use -o
to select(2,7,2 select_tut) another name.
GCC ignores any unrecognized input files (those that do not re-
quire compilation or assembly) with the -c option.
-S Stop after the stage of compilation proper; do not assemble.
The output is an assembler code file(1,n) for each non-assembler in-
put file(1,n) specified.
By default, GCC makes the assembler file(1,n) name for a source file(1,n)
by replacing the suffix `.c', `.i', etc., with `.s'. Use -o to
select(2,7,2 select_tut) another name.
GCC ignores any input files that don't require compilation.
-E Stop after the preprocessing stage; do not run the compiler
proper. The output is preprocessed source code, which is sent
to the standard output.
GCC ignores input files which don't require preprocessing.
-o file(1,n)
Place output in(1,8) file(1,n) file(1,n). This applies regardless to whatever
sort(1,3) of output GCC is producing, whether it be an executable
file(1,n), an object file(1,n), an assembler file(1,n) or preprocessed C code.
Since only one output file(1,n) can be specified, it does not make
sense to use `-o' when compiling more than one input file(1,n), un-
less(1,3) you are producing an executable file(1,n) as output.
If you do not specify `-o', the default is to put an executable
file(1,n) in(1,8) `a.out', the object file(1,n) for `source.suffix' in(1,8)
`source.o', its assembler file(1,n) in(1,8) `source.s', and all prepro-
cessed C source on standard output.
-v Print (on standard error(8,n) output) the commands executed to run
the stages of compilation. Also print the version(1,3,5) number of the
compiler driver program and of the preprocessor and the compiler
proper.
-pipe Use pipes rather than temporary files for communication between
the various stages of compilation. This fails to work on some
systems where the assembler cannot read(2,n,1 builtins) from a pipe(2,8); but the GNU
assembler has no trouble.
LANGUAGE OPTIONS
The following options control the dialect of C that the compiler ac-
cepts:
-ansi Support all ANSI standard C programs.
This turns off certain features of GNU C that are incompatible
with ANSI C, such as the asm, inline 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 ANSI trigraph feature, and disallows `$' as part of
identifiers.
The alternate keywords __asm__, __extension__, __inline__ and
__typeof__ continue to work despite `-ansi'. You would not want
to use them in(1,8) an ANSI C program, of course, but it is useful to
put them in(1,8) header files that might be included in(1,8) 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-ANSI programs to be re-
jected gratuitously. For that, `-pedantic' is required in(1,8) addi-
tion to `-ansi'.
The preprocessor predefines a macro __STRICT_ANSI__ when you use
the `-ansi' option. Some header files may notice this macro and
refrain from declaring certain functions or defining certain
macros that the ANSI standard doesn't call for; this is to avoid
interfering with any programs that might use these names for
other things.
-fno-asm
Do not recognize asm, inline or typeof as a keyword. These
words may then be used as identifiers. You can use __asm__,
__inline__ and __typeof__ instead. `-ansi' implies `-fno-asm'.
-fno-builtin
Don't recognize built-in functions that do not begin with two
leading underscores. Currently, the functions affected include
_exit, abort(3,7), abs, alloca, cos, exit(3,n,1 builtins), fabs, labs, memcmp, mem-
cpy, sin, sqrt, strcmp, strcpy, and strlen.
The `-ansi' option prevents alloca and _exit from being builtin
functions.
-fhosted
Compile for a hosted environment; this implies the `-fbuiltin'
option, and implies that suspicious declarations of main should
be warned about.
-ffreestanding
Compile for a freestanding environment; this implies the `-fno-
builtin' option, and implies that main has no special require-
ments.
-fno-strict-prototype
Treat a function declaration with no arguments, such as `int foo
();', as C would treat it--as saying nothing about the number of
arguments or their types (C++ only). Normally, such a declara-
tion in(1,8) C++ means that the function foo takes no arguments.
-trigraphs
Support ANSI C trigraphs. The `-ansi' option implies `-tri-
graphs'.
-traditional
Attempt to support some aspects of traditional C compilers. For
details, see the GNU C Manual; the duplicate list here has been
deleted so that we won't get complaints when it is out of date.
But one note about C++ programs only (not C). `-traditional'
has one additional effect for C++: assignment to this is permit-
ted. This is the same as the effect of `-fthis-is-variable'.
-traditional-cpp
Attempt to support some aspects of traditional C preprocessors.
This includes the items that specifically mention the preproces-
sor above, but none of the other effects of `-traditional'.
-fdollars-in-identifiers
Permit the use of `$' in(1,8) identifiers (C++ only). You can also
use `-fno-dollars-in-identifiers' to explicitly prohibit use of
`$'. (GNU C++ allows `$' by default on some target systems but
not others.)
-fenum-int-equiv
Permit implicit conversion of int to enumeration types (C++ on-
ly). Normally GNU C++ allows conversion of enum to int, but not
the other way around.
-fexternal-templates
Produce smaller code for template declarations, by generating
only a single copy of each template function where it is defined
(C++ only). To use this option successfully, you must also mark
all files that use templates with either `#pragma implementa-
tion' (the definition) or `#pragma interface' (declarations).
When your code is compiled with `-fexternal-templates', all tem-
plate instantiations are external. You must arrange for all
necessary instantiations to appear in(1,8) the implementation file(1,n);
you can do this with a typedef that references each instantia-
tion needed. Conversely, when you compile using the default op-
tion `-fno-external-templates', all template instantiations are
explicitly internal.
-fall-virtual
Treat all possible member functions as virtual(5,8), implicitly. All
member functions (except for constructor functions and new or
delete member operators) are treated as virtual(5,8) functions of the
class where they appear.
This does not mean that all calls to these member functions will
be made through the internal table of virtual(5,8) functions. Under
some circumstances, the compiler can determine that a call to a
given virtual(5,8) function can be made directly; in(1,8) these cases the
calls are direct in(1,8) any case.
-fcond-mismatch
Allow conditional expressions with mismatched types in(1,8) the sec-
ond and third arguments. The value of such an expression is
void.
-fthis-is-variable
Permit assignment to this (C++ only). The incorporation of us-
er-defined free store management into C++ has made assignment to
`this' an anachronism. Therefore, by default it is invalid to
assign to this within a class member function. However, for
backwards compatibility, you can make it valid with `-fthis-is-
variable'.
-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 un-
signed 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 ma-
chines 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
and unsigned char, even though its behavior is always just like
one of those two.
-fsigned-char
Let the type char be signed, like signed char.
Note that this is equivalent to `-fno-unsigned-char', which is
the negative form of `-funsigned-char'. Likewise,
`-fno-signed-char' is equivalent to `-funsigned-char'.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bitfield is signed or unsigned,
when declared with no explicit `signed' or `unsigned' qualifier.
By default, such a bitfield is signed, because this is consis-
tent: the basic integer types such as int are signed types.
However, when you specify `-traditional', bitfields are all un-
signed no matter what.
-fwritable-strings
Store string(3,n) constants in(1,8) the writable data segment and don't
uniquize them. This is for compatibility with old programs
which assume they can write(1,2) into string(3,n) constants. `-tradition-
al' also has this effect.
Writing into string(3,n) constants is a very bad idea; "constants"
should be constant.
PREPROCESSOR OPTIONS
These options control the C preprocessor, which is run on each C source
file(1,n) before actual compilation.
If you use the `-E' option, GCC does nothing except preprocessing.
Some of these options make sense only together with `-E' because they
cause the preprocessor output to be unsuitable for actual compilation.
-include file(1,n)
Process file(1,n) as input before processing the regular input file.
In effect, the contents of file(1,n) are compiled first. Any `-D'
and `-U' options on the command line are always processed before
`-include file(1,n)', regardless of the order in(1,8) which they are writ-
ten. All the `-include' and `-imacros' options are processed in(1,8)
the order in(1,8) which they are written.
-imacros file(1,n)
Process file(1,n) as input, discarding the resulting output, before
processing the regular input file. Because the output generated
from file(1,n) is discarded, the only effect of `-imacros file(1,n)' is to
make the macros defined in(1,8) file(1,n) available for use in(1,8) the main
input. The preprocessor evaluates any `-D' and `-U' options on
the command line before processing `-imacrosfile(1,n)', regardless of
the order in(1,8) which they are written. All the `-include' and
`-imacros' options are processed in(1,8) the order in(1,8) which they are
written.
-idirafter dir
Add the directory dir to the second include path. The directo-
ries on the second include path are searched when a header file(1,n)
is not found in(1,8) any of the directories in(1,8) the main include path
(the one that `-I' adds to).
-iprefix prefix
Specify prefix as the prefix for subsequent `-iwithprefix' op-
tions.
-iwithprefix dir
Add a directory to the second include path. The directory's
name is made by concatenating prefix and dir, where prefix was
specified previously with `-iprefix'.
-nostdinc
Do not search the standard system directories for header files.
Only the directories you have specified with `-I' options (and
the current directory, if(3,n) appropriate) are searched.
By using both `-nostdinc' and `-I-', you can limit the include-
file(1,n) search file(1,n) to only those directories you specify explicit-
ly.
-nostdinc++
Do not search for header files in(1,8) the C++-specific standard di-
rectories, but do still search the other standard directories.
(This option is used when building `libg++'.)
-undef Do not predefine any nonstandard macros. (Including architec-
ture flags).
-E Run only the C preprocessor. Preprocess all the C source files
specified and output the results to standard output or to the
specified output file.
-C Tell the preprocessor not to discard comments. Used with the
`-E' option.
-P Tell the preprocessor not to generate `#line' commands. Used
with the `-E' option.
-M [ -MG ]
Tell the preprocessor to output a rule suitable for make de-
scribing the dependencies of each object file. For each source
file(1,n), the preprocessor outputs one make-rule whose target is the
object file(1,n) name for that source file(1,n) and whose dependencies are
all the files `#include'd in(1,8) it. This rule may be a single line
or may be continued with `\'-newline if(3,n) it is long. The list of
rules is printed on standard output instead of the preprocessed
C program.
`-M' implies `-E'.
`-MG' says to treat missing header files as generated files and
assume they live in(1,8) the same directory as the source file. It
must be specified in(1,8) addition to `-M'.
-MM [ -MG ]
Like `-M' but the output mentions only the user header files in-
cluded with `#include file(1,n)"'. System header files included with
`#include <file(1,n)>' are omitted.
-MD Like `-M' but the dependency information is written to files
with names made by replacing `.o' with `.d' at the end of the
output file(1,n) names. This is in(1,8) addition to compiling the file(1,n) as
specified--`-MD' does not inhibit ordinary compilation the way
`-M' does.
The Mach utility `md' can be used to merge(1,8) the `.d' files into a
single dependency file(1,n) suitable for using with the `make' com-
mand.
-MMD Like `-MD' except mention only user header files, not system
header files.
-H Print the name of each header file(1,n) used, in(1,8) addition to other
normal activities.
-Aquestion(answer)
Assert the answer answer for question, in(1,8) case it is tested with
a preprocessor conditional such as `#if(3,n) #question(answer)'.
`-A-' disables the standard assertions that normally describe
the target machine.
-Aquestion
(answer) Assert the answer answer for question, in(1,8) case it is
tested with a preprocessor conditional such as `#if(3,n) #ques-
tion(answer)'. `-A-' disables the standard assertions that nor-
mally describe the target machine.
-Dmacro
Define macro macro with the string(3,n) `1' as its definition.
-Dmacro=defn
Define macro macro as defn. All instances of `-D' on the com-
mand line are processed before any `-U' options.
-Umacro
Undefine macro macro. `-U' options are evaluated after all `-D'
options, but before any `-include' and `-imacros' options.
-dM Tell the preprocessor to output only a list of the macro defini-
tions that are in(1,8) effect at the end of preprocessing. Used with
the `-E' option.
-dD Tell the preprocessor to pass all macro definitions into the
output, in(1,8) their proper sequence in(1,8) the rest of the output.
-dN Like `-dD' except that the macro arguments and contents are
omitted. Only `#define name' is included in(1,8) the output.
ASSEMBLER OPTION
-Wa,option
Pass option as an option to the assembler. If option contains
commas, it is split(1,n) into multiple options at the commas.
LINKER OPTIONS
These options come into play when the compiler links object files into
an executable output file. They are meaningless if(3,n) the compiler is not
doing a link(1,2) step.
object-file-name
A file(1,n) name that does not end in(1,8) a special recognized suffix is
considered to name an object file(1,n) or library. (Object files are
distinguished from libraries by the linker according to the file(1,n)
contents.) If GCC does a link(1,2) step, these object files are used
as input to the linker.
-llibrary
Use the library named(5,8) library when linking.
The linker searches a standard list of directories for the li-
brary, which is actually a file(1,n) named(5,8) `liblibrary.a'. The link-
er then uses this file(1,n) as if(3,n) it had been specified precisely by
name.
The directories searched include several standard system direc-
tories 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 ar-
chive file(1,n) by scanning through it for members which define sym-
bols that have so far been referenced but not defined. However,
if(3,n) the linker finds an ordinary object file(1,n) rather than a li-
brary, the object file(1,n) is linked in(1,8) the usual fashion. The only
difference between using an `-l' option and specifying a file(1,n)
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(1,8) order to link(1,2) an
Objective C program.
-nostartfiles
Do not use the standard system startup files when linking. The
standard libraries are used normally.
-nostdlib
Don't use the standard system libraries and startup files when
linking. Only the files you specify will be passed to the link-
er.
-static
On systems that support dynamic linking, this prevents linking
with the shared libraries. On other systems, this option has no
effect.
-shared
Produce a shared object which can then be linked with other ob-
jects to form an executable. Only a few systems support this
option.
-symbolic
Bind references to global symbols when building a shared object.
Warn about any unresolved references (unless overridden by the
link(1,2) 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 GNU CC 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 argu-
ment. For example, to pass `-assert definitions', you must
write(1,2) `-Xlinker -assert -Xlinker definitions'. It does not work
to write(1,2) `-Xlinker "-assert definitions"', because this passes
the entire string(3,n) as a single argument, which is not what the
linker expects.
-Wl,option
Pass option as an option to the linker. If option contains com-
mas, it is split(1,n) into multiple options at the commas.
-u symbol
Pretend the symbol symbol is undefined, to force linking of li-
brary modules to define it. You can use `-u' multiple times
with different symbols to force loading of additional library
modules.
DIRECTORY OPTIONS
These options specify directories to search for header files, for li-
braries and for parts of the compiler:
-Idir Append directory dir to the list of directories searched for in-
clude files.
-I- Any directories you specify with `-I' options before the `-I-'
option are searched only for the case of `#include "file(1,n)"'; they
are not searched for `#include <file(1,n)>'.
If additional directories are specified with `-I' options after
the `-I-', these directories are searched for all `#include' di-
rectives. (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(1,n) came from) as the first
search directory for `#include "file(1,n)"'. 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 directo-
ries for header files. Thus, `-I-' and `-nostdinc' are indepen-
dent.
-Ldir Add directory dir to the list of directories to be searched for
`-l'.
-Bprefix
This option specifies where to find the executables, libraries
and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms
`cpp', `cc1' (or, for C++, `cc1plus'), `as' and `ld(1,8)'. It tries
prefix as a prefix for each program it tries to run, both with
and without `machine/version(1,3,5)/'.
For each subprogram to be run, the compiler driver first tries
the `-B' prefix, if(3,n) any. If that name is not found, or if(3,n) `-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(1,8) a file(1,n) name that is found, the compiler
driver searches for the unmodified program name, using the di-
rectories specified in(1,8) your `PATH' environment variable.
The run-time support file(1,n) `libgcc.a' is also searched for using
the `-B' prefix, if(3,n) needed. If it is not found there, the two
standard prefixes above are tried, and that is all. The file(1,n) is
left out of the link(1,2) if(3,n) it is not found by those means. Most of
the time(1,2,n), on most machines, `libgcc.a' is not actually neces-
sary.
You can get a similar result from the environment variable
GCC_EXEC_PREFIX; if(3,n) it is defined, its value is used as a prefix
in(1,8) the same way. If both the `-B' option and the GCC_EXEC_PRE-
FIX variable are present, the `-B' option is used first and the
environment variable value second.
WARNING OPTIONS
Warnings are diagnostic messages that report constructions which are
not inherently erroneous but which are risky or suggest there may have
been an error.
These options control the amount and kinds of warnings produced by GNU
CC:
-fsyntax-only
Check the code for syntax errors, but don't emit any output.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-pedantic
Issue all the warnings demanded by strict ANSI standard C; re-
ject all programs that use forbidden extensions.
Valid ANSI standard C programs should compile properly with or
without this option (though a rare few will require `-ansi').
However, without this option, certain GNU extensions and tradi-
tional C features are supported as well. With this option, they
are rejected. There is no reason to use this option; it exists
only to satisfy pedants.
`-pedantic' does not cause warning messages for use of the al-
ternate keywords whose names begin and end with `__'. Pedantic
warnings are also disabled in(1,8) the expression that follows __ex-
tension__. However, only system header files should use these
escape routes; application programs should avoid them.
-pedantic-errors
Like `-pedantic', except that errors are produced rather than
warnings.
-W Print extra warning messages for these events:
A nonvolatile automatic variable might be changed by a call to
longjmp. These warnings are possible only in(1,8) optimizing compi-
lation.
The compiler sees only the calls to setjmp. It cannot know
where longjmp will be called; in(1,8) fact, a signal(2,7) handler could
call it at any point in(1,8) the code. As a result, you may get a
warning even when there is in(1,8) fact no problem because longjmp
cannot in(1,8) fact be called at the place which would cause a prob-
lem.
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 warn-
ing:
foo (a)
{
if(3,n) (a > 0)
return a;
}
Spurious warnings can occur because GNU CC does not realize that
certain functions (including abort(3,7) and longjmp) will never re-
turn.
An expression-statement or the left-hand side of a comma expres-
sion contains no side effects. To suppress the warning, cast
the unused expression to void. For example, an expression such
as `x[i,j]' will cause a warning, but `x[(void)i,j]' will not.
An unsigned value is compared against zero with `>' or `<='.
-Wimplicit-int
Warn whenever a declaration does not specify a type.
-Wimplicit-function-declaration
Warn whenever a function is used before being declared.
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration.
-Wmain Warn if(3,n) the main function is declared or defined with a suspi-
cious type. Typically, it is a function with external linkage,
returning int, and taking zero or two arguments.
-Wreturn-type
Warn whenever a function is defined with a return-type that de-
faults to int. Also warn about any return statement with no re-
turn-value in(1,8) a function whose return-type is not void.
-Wunused
Warn whenever a local variable is unused aside from its declara-
tion, whenever a function is declared static but never defined,
and whenever a statement computes a result that is explicitly
not used.
-Wswitch
Warn whenever a switch(1,n) statement has an index of enumeral type
and lacks a case for one or more of the named(5,8) codes of that enu-
meration. (The presence of a default label prevents this warn-
ing.) case labels outside the enumeration range also provoke
warnings when this option is used.
-Wcomment
Warn whenever a comment-start sequence `/*' appears in(1,8) a com-
ment.
-Wtrigraphs
Warn if(3,n) any trigraphs are encountered (assuming they are en-
abled).
-Wformat
Check calls to printf(1,3,1 builtins) and scanf, etc., to make sure that the ar-
guments supplied have types appropriate to the format string(3,n)
specified.
-Wchar-subscripts
Warn if(3,n) an array subscript has type char. This is a common
cause of error(8,n), as programmers often forget that this type is
signed on some machines.
-Wuninitialized
An automatic variable is used without first being initialized.
These warnings are possible only in(1,8) optimizing compilation, be-
cause they require data flow information that is computed only
when optimizing. If you don't specify `-O', you simply won't
get these warnings.
These warnings occur only for variables that are candidates for
register allocation. Therefore, they do not occur for a vari-
able 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(1,8)
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 GNU CC is not smart
enough to see all the reasons why the code might be correct de-
spite appearing to have an error. Here is one example of how
this can happen:
{
int x;
switch(1,n) (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 initial-
ized, but GNU CC doesn't know this. Here is another common
case:
{
int save_y;
if(3,n) (change_y) save_y = y, y = new_y;
...
if(3,n) (change_y) y = save_y;
}
This has no bug because save_y is used only if(3,n) it is set.
Some spurious warnings can be avoided if(3,n) you declare as volatile
all the functions you use that never return.
-Wparentheses
Warn if(3,n) parentheses are omitted in(1,8) certain contexts.
-Wtemplate-debugging
When using templates in(1,8) a C++ program, warn if(3,n) debugging is not
yet fully available (C++ only).
-Wall All of the above `-W' options combined. These are all the op-
tions which pertain to usage that we recommend avoiding and that
we believe is easy to avoid, even in(1,8) conjunction with macros.
The remaining `-W...' options are not implied by `-Wall' because they
warn about constructions that we consider reasonable to use, on occa-
sion, in(1,8) clean programs.
-Wtraditional
Warn about certain constructs that behave differently in(1,8) tradi-
tional and ANSI C.
Macro arguments occurring within string(3,n) constants in(1,8) the macro
body. These would substitute the argument in(1,8) traditional C, but
are part of the constant in(1,8) ANSI C.
A function declared external in(1,8) one block and then used after
the end of the block.
A switch(1,n) statement has an operand of type long.
-Wshadow
Warn whenever a local variable shadows another local variable.
-Wid-clash-len
Warn whenever two distinct identifiers match in(1,8) the first len
characters. This may help you prepare a program that will com-
pile with certain obsolete, brain-damaged compilers.
-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(1,8) calculations with void * pointers and pointers to
functions.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if(3,n) 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(3,n) a char * is
cast to an int * on machines where integers can only be accessed
at two- or four-byte boundaries.
-Wwrite-strings
Give string(3,n) constants the type const char[length] so that copy-
ing the address of one into a non-const char * pointer will get
a warning. These warnings will help you find at compile time(1,2,n)
code that can try to write(1,2) into a string(3,n) constant, but only if(3,n)
you have been very careful about using const in(1,8) declarations and
prototypes. Otherwise, it will just be a nuisance; this is why
we did not make `-Wall' request these warnings.
-Wconversion
Warn if(3,n) a prototype causes a type conversion that is different
from what would happen to the same argument in(1,8) the absence of a
prototype. This includes conversions of fixed point to floating
and vice versa, and conversions changing the width or signedness
of a fixed point argument except when the same as the default
promotion.
-Waggregate-return
Warn if(3,n) any functions that return structures or unions are de-
fined or called. (In languages where you can return an array,
this also elicits a warning.)
-Wstrict-prototypes
Warn if(3,n) a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if(3,n) preceded by a declaration which specifies
the argument types.)
-Wmissing-prototypes
Warn if(3,n) a global function is defined without a previous proto-
type declaration. This warning is issued even if(3,n) the definition
itself provides a prototype. The aim is to detect global func-
tions that fail to be declared in(1,8) header files.
-Wmissing-declarations
Warn if(3,n) a global function is defined without a previous declara-
tion. Do so even if(3,n) the definition itself provides a prototype.
Use this option to detect global functions that are not declared
in(1,8) header files.
-Wredundant-decls
Warn if(3,n) anything is declared more than once in(1,8) the same scope,
even in(1,8) cases where multiple declaration is valid and changes
nothing.
-Wnested-externs
Warn if(3,n) an extern declaration is encountered within an function.
-Wenum-clash
Warn about conversion between different enumeration types (C++
only).
-Wlong-long
Warn if(3,n) long long type is used. This is default. To inhibit
the warning messages, use flag `-Wno-long-long'. Flags
`-W-long-long' and `-Wno-long-long' are taken into account only
when flag `-pedantic' is used.
-Woverloaded-virtual
(C++ only.) In a derived class, the definitions of virtual(5,8)
functions must match the type signature of a virtual(5,8) function
declared in(1,8) the base class. Use this option to request warnings
when a derived class declares a function that may be an erro-
neous attempt to define a virtual(5,8) function: that is, warn when a
function with the same name as a virtual(5,8) function in(1,8) the base
class, but with a type signature that doesn't match any virtual(5,8)
functions from the base class.
-Winline
Warn if(3,n) a function can not be inlined, and either it was de-
clared as inline, or else the -finline-functions option was giv-
en.
-Werror
Treat warnings as errors; abort(3,7) compilation after any warning.
DEBUGGING OPTIONS
GNU CC has various special options that are used for debugging either
your program or GCC:
-g Produce debugging information in(1,8) the operating system's native
format (stabs, COFF, XCOFF, or DWARF). GDB can work with this
debugging information.
On most systems that use stabs format, `-g' enables use of extra
debugging information that only GDB can use; this extra informa-
tion makes debugging work better in(1,8) GDB but will probably make
other debuggers crash or refuse to read(2,n,1 builtins) the program. If you
want to control for certain whether to generate the extra infor-
mation, use `-gstabs+', `-gstabs', `-gxcoff+', `-gxcoff',
`-gdwarf+', or `-gdwarf' (see below).
Unlike most other C compilers, GNU CC allows you to use `-g'
with `-O'. The shortcuts taken by optimized code may occasion-
ally produce surprising results: some variables you declared may
not exist at all; flow of control may briefly move(3x,7,3x curs_move) where you did
not expect it; some statements may not be executed because they
compute constant results or their values were already at hand;
some statements may execute in(1,8) 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 GNU CC is generated with the ca-
pability for more than one debugging format.
-ggdb Produce debugging information in(1,8) the native format (if(3,n) that is
supported), including GDB extensions if(3,n) at all possible.
-gstabs
Produce debugging information in(1,8) stabs format (if(3,n) that is sup-
ported), without GDB extensions. This is the format used by DBX
on most BSD systems.
-gstabs+
Produce debugging information in(1,8) stabs format (if(3,n) that is sup-
ported), using GNU extensions understood only by the GNU debug-
ger (GDB). The use of these extensions is likely to make other
debuggers crash or refuse to read(2,n,1 builtins) the program.
-gcoff Produce debugging information in(1,8) COFF format (if(3,n) that is sup-
ported). This is the format used by SDB on most System V sys-
tems prior to System V Release 4.
-gxcoff
Produce debugging information in(1,8) XCOFF format (if(3,n) that is sup-
ported). This is the format used by the DBX debugger on IBM
RS/6000 systems.
-gxcoff+
Produce debugging information in(1,8) XCOFF format (if(3,n) that is sup-
ported), using GNU extensions understood only by the GNU debug-
ger (GDB). The use of these extensions is likely to make other
debuggers crash or refuse to read(2,n,1 builtins) the program.
-gdwarf
Produce debugging information in(1,8) DWARF format (if(3,n) that is sup-
ported). This is the format used by SDB on most System V Re-
lease 4 systems.
-gdwarf+
Produce debugging information in(1,8) DWARF format (if(3,n) that is sup-
ported), using GNU extensions understood only by the GNU debug-
ger (GDB). The use of these extensions is likely to make other
debuggers crash or refuse to read(2,n,1 builtins) the program.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel -gxcofflevel
-gdwarflevel
Request debugging information and also use level to specify how
much information. The default level is 2.
Level 1 produces minimal information, enough for making back-
traces in(1,8) parts of the program that you don't plan to debug.
This includes descriptions of functions and external variables,
but no information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro defi-
nitions present in(1,8) the program. Some debuggers support macro
expansion when you use `-g3'.
-p Generate extra code to write(1,2) profile information suitable for
the analysis program prof.
-pg Generate extra code to write(1,2) profile information suitable for
the analysis program gprof.
-a Generate extra code to write(1,2) profile information for basic
blocks, which will record the number of times each basic block
is executed. 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.
-ax Generate extra code to read(2,n,1 builtins) basic block profiling parameters
from file(1,n) `bb.in' and write(1,2) profiling results to file(1,n) `bb.out'.
`bb.in' contains a list of functions. Whenever a function on the
list is entered, profiling is turned on. When the outmost func-
tion is left, profiling is turned off. If a function name is
prefixed with `-' the function is excluded from profiling. If a
function name is not unique it can be disambiguated by writing
`/path/filename.d:functionname'. `bb.out' will list some avail-
able filenames. Four function names have a special meaning:
`__bb_jumps__' will cause jump frequencies to be written to
`bb.out'. `__bb_trace__' will cause the sequence of basic
blocks to be piped into `gzip' and written to file(1,n) `bbtrace.gz'.
`__bb_hidecall__' will cause call instructions to be excluded
from the trace. `__bb_showret__' will cause return instructions
to be included in(1,8) the trace.
-dletters
Says to make debugging dumps during compilation at times speci-
fied by letters. This is used for debugging the compiler. The
file(1,n) names for most of the dumps are made by appending a word to
the source file(1,n) name (e.g. `foo.c.rtl' or `foo.c.jump').
-dM Dump all macro definitions, at the end of preprocessing, and
write(1,2) no output.
-dN Dump all macro names, at the end of preprocessing.
-dD Dump all macro definitions, at the end of preprocessing, in(1,8) ad-
dition to normal output.
-dy Dump debugging information during parsing, to standard error.
-dr Dump after RTL generation, to `file(1,n).rtl'.
-dx Just generate RTL for a function instead of compiling it. Usu-
ally used with `r'.
-dj Dump after first jump optimization, to `file(1,n).jump'.
-ds Dump after CSE (including the jump optimization that sometimes
follows CSE), to `file(1,n).cse'.
-dL Dump after loop optimization, to `file(1,n).loop'.
-dt Dump after the second CSE pass (including the jump optimization
that sometimes follows CSE), to `file(1,n).cse2'.
-df Dump after flow analysis, to `file(1,n).flow'.
-dc Dump after instruction combination, to `file(1,n).combine'.
-dS Dump after the first instruction scheduling pass, to
`file(1,n).sched'.
-dl Dump after local register allocation, to `file(1,n).lreg'.
-dg Dump after global register allocation, to `file(1,n).greg'.
-dR Dump after the second instruction scheduling pass, to
`file(1,n).sched2'.
-dJ Dump after last jump optimization, to `file(1,n).jump2'.
-dd Dump after delayed branch scheduling, to `file(1,n).dbr'.
-dk Dump after conversion from registers to stack, to `file(1,n).stack'.
-da Produce all the dumps listed above.
-dm Print statistics on memory usage, at the end of the run, to
standard error.
-dp Annotate the assembler output with a comment indicating which
pattern and alternative was used.
-fpretend-float
When running a cross-compiler, pretend that the target machine
uses the same floating point format as the host(1,5) machine. This
causes incorrect output of the actual floating constants, but
the actual instruction sequence will probably be the same as GNU
CC would make when running on the target machine.
-save-temps
Store the usual "temporary" intermediate files permanently;
place them in(1,8) the current directory and name them based on the
source file. Thus, compiling `foo.c' with `-c -save-temps'
would produce files `foo.cpp' and `foo.s', as well as `foo.o'.
-print-file-name=library
Print the full absolute name of the library file(1,n) library that
would be used when linking--and do not do anything else. With
this option, GNU CC does not compile or link(1,2) anything; it just
prints the file(1,n) name.
-print-libgcc-file-name
Same as `-print-file-name=libgcc.a'.
-print-prog-name=program
Like `-print-file-name', but searches for a program such as
`cpp'.
OPTIMIZATION OPTIONS
These options control various sorts of optimizations:
-O
-O1 Optimize. Optimizing compilation takes somewhat more time(1,2,n), and
a lot more memory for a large function.
Without `-O', the compiler's goal is to reduce the cost of com-
pilation and to make debugging produce the expected results.
Statements are independent: if(3,n) 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(1,8) the function and get exactly the results you would
expect from the source code.
Without `-O', only variables declared register are allocated in(1,8)
registers. The resulting compiled code is a little worse than
produced by PCC without `-O'.
With `-O', the compiler tries to reduce code size and execution
time.
When you specify `-O', the two options `-fthread-jumps' and
`-fdefer-pop' are turned on. On machines that have delay slots,
the `-fdelayed-branch' option is turned on. For those machines
that can support debugging even without a frame pointer, the
`-fomit-frame-pointer' option is turned on. On some machines
other flags may also be turned on.
-O2 Optimize even more. Nearly all supported optimizations that do
not involve a space-speed tradeoff are performed. Loop un-
rolling and function inlining are not done, for example. As
compared to -O, this option increases both compilation time(1,2,n) and
the performance of the generated code.
-O3 Optimize yet more. This turns on everything -O2 does, along with
also turning on -finline-functions.
-O0 Do not optimize.
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'. The following list shows only one form--
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(1,8) registers. This pre-
vents undesirable excess precision on machines such as the 68000
where the floating registers (of the 68881) keep more precision
than a double is supposed to have.
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.
-fmemoize-lookups
-fsave-memoized
Use heuristics to compile faster (C++ only). These heuristics
are not enabled by default, since they are only effective for
certain input files. Other input files compile more slowly.
The first time(1,2,n) the compiler must build a call to a member func-
tion (or reference to a data member), it must (1) determine
whether the class implements member functions of that name; (2)
resolve which member function to call (which involves figuring
out what sorts of type conversions need to be made); and (3)
check the visibility of the member function to the caller. All
of this adds up to slower compilation. Normally, the second
time(1,2,n) a call is made to that member function (or reference to
that data member), it must go through the same lengthy process
again. This means that code like this
cout << "This " << p << " has " << n << " legs.\n";
makes six passes through all three steps. By using a software
cache, a "hit" significantly reduces this cost. Unfortunately,
using the cache introduces another layer of mechanisms which
must be implemented, and so incurs its own overhead. `-fmemo-
ize-lookups' enables the software cache.
Because access(2,5) privileges (visibility) to members and member
functions may differ from one function context to the next, g++
may need to flush(8,n) the cache. With the `-fmemoize-lookups' flag,
the cache is flushed after every function that is compiled. The
`-fsave-memoized' flag enables the same software cache, but when
the compiler determines that the context of the last function
compiled would yield the same access(2,5) privileges of the next
function to compile, it preserves the cache. This is most help-
ful when defining many member functions for the same class: with
the exception of member functions which are friends of other
classes, each member function has exactly the same access(2,5) privi-
leges as every other, and the cache need not be flushed.
-fno-default-inline
Don't make member functions inline by default merely because
they are defined inside the class scope (C++ only).
-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 may produce better code by making all
memory references potential common subexpressions. When they
are not common subexpressions, instruction combination should
eliminate the separate register-load. I am interested in(1,8) hear-
ing about the difference this makes.
-fforce-addr
Force memory address constants to be copied into registers be-
fore doing arithmetic on them. This may produce better code
just as `-fforce-mem' may. I am interested in(1,8) hearing about the
difference this makes.
-fomit-frame-pointer
Don't keep the frame pointer in(1,8) a register for functions that
don't need one. This avoids the instructions to save, set(7,n,1 builtins) up
and restore frame pointers; it also makes an extra register
available in(1,8) many functions. It also makes debugging impossible
on most machines.
On some machines, such as the Vax, this flag has no effect, be-
cause the standard calling sequence automatically handles the
frame pointer and nothing is saved by pretending it doesn't ex-
ist. The machine-description macro FRAME_POINTER_REQUIRED con-
trols whether a target machine supports this flag.
-finline-functions
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be
worth integrating in(1,8) this way.
If all calls to a given function are integrated, and the func-
tion is declared static, then GCC normally does not output the
function as assembler code in(1,8) its own right.
-fcaller-saves
Enable values to be allocated in(1,8) registers that will be clob-
bered 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(1,8) better code than would oth-
erwise be produced.
This option is enabled by default on certain machines, usually
those which have no call-preserved registers to use instead.
-fkeep-inline-functions
Even if(3,n) all calls to a given function are integrated, and the
function is declared static, nevertheless output a separate run-
time(1,2,n) callable version(1,3,5) of the function.
-fno-function-cse
Do not put function addresses in(1,8) registers; make each instruc-
tion that calls a constant function contain the function's ad-
dress explicitly.
This option results in(1,8) less(1,3) efficient code, but some strange
hacks that alter the assembler output may be confused by the op-
timizations performed when this option is not used.
-fno-peephole
Disable any machine-specific peephole optimizations.
-ffast-math
This option allows GCC to violate some ANSI or IEEE rules/speci-
fications in(1,8) the interest of optimizing code for speed. For ex-
ample, it allows the compiler to assume arguments to the sqrt
function are non-negative numbers.
This option should never be turned on by any `-O' option since
it can result in(1,8) incorrect output for programs which depend on
an exact implementation of IEEE or ANSI rules/specifications for
math functions.
The following options control specific optimizations. The `-O2' option
turns on all of these optimizations except `-funroll-loops' and `-fun-
roll-all-loops'.
The `-O' option usually turns on the `-fthread-jumps' and `-fde-
layed-branch' options, but specific machines may change the default op-
timizations.
You can use the following flags in(1,8) the rare cases when "fine-tuning" of
optimizations to be performed is desired.
-fstrength-reduce
Perform the optimizations of loop strength reduction and elimi-
nation of iteration variables.
-fthread-jumps
Perform optimizations where we check to see if(3,n) a jump branches
to a location where another comparison subsumed by the first is
found. If so, the first branch is redirected to either the des-
tination of the second branch or a point immediately following
it, depending on whether the condition is known to be true or
false.
-funroll-loops
Perform the optimization of loop unrolling. This is only done
for loops whose number of iterations can be determined at com-
pile time(1,2,n) or run time.
-funroll-all-loops
Perform the optimization of loop unrolling. This is done for
all loops. This usually makes programs run more slowly.
-fcse-follow-jumps
In common subexpression elimination, scan through jump instruc-
tions when the target of the jump is not reached by any other
path. For example, when CSE encounters an if(3,n) 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 fol-
low jumps which conditionally skip over blocks. When CSE en-
counters a simple if(3,n) statement with no else clause,
`-fcse-skip-blocks' causes CSE to follow the jump around the
body of the if(3,n).
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations
has been performed.
-felide-constructors
Elide constructors when this seems plausible (C++ only). With
this flag, GNU C++ initializes y directly from the call to foo
without going through a temporary in(1,8) the following code:
A foo (); A y = foo ();
Without this option, GNU C++ first initializes y by calling the
appropriate constructor for type A; then assigns the result of
foo to a temporary; and, finally, replaces the initial value of
`y' with the temporary.
The default behavior (`-fno-elide-constructors') is specified by
the draft ANSI C++ standard. If your program's constructors
have side effects, using `-felide-constructors' can make your
program act differently, since some constructor calls may be
omitted.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively ex-
pensive.
-fdelayed-branch
If supported for the target machine, attempt to reorder instruc-
tions to exploit instruction slots available after delayed
branch instructions.
-fschedule-insns
If supported for the target machine, attempt to reorder instruc-
tions to eliminate execution stalls due to required data being
unavailable. This helps machines that have slow floating point
or memory load(7,n) instructions by allowing other instructions to be
issued until the result of the load(7,n) or floating point instruc-
tion is required.
-fschedule-insns2
Similar to `-fschedule-insns', but requests an additional pass
of instruction scheduling after register allocation has been
done. This is especially useful on machines with a relatively
small number of registers and where memory load(7,n) instructions
take more than one cycle.
TARGET OPTIONS
By default, GNU CC 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 GNU CC, for different target machines, can be in-
stalled side by side. Then you specify which one to use with the `-b'
option.
In addition, older and newer versions of GNU CC 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 compila-
tion. This is useful when you have installed GNU CC as a cross-
compiler.
The value to use for machine is the same as was specified as the
machine type when configuring GNU CC as a cross-compiler. For
example, if(3,n) 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 compile for
the same type of machine that you are using.
-V version(1,3,5)
The argument version(1,3,5) specifies which version(1,3,5) of GNU CC to run.
This is useful when multiple versions are installed. For exam-
ple, version(1,3,5) might be `2.0', meaning to run GNU CC version(1,3,5) 2.0.
The default version(1,3,5), when you do not specify `-V', is controlled
by the way GNU CC is installed. Normally, it will be a version(1,3,5)
that is recommended for general use.
MACHINE DEPENDENT OPTIONS
Each of the target machine types can have its own special options,
starting with `-m', to choose among various hardware models or configu-
rations--for example, 68010 vs 68020, floating coprocessor or none. A
single installed version(1,3,5) of the compiler can compile for any model or
configuration, according to the options specified.
Some configurations of the compiler also support additional special op-
tions, usually for command-line compatibility with other compilers on
the same platform.
These are the `-m' options defined for the 68000 series:
-m68000
-mc68000
Generate output for a 68000. This is the default when the com-
piler is configured for 68000-based systems.
-m68020
-mc68020
Generate output for a 68020 (rather than a 68000). This is the
default when the compiler is configured for 68020-based systems.
-m68881
Generate output containing 68881 instructions for floating
point. This is the default for most 68020-based systems unless
-nfp was specified when the compiler was configured.
-m68030
Generate output for a 68030. This is the default when the com-
piler is configured for 68030-based systems.
-m68040
Generate output for a 68040. This is the default when the com-
piler is configured for 68040-based systems.
-m68020-40
Generate output for a 68040, without using any of the new in-
structions. This results in(1,8) code which can run relatively effi-
ciently on either a 68020/68881 or a 68030 or a 68040.
-mfpa Generate output containing Sun FPA instructions for floating
point.
-msoft-float
Generate output containing library calls for floating point.
WARNING: the requisite libraries are not part of GNU CC. Nor-
mally the facilities of the machine's usual C compiler are used,
but this can't be done directly in(1,8) cross-compilation. You must
make your own arrangements to provide suitable library functions
for cross-compilation.
-mshort
Consider type int to be 16 bits wide, like short int.
-mnobitfield
Do not use the bit-field instructions. `-m68000' implies `-mno-
bitfield'.
-mbitfield
Do use the bit-field instructions. `-m68020' implies `-mbit-
field'. This is the default if(3,n) you use the unmodified sources.
-mrtd Use a different function-calling convention, in(1,8) which functions
that take a fixed number of arguments return with the rtd in-
struction, which pops their arguments while returning. This
saves one instruction in(1,8) the caller since there is no need to
pop the arguments there.
This calling convention is incompatible with the one normally
used on Unix, so you cannot use it if(3,n) you need to call libraries
compiled with the Unix compiler.
Also, you must provide function prototypes for all functions
that take variable numbers of arguments (including printf(1,3,1 builtins)); oth-
erwise incorrect code will be generated for calls to those func-
tions.
In addition, seriously incorrect code will result if(3,n) you call a
function with too many arguments. (Normally, extra arguments
are harmlessly ignored.)
The rtd instruction is supported by the 68010 and 68020 proces-
sors, but not by the 68000.
These `-m' options are defined for the Vax:
-munix Do not output certain jump instructions (aobleq and so on) that
the Unix assembler for the Vax cannot handle across long ranges.
-mgnu Do output those jump instructions, on the assumption that you
will assemble with the GNU assembler.
-mg Output code for g-format floating point numbers instead of d-
format.
These `-m' switches are supported on the SPARC:
-mfpu
-mhard-float
Generate output containing floating point instructions. This is
the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point.
Warning: there is no GNU floating-point library for SPARC. Nor-
mally the facilities of the machine's usual C compiler are used,
but this cannot be done directly in(1,8) cross-compilation. You must
make your own arrangements to provide suitable library functions
for cross-compilation.
-msoft-float changes the calling convention in(1,8) the output file(1,n);
therefore, it is only useful if(3,n) you compile all of a program
with this option.
-mno-epilogue
-mepilogue
With -mepilogue (the default), the compiler always emits code
for function exit(3,n,1 builtins) at the end of each function. Any function ex-
it(3,n,1 builtins) in(1,8) the middle of the function (such as a return statement in(1,8)
C) will generate a jump to the exit(3,n,1 builtins) code at the end of the func-
tion.
With -mno-epilogue, the compiler tries to emit exit(3,n,1 builtins) code inline
at every function exit.
-mno-v8
-mv8
-msparclite
These three options select(2,7,2 select_tut) variations on the SPARC architecture.
By default (unless specifically configured for the Fujitsu SPAR-
Clite), GCC generates code for the v7 variant of the SPARC ar-
chitecture.
-mv8 will give you SPARC v8 code. The only difference from v7
code is that the compiler emits the integer multiply and integer
divide instructions which exist in(1,8) SPARC v8 but not in(1,8) SPARC v7.
-msparclite will give you SPARClite code. This adds the integer
multiply, integer divide step and scan (ffs) instructions which
exist in(1,8) SPARClite but not in(1,8) SPARC v7.
-mcypress
-msupersparc
These two options select(2,7,2 select_tut) the processor for which the code is op-
timised.
With -mcypress (the default), the compiler optimises code for
the Cypress CY7C602 chip, as used in(1,8) the SparcStation/SparcServ-
er 3xx series. This is also appropriate for the older SparcSta-
tion 1, 2, IPX etc.
With -msupersparc the compiler optimises code for the SuperSparc
cpu(5,8,8 cpu-ldap), as used in(1,8) the SparcStation 10, 1000 and 2000 series. This
flag also enables use of the full SPARC v8 instruction set.
These `-m' options are defined for the Convex:
-mc1 Generate output for a C1. This is the default when the compiler
is configured for a C1.
-mc2 Generate output for a C2. This is the default when the compiler
is configured for a C2.
-margcount
Generate code which puts(3,n) an argument count in(1,8) the word preceding
each argument list. Some nonportable Convex and Vax programs
need this word. (Debuggers don't, except for functions with
variable-length argument lists; this info(1,5,n) is in(1,8) the symbol ta-
ble.)
-mnoargcount
Omit the argument count word. This is the default if(3,n) you use
the unmodified sources.
These `-m' options are defined for the AMD Am29000:
-mdw Generate code that assumes the DW bit is set(7,n,1 builtins), i.e., that byte
and halfword operations are directly supported by the hardware.
This is the default.
-mnodw Generate code that assumes the DW bit is not set.
-mbw Generate code that assumes the system supports byte and halfword
write(1,2) operations. This is the default.
-mnbw Generate code that assumes the systems does not support byte and
halfword write(1,2) operations. This implies `-mnodw'.
-msmall
Use a small memory model that assumes that all function address-
es are either within a single 256 KB segment or at an absolute
address of less(1,3) than 256K. This allows the call instruction to
be used instead of a const, consth, calli sequence.
-mlarge
Do not assume that the call instruction can be used; this is the
default.
-m29050
Generate code for the Am29050.
-m29000
Generate code for the Am29000. This is the default.
-mkernel-registers
Generate references to registers gr64-gr95 instead of
gr96-gr127. This option can be used when compiling kernel code
that wants a set(7,n,1 builtins) of global registers disjoint from that used by
user-mode code.
Note that when this option is used, register names in(1,8) `-f' flags
must use the normal, user-mode, names.
-muser-registers
Use the normal set(7,n,1 builtins) of global registers, gr96-gr127. This is the
default.
-mstack-check
Insert a call to __msp_check after each stack adjustment. This
is often used for kernel code.
These `-m' options are defined for Motorola 88K architectures:
-m88000
Generate code that works well on both the m88100 and the m88110.
-m88100
Generate code that works best for the m88100, but that also runs
on the m88110.
-m88110
Generate code that works best for the m88110, and may not run on
the m88100.
-midentify-revision
Include an ident directive in(1,8) the assembler output recording the
source file(1,n) name, compiler name and version(1,3,5), timestamp, and com-
pilation flags used.
-mno-underscores
In assembler output, emit symbol names without adding an under-
score character at the beginning of each name. The default is
to use an underscore as prefix on each name.
-mno-check-zero-division
-mcheck-zero-division
Early models of the 88K architecture had problems with division
by zero; in(1,8) particular, many of them didn't trap. Use these op-
tions to avoid including (or to include explicitly) additional
code to detect division by zero and signal(2,7) an exception. All
GCC configurations for the 88K use `-mcheck-zero-division' by
default.
-mocs-debug-info
-mno-ocs-debug-info
Include (or omit) additional debugging information (about regis-
ters used in(1,8) each stack frame) as specified in(1,8) the 88Open Object
Compatibility Standard, "OCS". This extra information is not
needed by GDB. The default for DG/UX, SVr4, and Delta 88 SVr3.2
is to include this information; other 88k configurations omit
this information by default.
-mocs-frame-position
-mno-ocs-frame-position
Force (or do not require) register values to be stored in(1,8) a par-
ticular place in(1,8) stack frames, as specified in(1,8) OCS. The DG/UX,
Delta88 SVr3.2, and BCS configurations use `-mocs-frame-posi-
tion'; other 88k configurations have the default
`-mno-ocs-frame-position'.
-moptimize-arg-area
-mno-optimize-arg-area
Control how to store function arguments in(1,8) stack frames. `-mop-
timize-arg-area' saves space, but may break some debuggers (not
GDB). `-mno-optimize-arg-area' conforms better to standards.
By default GCC does not optimize the argument area.
-mshort-data-num
num Generate smaller data references by making them relative to
r0, which allows loading a value using a single instruction
(rather than the usual two). You control which data references
are affected by specifying num with this option. For example,
if(3,n) you specify `-mshort-data-512', then the data references af-
fected are those involving displacements of less(1,3) than 512 bytes.
`-mshort-data-num' is not effective for num greater than 64K.
-mserialize-volatile
-mno-serialize-volatile
Do, or do not, generate code to guarantee sequential consistency
of volatile memory references.
GNU CC always guarantees consistency by default, for the pre-
ferred processor submodel. How this is done depends on the sub-
model.
The m88100 processor does not reorder memory references and so
always provides sequential consistency. If you use `-m88100',
GNU CC does not generate any special instructions for sequential
consistency.
The order of memory references made by the m88110 processor does
not always match the order of the instructions requesting those
references. In particular, a load(7,n) instruction may execute be-
fore a preceding store instruction. Such reordering violates
sequential consistency of volatile memory references, when there
are multiple processors. When you use `-m88000' or `-m88110',
GNU CC generates special instructions when appropriate, to force
execution in(1,8) the proper order.
The extra code generated to guarantee consistency may affect the
performance of your application. If you know that you can safe-
ly forgo this guarantee, you may use the option `-mno-serialize-
volatile'.
If you use the `-m88100' option but require sequential consis-
tency when running on the m88110 processor, you should use
`-mserialize-volatile'.
-msvr4
-msvr3 Turn on (`-msvr4') or off (`-msvr3') compiler extensions related
to System V release 4 (SVr4). This controls the following:
Which variant of the assembler syntax to emit (which you can se-
lect(2,7,2 select_tut) independently using `-mversion-03.00').
`-msvr4' makes the C preprocessor recognize `#pragma weak'
`-msvr4' makes GCC issue additional declaration directives used
in(1,8) SVr4.
`-msvr3' is the default for all m88K configurations except the SVr4
configuration.
-mtrap-large-shift
-mhandle-large-shift
Include code to detect bit-shifts of more than 31 bits; respec-
tively, trap such shifts or emit code to handle them properly.
By default GCC makes no special provision for large bit shifts.
-muse-div-instruction
Very early models of the 88K architecture didn't have a divide
instruction, so GCC avoids that instruction by default. Use
this