gcc, g++ - GNU project C and C++ Compiler (gcc-2.95)
gcc [ option | filename ]...
g++ [ option | filename ]...
The information in this man page is an extract from the
full documentation of the GNU C compiler, and is limited
to the meaning of the options.
This man page is not kept up to date except when volunteers
want to maintain it. If you find a discrepancy
between the man page and the software, please check the
Info file, which is the authoritative documentation.
If we find that the things in this man page that are out
of date cause significant confusion or complaints, we will
stop distributing the man page. The alternative, updating
the man page when we update the Info file, is impossible
because the rest of the work of maintaining GNU CC leaves
us no time for that. The GNU project regards man pages as
obsolete and should not let them take time away from other
things.
For complete and current documentation, refer to the Info
file `gcc' or the manual Using and Porting GNU CC (for
version 2.0). Both are made from the Texinfo source file
gcc.texinfo.
The C and C++ compilers are integrated. Both process
input files through one or more of four stages: preprocessing,
compilation, assembly, and linking. Source filename
suffixes identify the source language, but which name
you use for the compiler governs default assumptions:
gcc assumes preprocessed (.i) files are C and assumes C
style linking.
g++ assumes preprocessed (.i) files are C++ and assumes
C++ style linking.
Suffixes of source file names indicate the language and
kind of processing 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; not usually named on command line
Files with other suffixes are passed to the linker. Common
cases include:
.o Object file
.a Archive file
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 stage, all
.o files corresponding to source files, -l libraries,
unrecognized filenames (including named .o object files
and .a archives) are passed to the linker in command-line
order.
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 nondefault
forms are shown here.
Here is a summary of all the options, grouped by type.
Explanations are in the following sections.
Overall Options [Toc] [Back]
-c -S -E -o file -pipe -v -x language
Language Options [Toc] [Back]
-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 [Toc] [Back]
-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 [Toc] [Back]
-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 [Toc] [Back]
-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 [Toc] [Back]
-Aassertion -C -dD -dM -dN -Dmacro[=defn] -E -H
-idirafter dir -include file -imacros file -iprefix
file -iwithprefix dir -M -MD -MM -MMD -nostdinc -P
-Umacro -undef
Assembler Option [Toc] [Back]
-Wa,option
Linker Options [Toc] [Back]
-llibrary -nostartfiles -nostdlib -static -shared
-symbolic -Xlinker option -Wl,option -u symbol
Directory Options [Toc] [Back]
-Bprefix -Idir -I- -Ldir
Target Options [Toc] [Back]
-b machine -V version
Configuration Dependent Options [Toc] [Back]
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 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 [Toc] [Back]
-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
-x language
Specify explicitly the language for the following
input files (rather than choosing a default based
on the file 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', `assem-
bler', and `assembler-with-cpp'.
-x none
Turn off any specification of a language, so that
subsequent files are handled according to their
file name suffixes (as they are if `-x' has not
been used at all).
If you want only some of the four stages (preprocess, compile,
assemble, link), 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 compiler output is an object file corresponding
to each source file.
By default, GCC makes the object file name for a
source file by replacing the suffix `.c', `.i',
`.s', etc., with `.o'. Use -o to select another
name.
GCC ignores any unrecognized input files (those
that do not require 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 for
each non-assembler input file specified.
By default, GCC makes the assembler file name for a
source file by replacing the suffix `.c', `.i',
etc., with `.s'. Use -o to select 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
Place output in file file. This applies regardless
to whatever sort of output GCC is producing,
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 you do not specify `-o', the default is to put
an executable file in `a.out', the object file for
`source.suffix' in `source.o', its assembler file
in `source.s', and all preprocessed C source on
standard output.
-v Print (on standard error output) the commands executed
to run the stages of compilation. Also print
the version number of the compiler driver program
and of the preprocessor and the compiler proper.
-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 from a pipe; but the GNU assembler
has no trouble.
The following options control the dialect of C that the
compiler accepts:
-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 an ANSI
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-ANSI programs
to be rejected gratuitously. For that, `-pedantic'
is required in addition 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, abs, allo-
ca, cos, exit, fabs, labs, memcmp, memcpy, 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 requirements.
-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 declaration in
C++ means that the function foo takes no arguments.
-trigraphs
Support ANSI C trigraphs. The `-ansi' option implies
`-trigraphs'.
-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 permitted. 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 preprocessor above, but
none of the other effects of `-traditional'.
-fdollars-in-identifiers
Permit the use of `$' in 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++ only). 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 im-
plementation' (the definition) or `#pragma inter-
face' (declarations).
When your code is compiled with `-fexternal-tem-
plates', all template instantiations are external.
You must arrange for all necessary instantiations
to appear in the implementation file; you can do
this with a typedef that references each instantiation
needed. Conversely, when you compile using
the default option `-fno-external-templates', all
template instantiations are explicitly internal.
-fall-virtual
Treat all possible member functions as virtual, implicitly.
All member functions (except for constructor
functions and new or delete member operators)
are treated as virtual 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 functions. Under some circumstances,
the compiler can determine that a call to a given
virtual function can be made directly; in these
cases the calls are direct in any case.
-fcond-mismatch
Allow conditional expressions with mismatched types
in the second and third arguments. The value of
such an expression is void.
-fthis-is-variable
Permit assignment to this (C++ only). The incorporation
of user-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 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 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-un-
signed-char', which is the negative form of `-fun-
signed-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 consistent:
the basic integer types such as int are signed
types.
However, when you specify `-traditional', bitfields
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. `-traditional' also has this effect.
Writing into string constants is a very bad idea;
"constants" should be constant.
These options control the C preprocessor, which is run on
each C source file 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
Process file as input before processing the regular
input file. In effect, the contents of file are
compiled first. Any `-D' and `-U' options on the
command line are always processed before `-include
file', regardless of the order in which they are
written. All the `-include' and `-imacros' options
are processed in the order in which they are written.
-imacros file
Process file as input, discarding the resulting
output, before processing the regular input file.
Because the output generated from file is discarded,
the only effect of `-imacros file' is to make
the macros defined in file available for use in the
main input. The preprocessor evaluates any `-D'
and `-U' options on the command line before processing
`-imacrosfile', regardless of the order in
which they are written. All the `-include' and
`-imacros' options are processed in the order in
which they are written.
-idirafter dir
Add the directory dir to the second include path.
The directories on the second include path are
searched when a header file is not found in any of
the directories in the main include path (the one
that `-I' adds to).
-iprefix prefix
Specify prefix as the prefix for subsequent
`-iwithprefix' options.
-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 appropriate) are searched.
By using both `-nostdinc' and `-I-', you can limit
the include-file search file to only those directories
you specify explicitly.
-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 `libg++'.)
-undef Do not predefine any nonstandard macros. (Including
architecture 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 describing the dependencies of each object
file. For each source file, the preprocessor outputs
one make-rule whose target is the object file
name for that source file and whose dependencies
are all the files `#include'd in it. This rule may
be a single line or may be continued with `\'-newline
if 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 the same directory
as the source file. It must be specified in
addition to `-M'.
-MM [ -MG ]
Like `-M' but the output mentions only the user
header files included with `#include file"'. System
header files included with `#include <file>'
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 names. This is
in addition to compiling the file as specified--`-MD'
does not inhibit ordinary compilation
the way `-M' does.
The Mach utility `md' can be used to merge the `.d'
files into a single dependency file suitable for
using with the `make' command.
-MMD Like `-MD' except mention only user header files,
not system header files.
-H Print the name of each header file used, in addition
to other normal activities.
-Aquestion(answer)
Assert the answer answer for question, in case it
is tested with a preprocessor conditional such as
`#if #question(answer)'. `-A-' disables the standard
assertions that normally describe the target
machine.
-Aquestion
(answer) Assert the answer answer for question, in
case it is tested with a preprocessor conditional
such as `#if #question(answer)'. `-A-' disables
the standard assertions that normally describe the
target machine.
-Dmacro
Define macro macro with the string `1' as its definition.
-Dmacro=defn
Define macro macro as defn. All instances of
`-D' on the command 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 definitions that are in effect at the end of
preprocessing. Used with the `-E' option.
-dD Tell the preprocessor to pass all macro definitions
into the output, in their proper sequence in the
rest of the output.
-dN Like `-dD' except that the macro arguments and contents
are omitted. Only `#define name' is included
in the output.
-Wa,option
Pass option as an option to the assembler. If op-
tion contains commas, it is split into multiple options
at the commas.
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.
object-file-name
A file name that does not end in a special recognized
suffix is considered to name an object file
or library. (Object files are distinguished from
libraries by the linker according to the file contents.)
If GCC does a link step, these object
files are used as input to the linker.
-llibrary
Use the library named library when linking.
The linker searches a standard list of directories
for the library, which is actually a file named
`liblibrary.a'. The linker then uses this file as
if it had been specified precisely by name.
The directories searched include several standard
system directories plus any that you specify with
`-L'.
Normally the files found this way are library
files--archive files whose members are object
files. The linker handles an archive file by scanning
through it for members which define symbols
that have so far been referenced but not defined.
However, if the linker finds an ordinary object
file rather than a library, the object file 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 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 linker.
-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 objects 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 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 argument. For example, to
pass `-assert definitions', you must write `-Xlink-
er -assert -Xlinker definitions'. It does not work
to write `-Xlinker "-assert definitions"', because
this passes the entire string as a single argument,
which is not what the linker expects.
-Wl,option
Pass option as an option to the linker. If option
contains commas, it is split into multiple options
at the commas.
-u symbol
Pretend the symbol symbol is undefined, to force
linking of library modules to define it. You can
use `-u' multiple times with different symbols to
force loading of additional library modules.
These options specify directories to search for header
files, for libraries and for parts of the compiler:
-Idir Append directory dir to the list of directories
searched for include files.
-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 `#in-
clude "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'.
-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'. 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 compiler driver searches for the unmodified
program name, using the directories specified
in your `PATH' environment variable.
The run-time support file `libgcc.a' is also
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.
Most of the time, on most machines, `libgcc.a' is
not actually necessary.
You can get a similar result from the environment
variable GCC_EXEC_PREFIX; if it is defined, its
value is used as a prefix in the same way. If both
the `-B' option and the GCC_EXEC_PREFIX variable
are present, the `-B' option is used first and the
environment variable value second.
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; reject 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 traditional 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 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.
-pedantic-errors
Like `-pedantic', except that errors are produced
rather than warnings.
-W Print extra warning messages for these events:
o A nonvolatile automatic variable might be changed
by a call to longjmp. These warnings are possible
only in optimizing compilation.
The compiler sees only the calls to setjmp. It
cannot know where longjmp will be called; in fact,
a signal handler could call it at any point in the
code. As a result, you may get a warning even when
there is in fact no problem because longjmp cannot
in fact be called at the place which would cause a
problem.
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;
}
Spurious warnings can occur because GNU CC does not
realize that certain functions (including abort and
longjmp) will never return.
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 `<='.
-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 the main function is declared or defined
with a suspicious 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 returntype
that defaults to int. Also warn about any re-
turn statement with no return-value in a function
whose return-type is not void.
-Wunused
Warn whenever a local variable is unused aside from
its declaration, 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 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.
-Wcomment
Warn whenever a comment-start sequence `/*' appears
in a comment.
-Wtrigraphs
Warn if any trigraphs are encountered (assuming
they are enabled).
-Wformat
Check calls to printf and scanf, etc., to make sure
that the arguments supplied have types appropriate
to the format string specified.
-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.
-Wuninitialized
An automatic variable is used without first being
initialized.
These warnings are possible only in optimizing compilation,
because they require data flow information
that is computed only when optimizing. If you
don't specify `-O', you simply won't get these
warnings.
These warnings occur only for variables that are
candidates for register allocation. Therefore,
they do not occur for a variable that is declared
volatile, or whose address is taken, or whose size
is other than 1, 2, 4 or 8 bytes. Also, they do
not occur for structures, unions or arrays, even
when they are in registers.
Note that there may be no warning about a variable
that is used only to compute a value that itself is
never used, because such computations may be deleted
by data flow analysis before the warnings are
printed.
These warnings are made optional because GNU CC 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 GNU CC 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.
Some spurious warnings can be avoided if you declare
as volatile all the functions you use that
never return.
-Wparentheses
Warn if parentheses are omitted in certain contexts.
-Wtemplate-debugging
When using templates in a C++ program, warn if debugging
is not yet fully available (C++ only).
-Wall All of the above `-W' options combined. These are
all the options which pertain to usage that we recommend
avoiding and that we believe is easy to
avoid, even in conjunction with macros.
The remaining `-W...' options are not implied by `-Wall'
because they warn about constructions that we consider
reasonable to use, on occasion, in clean programs.
-Wtraditional
Warn about certain constructs that behave differently
in traditional and ANSI C.
o Macro arguments occurring within string constants
in the macro body. These would substitute the argument
in traditional C, but are part of the constant
in ANSI C.
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.
-Wshadow
Warn whenever a local variable shadows another local
variable.
-Wid-clash-len
Warn whenever two distinct identifiers match in the
first len characters. This may help you prepare a
program that will compile 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 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 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
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. These warnings
will help you find at compile time code that can
try to write into a string constant, but only if
you have been very careful about using const in
declarations and prototypes. Otherwise, it will
just be a nuisance; this is why we did not make
`-Wall' request these warnings.
-Wconversion
Warn if a prototype causes a type conversion that
is different from what would happen to the same argument
in the absence of a prototype. This includes
conversions of fixed point to floating and
vice versa, and conversions changing the width or
signedness of a fixed point argument except when
the same as the default promotion.
-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
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
Warn if a global function is defined without a previous
prototype declaration. This warning is issued
even if the definition itself provides a prototype.
The aim is to detect global functions that
fail to be declared in header files.
-Wmissing-declarations
Warn if a global function is defined without a previous
declaration. Do so even if the definition
itself provides a prototype. Use this option to
detect global functions that are not declared in
header files.
-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
Warn if an extern declaration is encountered within
an function.
-Wenum-clash
Warn about conversion between different enumeration
types (C++ only).
-Wlong-long
Warn if 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 functions must match the type signature of
a virtual function declared in the base class. Use
this option to request warnings when a derived
class declares a function that may be an erroneous
attempt to define a virtual function: that is, warn
when a function with the same name as a virtual
function in the base class, but with a type signature
that doesn't match any virtual functions from
the base class.
-Winline
Warn if a function can not be inlined, and either
it was declared as inline, or else the -fin-
line-functions option was given.
-Werror
Treat warnings as errors; abort compilation after
any warning.
GNU CC has various special options that are used for debugging
either your program or GCC:
-g Produce debugging information in the operating system's
native format (stabs, COFF, XCOFF, or DWARF).
GDB can work with this debugging information.
On most systems that use stabs format, `-g' enables
use of extra debugging information that only GDB
can use; this extra information makes debugging
work better in GDB but will probably make other debuggers
crash or refuse to read the program. If
you want to control for certain whether to generate
the extra information, use `-gstabs+', `-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 occasionally produce surprising results:
some variables you declared may not exist at
all; flow of control may briefly move where you did
not expect it; some statements may not be executed
because they compute constant results or their values
were already at hand; some statements may execute
in different places because they were moved
out of loops.
Nevertheless it proves possible to debug optimized
output. This makes it reasonable to use the optimizer
for programs that might have bugs.
The following options are useful when GNU CC is generated
with the capability for more than one debugging format.
-ggdb Produce debugging information in the native format
(if that is 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.
-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.
-gcoff Produce debugging information in COFF format (if
that is supported). This is the format used by SDB
on most System V systems prior to System V Release
4.
-gxcoff
Produce debugging information in XCOFF format (if
that is supported). This is the format used by the
DBX debugger on IBM RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF 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.
-gdwarf
Produce debugging information in DWARF format (if
that is supported). This is the format used by SDB
on most System V Release 4 systems.
-gdwarf+
Produce debugging information in DWARF 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.
-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 backtraces in parts of the program that you
don't plan to debug. This includes descriptions of
functions and external variables, but no information
about local variables and no line numbers.
Level 3 includes extra information, such as all the
macro definitions present in the program. Some debuggers
support macro expansion when you use `-g3'.
-p Generate extra code to write profile
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