invoke.texi

gcc库的原代码,对编程有很大帮助.

* Optimize Options::    How much optimization?
* Preprocessor Options:: Controlling header files and macro definitions.
                         Also, getting dependency information for Make.
* Assembler Options::   Passing options to the assembler.
* Link Options::        Specifying libraries and so on.
* Directory Options::   Where to find header files and libraries.
                        Where to find the compiler executable files.
* Target Options::      Running a cross-compiler, or an old version of GNU CC.
@end menu

@node Overall Options
@section Options Controlling the Kind of Output

Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order.  The first three
stages apply to an individual source file, and end by producing an
object file; linking combines all the object files (those newly
compiled, and those specified as input) into an executable file.

@cindex file name suffix
For any given input file, the file name suffix determines what kind of
compilation is done:

@table @code
@item @var{file}.c
C source code which must be preprocessed.

@item @var{file}.i
C source code which should not be preprocessed.

@item @var{file}.ii
C++ source code which should not be preprocessed.

@item @var{file}.m
Objective-C source code.  Note that you must link with the library
@file{libobjc.a} to make an Objective-C program work.

@item @var{file}.h
C header file (not to be compiled or linked).

@item @var{file}.cc
@itemx @var{file}.cxx
@itemx @var{file}.cpp
@itemx @var{file}.C
C++ source code which must be preprocessed.  Note that in @samp{.cxx},
the last two letters must both be literally @samp{x}.  Likewise,
@samp{.C} refers to a literal capital C.

@item @var{file}.s 
Assembler code.

@item @var{file}.S
Assembler code which must be preprocessed.

@item @var{other}
An object file to be fed straight into linking.
Any file name with no recognized suffix is treated this way.
@end table

You can specify the input language explicitly with the @samp{-x} option:

@table @code
@item -x @var{language}
Specify explicitly the @var{language} for the following input files
(rather than letting the compiler choose a default based on the file
name suffix).  This option applies to all following input files until
the next @samp{-x} option.  Possible values for @var{language} are:
@example
c  objective-c  c++
c-header  cpp-output  c++-cpp-output
assembler  assembler-with-cpp
@end example

@item -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 @samp{-x}
has not been used at all).
@end table

If you only want some of the stages of compilation, you can use
@samp{-x} (or filename suffixes) to tell @code{gcc} where to start, and
one of the options @samp{-c}, @samp{-S}, or @samp{-E} to say where
@code{gcc} is to stop.  Note that some combinations (for example,
@samp{-x cpp-output -E} instruct @code{gcc} to do nothing at all.

@table @code
@item -c
Compile or assemble the source files, but do not link.  The linking
stage simply is not done.  The ultimate output is in the form of an
object file for each source file.

By default, the object file name for a source file is made by replacing
the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.

Unrecognized input files, not requiring compilation or assembly, are
ignored.

@item -S
Stop after the stage of compilation proper; do not assemble.  The output
is in the form of an assembler code file for each non-assembler input
file specified.

By default, the assembler file name for a source file is made by
replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.

Input files that don't require compilation are ignored.

@item -E
Stop after the preprocessing stage; do not run the compiler proper.  The
output is in the form of preprocessed source code, which is sent to the
standard output.

Input files which don't require preprocessing are ignored.

@cindex output file option
@item -o @var{file}
Place output in file @var{file}.  This applies regardless to whatever
sort of output is being produced, whether it be an executable file,
an object file, an assembler file or preprocessed C code.

Since only one output file can be specified, it does not make sense to
use @samp{-o} when compiling more than one input file, unless you are
producing an executable file as output.

If @samp{-o} is not specified, the default is to put an executable file
in @file{a.out}, the object file for @file{@var{source}.@var{suffix}} in
@file{@var{source}.o}, its assembler file in @file{@var{source}.s}, and
all preprocessed C source on standard output.@refill

@item -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.

@item -pipe
Use pipes rather than temporary files for communication between the
various stages of compilation.  This fails to work on some systems where
the assembler is unable to read from a pipe; but the GNU assembler has
no trouble.
@end table

@node Invoking G++
@section Compiling C++ Programs

@cindex suffixes for C++ source
@cindex C++ source file suffixes
C++ source files conventionally use one of the suffixes @samp{.C},
@samp{.cc}, @samp{cpp}, or @samp{.cxx}; preprocessed C++ files use the
suffix @samp{.ii}.  GNU CC recognizes files with these names and
compiles them as C++ programs even if you call the compiler the same way
as for compiling C programs (usually with the name @code{gcc}).

@findex g++
@findex c++
However, C++ programs often require class libraries as well as a
compiler that understands the C++ language---and under some
circumstances, you might want to compile programs from standard input,
or otherwise without a suffix that flags them as C++ programs.
@code{g++} is a program that calls GNU CC with the default language
set to C++, and automatically specifies linking against the GNU class
library libg++.
@cindex @code{g++ 1.@var{xx}}
@cindex @code{g++}, separate compiler
@cindex @code{g++} older version
@footnote{Prior to release 2 of the compiler,
there was a separate @code{g++} compiler.  That version was based on GNU
CC, but not integrated with it.  Versions of @code{g++} with a
@samp{1.@var{xx}} version number---for example, @code{g++} version 1.37
or 1.42---are much less reliable than the versions integrated with GCC
2.  Moreover, combining G++ @samp{1.@var{xx}} with a version 2 GCC will
simply not work.} On many systems, the script @code{g++} is also
installed with the name @code{c++}.

@cindex invoking @code{g++}
When you compile C++ programs, you may specify many of the same
command-line options that you use for compiling programs in any
language; or command-line options meaningful for C and related
languages; or options that are meaningful only for C++ programs.
@xref{C Dialect Options,,Options Controlling C Dialect}, for
explanations of options for languages related to C.
@xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
explanations of options that are meaningful only for C++ programs.

@node C Dialect Options
@section Options Controlling C Dialect
@cindex dialect options
@cindex language dialect options
@cindex options, dialect

The following options control the dialect of C (or languages derived
from C, such as C++ and Objective C) that the compiler accepts:

@table @code
@cindex ANSI support
@item -ansi
Support all ANSI standard C programs.

This turns off certain features of GNU C that are incompatible with ANSI
C, such as the @code{asm}, @code{inline} and @code{typeof} keywords, and
predefined macros such as @code{unix} and @code{vax} that identify the
type of system you are using.  It also enables the undesirable and
rarely used ANSI trigraph feature, disallows @samp{$} as part of
identifiers, and disables recognition of C++ style @samp{//} comments.

The alternate keywords @code{__asm__}, @code{__extension__},
@code{__inline__} and @code{__typeof__} continue to work despite
@samp{-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 @samp{-ansi}.  Alternate predefined macros
such as @code{__unix__} and @code{__vax__} are also available, with or
without @samp{-ansi}.

The @samp{-ansi} option does not cause non-ANSI programs to be
rejected gratuitously.  For that, @samp{-pedantic} is required in
addition to @samp{-ansi}.  @xref{Warning Options}.

The macro @code{__STRICT_ANSI__} is predefined when the @samp{-ansi}
option is used.  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.

The functions @code{alloca}, @code{abort}, @code{exit}, and
@code{_exit} are not builtin functions when @samp{-ansi} is used.

@item -fno-asm
Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
keyword, so that code can use these words as identifiers.  You can use
the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
instead.  @samp{-ansi} implies @samp{-fno-asm}.

In C++, this switch only affects the @code{typeof} keyword, since
@code{asm} and @code{inline} are standard keywords.  You may want to
use the @samp{-fno-gnu-keywords} flag instead, as it also disables the
other, C++-specific, extension keywords such as @code{headof}.

@item -fno-builtin
@cindex builtin functions
@findex abort
@findex abs
@findex alloca
@findex cos
@findex exit
@findex fabs
@findex ffs
@findex labs
@findex memcmp
@findex memcpy
@findex sin
@findex sqrt
@findex strcmp
@findex strcpy
@findex strlen
Don't recognize builtin functions that do not begin with two leading
underscores.  Currently, the functions affected include @code{abort},
@code{abs}, @code{alloca}, @code{cos}, @code{exit}, @code{fabs},
@code{ffs}, @code{labs}, @code{memcmp}, @code{memcpy}, @code{sin},
@code{sqrt}, @code{strcmp}, @code{strcpy}, and @code{strlen}.

GCC normally generates special code to handle certain builtin functions
more efficiently; for instance, calls to @code{alloca} may become single
instructions that adjust the stack directly, and calls to @code{memcpy}
may become inline copy loops.  The resulting code is often both smaller
and faster, but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library.

The @samp{-ansi} option prevents @code{alloca} and @code{ffs} from being
builtin functions, since these functions do not have an ANSI standard
meaning.

@item -trigraphs
Support ANSI C trigraphs.  You don't want to know about this
brain-damage.  The @samp{-ansi} option implies @samp{-trigraphs}.

@cindex traditional C language
@cindex C language, traditional
@item -traditional
Attempt to support some aspects of traditional C compilers.
Specifically:

@itemize @bullet
@item
All @code{extern} declarations take effect globally even if they
are written inside of a function definition.  This includes implicit
declarations of functions.

@item
The newer keywords @code{typeof}, @code{inline}, @code{signed}, @code{const}
and @code{volatile} are not recognized.  (You can still use the
alternative keywords such as @code{__typeof__}, @code{__inline__}, and
so on.)

@item
Comparisons between pointers and integers are always allowed.

@item
Integer types @code{unsigned short} and @code{unsigned char} promote
to @code{unsigned int}.

@item
Out-of-range floating point literals are not an error.

@item
Certain constructs which ANSI regards as a single invalid preprocessing
number, such as @samp{0xe-0xd}, are treated as expressions instead.

@item
String ``constants'' are not necessarily constant; they are stored in
writable space, and identical looking constants are allocated
separately.  (This is the same as the effect of
@samp{-fwritable-strings}.)

@cindex @code{longjmp} and automatic variables
@item
All automatic variables not declared @code{register} are preserved by
@code{longjmp}.  Ordinarily, GNU C follows ANSI C: automatic variables
not declared @code{volatile} may be clobbered.

@item
@kindex \x
@kindex \a
@cindex escape sequences, traditional
The character escape sequences @samp{\x} and @samp{\a} evaluate as the
literal characters @samp{x} and @samp{a} respectively.  Without
@w{@samp{-traditional}}, @samp{\x} is a prefix for the hexadecimal
representation of a character, and @samp{\a} produces a bell.

@item
In C++ programs, assignment to @code{this} is permitted with
@samp{-traditional}.  (The option @samp{-fthis-is-variable} also has
this effect.)
@end itemize

You may wish to use @samp{-fno-builtin} as well as @samp{-traditional}
if your program uses names that are normally GNU C builtin functions for
other purposes of its own.

You cannot use @samp{-traditional} if you include any header files that
rely on ANSI C features.  Some vendors are starting to ship systems with
ANSI C header files and you cannot use @samp{-traditional} on such
systems to compile files that include any system headers.