cpp.texi

gcc库的原代码,对编程有很大帮助.

where @samp{1} has been substituted for @samp{X} and @samp{2} for @samp{Y}.

Likewise, @samp{min (x + 28, *p)} expands into

@example
((x + 28) < (*p) ? (x + 28) : (*p))
@end example

Parentheses in the actual arguments must balance; a comma within
parentheses does not end an argument.  However, there is no requirement
for brackets or braces to balance, and they do not prevent a comma from
separating arguments.  Thus,

@example
macro (array[x = y, x + 1])
@end example

@noindent
passes two arguments to @code{macro}: @samp{array[x = y} and @samp{x +
1]}.  If you want to supply @samp{array[x = y, x + 1]} as an argument,
you must write it as @samp{array[(x = y, x + 1)]}, which is equivalent C
code.

After the actual arguments are substituted into the macro body, the entire
result is appended to the front of the remaining input, and the check for
macro calls continues.  Therefore, the actual arguments can contain calls
to other macros, either with or without arguments, or even to the same
macro.  The macro body can also contain calls to other macros.  For
example, @samp{min (min (a, b), c)} expands into this text:

@example
((((a) < (b) ? (a) : (b))) < (c)
 ? (((a) < (b) ? (a) : (b)))
 : (c))
@end example

@noindent
(Line breaks shown here for clarity would not actually be generated.)

@cindex blank macro arguments
@cindex space as macro argument
If a macro @code{foo} takes one argument, and you want to supply an
empty argument, you must write at least some whitespace between the
parentheses, like this: @samp{foo ( )}.  Just @samp{foo ()} is providing
no arguments, which is an error if @code{foo} expects an argument.  But
@samp{foo0 ()} is the correct way to call a macro defined to take zero
arguments, like this:

@example
#define foo0() @dots{}
@end example

If you use the macro name followed by something other than an
open-parenthesis (after ignoring any spaces, tabs and comments that
follow), it is not a call to the macro, and the preprocessor does not
change what you have written.  Therefore, it is possible for the same name
to be a variable or function in your program as well as a macro, and you
can choose in each instance whether to refer to the macro (if an actual
argument list follows) or the variable or function (if an argument list
does not follow).

Such dual use of one name could be confusing and should be avoided
except when the two meanings are effectively synonymous: that is, when the
name is both a macro and a function and the two have similar effects.  You
can think of the name simply as a function; use of the name for purposes
other than calling it (such as, to take the address) will refer to the
function, while calls will expand the macro and generate better but
equivalent code.  For example, you can use a function named @samp{min} in
the same source file that defines the macro.  If you write @samp{&min} with
no argument list, you refer to the function.  If you write @samp{min (x,
bb)}, with an argument list, the macro is expanded.  If you write
@samp{(min) (a, bb)}, where the name @samp{min} is not followed by an
open-parenthesis, the macro is not expanded, so you wind up with a call to
the function @samp{min}.

You may not define the same name as both a simple macro and a macro with
arguments.

In the definition of a macro with arguments, the list of argument names
must follow the macro name immediately with no space in between.  If there
is a space after the macro name, the macro is defined as taking no
arguments, and all the rest of the line is taken to be the expansion.  The
reason for this is that it is often useful to define a macro that takes no
arguments and whose definition begins with an identifier in parentheses.
This rule about spaces makes it possible for you to do either this:

@example
#define FOO(x) - 1 / (x)
@end example

@noindent
(which defines @samp{FOO} to take an argument and expand into minus the
reciprocal of that argument) or this:

@example
#define BAR (x) - 1 / (x)
@end example

@noindent
(which defines @samp{BAR} to take no argument and always expand into
@samp{(x) - 1 / (x)}).

Note that the @emph{uses} of a macro with arguments can have spaces before
the left parenthesis; it's the @emph{definition} where it matters whether
there is a space.

@node Predefined, Stringification, Argument Macros, Macros
@subsection Predefined Macros

@cindex predefined macros
Several simple macros are predefined.  You can use them without giving
definitions for them.  They fall into two classes: standard macros and
system-specific macros.

@menu
* Standard Predefined::     Standard predefined macros.
* Nonstandard Predefined::  Nonstandard predefined macros.
@end menu

@node Standard Predefined, Nonstandard Predefined, Predefined, Predefined
@subsubsection Standard Predefined Macros
@cindex standard predefined macros

The standard predefined macros are available with the same meanings
regardless of the machine or operating system on which you are using GNU C.
Their names all start and end with double underscores.  Those preceding
@code{__GNUC__} in this table are standardized by ANSI C; the rest are
GNU C extensions.

@table @code
@item __FILE__
@findex __FILE__
This macro expands to the name of the current input file, in the form of
a C string constant.  The precise name returned is the one that was
specified in @samp{#include} or as the input file name argument.

@item __LINE__
@findex __LINE__
This macro expands to the current input line number, in the form of a
decimal integer constant.  While we call it a predefined macro, it's
a pretty strange macro, since its ``definition'' changes with each
new line of source code.

This and @samp{__FILE__} are useful in generating an error message to
report an inconsistency detected by the program; the message can state
the source line at which the inconsistency was detected.  For example,

@smallexample
fprintf (stderr, "Internal error: "
                 "negative string length "
                 "%d at %s, line %d.",
         length, __FILE__, __LINE__);
@end smallexample

A @samp{#include} directive changes the expansions of @samp{__FILE__}
and @samp{__LINE__} to correspond to the included file.  At the end of
that file, when processing resumes on the input file that contained
the @samp{#include} directive, the expansions of @samp{__FILE__} and
@samp{__LINE__} revert to the values they had before the
@samp{#include} (but @samp{__LINE__} is then incremented by one as
processing moves to the line after the @samp{#include}).

The expansions of both @samp{__FILE__} and @samp{__LINE__} are altered
if a @samp{#line} directive is used.  @xref{Combining Sources}.

@item __DATE__
@findex __DATE__
This macro expands to a string constant that describes the date on
which the preprocessor is being run.  The string constant contains
eleven characters and looks like @samp{"Jan 29 1987"} or @w{@samp{"Apr
1 1905"}}.

@item __TIME__
@findex __TIME__
This macro expands to a string constant that describes the time at
which the preprocessor is being run.  The string constant contains
eight characters and looks like @samp{"23:59:01"}.

@item __STDC__
@findex __STDC__
This macro expands to the constant 1, to signify that this is ANSI
Standard C.  (Whether that is actually true depends on what C compiler
will operate on the output from the preprocessor.)

@item __STDC_VERSION__
@findex __STDC_VERSION__
This macro expands to the C Standard's version number,
a long integer constant of the form @samp{@var{yyyy}@var{mm}L}
where @var{yyyy} and @var{mm} are the year and month of the Standard version.
This signifies which version of the C Standard the preprocessor conforms to.
Like @samp{__STDC__}, whether this version number is accurate
for the entire implementation depends on what C compiler
will operate on the output from the preprocessor.

@item __GNUC__
@findex __GNUC__
This macro is defined if and only if this is GNU C.  This macro is
defined only when the entire GNU C compiler is in use; if you invoke the
preprocessor directly, @samp{__GNUC__} is undefined.  The value
identifies the major version number of GNU CC (@samp{1} for GNU CC
version 1, which is now obsolete, and @samp{2} for version 2).

@item __GNUC_MINOR__
@findex __GNUC_MINOR__
The macro contains the minor version number of the compiler.  This can
be used to work around differences between different releases of the
compiler (for example, if gcc 2.6.3 is known to support a feature, you
can test for @code{__GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 6)}).
The last number, @samp{3} in the
example above, denotes the bugfix level of the compiler; no macro
contains this value.

@item __GNUG__
@findex __GNUG__
The GNU C compiler defines this when the compilation language is
C++; use @samp{__GNUG__} to distinguish between GNU C and GNU
C++.

@item __cplusplus 
@findex __cplusplus 
The draft ANSI standard for C++ used to require predefining this
variable.  Though it is no longer required, GNU C++ continues to define
it, as do other popular C++ compilers.  You can use @samp{__cplusplus}
to test whether a header is compiled by a C compiler or a C++ compiler.

@item __STRICT_ANSI__
@findex __STRICT_ANSI__
This macro is defined if and only if the @samp{-ansi} switch was
specified when GNU C was invoked.  Its definition is the null string.
This macro exists primarily to direct certain GNU header files not to
define certain traditional Unix constructs which are incompatible with
ANSI C.

@item __BASE_FILE__
@findex __BASE_FILE__
This macro expands to the name of the main input file, in the form
of a C string constant.  This is the source file that was specified
as an argument when the C compiler was invoked.

@item __INCLUDE_LEVEL__
@findex __INCLUDE_LEVEL_
This macro expands to a decimal integer constant that represents the
depth of nesting in include files.  The value of this macro is
incremented on every @samp{#include} directive and decremented at every
end of file.  For input files specified by command line arguments,
the nesting level is zero.

@item __VERSION__
@findex __VERSION__
This macro expands to a string which describes the version number of
GNU C.  The string is normally a sequence of decimal numbers separated
by periods, such as @samp{"2.6.0"}.  The only reasonable use of this
macro is to incorporate it into a string constant.

@item __OPTIMIZE__
@findex __OPTIMIZE__
This macro is defined in optimizing compilations.  It causes certain
GNU header files to define alternative macro definitions for some
system library functions.  It is unwise to refer to or test the
definition of this macro unless you make very sure that programs will
execute with the same effect regardless.

@item __CHAR_UNSIGNED__
@findex __CHAR_UNSIGNED__
This macro is defined if and only if the data type @code{char} is
unsigned on the target machine.  It exists to cause the standard
header file @file{limit.h} to work correctly.  It is bad practice
to refer to this macro yourself; instead, refer to the standard
macros defined in @file{limit.h}.  The preprocessor uses
this macro to determine whether or not to sign-extend large character
constants written in octal; see @ref{#if Directive,,The @samp{#if} Directive}.

@item __REGISTER_PREFIX__
@findex __REGISTER_PREFIX__
This macro expands to a string describing the prefix applied to cpu
registers in assembler code.  It can be used to write assembler code
that is usable in multiple environments.  For example, in the
@samp{m68k-aout} environment it expands to the string @samp{""},
but in the @samp{m68k-coff} environment it expands to the string
@samp{"%"}.

@item __USER_LABEL_PREFIX__
@findex __USER_LABEL_PREFIX__
This macro expands to a string describing the prefix applied to
user generated labels in assembler code.  It can be used to write
assembler code that is usable in multiple environments.
For example, in the @samp{m68k-aout} environment it expands to the
string @samp{"_"}, but in the @samp{m68k-coff} environment it expands
to the string @samp{""}.
@end table

@node Nonstandard Predefined,, Standard Predefined, Predefined
@subsubsection Nonstandard Predefined Macros

The C preprocessor normally has several predefined macros that vary between
machines because their purpose is to indicate what type of system and
machine is in use.  This manual, being for all systems and machines, cannot
tell you exactly what their names are; instead, we offer a list of some
typical ones.  You can use @samp{cpp -dM} to see the values of
predefined macros; see @ref{Invocation}.

Some nonstandard predefined macros describe the operating system in use,
with more or less specificity.  For example,

@table @code
@item unix
@findex unix
@samp{unix} is normally predefined on all Unix systems.

@item BSD
@findex BSD
@samp{BSD} is predefined on recent versions of Berkeley Unix
(perhaps only in version 4.3).
@end table

Other nonstandard predefined macros describe the kind of CPU, with more or
less specificity.  For example,

@table @code
@item vax
@findex vax
@samp{vax} is predefined on Vax computers.

@item mc68000
@findex mc68000
@samp{mc68000} is predefined on most computers whose CPU is a Motorola
68000, 68010 or 68020.

@item m68k
@findex m68k
@samp{m68k} is also predefined on most computers whose CPU is a 68000,
68010 or 68020; however, some makers use @samp{mc68000} and some use
@samp{m68k}.  Some predefine both names.  What happens in GNU C
depends on the system you are using it on.

@item M68020
@findex M68020
@samp{M68020} has been observed to be predefined on some systems that
use 68020 CPUs---in addition to @samp{mc68000} and @samp{m68k}, which
are less specific.

@item _AM29K
@findex _AM29K
@itemx _AM29000
@findex _AM29000
Both @samp{_AM29K} and @samp{_AM29000} are predefined for the AMD 29000