cpp.texi

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@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.)

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

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} command 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} command, 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} command is used.  @xref{Combining Sources}.

@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} command and decremented at every
end of file.

@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 __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.

@item __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 __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{"1.18"}.  The only reasonable use of this
macro is to incorporate it into a string constant.

@item __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__
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 Command,,The @samp{#if} Command}.
@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; @pxref{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
CPU family.

@item ns32000
@findex ns32000
@samp{ns32000} is predefined on computers which use the National
Semiconductor 32000 series CPU.
@end table

Yet other nonstandard predefined macros describe the manufacturer of
the system.  For example,

@table @code
@item sun
@findex sun
@samp{sun} is predefined on all models of Sun computers.

@item pyr
@findex pyr
@samp{pyr} is predefined on all models of Pyramid computers.

@item sequent
@findex sequent
@samp{sequent} is predefined on all models of Sequent computers.
@end table

These predefined symbols are not only nonstandard, they are contrary to the
ANSI standard because their names do not start with underscores.
Therefore, the option @samp{-ansi} inhibits the definition of these
symbols.

This tends to make @samp{-ansi} useless, since many programs depend on the
customary nonstandard predefined symbols.  Even system header files check
them and will generate incorrect declarations if they do not find the names
that are expected.  You might think that the header files supplied for the
Uglix computer would not need to test what machine they are running on,
because they can simply assume it is the Uglix; but often they do, and they
do so using the customary names.  As a result, very few C programs will
compile with @samp{-ansi}.  We intend to avoid such problems on the GNU
system.

What, then, should you do in an ANSI C program to test the type of machine
it will run on?

GNU C offers a parallel series of symbols for this purpose, whose names
are made from the customary ones by adding @samp{__} at the beginning
and end.  Thus, the symbol @code{__vax__} would be available on a Vax,
and so on.

The set of nonstandard predefined names in the GNU C preprocessor is
controlled (when @code{cpp} is itself compiled) by the macro
@samp{CPP_PREDEFINES}, which should be a string containing @samp{-D}
options, separated by spaces.  For example, on the Sun 3, we use the
following definition:

@example
#define CPP_PREDEFINES "-Dmc68000 -Dsun -Dunix -Dm68k"
@end example

@noindent 
This macro is usually specified in @file{tm.h}.

@node Stringification, Concatenation, Predefined, Macros
@subsection Stringification

@cindex stringification
@dfn{Stringification} means turning a code fragment into a string constant
whose contents are the text for the code fragment.  For example,
stringifying @samp{foo (z)} results in @samp{"foo (z)"}.

In the C preprocessor, stringification is an option available when macro
arguments are substituted into the macro definition.  In the body of the
definition, when an argument name appears, the character @samp{#} before
the name specifies stringification of the corresponding actual argument
when it is substituted at that point in the definition.  The same argument
may be substituted in other places in the definition without
stringification if the argument name appears in those places with no
@samp{#}.

Here is an example of a macro definition that uses stringification:

@smallexample
#define WARN_IF(EXP) \
do @{ if (EXP) \
        fprintf (stderr, "Warning: " #EXP "\n"); @} \
while (0)
@end smallexample

@noindent
Here the actual argument for @samp{EXP} is substituted once as given,
into the @samp{if} statement, and once as stringified, into the
argument to @samp{fprintf}.  The @samp{do} and @samp{while (0)} are
a kludge to make it possible to write @samp{WARN_IF (@var{arg});},
which the resemblance of @samp{WARN_IF} to a function would make
C programmers want to do; @pxref{Swallow Semicolon}).