cpp.texinfo

这是完整的gcc源代码

@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 __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,

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

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 __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 __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}.
@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.

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} that
are less specific.

@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 is to 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 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

@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:

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

@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}).

The stringification feature is limited to transforming one macro argument
into one string constant: there is no way to combine the argument with
other text and then stringify it all together.  But the example above shows
how an equivalent result can be obtained in ANSI Standard C using the
feature that adjacent string constants are concatenated as one string
constant.  The preprocessor stringifies @samp{EXP}'s actual argument
into a separate string constant, resulting in text like

@example
do @{ if (x == 0) fprintf (stderr, "Warning: " "x == 0" "\n"); @} while (0)
@end example

@noindent
but the C compiler then sees three consecutive string constants and
concatenates them into one, producing effectively

@example
do @{ if (x == 0) fprintf (stderr, "Warning: x == 0\n"); @} while (0)
@end example

Stringification in C involves more than putting doublequote characters
around the fragment; it is necessary to put backslashes in front of all
doublequote characters, and all backslashes in string and character
constants, in order to get a valid C string constant with the proper
contents.  Thus, stringifying @samp{p = "foo\n";} results in @samp{"p =
\"foo\\n\";"}.  However, backslashes that are not inside of string or
character constants are not duplicated: @samp{\n} by itself stringifies to
@samp{"\n"}.

Whitespace (including comments) in the text being stringified is handled
according to precise rules.  All leading and trailing whitespace is ignored.
Any sequence of whitespace in the middle of the text is converted to
a single space in the stringified result.

@node Concatenation, Undefining, Stringification, Macros
@subsection Concatenation

@cindex concatenation
@dfn{Concatenation} means joining two strings into one.  In the context
of macro expansion, concatenation refers to joining two lexical units
into one longer one.  Specifically, an actual argument to the macro can be
concatenated with another actual argument or with fixed text to produce
a longer name.  The longer name might be the name of a function,
variable or type, or a C keyword; it might even be the name of another
macro, in which case it will be expanded.

When you define a macro, you request concatenation with the special
operator @samp{##} in the macro body.  When the macro is called,
after actual arguments are substituted, all @samp{##} operators are
deleted, and so is any whitespace next to them (including whitespace
that was part of an actual argument).  The result is to concatenate
the syntactic tokens on either side of the @samp{##}.

Consider a C program that interprets named commands.  There probably needs
to be a table of commands, perhaps an array of structures declared as
follows:

@example
struct command
@{
  char *name;
  void (*function) ();
@};

struct command commands[] =
@{
  @{ "quit", quit_command@},
  @{ "help", help_command@},
  @dots{}
@};
@end example

It would be cleaner not to have to give each command name twice, once in
the string constant and once in the function name.  A macro which takes the
name of a command as an argument can make this unnecessary.  The string
constant can be created with stringification, and the function name by
concatenating the argument with @samp{_command}.  Here is how it is done:

@example
#define COMMAND(NAME)  @{ #NAME, NAME ## _command @}

struct command commands[] =
@{
  COMMAND (quit),
  COMMAND (help),
  @dots{}
@};
@end example

The usual case of concatenation is concatenating two names (or a name and a
number) into a longer name.  But this isn't the only valid case.  It is
also possible to concatenate two numbers (or a number and a name, such as
@samp{1.5} and @samp{e3}) into a number.  Also, multi-character operators
such as @samp{+=} can be formed by concatenation.  In some cases it is even
possible to piece together a string constant.  However, two pieces of text
that don't together form a valid lexical unit cannot be concatenated.  For
example, concatenation with @samp{x} on one side and @samp{+} on the other
is not meaningful because those two characters can't fit together in any
lexical unit of C.  The ANSI standard says that such attempts at
concatenation are undefined, but in the GNU C preprocessor it is well
defined: it puts the @samp{x} and @samp{+} side by side with no particular
special results.

Keep in mind that the C preprocessor converts comments to whitespace before
macros are even considered.  Therefore, you cannot create a comment by
concatenating @samp{/} and @samp{*}: the @samp{/*} sequence that starts a
comment is not a lexical unit, but rather the beginning of a ``long'' space
character.  Also, you can freely use comments next to a @samp{##} in a
macro definition, or in actual arguments that will be concatenated, because
the comments will be converted to spaces at first sight, and concatenation
will later discard the spaces.

@node Undefining, Redefining, Concatenation, Macros
@subsection Undefining Macros

@cindex undefining macros
To @dfn{undefine} a macro means to cancel its definition.  This is done
with the @samp{#undef} command.  @samp{#undef} is followed by the macro
name to be undefined.

Like definition, undefinition occurs at a specific point in the source
file, and it applies starting from that point.  The name ceases to be a
macro name, and from that point on it is treated by the preprocessor as if
it had never been a macro name.

For example,

@example
#define FOO 4
x = FOO;