Standard Predefined Macros
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   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 `__GNUC__' in this table are standardized by ANSI C; the rest
are GNU C extensions.

`__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 `#include' or as the input file name
     argument.

`__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 `__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,

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

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

     The expansions of both `__FILE__' and `__LINE__' are altered if a
     `#line' directive is used.  See Combining Sources.

`__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 `"Jan 29 1987"' or `"Apr 1 1905"'.

`__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 `"23:59:01"'.

`__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.)

`__STDC_VERSION__'
     This macro expands to the C Standard's version number, a long
     integer constant of the form `YYYYMML' where YYYY and MM are the
     year and month of the Standard version.  This signifies which
     version of the C Standard the preprocessor conforms to.  Like
     `__STDC__', whether this version number is accurate for the entire
     implementation depends on what C compiler will operate on the
     output from the preprocessor.

`__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, `__GNUC__' is undefined.  The
     value identifies the major version number of GNU CC (`1' for GNU CC
     version 1, which is now obsolete, and `2' for version 2).

`__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 `__GNUC__ > 2 || (__GNUC__ == 2 &&
     __GNUC_MINOR__ >= 6)').  The last number, `3' in the example
     above, denotes the bugfix level of the compiler; no macro contains
     this value.

`__GNUG__'
     The GNU C compiler defines this when the compilation language is
     C++; use `__GNUG__' to distinguish between GNU C and GNU C++.

`__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
     `__cplusplus' to test whether a header is compiled by a C compiler
     or a C++ compiler.

`__STRICT_ANSI__'
     This macro is defined if and only if the `-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.

`__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.

`__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 `#include' directive and decremented at every
     end of file.  For input files specified by command line arguments,
     the nesting level is zero.

`__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 `"2.6.0"'.  The only reasonable use
     of this macro is to incorporate it into a string constant.

`__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.

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

`__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
     `m68k-aout' environment it expands to the string `""', but in the
     `m68k-coff' environment it expands to the string `"%"'.

`__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 `m68k-aout' environment it expands to the string
     `"_"', but in the `m68k-coff' environment it expands to the string
     `""'.