#### 3.7.1 Standard Predefined Macros

The standard predefined macros are specified by the relevant language standards, so they are available with all compilers that implement those standards. Older compilers may not provide all of them. Their names all start with double underscores.

`__FILE__`
  This macro expands to the name of the current input file, in the form of a C string constant. This is the path by which the preprocessor opened the file, not the short name specified in ``#include`' or as the input file name argument. For example, `"/usr/local/include/myheader.h"` is a possible expansion of this macro.
`__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.

`__FILE__` and `__LINE__` 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__);

An ``#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`').

A ``#line`' directive changes `__LINE__`, and may change `__FILE__` as well. See Line Control.

C99 introduces `__func__`, and GCC has provided `__FUNCTION__` for a long time. Both of these are strings containing the name of the current function (there are slight semantic differences; see the GCC manual). Neither of them is a macro; the preprocessor does not know the name of the current function. They tend to be useful in conjunction with `__FILE__` and `__LINE__`, though.

`__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 `"Feb 12 1996"`. If the day of the month is less than 10, it is padded with a space on the left.

  If GCC cannot determine the current date, it will emit a warning message (once per compilation) and `__DATE__` will expand to `"??? ?? ????"`.

`__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"`.

  If GCC cannot determine the current time, it will emit a warning message (once per compilation) and `__TIME__` will expand to `"??:??:??"`.

`__STDC__`
  In normal operation, this macro expands to the constant 1, to signify that this compiler conforms to ISO Standard C. If GNU CPP is used with a compiler other than GCC, this is not necessarily true; however, the preprocessor always conforms to the standard unless the `-traditional-cpp` option is used.

  This macro is not defined if the `-traditional-cpp` option is used.

  On some hosts, the system compiler uses a different convention, where `__STDC__` is normally 0, but is 1 if the user specifies strict conformance to the C Standard. CPP follows the host convention when processing system header files, but when processing user files `__STDC__` is always 1. This has been reported to cause problems; for instance, some versions of Solaris provide X Windows headers that expect `__STDC__` to be either undefined or 1. See Invocation.

`__STDC_VERSION__`
  This macro expands to the C Standard's version number, a long integer constant of the form yyyymm `L` where yyyy and mm are the year and month of the Standard version. This signifies which version of the C Standard the compiler conforms to. Like `__STDC__`, this is not necessarily accurate for the entire implementation, unless GNU CPP is being used with GCC.

  The value `199409L` signifies the 1989 C standard as amended in 1994, which is the current default; the value `199901L` signifies the 1999 revision of the C standard. Support for the 1999 revision is not yet complete.

  This macro is not defined if the `-traditional-cpp` option is used, nor when compiling C++ or Objective-C.

`__STDC_HOSTED__`
  This macro is defined, with value 1, if the compiler's target is a hosted environment. A hosted environment has the complete facilities of the standard C library available.
`__cplusplus`
  This macro is defined when the C++ compiler is in use. You can use `__cplusplus` to test whether a header is compiled by a C compiler or a C++ compiler. This macro is similar to `__STDC_VERSION__`, in that it expands to a version number. Depending on the language standard selected, the value of the macro is `199711L`, as mandated by the 1998 C++ standard; `201103L`, per the 2011 C++ standard; an unspecified value strictly larger than `201103L` for the experimental languages enabled by `-std=c++1y` and `-std=gnu++1y`.
`__OBJC__`
  This macro is defined, with value 1, when the Objective-C compiler is in use. You can use `__OBJC__` to test whether a header is compiled by a C compiler or an Objective-C compiler.
`__ASSEMBLER__`
  This macro is defined with value 1 when preprocessing assembly language.