26.5.7 Predefined Macros

Several object-like macros are predefined; you use them without supplying their definitions. Here we explain the ones user programs often need to use. Many other macro names starting with ‘__’ are predefined; in general, you should not define such macro names yourself.

__FILE__

This macro expands to the name of the current input file, in the form of a C string constant. This is the full name by which the GCC 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.

__func__

__FUNCTION__

These names are like variables that have as value a string containing the name of the current function definition. They are not really macros, but this is the best place to mention them.

__FUNCTION__ is the name that has been defined in GNU C since time immemorial; __func__ is defined by the C standard. With the following conditionals, you can use whichever one is defined.

#if __STDC_VERSION__ < 199901L
# if __GNUC__ >= 2
#  define __func__ __FUNCTION__
# else
#  define __func__ "<unknown>"
# endif
#endif

__PRETTY_FUNCTION__

This is equivalent to __FUNCTION__ in C, but in C++ the string includes argument type information as well. It is a GNU C extension.

Those features are useful in generating an error message to report an inconsistency detected by the program; the message can state the source line where the inconsistency was detected. For example,

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

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

__DATE__

This macro expands to a string constant that describes the date of compilation. The string constant contains eleven characters and looks like "Feb 12 1996". If the day of the month is just one digit, an extra space precedes it so that the date is always eleven characters.

If the compiler cannot determine the current date, it emits a warning messages (once per compilation) and __DATE__ expands to "??? ?? ????".

We deprecate the use of __DATE__ for the sake of reproducible compilation.

__TIME__

This macro expands to a string constant that describes the time of compilation. The string constant contains eight characters and looks like "23:59:01".

If the compiler cannot determine the current time, it emits a warning message (once per compilation) and __TIME__ expands to "??:??:??".

We deprecate the use of __TIME__ for the sake of reproducible compilation.

__STDC__

In normal operation, this macro expands to the constant 1, to signify that this compiler implements ISO Standard C.

__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 states which version of the C Standard the compiler implements.

The current default value is 201112L, which signifies the C 2011 standard.

__STDC_HOSTED__

This macro is defined, with value 1, if the compiler’s target is a hosted environment. A hosted environment provides the full facilities of the standard C library.

The rest of the predefined macros are GNU C extensions.

__COUNTER__

This macro expands to sequential integral values starting from 0. In other words, each time the program uses this acro, it generates the next successive integer. This, with the ## operator, provides a convenient means for macros to generate unique identifiers.

__GNUC__

__GNUC_MINOR__

__GNUC_PATCHLEVEL__

These macros expand to the major version, minor version, and patch level of the compiler, as integer constants. For example, GCC 3.2.1 expands __GNUC__ to 3, __GNUC_MINOR__ to 2, and __GNUC_PATCHLEVEL__ to 1.

If all you need to know is whether or not your program is being compiled by GCC, or a non-GCC compiler that claims to accept the GNU C extensions, you can simply test __GNUC__. If you need to write code that depends on a specific version, you must check more carefully. Each change in the minor version resets the patch level to zero; each change in the major version (which happens rarely) resets the minor version and the patch level to zero. To use the predefined macros directly in the conditional, write it like this:

/* Test for version 3.2.0 or later. */
#if __GNUC__ > 3 || \
    (__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \
                       (__GNUC_MINOR__ == 2 && \
                        __GNUC_PATCHLEVEL__ > 0))

Another approach is to use the predefined macros to calculate a single number, then compare that against a threshold:

#define GCC_VERSION (__GNUC__ * 10000 \
                     + __GNUC_MINOR__ * 100 \
                     + __GNUC_PATCHLEVEL__)
/* … */
/* Test for GCC > 3.2.0 */
#if GCC_VERSION > 30200

Many people find this form easier to understand.

__VERSION__

This macro expands to a string constant that describes the version of the compiler in use. You should not rely on its contents’ having any particular form, but you can count on it to contain at least the release number.

__TIMESTAMP__

This macro expands to a string constant that describes the date and time of the last modification of the current source file. The string constant contains abbreviated day of the week, month, day of the month, time in hh:mm:ss form, and the year, in the format "Sun Sep 16 01:03:52 1973". If the day of the month is less than 10, it is padded with a space on the left.

If GCC cannot determine that information date, it emits a warning message (once per compilation) and __TIMESTAMP__ expands to "??? ??? ?? ??:??:?? ????".

We deprecate the use of this macro for the sake of reproducible compilation.