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 introduced __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 yyyymmL 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; the value 201112L
signifies the 2011 revision of the C standard; the value
201710L signifies the 2017 revision of the C standard (which is
otherwise identical to the 2011 version apart from correction of
defects). An unspecified value larger than 201710L is used for
the experimental -std=c2x and -std=gnu2x modes.
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.
__cplusplusThis 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 for the 1998 C++ standard,
201103L for the 2011 C++ standard,
201402L for the 2014 C++ standard,
201703L for the 2017 C++ standard,
202002L for the 2020 C++ standard,
or an unspecified value strictly larger than 202002L for the
experimental languages enabled by -std=c++23 and
-std=gnu++23.
__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.