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.
"/usr/local/include/myheader.h"is a possible expansion of this macro.
__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
fprintf (stderr, "Internal error: " "negative string length " "%d at %s, line %d.", length, __FILE__, __LINE__);
An ‘#include’ directive changes the expansions of
__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
__LINE__ revert to the values they had before the
__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.
__func__, and GCC has provided
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
"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
"??? ?? ????".
If GCC cannot determine the current time, it will emit a warning message
(once per compilation) and
__TIME__ will expand to
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
__STDC__ to be either undefined or 1. See Invocation.
Lwhere 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.
199409L signifies the 1989 C standard as amended in
1994, which is the current default; the value
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.
__cplusplusto 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
201103Lfor the experimental languages enabled by -std=c++1y and -std=gnu++1y.
__OBJC__to test whether a header is compiled by a C compiler or an Objective-C compiler.