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.
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__);
’ directive changes the expansions of
to correspond to the included file. At the end of that file, when processing resumes on the input file that contained the ‘
’ directive, the expansions of
revert to the values they had before the ‘
is then incremented by one as processing moves to the line after the ‘
’ directive changes
, and may change
as well. See Line Control
, 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 with
"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
will expand to
"??? ?? ????"
If GCC cannot determine the current time, it will emit a warning message (once per compilation) and
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
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
is always 1. This has been reported to cause problems; for instance, some versions of Solaris provide X Windows headers that expect
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.
signifies the 1989 C standard as amended in 1994, which is the current default; the value
signifies the 1999 revision of the C standard; the value
signifies the 2011 revision of the C standard; the value
signifies the 2017 revision of the C standard (which is otherwise identical to the 2011 version apart from correction of defects).
This macro is not defined if the
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
199711Lfor the 1998 C++ standard,
201103Lfor the 2011 C++ standard,
201402Lfor the 2014 C++ standard,
201703Lfor the 2017 C++ standard, or an unspecified value strictly larger than
201703Lfor the experimental languages enabled by -std=c++2a and -std=gnu++2a .
__OBJC__to test whether a header is compiled by a C compiler or an Objective-C compiler.