GNU Info
(cpp-295.info)Standard Predefined
Standard Predefined Macros
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The standard predefined macros are available with the same meanings
regardless of the machine or operating system on which you are using
GNU C. Their names all start and end with double underscores. Those
preceding `__GNUC__' in this table are standardized by ANSI C; the rest
are GNU C extensions.
`__FILE__'
This macro expands to the name of the current input file, in the
form of a C string constant. The precise name returned is the one
that was specified in `#include' or as the input file name
argument.
`__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.
This and `__FILE__' 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__);
A `#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').
The expansions of both `__FILE__' and `__LINE__' are altered if a
`#line' directive is used. Note: Combining Sources.
`__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 1 1996"'.
`__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"'.
`__STDC__'
This macro expands to the constant 1, to signify that this is ANSI
Standard C. (Whether that is actually true depends on what C
compiler will operate on the output from the preprocessor.)
On some hosts, system include files use a different convention,
where `__STDC__' is normally 0, but is 1 if the user specifies
strict conformance to the C Standard. The preprocessor follows
the host convention when processing system include files, but when
processing user files it follows the usual GNU C convention.
This macro is not defined if the `-traditional' option is used.
`__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 preprocessor conforms to. Like
`__STDC__', whether this version number is accurate for the entire
implementation depends on what C compiler will operate on the
output from the preprocessor.
This macro is not defined if the `-traditional' option is used.
`__GNUC__'
This macro is defined if and only if this is GNU C. This macro is
defined only when the entire GNU C compiler is in use; if you
invoke the preprocessor directly, `__GNUC__' is undefined. The
value identifies the major version number of GNU CC (`1' for GNU CC
version 1, which is now obsolete, and `2' for version 2).
`__GNUC_MINOR__'
The macro contains the minor version number of the compiler. This
can be used to work around differences between different releases
of the compiler (for example, if gcc 2.6.3 is known to support a
feature, you can test for `__GNUC__ > 2 || (__GNUC__ == 2 &&
__GNUC_MINOR__ >= 6)'). The last number, `3' in the example
above, denotes the bugfix level of the compiler; no macro contains
this value.
`__GNUG__'
The GNU C compiler defines this when the compilation language is
C++; use `__GNUG__' to distinguish between GNU C and GNU C++.
`__cplusplus'
The draft ANSI standard for C++ used to require predefining this
variable. Though it is no longer required, GNU C++ continues to
define it, as do other popular C++ compilers. You can use
`__cplusplus' to test whether a header is compiled by a C compiler
or a C++ compiler.
`__STRICT_ANSI__'
GNU C defines this macro if and only if the `-ansi' switch was
specified when GNU C was invoked. Its definition is the null
string. This macro exists primarily to direct certain GNU header
files not to define certain traditional Unix constructs which are
incompatible with ANSI C.
`__BASE_FILE__'
This macro expands to the name of the main input file, in the form
of a C string constant. This is the source file that was specified
as an argument when the C compiler was invoked.
`__INCLUDE_LEVEL__'
This macro expands to a decimal integer constant that represents
the depth of nesting in include files. The value of this macro is
incremented on every `#include' directive and decremented at every
end of file. For input files specified by command line arguments,
the nesting level is zero.
`__VERSION__'
This macro expands to a string constant which describes the
version number of GNU C. The string is normally a sequence of
decimal numbers separated by periods, such as `"2.6.0"'.
`__OPTIMIZE__'
GNU CC defines this macro in optimizing compilations. It causes
certain GNU header files to define alternative macro definitions
for some system library functions. You should not refer to or
test the definition of this macro unless you make very sure that
programs will execute with the same effect regardless.
`__CHAR_UNSIGNED__'
GNU C defines this macro if and only if the data type `char' is
unsigned on the target machine. It exists to cause the standard
header file `limits.h' to work correctly. You should not refer to
this macro yourself; instead, refer to the standard macros defined
in `limits.h'. The preprocessor uses this macro to determine
whether or not to sign-extend large character constants written in
octal; see Note: The `#if' Directive.
`__REGISTER_PREFIX__'
This macro expands to a string (not a string constant) describing
the prefix applied to CPU registers in assembler code. You can
use it to write assembler code that is usable in multiple
environments. For example, in the `m68k-aout' environment it
expands to the null string, but in the `m68k-coff' environment it
expands to the string `%'.
`__USER_LABEL_PREFIX__'
Similar to `__REGISTER_PREFIX__', but describes the prefix applied
to user generated labels in assembler code. For example, in the
`m68k-aout' environment it expands to the string `_', but in the
`m68k-coff' environment it expands to the null string. This does
not work with the `-mno-underscores' option that the i386 OSF/rose
and m88k targets provide nor with the `-mcall*' options of the
rs6000 System V Release 4 target.
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