|The C Preprocessor|
The ‘#pragma’ directive is the method specified by the C standard for providing additional information to the compiler, beyond what is conveyed in the language itself. Three forms of this directive (commonly known as pragmas) are specified by the 1999 C standard. A C compiler is free to attach any meaning it likes to other pragmas.
GCC has historically preferred to use extensions to the syntax of the
language, such as
__attribute__, for this purpose. However, GCC
does define a few pragmas of its own. These mostly have effects on the
entire translation unit or source file.
In GCC version 3, all GNU-defined, supported pragmas have been given a
GCC prefix. This is in line with the
STDC prefix on all
pragmas defined by C99. For backward compatibility, pragmas which were
recognized by previous versions are still recognized without the
GCC prefix, but that usage is deprecated. Some older pragmas are
deprecated in their entirety. They are not recognized with the
GCC prefix. See Obsolete Features.
C99 introduces the
_Pragma operator. This feature addresses a
major problem with ‘#pragma’: being a directive, it cannot be
produced as the result of macro expansion.
_Pragma is an
operator, much like
defined, and can be embedded
in a macro.
Its syntax is
string-literal can be either a normal or wide-character string
literal. It is destringized, by replacing all ‘\\’ with a single
‘\’ and all ‘\"’ with a ‘"’. The result is then
processed as if it had appeared as the right hand side of a
‘#pragma’ directive. For example,
_Pragma ("GCC dependency \"parse.y\"")
has the same effect as
#pragma GCC dependency "parse.y". The
same effect could be achieved using macros, for example
#define DO_PRAGMA(x) _Pragma (#x) DO_PRAGMA (GCC dependency "parse.y")
The standard is unclear on where a
_Pragma operator can appear.
The preprocessor does not accept it within a preprocessing conditional
directive like ‘#if’. To be safe, you are probably best keeping it
out of directives other than ‘#define’, and putting it on a line of
This manual documents the pragmas which are meaningful to the preprocessor itself. Other pragmas are meaningful to the C or C++ compilers. They are documented in the GCC manual.
GCC plugins may provide their own pragmas.
#pragma GCC dependency
#pragma GCC dependencyallows you to check the relative dates of the current file and another file. If the other file is more recent than the current file, a warning is issued. This is useful if the current file is derived from the other file, and should be regenerated. The other file is searched for using the normal include search path. Optional trailing text can be used to give more information in the warning message.
#pragma GCC dependency "parse.y" #pragma GCC dependency "/usr/include/time.h" rerun fixincludes
#pragma GCC poison
#pragma GCC poisonis followed by a list of identifiers to poison. If any of those identifiers appears anywhere in the source after the directive, it is a hard error. For example,
#pragma GCC poison printf sprintf fprintf sprintf(some_string, "hello");
will produce an error.
If a poisoned identifier appears as part of the expansion of a macro which was defined before the identifier was poisoned, it will not cause an error. This lets you poison an identifier without worrying about system headers defining macros that use it.
#define strrchr rindex #pragma GCC poison rindex strrchr(some_string, 'h');
will not produce an error.
#pragma GCC system_header