The C Preprocessor: Pragmas

Next: Other Directives, Previous: Line Control, Up: Top [Contents][Index]

## 7 Pragmas

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. The forms of this directive (commonly known as pragmas) specified by C standard are prefixed with `STDC`. A C compiler is free to attach any meaning it likes to other pragmas. Most GNU-defined, supported pragmas have been given a `GCC` prefix.

C99 introduced 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 `sizeof` or `defined`, and can be embedded in a macro.

Its syntax is `_Pragma (string-literal)`, where 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 its own.

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 dependency` allows 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`

  Sometimes, there is an identifier that you want to remove completely from your program, and make sure that it never creeps back in. To enforce this, you can poison the identifier with this pragma. `#pragma GCC poison` is 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.

  For example,

      #define strrchr rindex
      #pragma GCC poison rindex
      strrchr(some_string, 'h');

  will not produce an error.

`#pragma GCC system_header`

  This pragma takes no arguments. It causes the rest of the code in the current file to be treated as if it came from a system header. See System Headers.

`#pragma GCC warning`
`#pragma GCC error`

  `#pragma GCC warning "message"` causes the preprocessor to issue a warning diagnostic with the text ‘`message`’. The message contained in the pragma must be a single string literal. Similarly, `#pragma GCC error "message"` issues an error message. Unlike the ‘`#warning`’ and ‘`#error`’ directives, these pragmas can be embedded in preprocessor macros using ‘`_Pragma`’.

`#pragma once`

  If `#pragma once` is seen when scanning a header file, that file will never be read again, no matter what. It is a less-portable alternative to using ‘`#ifndef`’ to guard the contents of header files against multiple inclusions.

Next: Other Directives, Previous: Line Control, Up: Top [Contents][Index]