What does #pragma once mean in C? [duplicate]

Possible Duplicate:
#pragma - help understanding

I saw the pragma many times,but always confused, anyone knows what it does?Is it windows only?

4 Answers

In the C and C++ programming languages, #pragma once is a non-standard but widely supported preprocessor directive designed to cause the current source file to be included only once in a single compilation. Thus, #pragma once serves the same purpose as #include guards, but with several advantages, including: less code, avoiding name clashes, and improved compile speed.

See the Wikipedia article for further details.

It's used to replace the following preprocessor code:

#ifndef _MYHEADER_H_ #define _MYHEADER_H_ ... #endif 

A good convention is adding both to support legacy compilers (which is rare tho):

#pragma once #ifndef _MYHEADER_H_ #define _MYHEADER_H_ ... #endif 

So if #pragma once fails the old method will still work.

Generally, the #pragma directives are intended for implementing compiler-specific preprocessor instructions. They are not standardized, so you shouldn't rely on them too heavily.

In this case, #pragma once's purpose is to replace the include guards that you use in header files to avoid multiple inclusion. It works a little faster on the compilers that support it, so it may reduce the compilation time on large projects with a lot of header files that are #include'ed frequently.

pragma is a directive to the preprocessor. It is usually used to provide some additional control during the compilation. For example do not include the same header file code. There is a lot of different directives. The answer depends on what follows the pragma word.