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

Question

Possible Duplicate:
#pragma - help understanding

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

Answer 1

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.

Answer 2

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.