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comp.lang.c FAQ list · Question 11.21

Q: What does ``#pragma once'' mean? I found it in some header files.

A: It is an extension implemented by some preprocessors to help make header files idempotent; that is, to make sure that their contents are processed exactly once even if they are #included multiple times. It is equivalent to the #ifndef trick mentioned in question 10.7, though less portable. Some people claim that #pragma once can be implemented ``more efficiently'' (of course only compilation efficiency is a factor here), but in fact a preprocessor that is serious about compilation efficiency can arrange for the portable #ifndef trick to be handled just as efficiently.

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