An Objective C preprocessor directive nearly identical to #include, except that it checks to see if the file has already been included.

gcc supports it, but doesn't like it, and will bark a rather large warning to stderr if you use it. Instead, they encourage you to do this in the header:

#ifndef __SOMEFILE_H__
#define __SOMEFILE_H__

-Wno-import gets rid of this warning.

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