Prevent multiple definitions in header files

When there definitions in a header file that can not be made twice, the code below should be used. A header file may be included twice other include files include it, or an included file includes it and the source file includes it again.

To prevent bad effects from a double include, it is common to surround the body in the include file with the following (where MYHEADER_H is replaced by a name that is appropriate for your program).

#ifndef MYHEADER_H
#define MYHEADER_H
	// This will be seen by the compiler only once 
#endif /* MYHEADER_H */

http://www.fredosaurus.com/notes-cpp/preprocessor/ifdef.html