Conditional text inclusions
A common use of #ifndef is in header files
It is usually harmful to include a header file more then once (p1.c includes h1.h that includes h2.h)
The way to prevent this is inserting this macro at the beginning of every header file you write: #ifndef _header_name #define _header_name /* prevents entering here in future inclusions*/ … and then insert this line at the end of file: #endif