#include <header name>
The include directive instructs the preprocessor to paste the text of the given file into the current file. Generally, it is necessary to tell the preprocessor where to look for header files if they are not placed in the current directory or a standard system directory. This can be done either at compile time or as part of your compiler's project file. This feature is implementation-specific, so see the compilers page for more information.

If an included file cannot be found, compilation will cease with an error.


C preprocessor tutorial