Each standard library has a corresponding header file containing the function prototypes for all the functions in that library and definitions of various data types and constants needed by those functions.

The header files ending in .h are "old style" header files .

The programmer can create custom header files that should end in .h. For example, the header file cube.h can be included in our program by placing the directive

#include "cube.h"

at the beginning of the program.

1. Create file with functions .

2. Save as filename.h

3. Load in other files with #include "filename.h"