The C Preprocessor cont.
these function prototype tell the compiler the types of arguments that get passed to the printf and scanf functions and the type of the value that get returned by these functions
to use other predefined function, other header file is included e.g.
#include <stdlib.h> /* random number generation */
#include <math.h> /* x^y, square root, sin, cos etc.*/
#include <string.h> /* string length, string copy */