Like any variable in a C program it is necessary to prototype or declare a function before its use, if it returns a value other than an int. It informs the compiler that the function would be referenced at a later stage in the program. / For example, / In program 1, the statement / void display_message(); / is a function prototype or declaration. Here void specifies that this function does not return any value, and the empty parenthesis indicate that it takes no parameters (arguments).