Explain the Function Prototype?

The Functions must be declared before they are used, ANSI C provides for the new function declaration syntax called as the function prototype, a function prototype tells the compiler the number and type of arguments that are to be passed to the function and the type of the value that is to be returned by the function.

An illustration is

double sqrt(double);

This tells the compiler that sqrt() is a function that acquire a single argument of type double and returns a double. The universal form of function prototype

type function_name(parameter type list);