C Pocket Reference

Function Prototypes

A function prototype is a declaration that indicates the types of the function’s parameters as well as its return value. For example:

double pow( double, double );    // prototype of pow()

This prototype informs the compiler that the function pow() expects two arguments of type double, and returns a result of type double. Each parameter type may be followed by a parameter name. This name has no more significance than a comment, however, since its scope is limited to the function prototype itself. For example:

double pow( double base, double exponent );

Functions that do not return any result are declared with the type specifier void. For example:

void func1( char *str ); // func1 expects one string
                         // argument and has no return 
                         // value.

Functions with no parameters are declared with the type specifier void in the parameter list:

int  func2( void );     // func2 takes no arguments and
                        // returns a value with type int.

Function declarations should always be in prototype form. All standard C functions are declared in one (or more) of the standard header files. For example, math.h contains the prototypes of the mathematical functions, such as sin(), cos(), pow(), etc., while stdio.h contains the prototypes of the standard input and output functions.