5.2 Function Types

A function has the derived type "function returning type". The type can be any data type except array types or function types, although pointers to arrays and functions can be returned. If the function returns no value, its type is "function returning void ", sometimes called a void function. A void function in C is equivalent to a procedure in Pascal or a subroutine in FORTRAN. A non-void function in C is equivalent to a function in these other languages.

Functions can be introduced into a program in one of two ways:

• A function definition can create a function designator, define its parameters and their type, define the type of its return value, and supply the body of the function. In the following example, power is a function returning int :

  int power(int base, int exp)
  {
   int n=1;
  
   if (exp < 0)
   {
    printf ("Error: Cannot handle negative exponent\n");
    return -1;
   }
  
   for ( ; exp; exp--)
       n = base * n;
  
   return n;
  }
  

  See Section 5.3 for more information on function definitions.

• A function declaration announces the properties of a function defined elsewhere. In the following example, the function main declares and calls the function power ; the definition of the function, where the code is defined, exists elsewhere:

  main()
  {
  int power(int base, int exp);       /* function declaration  */
  int x, n, y;
     .
     .
     .
  y = power(x,n);                     /* function call         */
  }
  

  This style of function declaration, in which the parameters are declared in a parameter type list, is called a function prototype. Function prototypes require the compiler to check function arguments for consistency with their parameters, and to convert arguments to the declared types of the parameters.

  See Sections 5.4 and 5.5 for more information on function declarations and prototypes.