5.4 Function Declarations

A function can be called without declaring it if the function's return value is int (although this practice is not recommended due to the loss of type-checking capability; all functions should be declared). If the return value is anything else, and if the function definition is located after the calling function in the source code, the function must be declared before calling it. For example:

char lower(int c);                    /* Function declaration */

caller()                              /* Calling function     */
{
int c;
char c_out;
      .
      .
      .
c_out = lower(c);                     /* Function call        */

}

char lower(int c_up)                  /* Function definition  */
{
   .
   .
   .
}

If the function definition for lower was located before the function caller in the source code, lower would not have to be declared again before calling it. In that case, the function definition would serve as its own declaration and would be in scope for any function calls from within all subsequently defined functions in the same source file.

Note that both the function definition and function declaration for lower are in the prototype style. Although C supports the old style of function declaration in which the parameter types are not specified in the function declarator, it is good programming practice to use prototype declarations for all user- defined functions in your program, and to place the prototypes before the first use of the function. Also note that it is valid for the parameter identifier in the function declaration to be different from the parameter identifier in the function definition.

In a function declaration, the void keyword should be used to specify an empty argument list. For example:

   char function_name(void);

As with function definitions, the void keyword can also be used in function declarations to specify the return value type for functions that do not return a value. For example:

main()
{
   void function_name( );
      .
      .
      .
}
void function_name( )
{ }