Just like variables, a function must be declared before it may be used. The function's declaration must occur before main() or other functions that use it. The declaration may then take two forms; the first one is to type in the entire function definition. The second option is to just type in a function prototype in which case the function definition itself may be placed anywhere in the program.
Function prototypes may take two different formats:
- An exact copy of the function header:
- Like the function header, but without the parameter names – only the types need be present for each parameter:
In this example the function is declared and defined before it is used in main().
In this example, the function is declared with a prototype before its use in main(). The function is then defined after it is used in main().