A function identifier preceded by its return type and followed by its parameter list is called a function declaration or function prototype. The prototype informs the compiler of the format and existence of a function prior to its use. The compiler checks for mismatches between the parameters of a function call and those in the function declaration. The compiler also uses the declaration for argument type checking and argument conversions.
Implicit declaration of functions is not allowed: you must explicitly declare every function before you can call it.
If a function declaration is not visible at the point at which a call to the function is made, the compiler assumes an implicit declaration of extern int func(); However, for conformance to C99, you should explicitly prototype every function before making a call to it.
All function declarations have the form:
Function declaration syntax >>-+-------------------------+--+--------------------+----------> '-storage_class_specifier-' '-function_specifier-' >--return_type_specifier--function_declarator--;---------------><
In addition, for compatibility with GNU C and C++, XL C/C++ allows you to use attributes to modify the properties of functions. They are described in Function attributes (IBM extension).