Function definitions

Programming Tools Guide
Chapter 3, C language compiler

A function definition includes the body of the function after the declaration of the function. As with declarations, a function may be defined as a function prototype definition or defined in the old style. The function prototype style includes type declarations for each parameter in the parameter list. This example shows how main would be defined in each style:

 -------------------------------------------------------------------
 Function prototype style       Old style
 -------------------------------------------------------------------
 int                            int
 main(int argc, char ∗argv[])   main(argc, argv)
 {                              int argc;
  ...                           char ∗argv[];
 }                              {
                                ...
                                }

Some important rules that govern function definitions:

An old style definition names its parameters in an identifier list, and their declarations appear between the function declarator and the ``{'' that begins the function body.
Under the old style, if the type declaration for a parameter was absent, the type defaulted to int. In the new style, all parameters in the parameter list must be type-specified and named. The exception to this rule is the use of ellipsis, explained in ``Function declarators''.
A function definition serves as a declaration.
Incomplete types are not allowed in the parameter list or as the return type of a function definition. They are allowed in other function declarations.