Functions must have a definition and should have a declaration, although a definition can serve as a declaration if the declaration appears before the function is called. The function definition includes the function body—the code that executes when the function is called.
A function declaration establishes the name, return type, and attributes of a function that is defined elsewhere in the program. A function declaration must precede the call to the function. This is why the header files containing the declarations for the run-time functions are included in your code prior to a call to a run-time function. If the declaration has information about the types and number of parameters, the declaration is a prototype. See “Function Prototypes” for more information.
The compiler uses the prototype to compare the types of arguments in subsequent calls to the function with the function's parameters and to convert the types of the arguments to the types of the parameters whenever necessary.
A function call passes execution control from the calling function to the called function. The arguments, if any, are passed by value to the called function. Execution of a return statement in the called function returns control and possibly a value to the calling function.
Obsolete Forms of Function Declarations and Definitions
The old-style function declarations and definitions use slightly different rules for declaring parameters than the syntax recommended by the ANSI standard. First, the old-style declarations don't have a parameter list. Second, in the function definition, the parameters are listed, but their types are not declared in the parameter list. The type declarations precede the compound statement constituting the function body. The old-style syntax is obsolete and should not be used in new code. Code using the old-style syntax is still supported, however. This example illustrates the obsolete forms of declarations and definitions:
double old_style(); /* Obsolete function declaration */
double alt_style( a , real ) /* Obsolete function definition */
double *real;
int a;
{
return ( *real + a ) ;
}
Functions returning an integer or pointer with the same size as an int are not required to have a declaration although the declaration is recommended.
The next section shows the syntax for function definitions, including the old-style syntax. The nonterminal for the list of parameters in the old-style syntax is identifier-list.