28.1 K&R-Style Functions

Classic C (also called K&R C after its authors, Brian Kernighan and Dennis Ritchie) uses a function header that's different from the one used in C++. In C++ the parameter types and names are included inside the ( ) defining the function. In Classic C, only the names appear. Type information comes later. The following code shows the same function twice, first as defined in C++, followed by its K&R definition:

int do_it(char *name, int function)   // C++ function definition
{
    // Body of the function

int do_it(name, function)             // Classic C definition
char *name;
int function;
{
    // Body of the function

When C++ came along, the ANSI C committee decided it would be a good idea if C used the new function definitions. However, because there was a lot of code out there using the old method, C accepts both types of functions. C++ does not.

28.1.1 Prototypes

Classic C does not require prototypes. In many cases, prototypes are missing from C programs. A function that does not have a prototype has an implied prototype of:

int funct(  );    // Default prototype for Classic C functions

The ( ) in C does not denote an empty argument list. Instead it denotes a variable length argument list with no type checking of the parameters. Also, Classic C prototypes have no parameter lists. The only "prototype" you'll see consists merely of "( )", such as:

int do_it(  );    // Classic C function prototype

This tells C that do_it returns an int and takes any number of parameters. C does not type-check parameters, so the following are legal calls to do_it:

i = do_it(  );
i = do_it(1, 2, 3);
i = do_it("Test", 'a');

C++ requires function prototypes, so you have to put them in. There are tools out there such as the GNU prototize utility that help you by reading your code and generating function prototypes. Otherwise, you will have to do it manually.