The scoping rules for functions are considerably simpler than for variables. Function declarations are very similar to external variable declarations. As we have variables that must be declared before they can be accessed, functions must be declared or prototyped before they can be called, and—like external variables—function declarations also have a file scope. They can be called anywhere within a source file after they have been prototyped or defined.
We have already seen how we can define functions in such a way that prototypes are not needed. We simply define them before they are ever called. Most often, however, it is far more convenient to simply declare function prototypes at the beginning of source files. When this is done, functions can be called from anywhere within the file, and there is no need to worry about whether a function has been declared before calling it.
To make functions extend beyond their compilation...
