As we have already seen in our single-file programs, we can put pretty much anything and everything in a source file. We use source files primarily to define functions and we put all the rest, or almost all of the rest, in a header file to be included in the source file.
A source file can be laid out in any number of ways. We can define all functions before they are called and have the main() function at the very end, or we can use function prototypes at the top of the program and place our function definitions in any order, with the main() function typically appearing immediately after the function prototypes. However, there are perfectly good reasons to keep some things in the source file only; we will explore these reasons fully in Chapter 25, Understanding Scope.
Since we know what a source file with everything in it looks like, we will focus our attention on which things properly go into a header file and which things do not go into one...
