In every program we have created thus far, functions have been available—which means callable—from everywhere else within the program. Even in the multi-file program of Chapter 24, Working with Multi-File Programs, every function in every file is available/callable from within every other file. This is not always appropriate, nor is it desirable. Likewise, some variables should only be accessed from within specific functions, or for use within a specific group of functions.

There are many instances where it is appropriate to limit the availability of a function or the accessibility of a variable. For instance, some functions may operate on a given structure and should only ever be called by other functions that also operate on that structure; these functions would never be called by any other functions. Similarly, we might want a value to be accessible to all functions within a program...