There are a few other things that you should keep in mind as you write functions in C.
The most obvious thing to remember is to use comments. Every function should be preceded by a comment that describes what the function does, what arguments the function takes, and what value the function returns. It may or may not be necessary to describe how the function does its job, depending on how complicated the function is.
Take a look at the ``rods.c'' program. Notice that the distance function uses the square function that was defined earlier in the file. What do you think would happen if you moved the definition of the square function to come after the definition of the distance function?
The C compiler would warn you that distance was calling a function that had not yet been defined. Always put the definition of a function before the statements that invoke the function.