C permits a function to call itself. This process is termed recursion. Recursion is a sometimes tricky, sometimes convenient tool. It's tricky to get recursion to end because a function that calls itself tends to do so indefinitely unless the programming includes a conditional test to terminate recursion.
Recursion often can be used where loops can be used. Sometimes the loop solution is more obvious, sometimes the recursive solution is more obvious. Recursive solutions tend to be more elegant and less efficient than loop solutions.
To see what's involved, let's look at an example. The function main() in Listing 9.6 calls the up_and_down() function. We'll term this the “first level of recursion.” Then up_and_down() ...