C Primer Plus, Fourth Edition

Functions and Pointers

As the discussion on declarations illustrated, it's possible to declare pointers to functions. You might wonder if such a beast has any usefulness. Typically, a function pointer is used as an argument to another function, telling the second function which function to use. For instance, sorting an array involves comparing two elements to see which comes first. If the elements are numbers, you can use the > operator. More generally, the elements may be a string or a structure, requiring a function call to do the comparison. The qsort() function from the C library is designed to work with arrays of any kind as long as you tell it what function to use to compare elements. For that purpose, it takes a pointer to a function as ...