Chapter 7. Functions
All the instructions of a C program are contained in functions . Each function performs a certain task. A special function name is main( ): the function with this name is the first one to run when the program starts. All other functions are subroutines of the main( ) function (or otherwise dependent procedures, such as call-back functions), and can have any names you wish.
Every function is defined exactly once. A program can declare and call a function as many times as necessary.