A main program is sub divided into small and manageable pieces which are calledsubprogram.It is a set of instructions which perform specific action/task. It can be reused anynumber of times in a program. It is called by its name and it returns a value to the position where it is called. It has two types.
i.Built-in functions ii.User defined functions
in functions:- Built in functions are the functions that are provided by C library. Many activities in C are carried out using library functions. These functions perform file access, mathematical computations, graphics, memory management etc. A library function is accessed simply by writing the function name, followed by an optional list of arguments and header file of used function should be included with the program. Definition of built in functions are defined in a special header file. A header file can contain definition of more than one library function but the same function can not be defined in two header files.
User defined function:-
Functions provided by library are not enough for user so to complete their needs user has to define some functions themselves, these are called user defined functions.Means except built in functions user can also define and write small programs as functions to do a task relevant to their programs, there functions should be codified by the user, so that such functions can perform the task as desired by user.