Function: The purpose of a function is to take in a number of values or arguments, do some calculations with those arguments and then return a single result.  Each language has different rules to define a function. In the C programming language the basic block for function is given as:  

return value function name (argument list) 

{ 

statement; 

}  

Functions can be called from the main program or from anywhere else, even from within itself. Program control will transfer to function definition statement as soon they are called and then return  back to next statement immediately after the calling point.