A function in C language is a block of code that performs a specific task. It has a name and it is reusable i.e. it can be executed from as many different parts in a C Program as required. It also optionally returns a value to the calling program.
Every function has a unique name. This name is used to call function from "main()" function. A function can be called from within another function.
A function is independent and it can perform its task without intervention from or interfering with other parts of the program.
A function performs a specific task. A task is a distinct job that your program must perform as a part of its overall operation, such as adding two or more integer, sorting an array into numerical order, or calculating a cube root etc.
A function returns a value to the calling program. This is optional and depends upon the task your function is going to accomplish. Suppose you want to just show few lines through function then it is not necessary to return a value. But if you are calculating area of rectangle and wanted to use result somewhere in program then you have to send back (return) value to the calling function.
Structure of Function
<return type> FunctionName (Argument1, Argument2, Argument3,_ _ _ _ _ _)
An example of function.
int sum (int x, int y)
result = x + y;