SEMAPHORE

First let me begin our topic by giving an insight to what is actually a semaphore. A semaphore is nothing but a term used in UNIX. That comes in mind is what for we need this variable. For instance there may be times when two processes try to access the same file simultaneously. In this event we must control the access of the file when the other process is accessing. This is done by assigning value to semaphore.
In other words, we also can say that 
In computer science, a semaphore is a protected variable or abstract data type that provides a simple but useful abstraction for controlling access by multiple processes to a common resource in a parallel environment.

A useful way to think of a semaphore is as a record of how many units of a particular resource are available, coupled with operations to safely (i.e. without race conditions) adjust that record as units are required or become free, and if necessary wait until a unit of the resource becomes available. Semaphores are a useful tool in the prevention of race conditions and deadlocks; however, their use is by no means a guarantee that a program is free from these problems. Semaphores which allow an arbitrary resource count are called counting semaphores, whilst semaphores which are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) are called binary semaphores.