Explain briefly the working of semaphore with example ?

The E.W. Dijkstra (1965) abstracted the key idea of mutual exclusion in his concepts of semaphores.

Definition
A semaphore is a confined variable whose value is able to be accessed and altered only by the operations P and V and initialization operation called as 'Semaphoiinitislize'.

Binary Semaphores is able to assume only the value 0 or the value 1 counting semaphores as well called general semaphores can assume only nonnegative values.

The P or sleep or wait or down operation on semaphores S written as P(S) or wait (S) operates as follows:

P(S):  IF  S > 0
                THEN S:= S-1
                 ELSE   (wait on S)

The V or wakeup or signal or up operation on semaphore S written as V(S) or signal (S) operates as follows:

V(S)   IF (one or more process is waiting on S)
                THEN (let one of these processes proceeds)
                ELSE   S: = S +1

Operations P and V are complete as indivisible, single, atomic action. It is guaranteed that formerly a semaphore operation has stared, no other process are able to access the semaphore until operation has completed. The Mutual exclusion on the semaphore S is enforced within P(S) and V(S).

If several processes attempt a P(S) concurrently only process will be allowed to proceed. The other processes will be set aside waiting however the implementation of P and V guarantees that processes will not suffer indefinite postponement.

Semaphores resolve the lost-wakeup problem.