These are like integers but has the following three difference from them:
1) Its value can be initialized as integer but only increment or decrement operations are allowed and its current value can't be read.
2) If any thread decrements its value and result is negative then thread itself can't continue until any other thread increments the Semaphore.
3) If any thread increments Semaphore and their are another threads waiting ,one of the thread gets blocked which was in waiting list.
Explanation : Consider the example of a Cyber Cafe which has 20 cabins and is used by one person at a time for the prevention of dispute user requests a cabin from front counter to use PC.When user work is over he returns to counter makes payment and indicates that now one cabin is free.If no cabin is free than user has to wait until any cabin is free.
The person sitting at front counter only knows the no. of occupied cabins ant not the particular cabin which is occupied.if somebody leaves the cabin the no. of occupied cabin's no. is decremented and if somebody requests a cabin the no. of occupied cabin's incremented.
