semaphore is used to avoid the effect of dead lock.

there are three types of semaphore .

1)binary semaphore->it is only to acquire and release the

semaphore.zero is used for unavailabel and

one is used for available.

2)counting semaphore->when semaphore is acquired counter is

incremented and when semaphore is released conter is

decremented.when counter is zero it is goes to blocked state.

3)mutual exclusion->it is same as binary semaphore.but it is

used for safty purpose.it is two state locked state and

unlocked state.when semaphore is acquired it is goes to

locked state and when semaphore is released it goes to

unlocked state.