UGC-NET | UGC NET CS 2017 Jan – II | Question 36

There are three processes P₁, P₂ and P₃ sharing a semaphore for synchronizing a variable. Initial value of semaphore is one. Assume that negative value of semaphore tells us how many processes are waiting in queue. Processes access the semaphore in following order :
(a) P₂ needs to access
(b) P₁ needs to access
(c) P₃ needs to access
(d) P₂ exits critical section
(e) P₁ exits critical section
The final value of semaphore will be :
(A) 0
(B) 1
(C) -1
(D) -2

Answer: (A)
Explanation: Initial value of semaphores S=1

1. P₂ needs to access decreases semaphore by 1, new value will be 0 (no one is waiting)
2. P₁ needs to access decreases semaphore by 1, new value will be -1 (one process is waiting)
3. P₃ needs to access decreases semaphore by 1, new value will be -2 ( 2 process are waiting)
4. P₂ exits critical section increases semaphore by 1, new value will be -1 ( one process is waiting)
5. P₁ exits critical section increases semaphore by 1, new value will be 0 ( no process is waiting)

So, option (A) is correct.
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