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UGC-NET | UGC NET CS 2017 Jan – II | Question 36
  • Last Updated : 26 Mar, 2018

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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