Operating Systems | Process Management | Question 6

A shared variable x, initialized to zero, is operated on by four concurrent processes W, X, Y, Z as follows. Each of the processes W and X reads x from memory, increments by one, stores it to memory, and then terminates. Each of the processes Y and Z reads x from memory, decrements by two, stores it to memory, and then terminates. Each process before reading x invokes the P operation (i.e., wait) on a counting semaphore S and invokes the V operation (i.e., signal) on the semaphore S after storing x to memory. Semaphore S is initialized to two. What is the maximum possible value of x after all processes complete execution? (GATE CS 2013)

(A) -2
(B) -1
(C) 1
(D) 2

Answer: (D)

Explanation: See http://quiz.geeksforgeeks.org/operating-systems-process-management-question-11/ for explanation.

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