Suppose we want to synchronize two concurrent processes P and Q using binary semaphores S and T. The code for the processes P and Q is shown below.

Process P:
while (1) { 
   W:
      print '0'; 
      print '0'; 
   X:
}
Process Q:
while (1) {
   Y:
      print '1';
      print '1'; 
   Z:
}

Synchronization statements can be inserted only at points W, X, Yand Z
Which of the following will ensure that the output string never contains a substring of the form 01n0 or 10n1 where n is odd?

A.

P(S) at W, V(S) at X, P(T) at Y, V(T) at Z, S and T initially 1

B.

P(S) at W, V(T) at X, P(T) at Y, V(S) at Z, S and T initially 1

C.

P(S) at W, V(S) at X, P(S) at Y, V(S) at Z, S initially 1

D.

V(S) at W, V(T) at X, P(S) at Y, P(T) at Z, S and T initially 1

Solution:

According to the question, the output shouldn't contain substrings of the form 01n0 or 10n1 where n is odd means any of the processes of P and Q should not be repeated twice concurrently. So, one semaphore is enough for the above problem.

So the correct answer is Option C.