# GATE - 2013 | OS | A certain computation generates two arrays a and b such that a[i]=f(i) ## Question:

A certain computation generates two arrays a and b such that a[i]=f(i) for 0 ≤ i < n and b[i]=g(a[i]) for 0 ≤ i < n. Suppose this computation is decomposed into two concurrent processes X and Y such that X computes the array a and Y computes the array b. The processes employ two binary semaphores R and S, both initialized to zero. The array a is shared by the two processes. The structures of the processes are shown below.

Process X:                         Process Y:

private i;                         private i;

for (i=0; i < n; i++) {            for (i=0; i < n; i++) {

a[i] = f(i);                       EntryY(R, S);

ExitX(R, S);                       b[i]=g(a[i]);

}                                 }

Which one of the following represents the CORRECT implementations of ExitX and EntryY?

 A ExitX(R,S) {P(R);V(S);}EntryY(R,S) {P(S);V(R);} B ExitX(R,S) {V(R);V(S);}EntryY(R,S) {P(R);P(S);} C ExitX(R,S) {P(S);V(R);}EntryY(R,S) {V(S);P(R);} D ExitX(R,S) {V(R);P(S);}EntryY(R,S) {V(S);P(R);}

## Solution:

### Option (C) is Correct.

The solution is using a binary semaphore. X and Y are two different processes whatever the values inserted by the processes in the array that will be used by process Y afterward.

Option C - Correct because each and every value inserted by process X in the array will be immediately consumed by process Y.

Thank You