A certain computation generates two arrays a and b such that a[i] = f(i) for 0…

GATE · 2013 · CS · Computer Science & ITBARC · Computer Science · 2013

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: X computes array a, and Y computes array b. The processes use two binary semaphores R and S, both initialized to zero. Array a is shared by the two processes.

Process X

private i;
for (i = 0; i < n; i++) {
    a[i] = f(i);
    ExitX(R, S);
}

Process Y

private i;
for (i = 0; i < n; i++) {
    EntryY(R, S);
    b[i] = g(a[i]);
}

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

ExitX(R, S) {
    P(R);
    V(S);
}

EntryY(R, S) {
    P(S);
    V(R);
}

ExitX(R, S) {
    V(R);
    V(S);
}

EntryY(R, S) {
    P(R);
    P(S);
}

ExitX(R, S) {
    P(S);
    V(R);
}

EntryY(R, S) {
    V(S);
    P(R);
}

ExitX(R, S) {
    V(R);
    P(S);
}

EntryY(R, S) {
    V(S);
    P(R);
}

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Correct answer: C

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