GATE | GATE CS Mock 2018 | Question 54

A program is called reentrant if it can be interrupted in the middle of its execution, and then be safely called again (“re-entered”) before its previous invocations complete execution. The interruption could be caused by an internal action such as a jump or call, or by an external action such as an interrupt or signal. Once the reentered invocation completes, the previous invocations will resume correct execution.

Which of the following program is not reentrant ?

(A)

int t;

void swap(int *x, int *y)
{
    t = *x;
    *x = *y;

    // hardware interrupt might invoke isr() here!
    *y = t;
}

void isr()
{
    int x = 1, y = 2;
    swap(&x, &y);
}

(B)

int t;

void swap(int *x, int *y)
{
    int s;

    s = t; // save global variable
    t = *x;
    *x = *y;

    // hardware interrupt might invoke isr() here!
    *y = t;
    t = s; // restore global variable
}

void isr()
{
    int x = 1, y = 2;
    swap(&x, &y);
}

(C)

void swap(int *x, int *y)
{
    int t = *x;
    *x = *y;

    // hardware interrupt might invoke isr() here!
    *y = t;
}

void isr()
{
    int x = 1, y = 2;
    swap(&x, &y);
}

(D) None of these


Answer: (A)

Explanation: Code given in option (A) of a swap() function that fails to be reentrant.

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