int x = 0; // global
Lock L1; // global
main() {
create thread T1 to execute foo();
create t

Q: What is GATE-CS?

GATE-CS is a competitive examination conducted in India. This question is from the 2021 GATE-CS Operating Systems paper.

Q: How can I prepare for GATE-CS Operating Systems?

To prepare for GATE-CS Operating Systems, practice previous year questions (PYQs), understand concepts thoroughly, and solve mock tests regularly. This 2021 question covers Concurrency and Synchronization topic.

Q: What is the difficulty level of this question?

This GATE-CS 2021 Operating Systems question has a medium difficulty level and is marked as an important question.

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Question

int x = 0; // global
Lock L1; // global
main() {
    create thread T1 to execute foo();
    create thread T2 to execute foo();
    wait for threads to finish;
    print(x);
}
foo() {
    int y = 0;
    Acquire L1;
    x = x + 1;
    y = y + 1;
    Release L1;
    print(y);
}Two processes P1 and P2 each spawn threads T1 and T2. Which statement(s) is/are correct?

Accepted Answer

Correct Answer: (A) and (B)Concept -- Threads, Locks, and Process Isolation (from Silberschatz, Galvin, Gagne, Operating System Concepts):(A) TRUE: Variable y is a LOCAL variable in foo() (declared as 'int y = 0' inside foo). Each thread call to foo() gets its own stack frame with its own copy of y starting at 0. When y = y + 1 executes, y becomes 1 in that thread's local frame. The lock L1 protects x (global), not y. Since y is local, each thread always prints y = 1. TRUE.(B) TRUE: In each process, x is a global variable initialized to 0. Two threads (T1 and T2) each execute x = x + 1 within the critical section protected by L1. Since L1 ensures mutual exclusion, x is incremented to 1 by T1, then to 2 by T2 (or vice versa). The final value of x printed by main() = 2 in each process. Note: each process has its own copy of global variables (process isolation), so x in P1 and x in P2 are separate. Both print 2. TRUE.(C) FALSE: y is a local variable. It starts at 0 and is incremented to 1 within each thread's foo() call. It can never reach 2 within a single foo() call. FALSE.(D) FALSE: Each process has its own address space and its own x. P1's x reaches 2 (from its own 2 threads). P2's x also reaches 2 (from its own 2 threads). Neither reaches 4 (that would require 4 increments in one process's x). FALSE.

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