Consider the following multi-threaded code segment (in a mix of C and… — GATE CS 2021 Set 2 Q42

Consider the following multi-threaded code segment (in a mix of C and pseudo-code), invoked by two processes $P_1$ and $P_2$ , and each of the processes spawns two threads $T_1$ and $T_2$ :

int x = 0;  // global
Lock L1;    // global
main () { 
    create a thread to execute foo(); // Thread T1
    create a thread to execute foo(); // Thread T2
    wait for the two threads to finish execution;
    print(x);}
    
    
foo() {
    int y = 0;
    Acquire L1;
    x = x + 1;
    y = y + 1;
    Release L1;
    print (y);}

Which of the following statement(s) is/are correct?

View worked solution

Answer: A;D

To determine the correct statements, let us analyze how processes and threads manage memory and variables:

1. Process Isolation (Independent Address Spaces)

Processes $P_1$ and $P_2$ run in completely separate address spaces. Therefore, they do not share global variables. Each process has its own independent copy of the global variable x (initialized to $0$ ) and the lock L1.

Changes made to x in $P_1$ have no effect on x in $P_2$ .

2. Thread Sharing within a Process

Threads $T_1$ and $T_2$ spawned by a process share that process's address space, including its global variables.

For process $P_1$ , both threads $T_1$ and $T_2$ access the same global variable x.
Since the increment of x is protected by the lock L1, the operations are mutually exclusive and free from race conditions.
Each thread increments x by $1$ . Since there are two threads, the final value of x in $P_1$ will be $0 + 1 + 1 = 2$ .
Similarly, in process $P_2$ , the two threads increment its independent copy of x, resulting in a final value of $2$ .
Thus, both $P_1$ and $P_2$ will print the value of $x$ as $2$ . (Option A is correct, Option B is incorrect)

3. Local Variables and Thread Stacks

The variable y is declared inside the function foo() as a local variable (int y = 0;).

Every thread has its own private stack where its local variables are allocated.
Therefore, each of the four threads (two in $P_1$ and two in $P_2$ ) has its own independent copy of y.
Each thread independently initializes its own y to $0$ , increments it to $1$ , and prints it.
Thus, all four threads will print the value of y as $1$ . (Option D is correct, Option C is incorrect)

Conclusion

The correct options are A and D.

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GATE CS 2021 Set 2 Q42 — Processes and Threads

1. Process Isolation (Independent Address Spaces)

2. Thread Sharing within a Process

3. Local Variables and Thread Stacks

Conclusion

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