GATE CSE concept:threads PYQs & Answers

Questions 1–10 of 10. Imported answers require independent review unless marked reviewed.

Which of the following statements about threads is/are TRUE?

Your answerSelect all correct options. You can choose more than one.

Threads can only be implemented in kernel space

Each thread has its own file descriptor table for open files

All the threads belonging to a process share a common stack

Threads belonging to a process are by default not protected from each other

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Consider the following two threads T1 and T2 that update two shared variables a and b. Assume that initially a = b = 1. Though context switching between threads can happen at any time, each statement of T1 or T2 is executed atomically.

T1: a = a + 1; b = b + 1;

T2: b = 2 * b; a = 2 * a;

Which one of the following options lists all the possible combinations of values of a and b after both T1 and T2 finish execution?

Your answerChoose one option.

(a = 4, b = 4); (a = 3, b = 3); (a = 4, b = 3)

(a = 3, b = 4); (a = 4, b = 3); (a = 3, b = 3)

(a = 4, b = 4); (a = 4, b = 3); (a = 3, b = 4)

(a = 2, b = 2); (a = 2, b = 3); (a = 3, b = 4)

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Consider a multi-threaded program with two threads T1 and T2. The threads share two semaphores: s1 (initialized to 1) and s2 (initialized to 0). The threads also share a global variable x (initialized to 0). The threads execute the code shown below.

// code of T1 // code of T2

wait(s1); wait(s1);

x = x+1; x = x+1;

print(x); print(x);

wait(s2); signal(s2);

signal(s1); signal(s1);

Which of the following outcomes is/are possible when threads T1 and T2 execute concurrently?

Your answerSelect all correct options. You can choose more than one.

T1 runs first and prints 1, T2 runs next and prints 2

T2 runs first and prints 1, T1 runs next and prints 2

T1 runs first and prints 1, T2 does not print anything (deadlock)

T2 runs first and prints 1, T1 does not print anything (deadlock)

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Consider the two functions inc and dec shown below.

inc(){ dec(){

wait(s); wait(s);

X = X+1; X = X-1;

signal(s); signal(s);

} }

There are 5 threads each invoking inc once, and 3 threads each invoking dec once, on the same shared variable X. The initial value of X is 10.

Suppose there are two implementations of the semaphore s, as follows:

I-1: s is a binary semaphore initialized to 1.

I-2: s is a counting semaphore initialized to 2.

Let V1, V2 be the values of X at the end of execution of all the threads with implementations I-1, I-2, respectively.

Which one of the following choices corresponds to the minimum possible values of V1, V2, respectively?

Your answerChoose one option.

15, 7

7, 7

12, 7

12, 8

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Consider the following threads, T₁, T₂, and T₃ executing on a single processor, synchronized using three binary semaphore variables, S₁, S₂, and S₃, operated upon using standard wait() and signal(). The threads can be context switched in any order and at any time.

Diagram for Computer Science and Information Technology question 19

Which initialization of the semaphores would print the sequence BCABCABCA....?

Your answerChoose one option.

S₁ = 1; S₂ = 1; S₃ = 1

S₁ = 1; S₂ = 1; S₃ = 0

S₁ = 1; S₂ = 0; S₃ = 0

S₁ = 0; S₂ = 1; S₃ = 1

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Consider the following multi-threaded code segment (in a mix of C and pseudo-code), invoked by two processes P1 and P2, and each of the processes spawns two threads T1 and T2:

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?

Your answerSelect all correct options. You can choose more than one.

Both P1 and P2 will print the value of x as 2.

At least one of P1 and P2 will print the value of x as 4.

At least one of the threads will print the value of y as 2.

Both T1 and T2, in both the processes, will print the value of y as 1.

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Consider the following pseudocode, where S is a semaphore initialized to 5 in line#2 and counter is a shared variable initialized to 0 in line#1. Assume that the increment operation in line#7 is not atomic.

int counter = 0;

Semaphore S = init(5);

void parop(void)

{

wait(S);

counter++;

signal(S);

} If five threads execute the function parop concurrently, which of the following program behavior(s) is/are possible?

Your answerSelect all correct options. You can choose more than one.

The value of counter is 5 after all the threads successfully complete the execution of parop.

The value of counter is 1 after all the threads successfully complete the execution of parop.

The value of counter is 0 after all the threads successfully complete the execution of parop.

There is a deadlock involving all the threads.

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Which of the following is/are shared by all the threads in a process?

I. Program counter II. Stack III. Address space IV. Registers

Your answerChoose one option.

I and II only

III only

IV only

III and IV only

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Threads of a process share

Your answerChoose one option.

global variables but not heap.

heap but not global variables.

neither global variables nor heap.

both heap and global variables.

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A multithreaded program P executes with

number of threads and uses

number of locks for ensuring mutual exclusion while operating on shared memory locations. All locks in the program are non-reentrant, i.e., if a thread holds a lock

, then it cannot re-acquire lock

without releasing it. If a thread is unable to acquire a lock, it blocks until the lock becomes available. The minimum value of

and the minimum value of

together for which execution of P can result in a deadlock are:

Your answerChoose one option.

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GATE CSE concept:threads PYQs

How to practice concept:threads

Question 24

Question 40

Question 46

Question 38

Question 19

Question 42

Question 46

Question 7

Question 18

Question 27