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GATE CSE 2026 Session 2 Q51: Possible output patterns of three processes using two semaphores

Question

Consider three processes P1, P2, and P3 running identical code, as shown in the pseudocode below. A and B are two binary semaphores initialized to 1 and 0, respectively. X is a shared variable initialized to 0. Each line in the pseudocode is executed atomically.

Pseudocode of P1, P2, and P3

Wait(A);
Print(*);
X = X+1;
If (X == 2)
  {
    Print($);
    Signal(B);
  }
Signal(A);
Wait(B);
Print(#);
Signal(B);

Assume that any of the three processes can start to execute first and context switching can happen between these processes at any arbitrary time and in any arbitrary order.

Which of the following patterns is/are possible to be generated as an outcome of the execution of these three processes?

Try it: multiple select: one or more correct options

Select one or more options
Select all correct options. No negative marking, no partial marks.
Show answer and explanation

Answer: A, B, C (official GATE 2026 final answer key)

Concept used: A guards the critical section; B is opened only by the second process to enter, and each # passes B on.

First step: Note that $ is printed by the second process inside the critical section, so it must come right after the second *; then check where the third * can fall.

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Source: official GATE 2026 Session 2 CS question paper and final answer key, organised by IIT Guwahati. Question paper · Answer key

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