Understanding Counting Semaphores: A Guide for GATE Exams

Prepare for GATE exams by mastering the fundamentals of counting semaphores. This tutorial provides a clear explanation of counting semaphores, wait/signal operations, and how to solve common GATE-style problems involving semaphore arithmetic.

Understanding Counting Semaphores

Counting Semaphore Problems

GATE exams often include simple questions on counting semaphores. These questions usually involve basic arithmetic—addition and subtraction.

Remember these key terms:

  • Wait (P): Decrements the semaphore value.
  • Signal (V): Increments the semaphore value.

Example Problem

Let's look at a typical GATE-style question:

A counting semaphore is initially set to 12. Ten wait (P) operations and four signal (V) operations are performed. What is the final value of the semaphore?

  1. Initial Value: S = 12
  2. 10 Wait (P) operations: Each P operation subtracts 1. So, S = S - 10 = 12 - 10 = 2
  3. 4 Signal (V) operations: Each V operation adds 1. So, S = S + 4 = 2 + 4 = 6

Therefore, the final value of the counting semaphore is 6.

C Code Example: Declaring a Character Variable

Syntax

char ch = 'a';

Example Output

Output

She said "Hello!" to me.

Next Topic: Binary Semaphores (Mutexes)