Semaphores - aryanjoshi0823/5143-Operating-System GitHub Wiki

Semaphores are just normal variables used to coordinate the activities of multiple processes in a computer system. They are used to enforce mutual exclusion, avoid race conditions, and implement synchronization between processes.

Semaphores

A semaphore is an integer variable that:

• Cannot be negative.
• Has a minimum value of 0.
• Can have a maximum value defined by the system.

Semaphores operate using two main operations:

1. Wait Operation (P operation)

• If the semaphore value > 0, the process enters the critical section.
• If the semaphore value = 0, the process waits.

2. Signal Operation (V operation)

• Increments the semaphore value when a process leaves the critical section.

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Types of Semaphores

1. Binary Semaphore:

   • Has values 0 and 1.
   • 1 allows access to the critical section.
   • 0 denies access.

2. Counting Semaphore:

   • Can have values ≥ 0.
   • Tracks the number of resources available.

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Solving Classical Synchronization Problems

Semaphores are widely used to solve classical synchronization problems such as:

1. Producer-Consumer Problem
2. Readers-Writers Problem
3. Dining Philosophers Problem