bg_image
header

Semaphore

A semaphore is a synchronization mechanism used in computer science and operating system theory to control access to shared resources in a parallel or distributed system. Semaphores are particularly useful for avoiding race conditions and deadlocks.

Types of Semaphores:

  1. Binary Semaphore: Also known as a "mutex" (mutual exclusion), it can only take values 0 and 1. It is used to control access to a resource by exactly one process or thread.
  2. Counting Semaphore: Can take a non-negative integer value and allows access to a specific number of concurrent resources.

How It Works:

  • Semaphore Value: The semaphore has a counter that represents the number of available resources.
    • If the counter is greater than zero, a process can use the resource, and the counter is decremented.
    • If the counter is zero, the process must wait until a resource is released.

Operations:

  • wait (P-operation, Proberen, "to test"):
    • Checks if the counter is greater than zero.
    • If so, it decrements the counter and allows the process to proceed.
    • If not, the process blocks until the counter is greater than zero.
  • signal (V-operation, Verhogen, "to increment"):
    • Increments the counter.
    • If processes are waiting, this operation wakes one of the waiting processes so it can use the resource.

Example:

Suppose we have a resource that can be used by multiple threads. A semaphore can protect this resource:

// PHP example using semaphores (pthreads extension required)

class SemaphoreExample {
    private $semaphore;

    public function __construct($initial) {
        $this->semaphore = sem_get(ftok(__FILE__, 'a'), $initial);
    }

    public function wait() {
        sem_acquire($this->semaphore);
    }

    public function signal() {
        sem_release($this->semaphore);
    }
}

// Main program
$sem = new SemaphoreExample(1); // Binary semaphore

$sem->wait();  // Enter critical section
// Access shared resource
$sem->signal();  // Leave critical section

Applications:

  • Access Control: Controlling access to shared resources like databases, files, or memory areas.
  • Thread Synchronization: Ensuring that certain sections of code are not executed concurrently by multiple threads.
  • Enforcing Order: Coordinating the execution of processes or threads in a specific order.

Semaphores are a powerful tool for making parallel programming safer and more controllable by helping to solve synchronization problems.