What are semaphores and how are they used in different programming languages?

What are semaphores and how are they used in different programming languages?

Semaphores are synchronization tools used to control access to shared resources in concurrent programming. They help prevent race conditions. Different programming languages implement semaphores in various ways. For example, in Java, you can use the Semaphore class from the java.util.concurrent package, while in Python, the threading module provides a Semaphore class.

Can you give me an example of how to implement a semaphore in Python?

Sure! Here's a simple example in Python using the threading module:

import threading
import time

# Create a semaphore with a limit of 2
semaphore = threading.Semaphore(2)

def access_resource(thread_id):
    print(f'Thread {thread_id} is trying to access the resource.')
    with semaphore:
        print(f'Thread {thread_id} has acquired the semaphore.')
        time.sleep(2)  # Simulate resource access
    print(f'Thread {thread_id} has released the semaphore.')

# Create multiple threads
threads = [threading.Thread(target=access_resource, args=(i,)) for i in range(5)]

# Start the threads
for thread in threads:
    thread.start()

# Wait for all threads to complete
for thread in threads:
    thread.join()

This code creates a semaphore that allows up to 2 threads to access the resource at the same time.

How does semaphore differ from mutex?

A semaphore allows multiple threads to access a resource concurrently, up to a specified limit, while a mutex (mutual exclusion) allows only one thread to access a resource at a time. Semaphores can be thought of as a generalized version of mutexes.