What are some examples of semaphore usage in programming?

What are some examples of semaphore usage in programming?

Semaphores are often used in concurrent programming to control access to shared resources. For example, in a multi-threaded application, a semaphore can limit the number of threads that access a particular resource at the same time.

Can you give me a specific code example?

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

import threading

# 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.')
    semaphore.acquire()
    print(f'Thread {thread_id} has accessed the resource.')
    # Simulate resource usage
    semaphore.release()
    print(f'Thread {thread_id} has released the resource.')

# 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()

What happens if more than two threads try to access the resource?

If more than two threads try to access the resource, the additional threads will be blocked until one of the currently accessing threads releases the semaphore. This ensures that only two threads can access the resource at the same time.