Semaphore: A lock with a counter (IRL: A flag that transmits a signal over a long distance)

• Contains: An atomic integer V, A queue of waiting threads
• init: Create a semaphore with an initial V
• wait (decrement): If V ≤ 0, block. Otherwise decrease V by 1
• signal (increment): Increase V by 1, unblock any waiting thread

Why semaphore?

• Some synchronization problems make more sense with semaphores.
• Multiple threads can call wait and signal

Types

Binary Semaphore: Count = 0/1, sem_init(1)

• Single access to a resource, mutex, same as lock

Counting Semaphore: sem_init(N > 1)

• N resources available, multiple threads can pass

Limitations

• Waiting reason is hidden in sem_wait
• sem_wait only checks count != 0, more complex conditions require CVs