Chapter 31: Semaphores

What is a semaphore?

An object with an integer value manipulated by wait and post

What does sem_wait() do?

Decrements the value; if negative, the thread waits (sleeps)

What does sem_post() do?

Increments the value; if threads are waiting, wakes one

To use a semaphore as a Lock (mutex), what value should it be initialized to?

1

To use a semaphore for Ordering (e.g., parent waits for child), what value should it be initialized to?

0

How many semaphores are typically used to solve the Bounded Buffer problem?

3 (mutex, empty, full)

In the Producer/Consumer solution, where should the mutex be acquired?

Inside the empty/full wait calls (around the critical section only)

What is the goal of a Reader-Writer lock?

To allow multiple concurrent readers but only a single writer

In the Dining Philosophers problem, what causes deadlock?

Every philosopher grabbing the fork to their left simultaneously

Can semaphores be built using locks and condition variables?

Yes, easily