31. Semaphores

Question 1

What is a semaphore?

Answer 1

A semaphore is an object with integer value that we can manipulate with two routines: sem_wait() and sem_post() .

Question 2

What does semaphore equal to when negative?

Answer 2

The value of the semaphore, when negative, is equal to the number of waiting threads.

Question 3

How is semaphore called when it’s used as a lock? What does its value equal to?

Answer 3

It’s called binary semaphore and its value is equal to 1.

Question 4

What value should semaphore have if it’s used for ordering?

Answer 4

0

Question 5

What value should semaphore be initialized to when used for consumer/producer problem?

Answer 5

One semaphore should be initialized to 0 (to identify emptiness) and the other one should be initialized to the size of buffer.

Question 6

What is other use case of semaphore?

Answer 6

Thread throttling. By surrounding resource-intensive region of code with semaphore, we can limit the amount of thread entering this area at the same time.