Semaphores

Question 1

Semaphores

Answer 1

synchronization primitive
-can use as both locks and condition variables
- object with an integer value that we can manipulate with two routines:
1. sem_wait()
2. sem_post()
must initialize it to some value

Question 2

Initializing a semaphore

Answer 2

sem_t s;
sem_init(&s, 0, 1)
sem_init(sem_t s, 0, value) // 0 to indicate that the semaphore is shared between threads in the same process

Question 3

sem_wait

Answer 3

decrement the value of semaphore s by one

wait if value of semaphore is negative

Question 4

sem_post()

Answer 4

-increments the value of the semaphore and if there is a thread waiting to be woken, wakes one of them up

Question 5

Value of semaphore

Answer 5

• when negative, is equal to the number of waiting threads

Question 6

Binary Semaphores (Locks)

Answer 6

-locks only have two states (held and not held), sometimes call a semaphore a binary semaphore

Question 7

Scheduler State of each thread

Answer 7

Run: Thread is running
Read: Runnable but not running
Sleep: Thread is blocked