Semaphores

Question 1

What is a semaphore?

Answer 1

A semaphore is a signal that indicates whether something can continue or must wait.

Semaphores consist of a queue and an integer value (sem).

Question 2

What are the two semaphore methods?

Answer 2

semWait:

If sem > 0

Then sem = sem - 1

Else make the process wait

semSignal:

If there is a process waiting in the queue for this semaphore

Then allow the process to resume processing

Else sem = sem + 1

semWait and semSignal should be implemented as atomic actions.

Question 3

(3 answers)

What are the different types of semaphore?

Answer 3

• Binary
  
  • Mutual exclusive access to a single shared resource
• Counting
  
  • Protect access to multiple instances of a single shared resource
• Blocking
  
  • Synchronise two processes (a producer and a consumer)

Question 4

(4 answers)

What are binary semaphores and how do they work?

Answer 4

• Several processes share one semaphore
• Semaphore is initialised to 1 (available)
• When a process enters the critical region, the semaphore value is changed to 0
• When the process leaves the critical region, a waiting process enters, or the semaphore value is changed back to 1

Question 5

(4 answers)

What are counting semaphores and how do they work?

Answer 5

• Several processes share multiple semaphores (or permits)
• Semaphore is initialised to the number of instances of the resource available
• When a process enters the critical region, the semaphore value is decreased by 1
• When the process leaves the critical region, a waiting process enters or the semaphore value is increased by 1

Question 6

(5 answers)

What are blocking semaphores and how do they work?

Answer 6

• Consumer and producer processes share one semaphore
• Semaphore is initialised to 0 (blocked)
• The consumer calls semWait to check whether data has been written
• When the producer finishes writing, it calls semSignal and the semaphore is changed to 1
• The consumer is then allowed to enter the critical region