Semaphores

Question 1

Semaphore

Answer 1

used to control access to a common resource by multiple threads and avoid critical section problems;
supports two atomic operations (acquire and release); never negative

Question 2

acquire()

Answer 2

if(counter > 0) then counter–, calling thread continue

if(counter ==0), block the thread on the semaphore

Question 3

release()

Answer 3

counter++;

if any blocked threads - unblock some such thread

Question 4

Monitor

Answer 4

used to achieve mutual exclusion and synchronization

Question 5

Lock

Answer 5

ensures at most one thread in any code that is synchronized on the object

Question 6

Wait set

Answer 6

maintains threads blocked on Monitor; provides coordination;

Question 7

wait()

Answer 7

suspends calling thread and adds it to Wait set; releases Lock

Question 8

notify()

Answer 8

the JVM takes a thread from the wait set and runs it

Question 9

Purpose of while loop

Answer 9

to stop the acquire() function from advancing and exiting