Synchronization

Problem

Output (program is called sem) The numbers in parentheses in the second column are the number of threads in the critical section.

sem 4 1	sem 8 3	sem 12 4	sem 12 8
ThreadId 0: acquired semaphore ThreadId 0: working... ThreadId 0: released semaphore ThreadId 1: acquired semaphore ThreadId 1: working... ThreadId 1: released semaphore ThreadId 2: acquired semaphore ThreadId 2: working... ThreadId 2: released semaphore ThreadId 3: acquired semaphore ThreadId 3: working... ThreadId 3: released semaphore	ThreadId 0: acquired semaphore (1) ThreadId 0: working... ThreadId 2: acquired semaphore (2) ThreadId 2: working... ThreadId 1: acquired semaphore (3) ThreadId 1: working... ThreadId 0: released semaphore (2) ThreadId 3: acquired semaphore (3) ThreadId 3: working... ThreadId 4: acquired semaphore (4) ThreadId 4: working... ThreadId 1: released semaphore (3) ThreadId 2: released semaphore (2) ThreadId 5: acquired semaphore (3) ThreadId 5: working... ThreadId 3: released semaphore (2) ThreadId 6: acquired semaphore (3) ThreadId 6: working... ThreadId 7: acquired semaphore (4) ThreadId 7: working... ThreadId 5: released semaphore (3) ThreadId 4: released semaphore (2) ThreadId 6: released semaphore (1) ThreadId 7: released semaphore (0)	ThreadId 1: acquired semaphore ThreadId 1: working... ThreadId 3: acquired semaphore ThreadId 2: acquired semaphore ThreadId 2: working... ThreadId 3: working... ThreadId 0: acquired semaphore ThreadId 0: working... ThreadId 1: released semaphore ThreadId 2: released semaphore ThreadId 5: acquired semaphore ThreadId 5: working... ThreadId 4: acquired semaphore ThreadId 4: working... ThreadId 3: released semaphore ThreadId 6: acquired semaphore ThreadId 6: working... ThreadId 0: released semaphore ThreadId 7: acquired semaphore ThreadId 7: working... ThreadId 6: released semaphore ThreadId 8: acquired semaphore ThreadId 8: working... ThreadId 5: released semaphore ThreadId 9: acquired semaphore ThreadId 9: working... ThreadId 4: released semaphore ThreadId 10: acquired semaphore ThreadId 10: working... ThreadId 7: released semaphore ThreadId 11: acquired semaphore ThreadId 11: working... ThreadId 8: released semaphore ThreadId 9: released semaphore ThreadId 10: released semaphore ThreadId 11: released semaphore	ThreadId 0: acquired semaphore ThreadId 0: working... ThreadId 2: acquired semaphore ThreadId 2: working... ThreadId 3: acquired semaphore ThreadId 5: acquired semaphore ThreadId 5: working... ThreadId 4: acquired semaphore ThreadId 4: working... ThreadId 3: working... ThreadId 7: acquired semaphore ThreadId 7: working... ThreadId 6: acquired semaphore ThreadId 1: acquired semaphore ThreadId 1: working... ThreadId 6: working... ThreadId 0: released semaphore ThreadId 3: released semaphore ThreadId 5: released semaphore ThreadId 4: released semaphore ThreadId 7: released semaphore ThreadId 8: acquired semaphore ThreadId 8: working... ThreadId 11: acquired semaphore ThreadId 11: working... ThreadId 2: released semaphore ThreadId 10: acquired semaphore ThreadId 10: working... ThreadId 6: released semaphore ThreadId 9: acquired semaphore ThreadId 9: working... ThreadId 1: released semaphore ThreadId 8: released semaphore ThreadId 11: released semaphore ThreadId 10: released semaphore ThreadId 9: released semaphore

/* sem.c                                             */
void *ThreadFn(void *p)
{
  struct ThreadParams *params = (struct ThreadParams *)p;
  sem_t *semaphore = params->semaphore;
  int tid = params->tid;
  
    /* Acquire semaphore */
  sem_wait(semaphore); 
  printf("ThreadId %i: acquired semaphore\n", tid);

    /* Pretend to do some work */
  printf("ThreadId %i: working...\n", tid);
  sleep(1);
  
    /* Release semaphore */
  sem_post(semaphore); 
  printf("ThreadId %i: released semaphore\n", tid);   ← This is not protected!
  
  return 0;
}

/* sem.c                                             */
void *ThreadFn(void *p)
{
  struct ThreadParams *params = (struct ThreadParams *)p;
  sem_t *semaphore = params->semaphore;
  int tid = params->tid;
    
  sem_wait(semaphore); /* Acquire semaphore */

    printf("ThreadId %i: acquired semaphore\n", tid);

      /* Pretend to do some work */
    printf("ThreadId %i: working...\n", tid);
    sleep(1);
  
    printf("ThreadId %i: released semaphore\n", tid);   ← Now this is protected.
    
  sem_post(semaphore); /* Release semaphore */
  
  return 0;
}