Recall the code below from class where we used a condition variable to make sure the child runs before the parent prints. Why did we include the int variable done if signalling and waiting is done automatically with condition variables?
int done = 0;
pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t c = PTHREAD_COND_INITIALIZER;

void thr_exit() {
Pthread_mutex_lock(&m);
done = 1;
Pthread_cond_signal(&c);
Pthread_mutex_unlock(&m);
}

void *child(void *arg) {
printf("child");
thr_exit();
return NULL;
}

void thr_join() {
Pthread_mutex_lock(&m);
while (done == 0)
Pthread_cond_wait(&c, &m);
Pthread_mutex_unlock(&m);
}

int main(int argc, char *argv[]) {
printf("parent: begin");
pthread_t p;
Pthread_create(&p, NULL, child, NULL);
thr_join();
printf("parent: end");
return 0;
}

Question

Recall the code below from class where we used a condition variable to make sure the child runs before the parent prints. Why did we include the  int  variable  done  if signalling and waiting is done automatically with condition variables? 
 int done = 0;
pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t c = PTHREAD_COND_INITIALIZER;

void thr_exit() {
  Pthread_mutex_lock(&m);
  done = 1;
  Pthread_cond_signal(&c);
  Pthread_mutex_unlock(&m);
}

void *child(void *arg) {
  printf("child");
  thr_exit();
  return NULL;
}

void thr_join() {
  Pthread_mutex_lock(&m);
  while (done == 0)
    Pthread_cond_wait(&c, &m);
  Pthread_mutex_unlock(&m);
}

int main(int argc, char *argv[]) {
  printf("parent: begin");
  pthread_t p;
  Pthread_create(&p, NULL, child, NULL);
  thr_join();
  printf("parent: end");
  return 0;
}

Accepted Answer

If the child runs immediately after is created and we did not use  done , then the child will signal before the parent puts itself to sleep. When the parent does sleep, it will then sleep forever since it missed its signal.

Answer

This was a buggy implementation of the code that introduced a race condition for setting and checking  done  that is unnecessary since  Pthread_cond_wait  and  Pthread_cond_signal  perform the equivalent of checking and setting  done , but atomically.

Answer

If the child runs immediately on another CPU core so that it runs concurrently with the thread then there is a race condition for acquiring the lock  m  that having the parent first check  done  will prevent.

Answer

If the child does not run immediately after is created (i.e., the parent continues running) and we did not use  done , then the parent will go to sleep still holding the lock  m  and therefore prevent the child from being able to signal it. The parent will therefore sleep forever and the child will wait forever.

Crowdly

Recall the code below from class where we used a condition variable to make sure...

Want instant access to all verified answers on moodle31.upei.ca?