Better, But Still Broken: While, Not If

Understand how replacing if statements with while loops in condition variables prevents race conditions when synchronizing threads. Explore the problem of signaling with a single condition variable in producer-consumer scenarios and why signals must be directed to wake appropriate threads to avoid deadlocks.

We'll cover the following...

- Why this works?

int loops; // must initialize somewhere...
cond_t  cond;
mutex_t mutex;
void *producer(void *arg) {
  int i;
  for (i = 0; i < loops; i++){
    Pthread_mutex_lock(&mutex);         // p1
    if (count == 1)                     // p2
      Pthread_cond_wait(&cond, &mutex); // p3
    put(i);                             // p4
    Pthread_cond_signal(&cond);         // p5
    Pthread_mutex_unlock(&mutex);       // p6
  }
} 
void *consumer(void *arg) {
 int i;
 for(i = 0; i < loops; i++) {
   Pthread_mutex_lock(&mutex);         // c1
   while (count == 0)                  // c2      
    Pthread_cond_wait(&cond, &mutex);  // c3
    int tmp = get();                   // c4
    Pthread_cond_signal(&cond);        // c5
    Pthread_mutex_unlock(&mutex);      // c6
    printf("%d\n", tmp);
  }
}

Better, But Still Broken: While, Not If

Why this works?