Operating Systems: Virtualization, Concurrency & Persistence/

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Better, But Still Broken: While, Not If

Better, But Still Broken: While, Not If

Let's try to work on the solution of the producer/consumer problem from the last lesson.

We'll cover the following...

In the last lesson, we saw how the solution we came up with for the producer/consumer problem did not work correctly for more than one consumer due to the race condition. Fortunately, fixing it is easy (see the code excerpt below): change the if to a while.

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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);
  }
}

Why this works?

Think about why this works; now consumer Tc1T_{c1} ...

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