C - producer/consumer deadlock issue

Question

I'm trying to work with producer/consumer threads in a bounded buffer. The buffer length is 5. I have 1 mutex and 2 semaphores, empty which starts out at the size of the buffer, and full, which starts out at 0.

When I run my code without sleep() at the end, it continually produces until the buffer is completely fully, the consumes until it's empty, so the output looks like this:

Placed 1 in the buffer at position 0.
Placed 2 in the buffer at position 1.
Placed 3 in the buffer at position 2.
Placed 4 in the buffer at position 3.
Placed 5 in the buffer at position 4.
The buffer now contains 0 at position 0.
The buffer now contains 0 at position 1.
The buffer now contains 0 at position 2.
The buffer now contains 0 at position 3.
The buffer now contains 0 at position 4.

However, when i run with sleep() at the end, it prints out:

Placed 1 in the buffer at position 0.
The buffer now contains 0 at position 0.

Then it appears to lock up, but I'm not really sure why it behaves the way it does regardless of whether or not sleep is there. Any suggestions? My main method essentially just makes some declarations then creates 1 thread to produce and 1 to consume, those methods are below.

void *producer()
{
        int k = 0; //producer index
        while (1)
        {
                sem_wait(&empty);
                pthread_mutex_lock(&mutex);
                buffer[k] = k+1;
                sem_post(&full);
                pthread_mutex_unlock(&mutex);
                printf("Placed %d in the buffer at position %d.\n", buffer[k], k);
                k = (k + 1) % BUFFER_SIZE;
                sleep(rand() * 10);
        }
}

void *consumer()
{
        int j = 0;   //consumer index
        while(1)
        {
                sem_wait(&full);
                pthread_mutex_lock(&mutex);
                buffer[j] = 0;
                sem_post(&empty);
                pthread_mutex_unlock(&mutex);
                printf("The buffer now contains %d at position %d.\n", buffer[j], j);
                j = (j + 1) % BUFFER_SIZE;
                sleep(rand() * 10);

        }
}

I suggest you to throw your code away and rewrite the task in pseudocode. Or you may rewrite it in C again, that usually helps me to find such hard bugs. — Lavir the Whiolet
One (unrelated) problem I can see, the consumer index j should be shared amongst consumers. It doesn't help if it's local to each thread. Make j global. — Jeff Mercado
Thanks, I was actually planning on doing that for when I implement multiple threads, but for now I only have 1 producer and 1 consumer each. — john

Adam Rosenfield Adam Rosenfield · Accepted Answer · 2010-11-05T19:18:41

The parameter to sleep() is the number of seconds to sleep for. rand() returns an integer between 0 and RAND_MAX (typically 32767 or 2³¹-1), and when you multiply that by 10, you're sleeping for an absurdly large amount of time. You're not deadlocking, just sleeping for a very long time.

C - producer/consumer deadlock issue

4 Answers