Strange deadlock in producer-consumer queue - c

Question

With C's pthread library, I'm trying to implement a simple producer-consumer schema.
The producer generates random numbers and puts them into a queue like this
typedef struct {
int q[MAX_QUEUE];
int head;
int tail;
} queue;
The consumer just takes numbers one by one and prints them to the standard output. Synchronisation is done with one mutex and two condition variables: empty_queue (to suspend the consumer if the queue is empty) and full_queue (to suspend the producer if the queue is full). The problem is that both threads suspend themselves when reaching MAX_QUEUEelements produced/consumed and so they enter a deadlock situation. I think I have done everything correct, I can't figure out what I'm doing wrong.
Producer:
void* producer(void* args) {
unsigned seed = time(NULL);
int random;
queue *coda = (queue *) args;
while(1) {
Pthread_mutex_lock(&queue_lock);
while(coda->head == MAX_QUEUE-1) { // Full Queue
printf("Suspending producer\n");
fflush(stdout);
Pthread_cond_wait(&full_queue, &queue_lock);
}
random = rand_r(&seed) % 21;
enqueue(coda, random);
Pthread_cond_signal(&empty_queue);
Pthread_mutex_unlock(&queue_lock);
sleep(1);
}
return NULL;
}
Consumer:
void* consumer(void* args) {
queue *coda = (queue *) args;
int elem;
while(1) {
Pthread_mutex_lock(&queue_lock);
while(coda->head == coda->tail) { // Empty Queue
printf("Suspending Consumer\n");
fflush(stdout);
Pthread_cond_wait(&empty_queue, &queue_lock);
}
elem = dequeue(coda);
printf("Found %i\n",elem);
Pthread_cond_signal(&full_queue);
Pthread_mutex_unlock(&queue_lock);
}
return NULL;
}
Enqueue/Dequeue routines
static void enqueue(queue *q, int elem) {
q->q[(q->tail)] = elem;
(q->tail)++;
if(q->tail == MAX_QUEUE)
q->tail = 0;
}
static int dequeue(queue *q) {
int elem = q->q[(q->head)];
(q->head)++;
if(q->head == MAX_QUEUE)
q->head = 0;
return elem;
}
Pthread_* are just wrapper functions to the standard pthread_* library functions.
Output (with MAX_QUEUE = 10):
Suspending Consumer
Found 16
Suspending Consumer
Found 7
Suspending Consumer
Found 5
Suspending Consumer
Found 6
Suspending Consumer
Found 17
Suspending Consumer
Found 1
Suspending Consumer
Found 12
Suspending Consumer
Found 14
Suspending Consumer
Found 11
Suspending Consumer
Suspending producer

Answer 1

coda->head == MAX_QUEUE-1
This does not check whether the queue is full. There are two variables that describe the state of the queue, head and tail.
coda->head == coda->tail
This properly checks that the queue is empty. Note how both variables are used in the check.