In this recipe, we are going to implement a typical producer/consumer program with multiple threads. The general idea is that there is one thread that produces items and puts them into a queue. Then there is another thread that consumes such items. If there is nothing to produce, the producer thread sleeps. If there is no item in the queue to consume, the consumer sleeps.
Since the queue that both threads have access to is also modified by both whenever an item is produced or consumed, it needs to be protected by a mutex.
Another thing to consider is: What does the consumer do if there is no item in the queue? Does it poll the queue every second until it sees new items? That is not necessary because we can let the consumer wait for wakeup events that are triggered by the producer, whenever there are new items...
