What does it mean for something to be thread-safe? By saying that X is thread-safe, we mean that if multiple threads use X at the same time, we don't have to worry about concurrency problems. The STL, for instance, is not thread-safe; if we were to create an STL queue and have two threads to operate on it simultaneously, we would have to manually perform all locking operations. The cout instruction is also not thread-safe.

Suppose now we want to build a thread-safe queue; the methods we want the queue to have are insert(item), remove() and empty(). The ?rst question to be asked is what should remove() do when the queue is empty. One solution would be for it to return a special value (such as NULL or -1, etc) or to throw an exception. It would be much more elegant and useful, however, to make that function call wait until something actually appears in the queue. By implementing this type of blocking system, we are in fact implementing part of a producer-consumer system.

Now let us think of how to make the function wait. We can spin, i.e. write something like while (empty()) ;. This, however, obviously doesn't work, since the test of emptiness needs to read shared data; we need to put locks somewhere! And if we lock around the while(empty()); line, the program will hang forever. The conclusion is that we need some way of going to sleep and at the same time having someone to wake us up when there's something interesting to do. Let us now present several possible implementations for this system and discuss why they do not work. The ?rst possibility is:

dequeue()
lock() // needs to lock before checking if it's empty
if (queue empty)
sleep()
remove_item()
unlock()
enqueue()
lock()
insert_item()
if (thread waiting)
wake up dequeuer
unlock()