Fair Solution to the Reader-Writer-Problem with Semaphores only

The reader-writer-problem is a standard problem in concurrent programming. A resource is shared by several processes which need either inclusive reading or exclusive writing access. The known solutions to this problem typically involve a number of global counters and queues. Here a very simple algorithm is presented which needs only two semaphores for synchronisation and no other global objects. The approach yields a fair solution without starving. The reader-writer-problem Assume the following situation: a shared resource, say a database, is accessed by several concurrent processes for reading or writing. Because the readers do not change the database, any number of them may be granted simultaneous access. On the other hand, only one writer is allowed to modify the database at a time.