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Semaphores

There are many ways to implement semaphores (especially the counting semaphores on posix), but I've kept it relatively simple here with the task at hand being just to provide a lock that can be used when accessing shared memory. Its usage is very similar to thread mutexes with the only difference being the instantiation via a string identifier (similar to the case for shared memory). Note, on windows semaphores are usually referred to as mutexes (confusing!).

   1 //...First process
   2 Semaphore semaphore("test_sem");
   3 semaphore.lock();
   4 // ... access shared memory here
   5 semaphore.unlock();
   6 
   7 //...Second process
   8 Semaphore semaphore("test_sem");
   9 semaphore.lock();
  10 // ... access shared memory here
  11 semaphore.unlock();

If another semaphore (doesn't have to be the same object, just a semaphore instantiated from the same name) tries to lock an already locked semaphore, it will wait until the first semaphore releases the lock. Be wary of deadlocks.