Quote Originally Posted by jcox23 View Post
you're saying that a construct like:
Qt Code: 
  1. if(!asyncqueue.isEmpty()) asyncqueue.pull()
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is not safe?
Yes, that's correct.

because between the actual test of emptiness and the actual pull, another thread could have pulled, emptying the queue possibly... and so my pull will fail...
Exactly.

and that's why you added the semaphore on top of the mutex protection?....
Yes, the semaphore makes sure you can't read from an empty queue or write to a full queue. If you try, you will be blocked until you can. This simplifies the code and improves resource usage as your threads won't be spinning around checking if they can access the queue.