GCD : How to write and read to variable from two threads

Question

this may sound a newbie question anyway Im new to GCD,

I\'m creating and running these two following threads. The first one puts data into ivar mMutableArray

Answer 1

You are using it wrong, by locking access to the variables you are simply losing any benefit from GCD. Create a single serial queue which is associated to the variables you want to modify (in this case the mutable array). Then use that queue to both write and read to it, which will happen in guaranteed serial sequence and with minimum locking overhead. You can read more about it in "Asynchronous setters" at http://www.fieryrobot.com/blog/2010/09/01/synchronization-using-grand-central-dispatch/. As long as your access to the shared variable happens through its associated dispatch queue, you won't have concurrency problems ever.