I am making an app that plays wav files. The way it works is to make an AudioQueue that requests data from a thread that is not the main thread. The audio queue callback calls a method in a class that handles opening and getting data from a wav file. The method to fill the buffer is always called from the AudioQueue thread, while the method that opens a new wav file, can be called both from the main thread and the AudioQueue thread. When I say thread, I probably mean run loop.

The problem is when I in the GUI of the app select another song to play. Usually, everything goes well, but sometimes it triggers some of my error checking code because the AudioQueue thread manages to sneak in a read from the NSInputStream in the middle of reading and parsing the wav file header.

I have solved the problem by wrapping the method run on the AudioQueue thread in a block like this:

    dispatch_sync(dispatch_get_main_queue(), ^{
        //Force anything within here to be run synchronously on the main thread
    });

When I do this, I can't reproduce my problem any more, but I lose the scrubbing effect I got from dragging the song position slider. I also feel that is a bit overkill for what I really need too.

When I try to wrap both methods in a @synchronized block like this:

    @synchronized(iStream) {
        //Trying to synchronise access to iStream
    }

It makes no difference. Probably because I don't change the actual variable itself.

My question is: How would I go about making sure that access to iStream is blocked from the AudioQueue thread while I am opening a new file from the main thread?

EDIT: I also noticed another problem with using the dispatch_sync, that when I quit the application, it goes into a spinning beach ball for a while before it quits. Not sure if the app was terminated, or actually quits normally.

It seems like I maybe have solved the problem.

I made a mutex using a NSLock that I initialised inside the init method. Then I use [mutex lock] before the critical code, then [mutex unlock] after the critical code for the method that is used by both threads. Then I did the same on the method that was used only By AudioQueue, making sure I unlock before calling the other method that can be used by both threads, then lock it again after that.

This seems to work exactly like I wants, no problems with scrubbing or beach ball when exiting the application, and I haven't managed to reproduce the bugs I had either.

Is this the right way to do it?

I am assuming with NSLock, if you call 'lock' twice, the second thread that tries to acquire a lock is blocked till the first thread calls 'unlock'? (And hence, if you call 'lock' twice on the same thread, you will get a deadlock?)

Edit:

I certainly did:

2014-03-09 22:24:35.880 Loud[4830:303] *** -[NSLock lock]: deadlock (<NSLock: 0x6080000ce690> '(null)')

2014-03-09 22:24:35.881 Loud[4830:303] *** Break on _NSLockError() to debug.