Question

I am running into a situation where the read and write operations (being done by two different threads and two different view controllers) to an XML file are overlapping.

I tried the following logic to use the same NSLock with the two view controllers :

ViewControllerOne:

(on a background thread using dispatch_async)
- (void)writeToXML {
    // get xmlLock (lock declared globally)
    // write
    // unlock
}

ViewControllerTwo:

(on the main thread)
- (void)readFromXML {
    // get xmlLock (lock referenced from ControllerOne)
    // read
    // unlock
}

However, while debugging, I am noticing that even when ControllerOne has locked the xmlLock, ControllerTwo is still able to get it.

What am I missing here ? Also, is there a better approach for doing something like this ?

Answer 1

Are you positive it’s the same NSLock instance? Regardless, it seems like a better way might be to have a dispatch queue for that; readFromXML can dispatch_sync onto it, and writeToXML can dispatch_async onto it.