Re: NSConditionLock console message: "unlocked from thread which did not lock it"

FROM : Chris Kane
DATE : Tue Nov 27 22:32:35 2007

On Nov 25, 2007, at 9:40 AM, Mark Alldritt wrote:
> Under Leopard, my application has begun logging console messages 
> like this:
>
> 2007-11-25 09:09:03.572 Script Debugger 4[66788:613] *** -
> [NSConditionLock
> unlockWithCondition:]: lock (<NSConditionLock: 0x2b5e7ed0> '(null)')
> unlocked from thread which did not lock it
>
> The message is correct: I lock the NSConditionLock from a worker 
> thread to
> block the thread until another thread (the main thread) receives a 
> response
> and the worker can continue on.
>
> This seems to me like a legitimate use of a lock.  Can anyone 
> explain what
> the problem here is.



POSIX does not require unlocking from a different thread to work 
properly, though it may, so as NSLock and related are built on top of 
the pthreads API, the same is true there as well.

What you want to do is lock and unlock in the same thread, in both 
threads.  NSConditionLock, which you're already using, makes this 
straightforward, whereas it is not possible with plain NSLocks.  For 
example, to block the background thread:

   [condLock lockWhenCondition:1];    // block until 1
   [condLock unlock];

In the main thread:

   [condLock lock];
   [condLock unlockWithCondition:1];  // 1 means background thread can 
proceed

assuming that the lock was created/initialized with -initWithCondition:
0.


Chris Kane
Cocoa Frameworks, Apple