Re: Thread question

[Banlu Kemiyatorn]

On Sat, Mar 5, 2011 at 1:03 AM, Richard Frith-Macdonald
<address@hidden> wrote:

Thanks! I switched to GNUstep objc runtime now and the problem is
gone. Thank you Richard and David.

> I suspect that what you have run into here is a bug in the gnu Objective-C 
> runtime.
>
> You have two threads started and immediately calling NSLog, which in turn 
> immediately tries to access the user defaults system.
> The user defaults system is protected by a lock 'classLock' created in its 
> +initialize method, which is run automatically by the runtime.
> The +initialize method is supposed to run in a thread-safe manner *before* 
> any other method in the class can run ... so this should mean that classLock, 
> created in +initialize, can safely be used later.
>
> What I think is happening is that one thread calls the +standarduserDefault 
> method, and the runtime calls +initialize, then before +initialize has 
> completed, the second thread calls +standardUserDefaults (and the runtime 
> incorrectly lets it), which tries to lock classLock before it has been 
> created.  This will send a _lock method to a nil object, which does nothing.  
> The thread then creates the shared user defaults object and tries to unlock 
> classLock, but by this time the first thread has created the lock, so an 
> -unlock message is sent to classLock when in has not been locked.
>
> This bug only rarely manifests, since it depends on a race condition when a 
> class is initialized, and in most non-trivial programs, most important 
> classes are used before any threads are started.  You could check to see if 
> this is your problem by calling any NSUserDefaults method before starting the 
> threads...  if it is, doing that will prevent the exception from occurring.
>
> Unless it's been fixed, this is probably the biggest outstanding bug in the 
> gnu runtime.
>
>



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