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Core Data crashed - a rarely happened error on the device

Answer 1

+4 A:

I'm surprised the exception being raised isn't printed. There isn't a lot of evidence to work with here.

In any case, the most likely cause of this is not correctly using threads with Core Data. That is, do you have multiple threads and, if so, are you sure you are properly managing your managed object context(s) across the threads?

bbum 2010-06-19 19:29:19

NSManagedObjectContext instances are not thread safe, this is noted in the ADC Class reference. Rule of thumb - use different NSManagedObjectContext instances for each thread while you're working on managed objects. When you're done, merge them together.

Denis 'Alpheus' Čahuk 2010-06-19 19:36:36

I agree that the explanation is almost certainly in your logs. You're throwing an exception; it should be printed out. Go look there rather than in the crash stack.

Rob Napier 2010-06-20 03:29:19

Thank you for your answers and suggestions, but I did not get the error when doing debug with the simulator, only with the device I got this crash log, try to save it with the Organizer to see why. It says that on the line where I query an instance of a class with its variable an NSArray of items (managedObject) Debug to this line there is no error :-s

sfa 2010-06-20 10:09:00

@bbum -- For my own curiosity, what makes you think this is thread related? The crash happened on thread 0 which IIRC is always present.

TechZen 2010-06-20 17:19:17

NSManagedObjects are inherently not thread safe and one of the most common causes of crashes at the faulting layer is when a managed object context is mis-used from multiple threads. I.e. if your context was used from a background thread, your main thread could easily crash when using same context.

bbum 2010-06-20 17:25:18

The full crash log would be helpful. That it happens on the device but not when debugging or in the simulator also makes it smell like a threading issue; still might not be, but given the lack of information we are "best guessing" the problem.

bbum 2010-06-20 17:30:20

I see, it was the location of the error within the Core Data stack itself that raised your suspicions.

TechZen 2010-06-20 17:31:28

sfa 2010-06-21 10:43:54

Answer 2

+3 A:

As a general rule of thumb, most crashes that happen on the device only result from some issue with the drive. (Especially if it obviously doesn't involve any other device specific function such as the accelerometer.) The most likely cause is a full drive. All kinds of weird things start happening when the disk gets full.

In this crash, Core Data is attempting to load the attributes of a fault (ghost) object and failing. This can happen when a cache gets out of synch with the store or if you have otherwise create a condition in which the store is altered without the knowledge of the context.

(1) I would start by seeing if the device is full and freeing up some room if it is. Conversely, you could intentionally fill the drive to try and trigger the error consistently.

(2) If you have multiple context, make sure they don't manipulate the same objects at the same time.

Edit:

The error could be caused by the object returned from the model.items not actually being of class "Item" it could be a superclass that lacks the attribute. That would explain why it fails when it tries to fault in (read in) the actual attribute.

Just a guess. Not sure why that would only occur on the device. Is there networking involved here?

More code from around the point of failure might be helpful.

TechZen 2010-06-20 17:30:13

hi techzen, thank you for your answer, I edited the question with more exception details. (1) I don't think that the disk is full, because my iPod still has 6GB available, I use a 8GB iPod only for development. (2) I don't use multiple context, only one in the AppDelegate I tried to call the delegate to get the managed object context any time when I need it.

sfa 2010-06-21 10:56:52

hi Techzen, the app does use network to download data, save the downloaded data into the context. I got the error randomly when I came back with the navigation controller to the step where it loads the data from the models.items. Debugging with the simulator does not help as it does not raise the error, even with the device, not always it happens. I guess, if the app remains in the current screen for too long and comes back to this screen, it tries to re-read the items but failed, so, there must be some timeouts somewhere with the core data (or the nsarray) to keep the items in memory.

sfa 2010-06-22 11:22:58

Core data doesn't have any kind of timeout. But, if your network operations are deleting objects from the context, then the objects stored in just an ordinary (non-core-data) array could become invalid. In any case, you need to run a fetch over again each time another source makes changes to the context. If you fully implement a NSFetchedResultsController's delegate protocol you can trigger that automatically.

TechZen 2010-06-22 13:49:14

You might try disabling the network operations on the device and then try to trigger the error. If you can't that would confirm the source of the problem.

TechZen 2010-06-22 13:50:17

disabling the network on the device does not help :-s it comes back to the previous screen without any problem :( I'm going to change to the normal NSObject for all the items instead of a core data managed objects and see if it still exists. I'll let you know the result.

sfa 2010-06-22 14:24:48

If disabling the networking prevents the crash then you should look at the parts of the code where the network data is inserted into the context.

TechZen 2010-06-22 14:36:26

it's my fault, each time the view is disappearing, I delete the old items created in this step, and as a result, when coming back to the view, the items are not there. But because I use navigation controller, so when comes back to the previous steps within a small time limit, the caches for these items are still being used, while for a bigger time limit, it tries to load again from these items, they are already being cleared from the Core Data. Thanks for your hints Techzen.

sfa 2010-06-24 10:36:36

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