Help me diagnose this crash


a lot of users of my app are sending in this crash but I'm unable to repeat it.. So I'm looking for tips on how I can troubleshoot it, here's the relevant section of the crash log:

Exception Codes: 0x0000000000000001, 0x0000000000000000
Crashed Thread:  0  Dispatch queue:

Application Specific Information:
objc[1535]: FREED(id): message retain sent to freed object=0x640ad0

Thread 0 Crashed:  Dispatch queue:
0   libobjc.A.dylib                     0x9116f4b4 _objc_error + 116
1   libobjc.A.dylib                     0x9116f4ea __objc_error + 52
2   libobjc.A.dylib                     0x9116d7dc _freedHandler + 58
3   ...my_company.my_app      0x00045635 -[MyObject mySelector] + 1494
4                0x90be18d4 __NSFireTimer + 141
5            0x93a38adb __CFRunLoopRun + 8059
6            0x93a36464 CFRunLoopRunSpecific + 452
7            0x93a36291 CFRunLoopRunInMode + 97
8                 0x92982e04 RunCurrentEventLoopInMode + 392
9                 0x92982bb9 ReceiveNextEventCommon + 354
10                 0x92982a3e BlockUntilNextEventMatchingListInMode     + 81
11                    0x9576e78d _DPSNextEvent + 847
12                    0x9576dfce -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 156
13                    0x95730247 -[NSApplication run] + 821
14                    0x957282d9 NSApplicationMain + 574
15  ...my_company.my_app      0x00002042 start + 54

So, from this line:

3   ...my_company.my_app      0x00045635 -[MyObject mySelector] + 1494

I can deduce that in mySelector a message is being sent to an object that's already being released, but I've looked through the code and I just can't see it and there are too many other dependencies to post the code here.

So my questions are:

  1. What do the numbers at the end of each line indicate, in the above example it's 1494.
  2. On the preceding line there's a reference to __NSFireTimer - does this indicate that mySelector is being called by a NSTimer in my code?
  3. I've tried running my app with Zombies enabled, aside from sprinkling my code with checks that each object != nil in mySelector, what else can I do if I can't repeat the crash?

Any help on this would be greatly appreciated!

asked on Stack Overflow May 1, 2011 by Bangy

2 Answers


Use instruments to watch allocation. I found it helpful.

answered on Stack Overflow May 1, 2011 by Hack Saw

I believe the numbers at the end of each line indicate the offset in terms of code bytes within the function at that point in the backtrace. A debugger would use this information to map the code offset to an actual line of source code (it needs debug information in the application to do this). In your case, the information is not particularly useful.

The reference to NSFireTimer means that your main thread's run loop has had a timer scheduled and it has fired calling [myObject mySelector]. I think from the stack trace we can assume that the timer has invoked "myObject" directly since there are no other intervening stack frames.

My guess would be that the object has been freed but it was the target of a scheduled timer in your main thread, which you have forgotten to remove.

How the object became freed in the first place depends on how your application is compiled. Does it use Garbage Collection, or does it rely on retain/release memory management?

If you are not using garbage collection, then you have probably missed a "retain" somewhere. Perhaps your object has been autoreleased and the runloop has cleaned it up when you didn't expect it to?

If you can identify the timer involved, you might want to consider disabling the timer in the dealloc or finalize method of your class. You could also add some logging there to show you when the object is freed.

answered on Stack Overflow Jun 9, 2011 by OzBandit

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