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Getting strange intermittent "unrecognized selector" exceptions in iPhone app

Answer 1

A:

This is often caused by memory management errors. Did you try zombies?

...with zombies enabled, messages to deallocated objects will no longer behave strangely or crash in difficult-to-understand ways, but will instead log a message and die in a predictable and debugger-breakpointable way. This is the tool to use when trying to track down over-releases and premature releases.

zoul 2010-09-17 20:19:50

This app is in the app store at the moment. Out of a couple thousand users I get an error report of this sort every few days. I've not encountered a zombie in my development environment.

Karl 2010-09-17 20:22:31

Am I the only one who finds "I've not encountered a zombie in my development environment." funny?

Douwe Maan 2010-09-17 20:33:25

I have not studied your samples too long, but such bugs are usually quite a pain to debug. Try the static analyzer, double-check your memory management, assert as much as you can, nil freed references and turn on the zombies in debug builds :)

zoul 2010-09-17 20:55:42

I just spent 1.5 days chasing down a really nasty bug where a Class object was being corrupted due to a rogue pointer - so it is possible. Mine was much more reproducible than yours - if yours is a memory corruption issue it could be tough to find. You can use gdb's "watch" command to set a breakpoint on write access to a given address, which is in my experience the only useful tool for finding these bugs. The hard bit is working out which location to watch. Before you do that, though, I'd definitely check all your code for warnings and run the static analyzer.

Echelon 2010-09-17 21:50:30

Yeah that's gonna be a bit of a problem. My code generates no warnings (I'm quite anal about that), and the static analyzer finds nothing (I'm anal about that, too). The static analyzer can never supplant good desk checking, of course. I only use factory constructors wherever possible, and if I'm forced to use the init/alloc idiom I make sure to autorelease. The program does no pointer walking, type coercion, or arithmetic, either. The worst part is, the issue has only ever surfaced in the wild, so all I ever get is a stack trace =/

Karl 2010-09-18 00:58:40

Answer 2

A:

Did you check some race conditions about multi-threads ? It seems like some resource freed by another thread, and current thread send a messaged to a deallocated object.

Error: NSInvalidArgumentException: -[NSCFString setObject:forKey:]: unrecognized selector sent to instance 0x3e793088
9  Mind                                0x0015de70 -[CCTextureCache textureFromFile:] + 528
10  Mind                                0x0015d9f4 -[CCTextureCache loadImageUncached:pixelFormat:] + 116
11  Mind                                0x0015d058 -[CCTextureCache addImage:pixelFormat:] + 152
12  Mind                                0x00080524 -[ImageLoader imageWithFile:pixelFormat:] + 84
13  Mind                                0x000854c4 -[ImageLoadOperation performLoad] + 68
14  Mind                                0x00085800 -[ResourceLoadOperation main] + 112
15  Foundation                          0x30c4c8b5 -[__NSOperationInternal start] + 664
16  Foundation                          0x30c4c613 -[NSOperation start] + 22
17  Foundation                          0x30cbdb63 ____startOperations_block_invoke_2 + 46
18  libSystem.B.dylib                  0x31227858 _dispatch_call_block_and_release + 20
19  libSystem.B.dylib                  0x3122863c _dispatch_worker_thread2 + 128
20  libSystem.B.dylib                  0x311b1544 _pthread_wqthread + 400
21  libSystem.B.dylib                  0x311a8b74 __stack_chk_fail + 4294967295

Toro 2010-09-17 22:18:21

The thing is, the objects that are getting overwritten are singletons that by this time have already existed for a long time (in the third example I cited, the singleton object class gets overwritten WHILE it's 3 levels deep in self-referencing calls). A race condition could only cause allocs to overwrite other allocs that happen very close to each other in different threads. It wouldn't overwrite long established objects. It seems that it would have to be a rogue pointer somewhere, but it's odd that the memory corruption always leads to a valid object class descriptor.

Karl 2010-09-19 15:31:59

At the 16. I see NSOperation start, so I guess it runs under multi-threads.

Toro 2010-09-20 23:34:02

And the textures means OpenGL-ES texture? Can the OpenGL-ES run under multi-thread env?

Toro 2010-09-20 23:39:48

Yes. The OpenGL stuff is in the cocos2d library. The code that is running at that time is a background texture load operation (the only thing that runs in a different thread).

Karl 2010-09-21 17:52:42

I remember it needs some skill to generate textures under multi-thread, something like to make another opengl context for child thread. As for me, if it runs not under main thread to generate textures, it always crash my application.

Toro 2010-09-21 22:12:03

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Getting strange intermittent "unrecognized selector" exceptions in iPhone app

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