Wednesday, March 13, 2013

Cocoa Debugging Tip

I'm attending my local Ann Arbor CocoaHeads meeting tomorrow, and the topic is
Objective Tips.  So I thought I would share a tip.

What do you do when you get a crash due to an uncaught exception such as:

2013-03-13 13:30:10.186 Picross[43233:1303] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[AppDelegate crash]: unrecognized selector sent to instance 0xc04de10'*** First throw call stack:
(0x355b012 0x32ffe7e 0x35e64bd 0x354abbc 0x354a94e 0x3313663 0x12f54 0x3f0153f 0x3f13014 0x3f042e8 0x3f04450 0x926b5e12 0x9269dcca)
libc++abi.dylib: terminate called throwing an exception

This can get really frustrating as you need to figure out where in your code it crashed.  Debugger to the rescue.  In the call stack, you can find the first "low" value.  This usually represents your code.  Then you just do a symbol lookup on that value.  Such as the following when using LLDB.
im loo -a 0x12f54
 This does a image lookup which gives a nice dump including the file and line number of the offending code:

Address: Picross[0x00012f54] (Picross.__TEXT.__text + 67540)
Summary: Picross`__57-[AppDelegate application:didFinishLaunchingWithOptions:]_block_invoke112 + 52 at AppDelegate.m:216

You can see from this dump that the offending code was at line 216 in the AppDelegate.m and was from a block where I was calling a selector that didn't exist.

I also find this GDB to LLDB guide to be a handy reference of the commands available in LLDB.

I hope this short quick tip was helpful.  Please feel free to follow me on twitter at @fivelakesstudio. I would love to hear about your experiences with the debugger or any tips you might have.

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