How can I track down a segmentation fault in a Cocoa application?

Question

I'm having a problem with a Cocoa application I am writing. It has to parse a timestamped file that is updated every hour, and during testing it keeps crashing consistently at around 11:45 PM due to a segmentation fault. I'm assuming I must be messaging an object that has been deallocated. What tools are provided with the Xcode install to track object allocations and (hopefully) tell me if I am messaging an object that has been deallocated?

I am using Mac OS X 10.5.

Answer 1

The way I do it is by using a command line tool called gdb. Here is a tutorial on how to use it. You'll have to learn a few of it's commands, but once you do it's almost a pleasure to use.

Note: gbd can be used on C, C++, and Objective-C programs.

Answer 2

Have you run the program under gdb? This should allow you to inspect the stack and variables when it SIGSEGVs.

To track allocations, use malloc_history. This requires the MallocStackLogging environment variable to be set.

Answer 3

I would recommend the following:

• Use NSZombieEnabled to monitor when messages are sent to deallocated NSObjects
• Use Instruments to track object allocations and/or memory leaks

Answer 4

A quick point: using a deallocated memory location usually results in a EXC_BAD_ACCESS exception. If that's the crash reason you're seeing then you're correct in assuming it's a deallocation problem.

Answer 5

Run it in Xcode's debugger (which is gdb with a GUI on top) and reproduce the crash. Then, look at the stack trace.

Messaging a deallocated object usually has the top frame in objc_msgSend. The next step then is to run the app with NSZombieEnabled and reproduce the crash; the zombie will identify itself.