How To Loop Through All Active Thread in iPad app

Question

In the iPad app that I'm creating, I'm trying to handle the uncaught Exceptions by outputting the callStackSymbols of the exception. This can be done with [NSException callStackSymbols]

However, I'd like to be able to see the callStackSymbols on all the other active threads as well. I know I can use [NSThread callStackSymbols] on any thread, but I need to loop through all the active threads to do so.

Is this possible?

Answer 1

This is a complex area, as Sedate Alien noted. You will need to implement your own stack walking to retrieve a stack trace from the other active threads; APIs such as backtrace(3) and +[NSThread callStackSymbols] will only produce a backtrace for the current thread.

Since I'm familiar with the PLCrashReporter code, I'll just use it for examples:

• To fetch the thread list, you'll need to use task_threads(): https://opensource.plausible.coop/stash/projects/PLCR/repos/plcrashreporter/browse/Source/PLCrashLogWriter.m?at=refs%2Ftags%2F1.0#694

• Once you have the thread list, you can fetch the thread state via thread_get_state(): https://opensource.plausible.coop/stash/projects/PLCR/repos/plcrashreporter/browse/Source/PLCrashFrameWalker_arm.c?at=refs%2Ftags%2F1.0#73

• Using the thread state, you can walk the target's stack: https://opensource.plausible.coop/stash/projects/PLCR/repos/plcrashreporter/browse/Source/PLCrashFrameWalker_arm.c?at=refs%2Ftags%2F1.0#104

Note that all this code is a bit unusual, as it executes within a signal handler and is written to be async-safe; if you're unfamiliar with the complications around signal handling, this is a good starting point: http://www.mikeash.com/pyblog/friday-qa-2011-04-01-signal-handling.html

Implementing this kind of thing correctly is a gigantic headache; I would really recommend that you make use of PLCrashReporter, or one of the products built on top of it (QuincyKit, HockeyApp, Atlassian JMC, etc).

Answer 2

This seems fraught with peril, doubly so if you need to ask here for help. May I suggest PLCrashReporter? Its listed features are:

• Implemented as a in-process fully async-safe signal handler.
• Does not interfere with debugging in gdb
• Handles both uncaught Objective-C exceptions and fatal signals (SIGSEGV, SIGBUS, etc)
• Backtraces for all active threads are provided. (emphasis my own)
• Provides full register state for the crashed thread.

Better yet, have a look at QuincyKit, a very handy wrapper around PLCrashReporter.