iphone - Printing a stack trace from another thread

Question

I know I can get the stack trace of the current thread using backtrace() or [NSThread callStackSymbols], but how would I get at the stack trace of a DIFFERENT thread (assuming it's been frozen)?

Answer 1

EDIT: My original answer will not print from an arbitrary thread. I've since written a proper implementation in my crash handler project: https://github.com/kstenerud/KSCrash

Specifically, these files:

• https://github.com/kstenerud/KSCrash/blob/master/KSCrash/KSCrash/Reporting/Tools/KSBacktrace.h
• https://github.com/kstenerud/KSCrash/blob/master/KSCrash/KSCrash/Reporting/Tools/KSBacktrace.c

With some help from:

• https://github.com/kstenerud/KSCrash/blob/master/KSCrash/KSCrash/Reporting/Tools/KSMach.h
• https://github.com/kstenerud/KSCrash/blob/master/KSCrash/KSCrash/Reporting/Tools/KSMach.c

What you do is:

• Make a new machine context structure (_STRUCT_MCONTEXT)
• Fill in its stack state using thread_get_state()
• Get the program counter (first stack trace entry) and frame pointer (all the rest)
• Step through the stack frame pointed to by the frame pointer and store all instruction addresses in a buffer for later use.

Note that you should pause the thread before doing this or else you can get unpredictable results.

The stack frame is filled with structures containing two pointers:

• Pointer to the next level up on the stack
• instruction address

So you need to take that into account when walking the frame to fill out your stack trace. There's also the possibility of a corrupted stack, leading to a bad pointer, which will crash your program. You can get around this by copying memory using vm_read_overwrite(), which first asks the kernel if it has access to the memory, so it doesn't crash.

Once you have the stack trace, you can just call backtrace() on it like normal (The crash handler has to be async-safe so it implements its own backtrace method, but in normal cases backtrace() is fine).