Use GDB to examine Objective C class from crash (HandleDelegateSource bad access)

Question

I am trying to debug an intermittent error on the iPhone, a crash with a trace that looks like:

objc_message_send
__invoking__
[NSInvocation invoke]
HandleDelegateSource
MainRunLoop
....

When GDB stops, I'd like to be able to determine details about what selector the system is attempting to be invoked - I've set a break point now around [NSInvocation Invoke], but from that point cannot figure out how to examine details of the NSInvocation object I am stopped in.

Answer 1

If you look at the reference information for objc_msgSend you will see that the selector is the second argument. Now it should be easier to decipher the debugger output.

objc_msgSend
Sends a message with a simple return value to an instance of a class.
id objc_msgSend(id theReceiver, SEL theSelector, ...)

Parameters
theReceiver
A pointer that points to the instance of the class that is to receive the message.
theSelector
The selector of the method that handles the message.
...
A variable argument list containing the arguments to the method.

Answer 2

This article, recently published at Phrack should answer all your questions. Enlightening reading.

Answer 3

A simple final answer - in GDB you can simply view the register with the name of the selector being called (theSelector parameter in lothar's answer). It's a C string, so you observe it using one of the following commands (depending on if you are running in the simulator or the device):

Simulator: display /s $ecx
Device: display /s $r1