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

<pre><code>objc_message_send
__invoking__
[NSInvocation invoke]
HandleDelegateSource
MainRunLoop
....
</code></pre>

<p>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.</p>

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.