An lldb command that prints the description of the exception being raised, when run while the target is stopped at the first instruction of objc_exception_throw. Many thanks to Enrico Granata and Sean Callanan for basically writing this to my specifications at WWDC 2013.

.lldbinit

command script import ~/Library/lldb/sniff_objc_exception_throw.py

sniff_objc_exception_throw.py

import lldb
 
def GetFirstArgumentAsValue(target, frame):
    # Note: I assume the PC is at the first instruction of the function, before the stack and registers have been modified.
    if target.triple.startswith('x86_64'):
        return frame.regs[0].GetChildMemberWithName("rdi")
    elif target.triple.startswith('i386'):
        espValue = frame.regs[0].GetChildMemberWithName("esp")
        address = espValue.GetValueAsUnsigned() + target.addr_size
        return espValue.CreateValueFromAddress('arg0', address, target.FindFirstType('id'))
    else:
        return frame.regs[0].GetChildMemberWithName("r0")
 
def command(debugger, user_input, result, unused):
    target = debugger.GetSelectedTarget()
    frame = target.GetProcess().GetSelectedThread().GetFrameAtIndex(0)
    description = GetFirstArgumentAsValue(target, frame).GetObjectDescription()
    if description is None:
        output = "I couldn't get the description of the exception being thrown."
    else:
        output = "Description of exception being thrown: " + repr(description)
    result.PutCString(output)
    return None
 
def __lldb_init_module(debugger, unused):
    debugger.HandleCommand('command script add --function sniff_objc_exception_throw.command sniff_objc_exception_throw')