Phil Calvin Phil Calvin - 3 days ago 5
iOS Question

Ignore certain exceptions when using Xcode's All Exceptions breakpoint

I have an All Exceptions breakpoint configured in Xcode:

screenshot of an exception breakpoint configured in Xcode breakpoint pain, configured to make a sound when an exception is thrown

Sometimes Xcode will stop on a line like:



[managedObjectContext save:&error];


with the following backtrace:

backtrace showing NSPersistentStoreCoordinator throwing an exception inside the call to save:

but the program continues on as if nothing happened if you click Continue.

How can I ignore these "normal" exceptions, but still have the debugger stop on exceptions in my own code?

(I understand that this happens because Core Data internally throws and catches exceptions, and that Xcode is simply honoring my request to pause the program whenever an exception is thrown. However, I want to ignore these so I can get back to debugging my own code!)

Moderators: this is similar to "Xcode 4 exception breakpoint filtering", but I think that question takes too long to get around to the point and doesn't have any useful answers. Can they be linked?

Answer

I wrote an lldb script that lets you selectively ignore Objective-C exceptions with a much simpler syntax, and it handles both OS X, iOS Simulator, and both 32bit and 64bit ARM.

Installation

  1. Put this script in ~/Library/lldb/ignore_specified_objc_exceptions.py or somewhere useful.
import lldb
import re
import shlex

# This script allows Xcode to selectively ignore Obj-C exceptions
# based on any selector on the NSException instance

def getRegister(target):
    if target.triple.startswith('x86_64'):
        return "rdi"
    elif target.triple.startswith('i386'):
        return "eax"
    elif target.triple.startswith('arm64'):
        return "x0"
    else:
        return "r0"

def callMethodOnException(frame, register, method):
    return frame.EvaluateExpression("(NSString *)[(NSException *)${0} {1}]".format(register, method)).GetObjectDescription()

def filterException(debugger, user_input, result, unused):
    target = debugger.GetSelectedTarget()
    frame = target.GetProcess().GetSelectedThread().GetFrameAtIndex(0)

    if frame.symbol.name != 'objc_exception_throw':
        # We can't handle anything except objc_exception_throw
        return None

    filters = shlex.split(user_input)

    register = getRegister(target)


    for filter in filters:
        method, regexp_str = filter.split(":", 1)
        value = callMethodOnException(frame, register, method)

        if value is None:
            output = "Unable to grab exception from register {0} with method {1}; skipping...".format(register, method)
            result.PutCString(output)
            result.flush()
            continue

        regexp = re.compile(regexp_str)

        if regexp.match(value):
            output = "Skipping exception because exception's {0} ({1}) matches {2}".format(method, value, regexp_str)
            result.PutCString(output)
            result.flush()

            # If we tell the debugger to continue before this script finishes,
            # Xcode gets into a weird state where it won't refuse to quit LLDB,
            # so we set async so the script terminates and hands control back to Xcode
            debugger.SetAsync(True)
            debugger.HandleCommand("continue")
            return None

    return None

def __lldb_init_module(debugger, unused):
    debugger.HandleCommand('command script add --function ignore_specified_objc_exceptions.filterException ignore_specified_objc_exceptions')
  1. Add the following to ~/.lldbinit:

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

    replacing ~/Library/lldb/ignore_specified_objc_exceptions.py with the correct path if you saved it somewhere else.

Usage

  • In Xcode, add a breakpoint to catch all Objective-C exceptions
  • Edit the breakpoint and add a Debugger Command with the following command: ignore_specified_objc_exceptions name:NSAccessibilityException className:NSSomeException
  • This will ignore exceptions where NSException -name matches NSAccessibilityException OR -className matches NSSomeException

It should look something like this:

Screenshot showing a breakpoint set in Xcode per the instructions

In your case, you would use ignore_specified_objc_exceptions className:_NSCoreData

See http://chen.do/blog/2013/09/30/selectively-ignoring-objective-c-exceptions-in-xcode/ for the script and more details.

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