Add a symbolic breakpoint on a selector in Xcode

Question

There's a bug in my app which shows up with the following (partial) stacktrace:

2011-11-25 01:55:59.760 Events2[6650:403] -[Event boolValue]: unrecognized selector sent to instance 0x7fb903928670

To debug this I decided to add a symbolic breakpoint on -[Event boolValue] reasoning that when that selector is sent, the debugger would halt.

However, nothing happens. After setting the breakpoint the app just soldiers on and generates the same exception without halting.

I have defined the breakpoint as follows:

I'm using the LLDB debugger with Xcode 4.2

Answer 1

Setting a breakpoint on a selector causes lldb to halt when that selector is executed, not when it is sent. In your case, there is no selector "-[Event boolValue]", therefore this breakpoint will never be hit.

I would set an exception breakpoint on "All Objective-C Exceptions". This will be hit when the "unrecognized selector sent" exception is thrown and you can see where the problem occurs.

Answer 2

I was looking for the same answer (symbolic breakpoints) and this link helped: http://www.cocoabuilder.com/archive/cocoa/308967-symbolic-breakpoints.html#308970

You have to follow this pattern (it is also given as a placeholder in Xcode breakpoint editor):

- [name_of_the_class name_of_the_method:]

For example I was looking to see who does set my left bar item and overrides my settings, I used -[UINavigationItem setLeftBarButtonItem:]

and it worked. Or this one

-[UINavigationController pushViewController:animated:]

Answer 3

I would set an Symbolic breakpoint with this symbol -[NSObject doesNotRecognizeSelector:]

which will help us to capture situations where a selector is being invoked against the wrong object.

Answer 4

It looks to me like symbolic breakpoints don't work right in LLDB (I'm running the most recent released version of Xcode as of this writing, 4.3.3).

I set a symbolic breakpoint at addAnimation:forKey: in LLDB, and it never gets hit. If I switch my project to GDB, the breakpoint works as expected.

Answer 5

Best way to find unrecognized selector call is to create this selector (as category) and put a break point in it.