Comments on: Objective-C: Logging Messages to Nil

By: Macinsoft - Logging Messages to Nil

Macinsoft - Logging Messages to Nil — Tue, 15 Jan 2008 07:03:11 +0000

[...] bbum’s weblog-o-mat: “Some mechanism for catching, logging, and breaking on messages to nil (and only messages to nil) is needed for the rare—but often insanity inducing—case where you need to track down a bug of this nature.” Comments RSS | Trackback URL [...]

By: leeg

leeg — Thu, 03 Jan 2008 11:58:01 +0000

Nice example, and thanks to Adrian for that D script . BTW bbum, HTML has swallowed the filenames in your #import lines.

By: Kevin Ballard

Kevin Ballard — Thu, 03 Jan 2008 08:40:51 +0000

Sure, it traps every call, but if the dtrace kernel buffer gets filled it will start discarding events. But trapping messages to nil should be fine. Logging a message for every single invocation of objc_msgSend, on the other hand, would probably start dropping them on the floor.

By: Adrian Milliner

Adrian Milliner — Wed, 02 Jan 2008 22:19:37 +0000

‘Keep up?’ it’ll trap every single call; dtrace is not (necessarily) a sampling profiler.

a dtrace script like:

pid$1::objc_msgSend:entry
/arg0==0/
{
  ustack();
}

make little difference to the Finder’s performance, but does really slow down Address Book (as it sends messages to nil tens of thousands of times in a few seconds).

By: Kevin Ballard

Kevin Ballard — Wed, 02 Jan 2008 20:32:43 +0000

dtrace executes faster, but objc_msgSend may be called too frequently for dtrace to keep up. It’s not meant for incredibly high-volume stuff. That said, it may still be worth a try.

By: Michael Tsai - Blog - Logging Messages to Nil

Michael Tsai - Blog - Logging Messages to Nil — Wed, 02 Jan 2008 15:28:21 +0000

[...] Bill Bumgarner shows how to use the private _objc_setNilReceiver() function and -resolveInstanceMethod: to log the first time each selector is sent to nil. I imagine that by uncommenting the print statement in -forwardInvocation: this could be made to work with Objective-C 1.x, but you’d get a log entry for every message sent to nil. [...]

By: Adrian Milliner

Adrian Milliner — Wed, 02 Jan 2008 08:12:04 +0000

I’ve not tried this but could you use dtrace (and Instruments) to trace entry to objc_msgSend where arg0 is null ?

The probe/instrument would look something like: pid::obj_msgSend:entry and arg0 would be available to the script that runs. You also have the dtrace ustack() method to show where you are.

Sorry, I haven’t done any D for a while and almost none on OSX, so little vague on detail – but a good google should help.

Note that dtrace probes execute *much* faster than a debugger or profiler, so maybe worth doing.