Friday, March 13, 2009


I've been wanting do to a post on using the Debugger in Xcode, since that's a topic that had to be cut from the book. Knowing how to debug gets to be rather important as you start writing your own programs, especially as those programs get to be bigger and more complex. Unfortunately, I'm heads down right now trying to get caught up on a couple of different things, and just haven't had the time to do full blog post on the topic.

I did, however, take a break last night to record a short screencast on Debugging in Xcode. There's very little editing in this, so it's not as polished as some screencasts, but if you're new to Xcode or to programming in general, I think you'll benefit from watching it. It shows the basics of using the debugger, including breakpoints, conditional breakpoints, and using breakpoint actions. Click the image to play the movie.


John Keyes said...

This was great -- thanks Jeff!

karstenbriksoft said...

nice screencast, but the mouse-click sound is really distracting. You already enabled the mouse click highlighting so the sound is really a bit too much.


Jeff LaMarche said...

I'll see if I can't turn it off... it's a new program for me, not very familiar with its features yet.

Cyril said...

As usual, great post, and great screencast.

Two or three personal comments :
- I use the all in one view, and get the debug + log window all in one.
- print description to console int he context menu seems to be the same as po.
- you should add obc_exception_throw in your global breakpoints.

Looking forward to the Instruments screencast!

Jeff LaMarche said...

D'oh! Cyril, now I feel stupid. I didn't even talk about symbolic breakpoints!

I'll have to do an addendum posting to show how to set a symbolic breakpoint using objc_exception_throw as an example.

jozero said...

That was super informative, thanks. I've been looking for the one step beyond 'caveman debugging' explanation. One item I'm having difficulty with is since items can run asynchronously (say a gui updated, nsurl call, nsthread, etc) if an app crashes its hard to find exactly what is causing the crash - here caveman debugging is of no value. I tried to use 'info symbol' to print out the lowest value of a stack crash but more often than not get 'line is junk' message. How do you narrow down a crash ? Am I missing something obvious - is there a way to get an explicit 'the app crashed in this object in this method on this line' information ?

thanks. I've enjoyed reading your book.

Jeff LaMarche said...


If it's a overreleasing error, it's going to be hard to set a breakpoint for it, because the error's often going to happen when the autorelease pool is drained, regardless of where the actual problem is in your code.

Cyril's suggestion above, is a good one, though - set a symbolic breakpoint for objc_exception_throw, which will stop execution at the line of code where the exception happened. Very helpful, and I'm going to do a follow up showing how to do symbolic breakpoints in a day or two if I can find the time, but it's pretty easy to do, if you Google it, there are a few blogs that show how to do it.


nicedexter said...

Great, thanks

In a Breakpoint Log, how would you print out the description, says of an NSURL *url, like it would do with "po url" ?


Frederik W said...

I just posted a (positive) review on Amazon for your book, pointing out that I felt some information on debugging was missing. I felt pretty lost when I had done a typo and all I got was a blank simulator. So I was really psyched to find this post. Thanks!, and also thanks for an excellent book.

Joe said...

Excellent, very informative, love your book as well. The conditional breakpoint info was worth the watch alone for me! Thanks for your efforts. :-)

Sebastian said...

How would you realize something like
NSLog(@"x %f",;
with this breakpoint logging ?

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John Hsu said...

Best thing I've ever seen on getting started with Xcode debugging - you solved several mysteries for me in a few short minutes.

Many thanks!

bamhm182 said...

I've been doing "Caveman debugging" since I started with XCode in 2009. Never felt the need to actually learn how to properly debug, but your video left me going ":O... WHAT have I been DOING all these years!?!" haha. Great video, Part 2 is amazing as well.