Vladimir Prus


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Thursday, February 01, 2007

Debugger Stories: Stack widget

In KDevelop 3.4, the stack widget was not changed much. I can remember just two changes­—one that is apparent and one that is subtle.

The apparent change is that we actually parse gdb output, and show it it a readable way, while in KDevelop 3.3 the stack frame formatting was entirely at mercy of gdb's "backtrace" command.

The subtle change is at the bottom of the screenshot—that "(click to get more frame)" thing. When a program stops, KDevelop fetches very few frames from gdb. If you click on that last item, then another chunk of frames will be fetched.

This behaviour is needed for two reasons. First, if your program is stuck in infinite recursion, and you try to interrupt it from KDevelop, in KDevelop 3.3 you're out of luck. As soon as the program is interrupted, KDevelop asks gdb for the list of all frames. Since your program is in infinite recursion, the number of frames is very large, and gdb is not very speedy stack-walker. So, you get to wait 5 mins for the stack to be shown. With incremental display, in a few clicks you'll see what function went astray.

The second reason is embarassing. Even without infinite recursion, getting the list of frames from gdb takes a lot of time. Something like half-a-second for getting 30 frames is not unheard of. Ideally, we'd fix gdb, but since we need incremental fetch anyway, fetching sufficiently small number of frames initially greatly improves responsiveness.

1 comment:

Anonymous said...

I really love KDevelop 3.4 which I'm using at work. One of the nicest features is this stack widget..!!!!
It make things really tidy, works perfectly and the 'show more frames' is really user friendly!
Thanks a lot for this candy!!