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doubi (9222)

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Journal of doubi (9222)

Wednesday August 19, 2009
06:19 PM

XS debugging and "holding out for the pretty one"

[ #39497 ]
In the last couple of days I used gdb a little for the first time as a result of looking around a bit for information on debugging XS, some of which I want to share here.

First is this journal entry by Shlomi Fish where he gives some useful settings for debugging an XS function with gdb.

I also found this very useful message from Eric Maland in a thread in the org.perl.perl-xs archives, which I will reproduce here in full:

It seems like real overkill, to me, to build a static version of perl with debugging symbols just to debug one module. maybe i'm just lazy.

The approach I take to debugging XS modules (and this is entirely home-brewed, since I've found virtually zero useful information on the subject) is this:

  • Build your module, test it, watch it segfault or whatever.
  • Once this has occurred, rebuild it with debugging symbols and don't strip symbols from the module (MakeMaker defaults to using ld -s).
  • Load perl into gdb, and get into the perl debugger. From there, load your module (use DynaLoader, add the module path to your @INC and 'use' it, do it from the command line, whatever). Now we're "meta-debugging".
  • Execute your perl/XS code, and watch it die. Nicely. In gdb. Where you can debug it.

You also have the option of loading the symbols from the shared library in gdb, and from there you can set breakpoints, which tends to be useful. I've never had a need for a staticly linked module just to debug it(!). Nor have I ever required a version of perl with debugging symbols included. I'm glad. Who has time for that? Maybe if you're debugging perl that would be useful, but aside from that I see no real benefit (maybe i'm ignorant of the limitations of other platforms, though, some on which I know you would have to statically link, so please don't flame me on that).

I've attached a dinky module that just calls one XS function that strcpy()'s into a NULL pointer, and included in it a 'debuglog' file that shows how I approach debugging this type of problem in XS modules (just a screen snapshot of my debugging session, with some minor comments at the top). This seems to work 9 times out of 10, for me. Your results may vary. I suspect the platform you're working on and your compiler and linker may be quite different from mine so out-of-the-box my method may not work for you - in fact, it probably won't. But if you get the general idea, it should be easy to tweak to work on your platform.

Hope this helps.


He did help a number of people on the list, and I hope reposting his wisdom here might make it even more findable. I didn't use all the techniques he details but he pointed me in the right direction for what I needed to do.

What I was doing was trying to work out the answer to issue number four in my Wx::WebKit mentioned a couple of posts ago. I spent a day or so trying to narrow down ever further the exact place where things were going wrong. I eventually had my finger on it, but no idea what to do about it. When I eventually took it to the list, Mattia informed me that "Everything in wxPerl assumes that the C++ class name matches the Perl name" and suggested I subclass at the Perl level, as another person who should know had suggested to me before I spent a lot of extra time looking deeper into the problem :-p

It had occurred to me as well of course, but it seemed somehow an untidy solution, ending up with one more .pm than I thought I should need floating around, and I was sure if I really got to grips with the problem properly there would be a more elegant solution to be had in the C / XS layer. That snobbery wasted a day of development time at least - the danger of holding out for the prettier solution that's just around the corner.

Anyway, the upshot of all this is my Wx::WebKit has been updated so you call with Wx::WebKit->new rather than Wx::WebView->new (because wtf's WebView and why should you care?) and is available at the git repo.

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