Category Archives: gtkmm

How to waste an evening debugging the internals of glib’s qsort implementation

Working on a personal itch-scratching project, I found myself wanting a sortable treeview. So I stuffed my treemodel into a TreeModelSort and set up my sort functions, and everything was happy. Except, when running my application, I kept getting strange segfaults within glib’s qsort implementation. I had set the following function as my sort_func (yes, it’s C++, but it should be fairly self-explanatory):

static int
compare_foo (const Gtk::TreeModel::iterator& a, const Gtk::TreeModel::iterator& b)
    std::tr1::shared_ptr<Foo> l1, l2;
    l1 = a->get_value (COLUMN_FOO);
    l2 = b->get_value (COLUMN_FOO);

    if (!l1)
        return -1;

    if (!l2)
        return 1;

    return l1->get_id () - l2->get_id ();

Can you spot the error? Running it under valgrind indicated that I was reading data from *before* the start of the array that was being sorted. In fact, the following lines of code in g_qsort_with_data() were causing tmp_ptr to be decreased back past the start of the array:

        while ((*compare_func) ((void *) run_ptr, (void *) tmp_ptr, user_data) < 0)
          tmp_ptr -= size;

Hmm. Why is there no guard to prevent tmp_ptr from being decreased past the start? Then I noticed the following comment up a few lines:

    /* Find smallest element in first threshold and place it at the
       array's beginning.  This is the smallest array element,
       and the operation speeds up insertion sort's inner loop. */

So at this point, the code is assuming that the very first element of the array is the smallest, so compare_func should always return >= 0 when it reaches the start of the array. Aha! So the problem is my compare_func. It turns out I forgot to handle the single case of both values being NULL.

    l1 = a->get_value (COLUMN_FOO);
    l2 = b->get_value (COLUMN_FOO);
+   if (!l1 && !l2)
+       return 0;

    if (!l1)
        return -1;

So there’s not really a big lesson to be learned here, but if you ever hit strange segfaults deep inside glib’s qsort while sorting a treemodel, do yourself a favor and double-check that your sort function is sane first. Also, do yourself a favor and run it under valgrind as soon as you get a strange segfault. Knowing the exact point of the invalid read is infinitely more helpful than waiting until that invalid read causes a segfault.


As a quick follow-on to my last post, I’ve put a webkitmm git repository up for those that might want to play around with it. It’s certainly still early, but it is mostly complete. You’ll probably need a recent checkout WebKit/Gtk+, I’ve only tested it on trunk. At some point, I’ll probably import it into GNOME subversion, though I really don’t look forward to that.

There is a very simple browser example in the source tree as well:



A belated happy first birthday to my beautiful daughter. It’s been a fantastic year.


Since then it seems like I’ve been sick most of the time. Instead of doing something useful, I’ve taken to fidding on a re-write of my Agave colorscheme designer, and I’ve made a decent amount of progress. It still lacks a lot of features of the original, but it benefits from my vastly better grasp of gtkmm and related technologies. I still don’t know if I’ll ever actually get around to releasing it. It’s currently serving as a way for me to relax and take breaks from my other projects. It’s become sort of a playground for me to try out new technologies, and I think I’ve succeeded in making it nearly impossible for normal users to build as it requires quite a few very new or unreleased libraries (goocanvasmm, giomm, glibmm-utils, etc).

Here’s a little screencast of what I’ve done so far:

video thumbnail

I’ve set up a repository on github for anybody that’s interested in playing around with it.

Also, I’ll be taking over some glibmm maintainer duties from murray after the 2.16.0 release.