Debugging reference counting in GNOME C++ bindings with GDB
Overview
For the last couple days I’ve been tracking down some reference counting problems in cluttermm. In the course of this I’ve been working on figuring out how GDB can help me do this. Here is what I’ve come up with. If you have improvements please let me know.
Watching the reference count variable
The first small obstacle is that pretty much all objects are wrapped in a RefPtr, glibmm‘s reference-counting shared smartpointers. This is a good thing and gdb makes accessing the underlying C++ object pretty painless. The following command demonstrates this.
p alpha.pCppObject_
For this example I’m using the alpha variable of type Glib::RefPtr<Clutter::Alpha>. In this command we used the fact that gdb gives you access to any member of a class, whether public or private. You should have gotten output similar to the following.
$1 = (Clutter::Alpha *) 0x9cfe40
Now the interesting part. We want to detect any changes in the ref_count variable. To do this we need to watch the ref_count field. The following command does this.
watch alpha.pCppObject_->gobject_->ref_count
To make this a bit more convenient, I’ve used the gdb define command to create a shortcut. Here is an example session.
(gdb) define watchRefCnt
Type commands for definition of "watchRefCnt".
End with a line saying just "end".
>watch ($arg0).pCppObject_->gobject_->ref_count
>end
Now you just need to use the shortcut like this.
watchRefCnt alpha
Which should give you a confirmation similar to the following.
Hardware watchpoint 2: (alpha).pCppObject_->gobject_->ref_count
The obvious thing to do next is to place this command in a .gdbinit file so that it can be pulled in automatically.
Breaking on last unreference
One of things I was most interested in catching was when the g_object_unref function was called with the condition that the _object pointer was equal to a certian pointer and the ref_count variable == 1. This would indicate that the object is ready to be disposed. To do this we first need to get the address of the underlying GObject like this.
(gdb) p alpha.pCppObject_->gobject_
$3 = (GObject *) 0x9c91e0
You now have a convenience variable, $3, which you can use in the conditions. You then need to set a breakpoint on g_object_unref and set the conditions like this.
b g_object_unref
cond 4 _object==$3&&((GObject*)$3)->ref_count==1
This, of course, would also be nice as a custom command. I’ll leave that as an exercise for the reader. 
Future explorations
I forsee future blog entries along these lines so stay tuned. There are quite a few things that interest me in gdb right now.
The Archer project is quickly improving C++ support in gdb. I’ve been using gdb 7.2 for my debugging as I wanted to have a stable debugging platform with all the new C++ enhancements. They have a git repository for those who want the bleeding edge.
Python scripting has been available in gdb since 7.0. I plan on looking into this as a way of streamlining debugging tasks and honing my Python skills which are pretty rusty.
Please let me know if you see any areas for improvement.
Learnt some advanced gdb stuff. 
right now.
