Hi all,
I'm trying to debug a segfault relating to the memory management in
Gtk2Hs. Rather than make you read the ticket http://hackage.haskell.org/trac/gtk2hs/ticket/1183
, I'll describe the problem:
- compiler 6.12.1 or 6.12.3
- darcs head of Gtk2Hs with #define DEBUG instead of #undef DEBUG in
gtk/Graphics/UI/Gtk/General/hsthread.c
- platform Ubuntu Linux, x86-64
- to reproduce: cd gtk2hs/gtk/demo/hello and run ghci World.hs and
type 'main'
A window with the "Hello World" button appears. After a few seconds,
the GC runs and the finaliser of the GtkButton is run since the
Haskell program no longer holds a reference to that object (only the
GtkWindow in C land has).
Thus, the GC calls a C function gtk2hs_g_object_unref_from_mainloop
which is supposed to enqueue the object into a global data structure
from which objects are later taken and g_object_unref is called on them.
This global data structure is protected by a mutex, which is acquired
using g_static_mutex_lock:
void gtk2hs_g_object_unref_from_mainloop(gpointer object) {
int mutex_locked = 0;
if (threads_initialised) {
#ifdef DEBUG
printf("acquiring lock to add a %s object at %lx\n",
g_type_name(G_OBJECT_TYPE(object)), (unsigned long)
object);
printf("value of lock function is %lx\n",
(unsigned long)
g_thread_functions_for_glib_use.mutex_lock);
#endif
g_rand_new();
#if defined( WIN32 )
EnterCriticalSection(>k2hs_finalizer_mutex);
#else
g_static_mutex_lock(>k2hs_finalizer_mutex);
#endif
mutex_locked = 1;
}
[..]
The program prints:
acquiring lock to add a GtkButton object at 22d8020
value of lock function is 0
zsh: segmentation fault ghci World
Now the debugging weirdness starts. Whatever I do, I cannot get gdb to
find the symbol gtk2hs_g_object_unref_from_mainloop.
Since the function above is contained in a C file that comes with our
Haskell library, I tried to add "cc-options: -g" and "cc-options: -
ggdb -O0", but maybe somewhere symbols are stripped. So I added the
bogus function call to "g_rand_new()" which is not called anywhere
else and gdb stops as follows:
acquiring lock to add a GtkButton object at 2105020
value of lock function is 0
[Switching to Thread 0x7ffff41ff710 (LWP 15735)]
Breakpoint 12, 0x00007ffff115bfa0 in g_rand_new () from /usr/lib/
libglib-2.0.so
This all seems reasonable, but:
(gdb) bt
#0 0x00007ffff115bfa0 in g_rand_new () from /usr/lib/libglib-2.0.so
#1 0x00000000419b3792 in ?? ()
#2 0x00007ffff678f078 in ?? ()
i.e. the calling context is broken. I'm very, very sure that the
caller is indeed the above mentioned function and since g_rand_new
isn't called anywhere in my Haskell program (and otherwise the calling
context would be sane).
I'm also passing the address of gtk2hs_g_object_unref_from_mainloop as
FinalizerPtr to all my ForeignPtrs, so there is no inlining going on.
Back to the culprit, the call to g_static_mutex_lock. This is a macro
that expands to
*g_thread_functions_for_glib_use.mutex_lock
where g_thread_functions_for_glib is a global variable that contains a
lot of function pointers. At the break point, it contains this:
(gdb) print g_thread_functions_for_glib_use
$33 = {mutex_new = 0x7ffff0cd9820