Re: Attaching an event source to a GMainLoop in a thread



On Sun, 2006-12-03 at 19:05 +0000, Chris Vine wrote:
On Sunday 03 December 2006 10:26, Braden McDaniel wrote:
I have a GMainLoop that I want to run in its own thread. But I seem to b
doing something horribly wrong with the event source I'm trying to
attach to it (all as part of the same thread). My thread function
includes this code:

        GMainContext * main_context = g_main_context_new();
        GMainLoop * main_loop = g_main_loop_new(main_context, false);

        GSource * quit = g_idle_source_new();
        g_source_set_callback(quit,
                              ::openvrml_player_command_channel_loop_quit_event,
                              main_loop,
                              notify);
        guint source_id = g_source_attach(quit, main_context);
        g_assert(source_id != 0);

        g_main_loop_run(main_loop);

        g_source_unref(quit);
        g_main_loop_unref(main_loop);
        g_main_context_unref(main_context);

If I do not attach the event source, the code runs. But if I do attach
it, as above, I crash as soon as I start to run the main loop:

Wouldn't unreferencing main_context cause it to be freed?  (And does 
g_main_loop_run() increement the reference count of main_loop?  If not then 
unreferencing main_loop that will also destroy the main loop).  The call to 
g_source_unref() is correct (g_source_attach will increment the reference 
count of quit).

I'm not getting past the call to g_main_loop_run, so the *_unref calls
aren't the problem.

I've gotten closer to the problem; but it's just mysteriouser.

The callback looks like this:

        gboolean openvrml_player_command_channel_loop_quit_event(const gpointer data)
        {
            GMainLoop * const main_loop = static_cast<GMainLoop *>(data);
        
            if (::quit_flag.value()) {
                g_main_loop_quit(main_loop);
                return false;
            }
            return true;
        }

quit_flag is just a boolean flag protected by a mutex:

        class flag : boost::noncopyable {
            mutable boost::mutex mutex_;
            bool value_;
        
        public:
            flag(bool init = false);
        
            bool value() const;
            void value(bool val);
        };

It appears that the core dump happens in the first call to
openvrml_player_command_channel_loop_quit_event. But here things get
murky because the problem seems to be very timing-sensitive. If I put a
print statement at the beginning of
openvrml_player_command_channel_loop_quit_event, it starts working. And
if I instead put a print statement at the beginning of flag::value(), it
starts working.

This points, I think, to something being wrong with the state of
quit_flag at the time I try to use it in
openvrml_player_command_channel_loop_quit_event--like it's not fully
initialized. Though I don't understand why that would be the case--my
thread is being started from main() quit_flag is a global variable (in
the same translation unit as main()).

I'll continue to experiment.

-- 
Braden McDaniel                           e-mail: <braden endoframe com>
<http://endoframe.com>                    Jabber: <braden jabber org>





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