Re: Providing gtk_main() with asynchronous data

[Owen Taylor <otaylor redhat com>]

On Tue, 2005-08-02 at 06:36 -0600, Pat Mahoney wrote:
> On Tue, Aug 02, 2005 at 07:47:32AM -0400, Owen Taylor wrote:
> > On Sun, 2005-07-31 at 15:44 -0600, Pat Mahoney wrote:
> > > (not subscribed; please CC replies)
> > >
> > > I'm trying to find a way to provide my mainloop with asynchronous data
> > > from a thread.  I have read this:
> > >
> > > http://mail.gnome.org/archives/gtk-app-devel-list/2004-July/msg00181.html
> > >
> > > which describes how to use a GAsyncQueue and GSource to watch the queue
> > > for available data (and to have the thread wake up the mainloop when data
> > > is available).  A callback is run in the context of gtk_main when
> > > data is available.  It works perfectly except for one show-stopper
> > > problem:
> >
> > Note that using a custom GSource here is perhaps overcomplicated. The
> > simplest method is:
> >
> >  g_idle_add (function_to_call_in_main_thread, data_to_pass_to_function);
> >  
> > (Or use g_idle_add_full() if you want to prioritize the callback higher
> > than the default idle priority.)
>
> Yes, that is simplest.  In my case, the CPU usage jumps to 100% (though
> the app is still responsive).  My particular app is reading frames from
> one or more webcams (select() doesn't work with video4linux devices, so
> this must be done in a seperate thread or process).  The camera_thread
> is triggered by the mainloop to snap a picture which it pushes onto a
> GAsyncQueue.
>
> My callback on_camera_frame_available() checks the queue and returns
> immediately if there is no data.  Similarly, the GSourceFuncs check the
> queue.  However, by using the GSource, the check is performed about 2-3
> times per frame (using printf() to watch that, so timing may be skewed),
> while the g_idle_add() method runs my callback continually, and so the
> GAsyncQueue is constantly being checked for data, sucking down the CPU.

Your idle function presumably has the wrong return value. To quote:

http://developer.gnome.org/doc/API/2.0/glib/glib-The-Main-Event-Loop.html#g-idle-add

  If the function returns FALSE it is automatically removed from the list of 
  event sources and will not be called again.

Regards,
                                                        Owen