Re: Multi-threading

[Paul Davis <paul linuxaudiosystems com>]

On Wed, 2006-04-05 at 15:58 -0400, Douglas C. MacKenzie wrote:
> Along these same lines, I would like to do:
> 
>   while( Gtk::Main::wait_until_events_pending())
>   {
>      synchronized( gtk_lock )
>      {
>         while( Gtk::Main::events_pending())
>            Gtk::Main::iteration();
>      }
>   }
> 
> But I can't find a method that will block until
> an event is pending, without processing that event.
> Or, is it a problem to even wait for events to be
> pending if another thread is updating gtk stuff
> inside another block synchronized to my gtk_lock?

this falls into a common trap/misunderstanding of how the global gtk
thread lock works. it took me a while to understand it and by then i
gave up on using the lock at all.

the lock is acquired by gtk when the event loop returns from blocking
(in the kernel) on some set of descriptors (e.g. the socket connecting
you to the X server). it is released before re-entering that blocking
call. 

so, you do not hold the lock unless you are handling events *or* making
"out of bounds" calls to GTK functions. this means that if you want to
make a GTK call (e.g. gtk_widget_queue_draw()) from another thread (say,
because some thing happened in that thread that isn't handled by GTK),
you must first acquire the lock to make sure that a different thread is
not processing GTK events and thereby prevent any parallel execution of
GTK functions. when you are done, you drop the lock, allowing other
threads to process GTK events in the normal way.

--p