DBus Threading in a Late-loading module
thiago at kde.org
Tue Apr 3 12:19:45 PDT 2007
>Thiago Macieira wrote:
>> Havoc Pennington wrote:
>>> There's just no way this can work AFAIK unless Qt and GLib in fact
>>> use the same main loop. And then there should be no problem with
>>> dbus, right?
>> The mainloop isn't enough: the functions handling watches and timeouts
>> There's a thin layer of code between libdbus and the toolkit that
>> handles that, including a mapping of the structures.
>I thought Qt>=4.2 can use glib main loop
>(http://doc.trolltech.com/4.2/qt4-2-intro.html). Thus watches and timers
>should work, no matter if they are registered with the Qt API or the
>glib main loop API... (because the Qt mainloop API just wraps glib main
It can, but you misunderstand the problem. And this is not mentioning that
the glib main loop integration is optional and can be disabled at
libdbus has two structures called DBusWatch and DBusTimeout. The bindings
are required to keep a mapping of those structures to their own
representations of timers and file-descriptor pollings. This glue is what
breaks if you have two bindings running in the same application -- or
Even if an application using QtDBus is indirectly using glib's own
structures (GSources for polls and timeouts), QtDBus doesn't know
anything about them: all it knows are the Qt equivalents (QSocketNotifier
and QObject timers). If a second binding were to re-initialise the
callback functions, the old associations would be lost and behaviour
would be unpredictable.
If no binding associates said functions, libdbus may simply not work
(again, unpredictable behaviour).
What I am trying to point out is that we're telling library and plugin
developers that they are only allowed to use D-Bus if they are certain
that the application is using one D-Bus binding. That sounds rather
unacceptable to me.
PS: as an implementation detail, all Qt timers are handled by a single
glib timeout GSource, so there's not even a direct 1-to-1 object mapping.
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
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