Using dbus_message_set_data() and dbus_message_get_data() in conjunction with dbus_connection_send_with_reply

Fri Jul 24 09:46:51 PDT 2015

As an aside, if I understand what you are doing: round-trippng a pointer in 
the presence of such things as async messaging or threads is fraught with 
danger.

If I didn't understand, never mind.

td

On Friday, July 24, 2015 11:08:44 Simon McVittie wrote:
> On 24/07/15 14:45, Sahana Vadwa wrote:
> > I'm new to Dbus
> 
> Before I answer your actual question, if you are new to D-Bus, perhaps
> you didn't notice these sentences in the front page of the libdbus
> documentation:
> 
>     This manual documents the low-level D-Bus C API. **If you use this
>     low-level API directly, you're signing up for some pain.**
> 
> I would strongly recommend using a more friendly D-Bus library, such as
> one of these:
> 
> * GDBus (part of Gio, shipped with GLib; C API; independent
>   implementation; portable; LGPL)
> * QtDBus (part of Qt; C++ API; wraps libdbus; portable; LGPL)
> * sd-bus (part of systemd; C API; reimplementation; Linux-only; LGPL)
> 
> libdbus is the reference implementation. That does not imply that it's
> the *best* implementation, only that it was the *first* implementation,
> and is the one that was used to implement low-level infrastructure like
> dbus-daemon.
> 
> > I'm trying to send a particular payload data from my
> > application to dbus via |dbus_message_set_data()|with an intention to
> > get the same data upon calling |dbus_message_get_data()|.
> 
> To avoid any misinterpretations: dbus_message_set_data() and
> dbus_message_get_data() are about the message object in memory, not the
> serializable data that can be transmitted across the bus. They are
> mostly intended for language bindings like dbus-python, rather than
> "normal" C API users.
> 
> >  1. Create a new dbus method call using |dbus_message_new_method_call()|
> >  2. Allocate a data slot for the DbusMessage
> >  
> >     using |dbus_message_allocate_data_slot()|
> >  
> >  3. Set application payload using |dbus_message_set_data()|
> >  4. Send method call using |dbus_connection_send_with_reply()| ///this
> >  
> >     is a pending call for which I have a callback registered./
> 
> So far, you have one DBusMessage object, which is a method call. However...
> 
> >  5. Upon receiving the response for the pending call, get the data set
> >  
> >     earlier using using |dbus_message_get_data()|
> 
> ... the response is a separate DBusMessage object, representing a
> different message. You sent a message of type METHOD_CALL; you got a
> message of type METHOD_REPLY back. They are different serialized
> messages travelling across the bus, and they are also different
> DBusMessage objects in memory. If you attached data to the method call,
> you can only get it back by inspecting the method call, and not by
> inspecting the reply.
> 
> If you want to associate in-memory data with a particular
> method-call/reply transaction, the usual way is to pass it to
> dbus_pending_call_set_notify() as the user_data. It will come back into
> your code in the DBusPendingCallNotifyFunction. This should be a
> familiar pattern if you have used asynchronous functions in a library
> like GLib.
> 
> Another way to achieve similar results is with
> dbus_pending_call_set_data(), but as with dbus_message_set_data(), you
> don't normally need that unless you are implementing a language binding
> like dbus-python.
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