Re: [mosquitto-dev] does calling mosquitto_subscribe twice add a new sub

[Steve Prior <sprior@xxxxxxxxxxxx>]

Just to follow up on this, over the weekend I spent some time on this and it 
seems that the mqtt lib for Prolog I
was using included an old version of the mosquitto library in its distribution.  
Once I removed that and recompiled
with a current version the subscriptions worked as expected.  I ended up forking 
the repository for the
interface (it appears the original is inactive) and I'm now working to allow 
binary blob message payloads via the library, plain string payloads already work.

My fork is at: https://github.com/sprior/swi-mqtt-pack

I think that there are some pretty cool possibilities using Prolog in 
combination with IOT devices, but I may just be weird.

Steve

On 5/30/2017 5:07 AM, Roger Light wrote:
> Hi Steve,
>
> If you call
>
> mosquitto_subscribe(client, NULL, "topic1", 1);
> mosquitto_subscribe(client, NULL, "topic2", 0);
>
> then you should end up subscribed to both topic1 (with qos 1) and
> topic2 (with qos0).
>
> If you subsequently call
>
> mosquitto_subscribe(client, NULL, "topic2", 1");
>
> then your subscription for topic2 will be updated to be qos 1 instead.
> It will not add an extra subscription for topic2.
>
> I hope that clears things up.
>
> Regards,
>
> Roger
>
>
>
> On 30 May 2017 at 03:39, Steve Prior <sprior@xxxxxxxxxxxx> wrote:
> > I'm getting started using the mqtt pack for SWI-Prolog to subscribe to
> > topics on a mosquitto message broker.  The library under the covers uses the
> > c mosquitto API.
> >
> > I've been able to subscribe to a topic and process incoming messages in
> > Prolog - this is working.  What I'm trying to understand is whether I can
> > subscribe to two completely different topics using the same mqtt connection.
> > I was under the impression that this was possible, but what I think I saw
> > was than when I called subscribe a second time I started seeing the messages
> > under the new topic, but no longer saw the messages I had previously
> > subscribed to.  Then when I called subscribe again with the original topic I
> > saw only those and not the ones for the second topic.
> >
> > I've gone through the prolog -> mosquitto interface code and the interface
> > code is doing very little beyond retrieving the connection and passing the
> > call to the mosquitto c API, so I don't think that interface is adding any
> > new restrictions.
> >
> > Looking at the c API documentation for mosquitto_subscribe it does not
> > specify the behavior if it is called twice.  Can anyone clarify if the new
> > subscription topic is supposed to replace (or add) to any previous
> > subscribed topics?
> >
> > Thanks
> > Steve Prior