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

[Roger Light <roger@xxxxxxxxxx>]

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
>
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