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Re: [paho-dev] MQTT C asynchronous token

Hi Frank,

the asynchronous mode of the synchronous client (we should find some more terms :-) does not behave the same way as the asynchronous client apart from the connect/disconnect callbacks.  The publish calls still block until the publish packet has been written to the wire, and you can't make other API calls in the callbacks (apart from connect in connectionLost).  It is still a bit more synchronous than the asynchronous API.  I've tried to explain more here:

The async C API is different to the Java API in that it allows more messages to be queued, and blocks less.  This is a reason for the differences.  It may be possible to do what you need, but I'll have to think about it.   Raising a bug would be useful.


On 04/23/2014 07:22 PM, Frank Pagliughi wrote:
Hello Ian, and all,

I'm trying to wrap up the next rev of the C++ library, and have one nagging issue that I believe I inquired about last time. In the C library, the MQTTAsync_send() and MQTTAsync_sendMessage() functions claim:
An MQTTAsync_token is issued when this function returns successfully.
I can't find how to get this MQTTAsyncToken value until the callback which happens after the message is delivered.

The problem is that, like the Java client, the C++ client has a publish() function which returns a token, and that token is supposed to know the message ID of the outgoing packet. It is reported via:
int     itoken::get_message_id();
So, to implement this I need the Message ID (MQTTAsyncToken) as soon as the message is queued - before returning from the publish() function.

It seems that the latest version of the ...err... asynchronous mode of the synchronous library has everything I would need to publish messages, since the publish functions take a pointer to an MQTTClient_deliveryToken
int MQTTClient_publish(MQTTClient handle, char *topicName, int payloadlen,
                       void *payload, int qos, int retained,

                       MQTTClient_deliveryToken *dt)
That would be perfect!  I would switch over to the synchronous library, but there are not corresponding asynchronous  versions of connect() and disconnect().

If those existed, I (we?) wouldn't need the asynchronous library at all.


Ian Craggs                          
icraggs@xxxxxxxxxx                 IBM United Kingdom
Committer on Paho, Mosquitto

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