• Registered: 2018-03-27
  • Posts: 7

Topic: MqttMessage and handling large messages

I apologize if I missed this in the WolfMQTT manual, but I'm not clear how to receive a large MQTT message from the broker (received message is larger than the buffer passed to MqttClient_Init()).  What is the sequence of calls to the callback function for such a message, including the values of callback function parameters msg_new, msg_done, msg->buffer, msg->buffer_len and msg->buffer_pos.

If this info is in the manual, a pointer to where it is would be appreciated.

Thanks!
Anthony Jenkins

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  • From: Mobile, AL
  • Registered: 2013-12-11
  • Posts: 349

Re: MqttMessage and handling large messages

Hello Anthony,

The callback description is here:
https://github.com/wolfSSL/wolfMQTT/blo … ient.h#L49

The callback is registered during `MqttClient_Init`, and it will be called when a published message is received on a topic the client is subscribed to. The topic name and total payload size are available in the `MqttMessage` parameter. The msg_new and msg_done parameters are used to indicate the progress of the payload.

There is an excellent example that demonstrates receiving a large payload using the message callback functionality. It a two part example, the first client creates and publishes a signed "firmware" blob. The second client is subscribed to the topic waiting for the firmware message. When it receives the message, it saves the payload and authenticates the firmware package.

https://github.com/wolfSSL/wolfMQTT/tre … s/firmware

Instructions:
https://github.com/wolfSSL/wolfMQTT/blo … re-example

Thanks,
Eric @ wolfSSL Support

embhorn's Website

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3 Reply by Scoobi_FreeBSD (edited by Scoobi_FreeBSD 2020-09-14 08:04:05)

  • Registered: 2018-03-27
  • Posts: 7

Re: MqttMessage and handling large messages

Awesome, thanks!!!

Okay, to summarize for anyone also looking for this information, it looks like:

for callback function message_cb with prototype

int message_cb(MqttClient *client, MqttMessage *msg, byte msg_new, byte msg_done)

  • msg->buffer points to the start of the i'th chunk of the message received from the broker

  • msg->buffer_pos is the absolute position of the i'th chunk in the total message

  • msg->buffer_len is the length of the i'th chunk of the message received from the broker

  • msg->total_len is the sum of all msg->buffer_len (i.e. the total length of the message)

  • msg_new = nonzero for i = 0

  • msg_done = nonzero for i = n-1, where n = the number of message chunks sent (and the number of calls to the callback)

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