How to receive an full binary data using mqtt callback function

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Gavrila Pi 2024-8-3

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https://ww2.mathworks.cn/matlabcentral/answers/2142626-how-to-receive-an-full-binary-data-using-mqtt-callback-function

评论： dpb 2024-8-5

I have sent a binary blob (for example 4k) from C++ app by mqtt mosquitto. In matlab only a chunk of sending data is received. This chunk is limited by the first zero byte which is in the blob. How to receive a full message?

Short code snippest below:

mqttClient = mqttclient("tcp://127.0.0.1");
mySub = subscribe(mqttClient, "topic", Callback=@MsgCallvBack)
%
function MsgCallvBack(topic, data)
 fprintf("topic=%s, data size=%u\n", topic, length(data));
end

Result for 4 sending attempts of random 4k blob:

>topic=testtopic, data size=40

>topic=testtopic, data size=22

>topic=testtopic, data size=3

>topic=testtopic, data size=241

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dpb 2024-8-4

OK, I didn't fully grasp how the callback functioned before -- I'll agree the implementation appears to not support anything but the text string. I presume at this point you are in control of the publishing side while testing at least? If so, I'd just do one last "prove it to myself" exercise in formatting the array as a string and then sending it and ensuring it comes over. Just a simple num2str(0:N) should be enough with a random N.

Presuming that does work as expected, it appears to be an implementation limitation as you've thought and the only recourse would be to request an enhancement from Mathworks or report it as a bug (although I'm pretty sure in this case they would use the "WAD" (Working As Designed) fallback.

I did find a <link to a Simulink MQTT block> that does support numeric arrays, but that requires Simulink, not just MATLAB.

I don't know of another workaround other than by using another protocol instead that does support binary data in MATLAB or try to link to Python or find C opensource and create a mex file of your own.

Gavrila Pi 2024-8-5

> I don't know of another workaround other than by using another protocol instead that does support binary data in MATLAB or try to link to Python or find C opensource and create a mex file of your own.

MATLAB cannot be proud of a large number of interaction protocols. Writing your own MEX will require significant time investment.

Thank you for your help.

dpb 2024-8-5

They (new protocols) seem to spring up like weeds; trying to support them all is a Herculean task it would seem and requires resources that must divert away from core development...

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dpb 2024-8-3

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此回答的直接链接

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编辑：dpb 2024-8-3

mqttclient and subsequent subscribe shows the callback function is to read the subscribed-to channel and the input function is an overloaded read method. Never heard of it before so no experience, but it appears it is a text-based messaging system so null bytes will be interpreted as character terminators unless treated specifically as text (like embedding as char(0) and the like for construction), but are highly likely to be considered terminators on reading internally.

From a quick look, it wouldn't seem to be a suitable transmission choice for arbitrary binary data...