Over the last week or so I have been trying to get an Arduino application working which supports Message Queue Telemetry Transport(MQTT) properties which were added in V5. This specification was released March 2019 so I figured it would be commonly supported. In the Arduino ecosystem I was wrong and there is going to be a series of pastes about my “epic fail”.

The post is not about WolfMQTT but what I learnt about configuring it (and other) libraries for Arduino.

I wanted to set the message properties (for reasons) and in the .NET nanoFramework (and other .NET libraries) it wasn’t a problem.

 while (true)
 {
    Console.WriteLine("MQTT publish message start...");


    var payload = new MessagePayload() { ClientID = Secrets.MQTT_CLIENTID, Sequence = sequenceNumber++ };

    string jsonPayload = JsonSerializer.SerializeObject(payload);

    var result = mqttClient.Publish(topicPublish, Encoding.UTF8.GetBytes(jsonPayload), "application/json; charset=utf-8", null);

    Debug.WriteLine($"MQTT published ({result}): {jsonPayload}");

    Thread.Sleep(60000);
 }

One of the options suggested by copilot was wolfMQTT(which uses an open/closed source model) but I found there was not an Arduino library for of this product. This was not unexpected due to the target audience of their products (but I did find there is a wolfSSL prebuilt library)

The first step was to grab a copy of the wolfMQTT library from GitHub and unpack it.

There were instructions on how to “re-organise” the source files into and Arduino friendly library

I don’t have an “easy” way of running bash on my dev box so I tried to do it manually (first of a series of mistakes).

#!/bin/sh

# this script will reformat the wolfSSL source code to be compatible with
# an Arduino project
# run as bash ./wolfssl-arduino.sh

DIR=${PWD##*/}

if [ "$DIR" = "ARDUINO" ]; then
    rm -rf wolfMQTT

    mkdir wolfMQTT
    cp ../../src/*.c ./wolfMQTT

    mkdir wolfMQTT/wolfmqtt
    cp ../../wolfmqtt/*.h ./wolfMQTT/wolfmqtt

    echo "/* Generated wolfMQTT header file for Arduino */" >> ./wolfMQTT/wolfMQTT.h
    echo "#include <wolfmqtt/mqtt_client.h>" >> ./wolfMQTT/wolfMQTT.h
else
    echo "ERROR: You must be in the IDE/ARDUINO directory to run this script"
fi

I copied the files into the specified folder tree and my compilation failed. I used a minimalist application to debug the compilation error.

I went back to the script file and realised that has missed creating a header file

    echo "/* Generated wolfMQTT header file for Arduino */" >> ./wolfMQTT/wolfMQTT.h
    echo "#include <wolfmqtt/mqtt_client.h>" >> ./wolfMQTT/wolfMQTT.h

I used a text editor to create the file which I saved into the src folder

I also checked that I had the same structure as wolfssl

The compilation was still failing so I turned on the Arduino verbose compiler output

After compilations I went back through the compiler output look for clues.

After a couple of failed compilations I paid attention to the error message.

I had the “case” of the header file name wrong, so I changed it to wolfMQTT.h

The compile then worked and the code executed.

After several hours of “fail” I now understand that case is important for header file names in Arduino(maybe due to Unix origins of some of the tools used). The naming of header file is also important so the library can be discovered.

In another post I will try and build an Azure Event Grid MQTT Broker client that uses wolfMQTT.