This repository contains an MQTT client and an MQTT server originally created for debugging connectivity problems. I published it since it is a rather minimal example of how to make an MQTT client and server pair using lmqtt, which is a fork of the gmqtt library that we've tried to clean up a bit.
makeThis produces six binaries in the bin directory. A client and server binary for the platform you are on, a amd64/linux binary and an ARM/linux binary.