This is a small example using mosquitto's MQTT client library.
It creates a client that connects to an MQTT broker at localhost:1883
(if running), then subscribes to the topics tick, control/#{PID},
and control/all. Every time it gets a tick message via the tick
topic, it publishes its pid and uptime on tock/#{PID}. If the message
halt is received on control/all or control/#{PID}, the client will
free its resources and halt.
Note: If you are running a Linux distribution that uses apt-based
packaging (e.g. Debian or Ubuntu) you may need to install the -dev
packages as well as the basic mosquitto packages to get the headers.
It was written four years ago and the Mosquitto C library has had API changes. This example is no longer actively maintained (but pull requests or requests to take over maintainership are welcome).