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.