The wolfMQTT library is a client implementation of MQTT written in C for embedded use. It supports SSL/TLS via the wolfSSL library. It was built from the ground up to be multi-platform, space conscience and extensible. It supports all Packet Types, all Quality of Service (QoS) levels 0-2 and supports SSL/TLS using the wolfSSL library. This implementation is based on the MQTT v3.1.1 specification, but supports up to the v5 specification.
wolfMQTT is open source, commercial grade, embedded security software.