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This is an implementation of the MQTT (Message Queuing Telemetry Transport) Client written in C. This library 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.

Protocol Overview

MQTT is a lightweight open messaging protocol that was developed for constrained environments such as M2M (Machine to Machine) and IoT (Internet of Things), where a small code footprint is required. MQTT is based on the Pub/Sub messaging principle of publishing messages and subscribing to topics. The protocol efficiently packs messages to keep the overhead very low. The MQTT specification recommends TLS as a transport option to secure the protocol using port 8883 (secure-mqtt). Constrained devices can benefit from using TLS session resumption to reduce the reconnection cost.

MQTT defines QoS levels 0-2 to specify the delivery integrity required: 0 = At most once delivery: No acknowledgment. 1 = At least once delivery: Sends acknowledgment (PUBLISH_ACK). 2 = Exactly once delivery: Sends received (PUBLISH_REC), gets back released (PUBLISH_REL) and then sends complete (PUBLISH_COMP).

Highlights :

  • A publish message payload can be up to 256MB (28 bits).
  • Packet header remaining length is encoded using a scheme where the most significant bit (7) indicates an additional length byte.
  • Packets which require a response must include a 16-bit packet Id. This needs to be unique for any outstanding transactions. Typically an incremented value.
  • A client can provide a last will and testament upon connect, which will be delivered when the broker sees the client has disconnected or network keep-alive has expired.
  • The connect packet contains the ASCII string “MQTT” to define the protocol name. This can be useful for wireshark/sniffing.
  • Multiple topics can be subscribed or unsubscribed in the same packet request.
  • Each subscription topic must define a QoS level. The QoS level is confirmed in the subscription acknowledgment.
  • A publish message can be sent or received by either the client or the broker.
  • Publish messages can be flagged for retention on the broker.
  • A QoS level 2 requires two round-trips to complete the delivery exchange confirmation.
  • Strings are UTF-8 encoded.

See for additional MQTT documentation.

Broker compatibility

wolfMQTT client library has been tested with the following brokers:

  • Adafruit IO by Adafruit

  • AWS by Amazon

  • Azure by Microsoft

  • flespi by Gurtam

  • HiveMQ and HiveMQ Cloud by HiveMQ GmbH

  • IBM WIoTP Message Gateway by IBM

  • Mosquitto by Eclipse

  • Paho MQTT-SN Gateway by Eclipse

  • VerneMQ by VerneMQ/Erlio

Specification Support

MQTT v3.1.1 Specification Support

The initially supported version with full specification support for all features and packets type such as:

  • QoS 0-2

  • Last Will and Testament (LWT)

  • Client examples for: AWS, Azure IoT, IBM Watson, Firmware update, non-blocking and generic.

MQTT v5.0 Specification Support

The wolfMQTT client supports connecting to v5 enabled brokers when configured with the --enable-mqtt5 option. Handling properties received from the server is accomplished via a callback when the --enable-propcb option is set. The following v5.0 specification features are supported by the wolfMQTT client:

  • AUTH packet

  • User properties

  • Server connect ACK properties

  • Format and content type for publish

  • Server disconnect

  • Reason codes and strings

  • Maximum packet size

  • Server assigned client identifier

  • Subscription ID

  • Topic Alias

The v5 enabled wolfMQTT client was tested with the following MQTT v5 brokers:

  • Mosquitto

** Runs locally.

** ./examples/mqttclient/mqttclient -h localhost

  • Flespi

** Requires an account tied token that is regenerated hourly.

** ./examples/mqttclient/mqttclient -h "" -u "<your-flespi-token>"

  • VerneMQ MQTTv5 preview

** Runs locally.

** ./examples/mqttclient/mqttclient -h localhost

  • HiveMQ 4.0.0 EAP

** Runs locally.

** ./examples/mqttclient/mqttclient -h localhost

  • HiveMQ Cloud

** ./examples/mqttclient/mqttclient -h -t -S -u wolf1 -w NEZjcm7i8eRjFKF -p 8883

  • Watson IoT Quickserver

** ./examples/wiot/wiot

MQTT Sensor Network (MQTT-SN) Specification Support

The wolfMQTT SN Client implementation is based on the OASIS MQTT-SN v1.2 specification. The SN API is configured with the --enable-sn option. There is a separate API for the sensor network API, which all begin with the "SN_" prefix. The wolfMQTT SN Client operates over UDP, which is distinct from the wolfMQTT clients that use TCP. The following features are supported by the wolfMQTT SN Client:

  • Register

  • Will topic and message set up

  • Will topic and message update

  • All QoS levels

  • Variable-sized packet length field

Unsupported features:

  • Automatic gateway discovery is not implemented

  • Multiple gateway handling

The SN client was tested using the Eclipse Paho MQTT-SN Gateway ( running locally and on a separate network node. Instructions for building and running the gateway are in the project README.