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This is an implementation of the MQTT Client written in C for embedded use, which supports SSL/TLS via the wolfSSL library. This library was built from the ground up to be multi-platform, space conscience and extensible. Integrates with wolfSSL to provide TLS support.



  1. ./ (if cloned from GitHub)
  2. ./configure (to see a list of build options use ./configure --help)
  3. make
  4. sudo make install


  • If wolfssl was recently installed run sudo ldconfig to update the linker cache.
  • Debug messages can be enabled using --enable-debug or --enable-debug=verbose (for extra logging).
  • For a list of build options run ./configure --help.
  • The build options are generated in a file here: wolfmqtt/options.h.

Windows Visual Studio

For building wolfMQTT with TLS support in Visual Studio:

  1. Open the <wolfssl-root>/wolfssl64.sln.
  2. Re-target for your Visual Studio version (right-click on solution and choose Retarget solution).
  3. Make sure the Debug DLL or Release DLL configuration is selected. Make note if you are building 32-bit x86 or 64-bit x64.
  4. Build the wolfSSL solution.
  5. Copy the wolfssl.lib and wolfssl.dll files into <wolfmqtt-root>.
    • For DLL Debug with x86 the files are in: DLL Debug.
    • For DLL Release with x86 the files are in: DLL Release.
    • For DLL Debug with x64 the files are in: x64/DLL Debug.
    • For DLL Release with x64 the files are in: x64/DLL Release.
  6. Open the <wolfmqtt-root>/wolfmqtt.sln solution.
  7. Make sure you have the same architecture (x86 or x64 selected) as used in wolfSSL above.
  8. By default the include path for the wolfssl headers is ./../wolfssl/. If your wolfssl root location is different you can go into the project settings and adjust this in C/C++ -> General -> Additional Include Directories.
  9. Configure your Visual Studio build settings using wolfmqtt/vs_settings.h.
  10. Build the wolfMQTT solution.


See at IDE/


export PATH="/opt/mingw-w32-bin_i686-darwin/bin:$PATH"
export PREFIX=$PWD/build

# wolfSSL
cd wolfssl
./configure --host=i686 CC=i686-w64-mingw32-gcc LD=i686-w64-mingw32-ld CFLAGS="-DWIN32 -DMINGW -D_WIN32_WINNT=0x0600" LIBS="-lws2_32 -L$PREFIX/lib -lwolfssl" --prefix=$PREFIX
make install

# wolfMQTT
cd ../wolfmqtt
./configure --host=i686 CC=i686-w64-mingw32-gcc LD=i686-w64-mingw32-ld CFLAGS="-DWIN32 -DMINGW -D_WIN32_WINNT=0x0600 -DBUILDING_WOLFMQTT -I$PREFIX/include" LDFLAGS="-lws2_32 -L$PREFIX/lib -lwolfssl" --prefix=$PREFIX --disable-examples


The library has three components.

1. mqtt_client

This is where the top level application interfaces for the MQTT client reside.

  • int MqttClient_Init(MqttClient *client, MqttNet *net, MqttMsgCb msg_cb, byte *tx_buf, int tx_buf_len, byte *rx_buf, int rx_buf_len, int cmd_timeout_ms);

These API's are blocking on until error/timeout (cmd_timeout_ms):

  • int MqttClient_Connect(MqttClient *client, MqttConnect *connect);
  • int MqttClient_Publish(MqttClient *client, MqttPublish *publish);
  • int MqttClient_Subscribe(MqttClient *client, MqttSubscribe *subscribe);
  • int MqttClient_Unsubscribe(MqttClient *client, MqttUnsubscribe *unsubscribe);
  • int MqttClient_Ping(MqttClient *client);
  • int MqttClient_Disconnect(MqttClient *client);

This function blocks waiting for a new publish message to arrive for a maximum duration of timeout_ms.

  • int MqttClient_WaitMessage(MqttClient *client, MqttMessage *message, int timeout_ms);

These are the network connect / disconnect interfaces that wrap the MqttNet callbacks and handle WolfSSL TLS:

  • int MqttClient_NetConnect(MqttClient *client, const char* host, word16 port, int timeout_ms, int use_tls, MqttTlsCb cb);
  • int MqttClient_NetDisconnect(MqttClient *client);

Helper functions:

  • const char* MqttClient_ReturnCodeToString(int return_code);

2. mqtt_packet

This is where all the packet encoding/decoding is handled.

The header contains the MQTT Packet structures for:

  • Connect: MqttConnect
  • Publish / Message: MqttPublish / MqttMessage (they are the same)
  • Subscribe: MqttSubscribe
  • Unsubscribe: MqttUnsubscribe

3. mqtt_socket

This is where the transport socket optionally wraps TLS and uses the MqttNet callbacks for the platform specific network handling.

The header contains the MQTT Network structure MqttNet for network callback and context.


Here are the steps for creating your own implementation.

  1. Create network callback functions for Connect, Read, Write and Disconnect. See examples/mqttnet.c and examples/mqttnet.h.
  2. Define the callback functions and context in a MqttNet structure.
  3. Call MqttClient_Init passing in a MqttClient structure pointer, MqttNet structure pointer, MqttMsgCb function pointer, TX/RX buffers with maximum length and command timeout.
  4. Call MqttClient_NetConnect to connect to broker over network. If use_tls is non-zero value then it will perform a TLS connection. The TLS callback MqttTlsCb should be defined for wolfSSL certificate configuration.
  5. Call MqttClient_Connect passing pointer to MqttConnect structure to send MQTT connect command and wait for Connect Ack.
  6. Call MqttClient_Subscribe passing pointer to MqttSubscribe structure to send MQTT Subscribe command and wait for Subscribe Ack (depending on QoS level).
  7. Call MqttClient_WaitMessage passing pointer to MqttMessage to wait for incoming MQTT Publish message.


Client Example

The example MQTT client is located in /examples/mqttclient/. This example exercises many of the exposed API’s and prints any incoming publish messages for subscription topic “wolfMQTT/example/testTopic”. This client contains examples of many MQTTv5 features, including the property callback and server assignment of client ID. The mqqtclient example is a good starting template for your MQTT application.

Simple Standalone Client Example

The example MQTT client is located in /examples/mqttsimple/. This example demonstrates a standalone client using standard BSD sockets. This requires HAVE_SOCKET to be defined, which comes from the ./configure generated wolfmqtt/config.h file. All parameters are build-time macros defined at the top of /examples/mqttsimple/mqttsimple.c.

Non-Blocking Client Example

The example MQTT client is located in /examples/nbclient/. This example uses non-blocking I/O for message exchange. The wolfMQTT library must be configured with the --enable-nonblock option (or built with WOLFMQTT_NONBLOCK).

Firmware Example

The MQTT firmware update is located in /examples/firmware/. This example has two parts. The first is called “fwpush”, which signs and publishes a firmware image. The second is called “fwclient”, which receives the firmware image and verifies the signature. This example publishes message on the topic “wolfMQTT/example/firmware”. The "fwpush" application is an example of using a publish callback to send the payload data.

Azure IoT Hub Example

We setup a wolfMQTT IoT Hub on the Azure server for testing. We added a device called demoDevice, which you can connect and publish to. The example demonstrates creation of a SasToken, which is used as the password for the MQTT connect packet. It also shows the topic names for publishing events and listening to devicebound messages. This example only works with ENABLE_MQTT_TLS set and the wolfSSL library present because it requires Base64 Encode/Decode and HMAC-SHA256. Note: The wolfSSL library must be built with ./configure --enable-base64encode or #define WOLFSSL_BASE64_ENCODE. The wc_GetTime API was added in 3.9.1 and if not present you'll need to implement your own version of this to get current UTC seconds or update your wolfSSL library. NOTE The Azure broker only supports MQTT v3.1.1

AWS IoT Example

We setup an AWS IoT endpoint and testing device certificate for testing. The AWS server uses TLS client certificate for authentication. The example is located in /examples/aws/. The example subscribes to $aws/things/"AWSIOT_DEVICE_ID"/shadow/update/delta and publishes to $aws/things/"AWSIOT_DEVICE_ID"/shadow/update. NOTE The AWS broker only supports MQTT v3.1.1

Watson IoT Example

This example enables the wolfMQTT client to connect to the IBM Watson Internet of Things (WIOT) Platform. The WIOT Platform has a limited test broker called "Quickstart" that allows non-secure connections to exercise the component. The example is located in /examples/wiot/. Works with MQTT v5 support enabled.

MQTT-SN Example

The Sensor Network client implements the MQTT-SN protocol for low-bandwidth networks. There are several differences from MQTT, including the ability to use a two byte Topic ID instead the full topic during subscribe and publish. The SN client requires an MQTT-SN gateway. The gateway acts as an intermediary between the SN clients and the broker. This client was tested with the Eclipse Paho MQTT-SN Gateway, which connects by default to the public Eclipse broker, much like our wolfMQTT Client example. The address of the gateway must be configured as the host. The example is located in /examples/sn-client/.

A special feature of MQTT-SN is the ability to use QoS level -1 (negative one) to publish to a predefined topic without first connecting to the gateway. There is no feedback in the application if there was an error, so confirmation of the test would involve running the sn-client first and watching for the publish from the sn-client_qos-1. There is an example provided in /examples/sn-client/sn-client_qos-1. It requires some configuration changes of the gateway.

  • Enable the the QoS-1 feature, predefined topics, and change the gateway name in gateway.conf:
  • Comment out all entries and add a new topic in predefinedTopic.conf:
WolfGatewayQoS-1,wolfMQTT/example/testTopic, 1

Multithread Example

This example exercises the multithreading capabilities of the client library. The client implements two tasks: one that publishes to the broker; and another that waits for messages from the broker. The publish thread is created NUM_PUB_TASKS times (10 by default) and sends unique messages to the broker. This feature is enabled using the --enable-mt configuration option. The example is located in /examples/multithread/.

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 by dc-square 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
  • 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.

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