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.

The library has three components.

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 MqttNet.read 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);

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

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.

The example MQTT client is located in /examples/mqttclient/. This example exercises all exposed API’s and prints any incoming publish messages for subscription topic “wolfMQTT/example/testTopic”.

The MQTT firmware update is located in /examples/firmware/. This example has two parts. The first is called “fwpush”, which publishes a signed 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”.

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.

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.

• Fixes issue with read timeout in non-blocking mode with TLS enabled being teated as socket error.
• Fixed issue with “msg->stat” not getting reset on failure or timeout.
• Fix to not link libwolfssl with ./configure --disable-tls.
• Added AWS IoT Example and test script.

• Fix for building MQTT client example without the wolfSSL headers present.
• Fix for Microchip Harmony IP check so it works with non 192 subnets.

• Enabled big endian support.
• Fixes for building with Visual Studio.

• Added Microchip Harmony support (see new readme in IDE/Microchip-Harmony/README.md).
• Added non-blocking mode --enable-nonblock or WOLFMQTT_NONBLOCK, which uses new MQTT_CODE_CONTINUE response code.
• Added scripts/azureiothub.test.
• Added ./commit-tests.sh for testing all configurations.
• Added git pre-commit hook to run commit-tests.sh.
• Combined duplicate code in the examples into examples/mqttexample.c.
• Examples now use MQTTCtx structure as argument for tracking info/state.

• Fixed stdin capture bug and improved signal (ctrl+c) handling.
• Added Azure IoT hub MQTT client example.
• Added support for MQX / RTCS.
• Added "--disable-tls" and "--disable-examples" configure options.
• Added comment about max packet size.
• Added example for how to load a client certificate to mqttclient example.
• Added return code for firmware and azure examples that are not compiled in due to older / incompatible version of wolfSSL.
• Moved the support for custom printf/line endings into the mqtt_types.h for use throughout the project.
• Updated README.md with information about the examples.

• Fixes to support MinGW compiler.
• Fixed bug with include of the wolfSSL include of options.h.
• Fix to properly handle negative return code from wc_SignatureGetSize.
• Added Arduino IDE example in IDE/ARDUINO. See IDE/ARDUINOREADME.md for details.
• Added example UART interface for wolfMQTT. See examples/mqttuart.c.
• Added the ability to pass additional arguments to the scripts. Example: ./scripts/client.test "-h localhost"

• Fixed build error when using older wolfSSL in firmware examples.
• Updated the get error string function in mqtt_socket.c from wc_GetErrorString to wolfSSL_ERR_reason_error_string so both wolfSSL and wolfCrypt error codes are resolved.
• Added -n <str> option so a custom topic name can be used.
• The mqttclient example now listens to stdin and will send a publish message with the data entered on the console when end-of-line (return) is detected (Linux only).
• Added keep-alive ping to the mqttclient and fwclient examples.
• Moved the TLS callback prior to the client->tls.ctx creation, allowing the callback function to implement its own client method cert verification.
• Enhanced MqttClient_WaitMessage so it will return if we get a message, not just on timeout
• Added make check/test scripts (scripts/client.test and scripts/firmware.test) to validate client TLS (with and without) plus QoS 0-2 levels and the firmware update example.
• Adjusted the example include paths for more flexibility.
• Added new -T option for using examples to test.
• Added new -C option to allow custom command timeout.
• Combined duplicate example code into new header mqttexample.h.
• Added a PRINTF helper macro to the examples for easier porting.
• Added better error trapping in examples so return code is populated for testing.
• Changed the example test functions to return int.

• Fixed bug with subscribe not populating acknowledgment return code(s) properly.
• Fixed build error if using wolfSSL 3.7.1 or older due to missing signature.c/.h wrappers. This fix disables the firmware examples if the wolfSSL version isn't greater than 3.7.1.
• Fix to ensure topic_name pointer is reset when publish callback message is not new msg_new = 0.
• Fixes to suppress possible warning "Value stored to [] is never read".
• Fixed firmware example to trap case where file isn't found.
• Fixed possible ./autogen.sh error with missing "config.rpath".
• Fixed Windows issue with SetConsoleCtrlHandler incorrectly reporting error.
• Fixed issue with Visual Studio 2015 wolfssl.lib reference.
• Fixed build errors with G++ (./configure CC=g++).
• Fixed "FirmwareHeader" to use WOLFMQTT_PACK macro.
• Added helper macro's and comments for topic names/filters.
• Added TLS certification verification reference implementation to examples.
• Updated the topic names in examples to use "wolfMQTT/example/".
• Added QoS level to example console output.
• Added memset to initialize some of the example stack variables.
• Removed the LWT from the firmware examples.
• Added retain flag "-r" option on the "fwpush" example.
• Updated the examples to use macros for all memory and string functions, so they are more portable.
• Added Visual Studio projects for "fwpush" and "fwclient".

• Fixes bug with first byte of payload being null'd if QoS level was 0.
• Fixed issue with stdint types (uint#_t) being used.
• Fixes for remaining length encoding/decoding for large packets.
• Added support for large payloads using new message callback flags msg_done and msg_new along with MqttMessage buffer_pos and buffer_len.
• Added example for secure firmware upgrade. Uses the MQTT client library to push a signed payload fwpush to a broker, then uses another client fwclient to receive the signed payload and verify its signature using a provided public key.

• Fixes to handle receival of publish and QoS messages while performing packet writes/waits.
• Added support / tested with Windows.
• Added Visual Studio 2015 solution and projects.
• Added support / tested with FreeRTOS and Lwip.
• Fixes for compiler warnings.

• Initial release with support for MQTT v3.1.1, QoS 0-2, TLS and example client.