// the find
IoTSharp/mqttclient
A high-performance, high-stability, cross-platform MQTT client, developed based on the socket API, can be used on embedded devices (FreeRTOS / LiteOS / RT-Thread / TencentOS tiny), Linux, Windows, Mac, with a very concise The API interface realizes the quality of service of QOS2 with very few resources, and seamlessly connects the mbedtls encryption library.
A C MQTT client library targeting constrained embedded systems (FreeRTOS, RT-Thread, TencentOS tiny) while also running on Linux, Windows, and Mac. It implements the full MQTT spec including QoS2, auto-reconnect, and optional TLS via mbedtls, all in under 11KB of ROM. Aimed at IoT firmware developers who need something lighter than Paho C with less setup friction.
The resource footprint is genuinely impressive — 10KB ROM, ~3.7KB RAM at QoS0 without TLS is workable on an ESP8266 or similar. The platform abstraction layer (thread, mutex, socket, timer) is clean and well-separated, which is why porting to five different RTOSes actually worked. QoS1/2 retransmission uses an async record-and-cancel approach rather than blocking, which is the right call for constrained systems where you can't afford to stall a thread. Auto-resubscription on reconnect is implemented correctly and not forgotten as in many smaller clients.
The bundled mbedtls is version 2.x, which is end-of-life — mbedtls 3.x has breaking API changes and security fixes that aren't here. Last push was February 2026 but activity has been sparse for a while, and there's no MQTT v5 support at all; if you need user properties, reason codes, or topic aliases you're blocked. The online code generation tool points to a gitee.io URL that may or may not be alive, and the README is clearly machine-translated from Chinese in places, making some configuration docs harder to follow than the code itself.