MQTT-C

MQTT-C explained simply

Imagine you want to collect data from industrial sensors – and do so easily on devices with little memory and processing power. This is exactly where MQTT-C comes into play. It is nothing more than an ultra-lightweight implementation of the MQTT protocol in the C programming language. Imagine it as a minimalist, fast postal system that has nothing in common with things like Python or Java – but is optimised for microcontrollers and embedded systems with limited memory. Very straightforward: you write a little C code, connect via MQTT, and your data flows steadily, efficiently and resource-conservingly.

Background information

MQTT-C is a portable MQTT client library written in C, suitable for both embedded systems and desktop PCs. It follows a minimalist approach: with only two source files (under 2000 lines of code) MQTT-C provides everything necessary to implement an MQTT publisher or subscriber. Thanks to a transparent Platform Abstraction Layer (PAL) it can be easily ported to new systems – practical in scenarios with limited hardware resources.

The project is open source under the MIT licence on GitHub. There, alongside the main code, you will also find examples, documentation and unit tests – the latter based on the cmocka framework. MQTT-C supports both multi-thread and single-thread environments, which makes it particularly suitable for embedded applications on microcontroller platforms.

In IIoT scenarios, MQTT-C plays a valuable role: it enables robust messaging via the publish/subscribe model, ideal for industrial environments with unstable networks, low bandwidth and limited computing resources. It is based on the proven MQTT protocol, which exchanges messages via a central broker – a pattern long established in IoT and IIoT.

Architecture and platform independence of MQTT-C

The architecture of MQTT-C is consistently geared towards minimalism and portability. At its core lies the so-called Platform Abstraction Layer (PAL), which serves as the interface between the actual MQTT logic and the operating system. This abstraction layer encapsulates all operating system-specific functions such as sockets, timers or threads. As a result, MQTT-C can be transferred to new platforms with only minor adjustments – whether a Linux-based system, an RTOS (Real-Time Operating System) or even a bare-metal microcontroller without an operating system.

Another architectural advantage is the clear separation between protocol implementation and platform integration. Developers do not need to deal with the internal details of the MQTT protocol, but merely adapt the PAL to their hardware. This design makes the library extremely lean and flexible. Typical use cases include sensor gateways, edge devices or small controllers in industrial environments that may be equipped with very different hardware architectures.

Technical features and functionality

Technically speaking, MQTT-C is a fully-fledged MQTT client library with support for the core functions connect, publish, subscribe and disconnect. Developers can use a compact API to send messages to a broker (publish) or receive them (subscribe). The underlying publish/subscribe model ensures that end devices communicate decoupled from one another – a decisive advantage in complex IIoT environments.

A special feature of MQTT-C is the choice between thread-safe and single-thread operation. This allows the library to adapt both to resource-constrained systems with only one task and to more powerful platforms with parallel processing. In addition, the implementation is non-blocking: network operations can be executed asynchronously, so that the main task of the device is not unnecessarily delayed.

The functional procedure in a practical example usually looks like this:

  1. Initialisation – an MQTT client is configured with the necessary parameters (broker address, port, QoS level).
  2. Connection – the device establishes a connection to the broker via TCP/IP.
  3. Publish/Subscribe – sensor data is published or control commands subscribed to.
  4. Event handling – incoming messages are processed via callback functions.
  5. Disconnect – the connection is cleanly terminated to release resources.

This lean, robust approach makes MQTT-C an ideal solution for IIoT applications where reliability, efficiency and adaptability are crucial.

Further information and links

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