Very Basic MQTT Broker

A lightweight MQTT broker in C for learning and experimentation.

Overview

This project implements a minimal MQTT v3.1.1 broker with multi-client handling via POSIX threads. It is intentionally simple and focused on core publish/subscribe behavior.

Features

• MQTT v3.1.1 handshake (CONNECT/CONNACK)
• Multi-client support using one thread per client
• SUBSCRIBE and topic-based message forwarding
• PUBLISH routing to matching subscribers
• Keep-alive via PINGREQ/PINGRESP
• Configurable broker limits in header files

Requirements

• GCC (or compatible C compiler)
• POSIX-compatible system (Linux/Unix)
• pthread
• make

Quick Start

Build:

make

Run:

./build/bin/mqtt_broker

The broker listens on port 1883 by default.

Optional Local Test

If mosquitto_pub and mosquitto_sub are installed:

Terminal 1:

./build/bin/mqtt_broker

Terminal 2:

mosquitto_sub -h 127.0.0.1 -p 1883 -t demo/topic

Terminal 3:

mosquitto_pub -h 127.0.0.1 -p 1883 -t demo/topic -m "hello broker"

Supported MQTT Packets

• CONNECT
• CONNACK
• PUBLISH
• SUBSCRIBE
• SUBACK
• PINGREQ
• PINGRESP
• DISCONNECT

Project Structure

.
├── include/
│   ├── broker.h             # Broker API and limits
│   └── mqtt.h               # MQTT constants and packet handlers
├── src/
│   ├── main.c               # TCP server and client threading
│   ├── broker.c             # Client/subscription registry and routing
│   └── mqtt.c               # MQTT packet parsing and handlers
├── build/                   # Generated build output
├── Makefile                 # Build targets: all, run, clean
├── README.md
└── LICENSE

Configuration

Defined in source headers:

• MAX_CLIENTS in include/broker.h
• MAX_SUBSCRIPTIONS in include/broker.h
• MAX_TOPIC_LEN in include/broker.h
• MQTT_PORT in include/mqtt.h

Current Limitations

• No authentication/authorization
• No TLS support
• No retained messages
• No wildcard topic matching (+, #)
• Simplified QoS behavior intended for learning

License

Licensed under the MIT License. See LICENSE for details.