emMQTT

SEGGER emMQTT is a compact MQTT client implementation built for embedded IoT devices where memory and power budgets are tight. Implementing the MQTT 3.1.1 specification, emMQTT provides full publish/subscribe messaging with all three Quality of Service levels, QoS 0 (at most once), QoS 1 (at least once), and QoS 2 (exactly once), enabling developers to choose the right delivery guarantee for each message type. At approximately 2.4 KB of ROM on a Cortex-M4 target and just 84 bytes of RAM per client context, emMQTT adds negligible overhead to even the most constrained sensor nodes and edge devices.

Security and Broker Compatibility

emMQTT supports SSL/TLS encryption and client certificate authentication when used with SEGGER emSSL, securing MQTT traffic end-to-end without requiring a separate security layer. The client supports will messages for graceful handling of unexpected disconnections and the retain flag for persistent message delivery. It has been tested with popular brokers including Mosquitto, HiveMQ, and AWS IoT Message Broker, ensuring interoperability across cloud and on-premise MQTT infrastructure.

Feature	Detail
Publish / Subscribe	Full client with topic filtering
QoS	Level 0, 1, and 2
Will messages	Registered on connect
Retain flag	Supported
TLS	Via emSSL integration
Client auth	X.509 certificates
Tested brokers	Mosquitto, HiveMQ, AWS IoT

TCP/IP Stack Independence

emMQTT is independent of the underlying TCP/IP stack, any stack providing a socket interface can be used through a simple callback API. It is optimised for SEGGER emNet but works equally well with third-party stacks, giving teams flexibility to integrate emMQTT into existing network architectures. The royalty-free, one-time-payment licensing model makes it straightforward to deploy across high-volume IoT product lines without per-unit cost concerns.