This is a very simple C99 implementation of the MQTT protocol from the client side only, for the purpose of exploring just how light-weight such an implementation can be made, for embedded use on small devices, eventually including even micro-controllers.
It has lots of limitations, including:
- It only implements version 3.1 of the MQTT protocol, and
any failures to produce or understand messages
of that version should be treated as a bug, except that:
- Although the protocols support UTF-8 messages, this client only supports ASCII.
- Very long messages are not supported and the limit is currently ill-defined.
- It doesn't yet implement the QoS aspects -- doesn't retry and doesn't deal with the server retrying.
- The approach to networking is currently very naive, assuming that connections are reliable.
- There is no support for SSL/TLS.
- In addition to the unit tests, it has been tested against the Mosquitto server. Testing is very far from complete, especially in edge cases like long messages.
To keep this a small, portable library across a range of devices, it is written in C, does not rely on threading, and avoids external libraries. Currently it does rely on the C library, and avoids multi-threading in an unnecessarily simple minded way -- each publisher and each subscriber requires a different connection, and all operations on a connection are assumed to take place in a single thread.
Note that the protocol level declared in a message packet is an unsigned integer, so its relationship with the official protocol version is not completely obvious.
| Version | Protocol Level |
|---|---|
| 3.1 | 3 |
| 3.1.1 | 4 |
| 5 | 5 |
Differences between 3.1 and 3.1.1 are described here.