Nanopb is a plain-C implementation of Google's Protocol Buffers data format. It is targeted at 32 bit microcontrollers, but is also fit for other embedded systems with tight (2-10 kB ROM, <1 kB RAM) memory constraints.
Latest stable releases
The development happens in Google Project Hosting git repository (browse repository):
git clone https://code.google.com/p/nanopb/
Bugs can be reported on the Google Project Hosting issue tracker.
There is a discussion forum on Google Groups. Please ask any questions there.
The zlib license:
Copyright (c) 2011 Petteri Aimonen <jpa at nanopb.mail.kapsi.fi>
This software is provided 'as-is', without any express or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software.
Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:
As of January 2012, protobuf-c is the most viable alternative, should you require something more than what nanopb can provide. It has more complete support of Protocol Buffers features (such as easy to use unbounded recursive messages), but relies on dynamic memory allocation and the code size is quite large (more than 20kB binary).
There is also a project called Protobuf-Embedded-C. It has a code size in the same range as nanopb. Currently it does not seem to be as stable as nanopb, but rather a work in progress. Most important difference is that it relies on more generated code than nanopb; this increases the code size somewhat, but on the other hand avoids many nasty pointer tricks that nanopb uses.
Yet another take on the subject is empb. It has a design that seems similar to nanopb, but most documentation is in Chinese.