> MessagePack ... official C library was not optimized for memory usage and code size

But libmpack is: https://github.com/libmpack/libmpack

- libmpack serialization/deserialization API is callback-based, making it simple to serialize/deserialize directly from/to application-specific objects

- libmpack does no allocation at all, and provides some helpers to simplify dynamic allocation by the user, if required.

- C89