If you want small code size, CBOR seems like a good bet:
> The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. [1]
This [2] C-implementation fits in under 1KiB of ARM code.
[1]: https://cbor.io/
And Amazon has picked it up as a first class citizen in some of their IoT Core features. It’s definitely here to stay.
Anybody know if (and why) U2F uses CBOR?
They’re extremely similar.
Also I don't know how they got "code sizes appreciably under 1 KiB". On my STM32F1 release mode with -Os it adds about 12kB.
But yeah, maybe I should reevaluate CBOR.
You’re on your own for malloc, which for me is great because FreeRTOS Heap4 management is quite good. So I malloc an object I’m decoding into and parse away.
There are two options parsing arrays and strings/bytestrings and I just chose the option where I specify the pointer to use, vs them using normal malloc then free() later.
I really like this setup. I made a deinit(bad_message) that works anywhere it failed (parse, validate, eval, etc), goes through and looks for pointers that I previously would have malloc’ed.
There is another popular library but I forget what it’s called.