[liamdiprose]

Protobufs/nanopb would be my go-to for minimal message size.

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/

[2]: https://github.com/cabo/cn-cbor

[RantyDave]

I, too, love cbor. https://github.com/RantyDave/cppbor

[jopsen]

CBOR is also used on WebAuthn, usage in a web spec means to me that someone smart considered it a sane choice -- and more importantly that the format is here to stay.

[kbumsik]

It's great CBOR is accepted in wider area, but I am personally curious why WebAuthn choose CBOR instead of JSON. WebAuthn is a web browser feature, and why W3C would introduce a new data exchange format in their specs? Maybe WebAuthn needed a binary data type?

[jopsen]

I'm guessing a binary format is nice when interacting with a device..

Anybody know if (and why) U2F uses CBOR?

[SlowRobotAhead]

CBOR’s RFC: https://tools.ietf.org/html/rfc7049

And Amazon has picked it up as a first class citizen in some of their IoT Core features. It’s definitely here to stay.

[kragen]

CBOR was originally part of the MsgPack project, by the way, before its designer forked it and renamed it after himself.

[magicalhippo]

Ah yes I looked at CBOR too, but I dismissed it for reasons I can't recall right now. Will have to take another look.

[SlowRobotAhead]

That’s strange because CBOR is almost literally msgpack that got an RFC and has extensions. I cant remember what MsgPack does for online streaming and indefinite lengths.

They’re extremely similar.

[magicalhippo]

Looked at it again, seems memory management is a bit of an issue, it supports memory allocation callback but not just handing it a buffer to work with (though I guess allocation should be predictable).

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.

[SlowRobotAhead]

For reference, I’m using TimyCBOR because it’s include with Amazon FreeRTOS.

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.

[kbumsik]

Yes. CBOR is designed for IoT especially in mind.