I would probably go for rust over c if the only reason for c is "os level". But either way, this is for requests from "real" http clients, eg: web browsers? Presumably you're already "trapped" at a pretty high level, like tls, http/2 etc?

Otoh, if low level is what you want, maybe look at:

https://2ton.com.au/rwasa/

and/or the library it is built on.

On the other hand, if you just want to log headers to a file, caddy can do that in a few lines:

https://caddyserver.com/docs/caddyfile/directives/log

AFAIK it can't log request bodies out of the box (like the content of a POST).

this is a 500 line server running in kernel mode on nanos

https://github.com/nanovms/nanos/blob/master/src/http/http.c