Both APIs allow you to associate arbitrary data (a `void *` in kqueue, or a union of `int/uin32_t/uint65_/void *` in epoll) with the event that you're registering. So when you're notified of the event, you can access this data. In my case, it's a big Conn struct. It contains things like the # of requests on this connection (to enforce a configured max request per connection), a timestamp where it should timeout if there's no activity. The Conn is part of an intrusive linked list, so it has a next: *Conn and prev: *Conn. But, what you're probably most curious about, is that it has a Request.State. This has a static buffer ([]u8) that can grow as needed to hold all the received data up until that point (or if we're writing the data, then the buffered data that we have to write). It's important to have a max # of connections and a max request size so you can enforce an upper limit on the maximum memory the library might use. It acts as a state machine to track up to what point it's parsed the request. (since you don't want to have to re-parse the entire request as more bytes trickle in).
It's all half-baked. I can do receiving/sending asynchronously, but the application handler is called synchronously, and if that, for example, calls PG, that's probably also synchronous (since there's no async PG library in Zig). Makes me feel that any modern language needs a cohensive (as in standard library, or de facto standard) concurrency story.