Request procedure: `fd-peek`

[jetjinser]

(fd-peek handle amount) → (or/c string? eof)
  handle : handle?
  amount : (or/c integer? eof 'avail)

I hope that the handle can also be peeked. Unlike fd-read, which consumes data, it retains the data and can be read/peek next time.

I don't know if this makes sense, if I got anything wrong, please correct me.
Thank you for your consideration!

[mflatt]

Unfortunately, this would be difficult to add, because file descriptors at the OS level generally do not support peeking. Peeking requires a layer that buffers input from a file descriptor, and adding that to Zuo would be a big change and have all sorts of complicated interactions with processes and pipes.

[jetjinser]

Ah, that's a pity 🥲. I still think this would be a very useful procedure, can we keep it open for the possibility of implementing it in the future? Thank you very much for your efforts anyway :)

[lassik]

The closest Unix system call is probably poll(). It does not tell how many bytes are available to read.

If amount is characters, it cannot be done with variable-length character encodings (e.g. UTF-8) without reading into a buffer.