[JoachimSchipper]

I may just be confused, but couldn't you pass a pipe into the whole (server) process, instead of having it open its own sockets? (In https://github.com/jart/cosmopolitan/blob/48b2afb192ec18eca4..., I gather?)

I haven't followed cosmopolitan / Linux pledge() too closely; but I notice your SERVER_PLEDGE and CLIENT_PLEDGE include stdio, so I expect you have write(2) (and could thus handle a pipe, and maybe even a passed-in AF_UNIX socket?)

Or is making the challenge more interesting - by actually including some networking - part of the fun?