The "how does it work" section doesn't make sense for macOS since there is no /proc there. How does it work on macOS?! I tested it and it works like a charm!
Look at progress.c, start with "#ifdef __APPLE__" and keep looking from there. Something called libproc, there's some headers[1] I've found but I couldn't find any man pages unfortunately. You need some way to look at open FDs and every system will have such an API, even if it looks slightly (or very) different from the next one.
