Sebastian Kuschel reports that pfd_closing can be called for a socket

[u/mdw/putty] / contrib / cygtermd / README

This directory contains 'cygtermd', a small and specialist Telnet
server designed to act as middleware between PuTTY and a Cygwin shell
session running on the same machine, so that PuTTY can act as an
xterm-alike for Cygwin.

To install it, you must compile it from source using Cygwin gcc,
install it in Cygwin's /bin, and configure PuTTY to use it as a local
proxy process. For detailed instructions, see the PuTTY Wishlist page
at

http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/cygwin-terminal-window.html