>Probably irrelevant here, but this approach wouldn't work under Windows
>(Unix.file_descr is the Win32 file handle at that point which is often
>larger than 1024). More relevantly, Unix can be reconfigured to allow for
>more than 1024 open files.

I think platform-dependent code is required here. The common way of
doing this under Linux (and Solaris, probably) is to readdir /proc/PID/fd/.
Windows is of course very different.