Thomas Munro <thomas.munro@gmail.com> writes:
> Oops, that was after it had been cleared already by the OS; duh. On
> entry to my wrapper, writefds does in fact contain the bit pattern for
> fd 13. That led me to try a very simply C program which runs to
> completion on Linux and FreeBSD, but hangs forever on Dragonfly.
For completeness, I checked this on macOS, OpenBSD, and NetBSD,
and they all give the expected results.
regards, tom lane
