Thanks Charles.  i had looked at  /sys/src/9/port/sysfile.c, but couldn't quite connect the dots.

On Wed, Apr 1, 2015 at 7:03 AM, Charles Forsyth <charles.forsyth@gmail.com> wrote:

On 1 April 2015 at 00:23, Skip Tavakkolian <skip.tavakkolian@gmail.com> wrote:
 trying 9pcon on the same fd (e.g. aux/9pcon /srv/foo) results in "inappropriate use of fd".

You aren't allowed to read or write an fd directly once it has been used for 9P traffic by mount (devmnt).