[Coco] single user mode for SCF devices?

[Aaron Wolfe]

Hello,

I have been trying without any luck to make an SCF device allow only
one process to have a path open to it at a time.
I see there is a mode byte in the device descriptor and one used in
the F$Open call, and a bit for "Shared" or "Single User" depending on
which documentation you read.
In the OS9Defs file it is called SHARED.  Setting this in the
descriptor doesn't do anything (multiple processes can open and write
to the device), and setting it in the Open call causes the call to
return error 203 - Illegal Mode.  The only documentation I see on
single user mode is in the RBF section of the NitrOS-9 tech reference,
no discussion in the SCF section, although it seems that the two are
intertwined in some ways that I don't yet understand.

Does anyone know if it's possible to make an SCF device fail for
additional opens until the first path is closed?  Or failing that, a
simple way to detect that a device is already in use and bail out?

-Aaron