Multiplexed Special Devices

[Guy Harris]

>Does anyone know the origin of the "Multiplexed Character Special" (IFMPC)
>and "Multiplexed Block Special" (IFMPB) device types?

It comes from "The UNIX Time-Sharing System, Seventh Edition" - more
commonly known as "V7".  I don't remember any "multiplexed block
special" files being supported, but the multiplexed character special
files were a sort of IPC mechanism.  You'll have to find somebody who
still has a V7 manual handy in order to get a full description; look
for, as I remember, the "mpx" man page.

I think they were rather buggy.  I also think Berkeley fixed some of the
bugs in 4.1BSD, and used them for some IPC stuff (did "comsat" use
them?), but nuked them in 4.2BSD in favor of sockets.

>These are defined in the inode.h header file as it comes in all Releases
>of System V, but I've never seen it used in any kernel source.

It's somewhat amusing that the #defines survive in S5.