Multiplexed Special Devices

[John F. Haugh II]

In article <4171 at auspex.auspex.com> guy at auspex.auspex.com (Guy Harris) writes:
>>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.

IBM has them in AIX v3.1 as multiplexed character devices.  each
open returns a different channel.  Thus, the PTY device driver is
a multiplexed character device.  When you want a PTY pair you open
/dev/pts and see what you got.  Then you open the /dev/ptc that
matches, and you have a PTY all to yourself.  The HFT device driver
works this way as well - when you want a new virtual native display
you just open /dev/hft, and poof, instant channel.

I've seen references in old V7 and such manuals about multiplexed
pipes.  I can't imagine what one of those must have smelled like.
-- 
John F. Haugh II                             UUCP: ...!cs.utexas.edu!rpp386!jfh
Ma Bell: (512) 832-8832                           Domain: jfh at rpp386.cactus.org
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