[TUHS] Unix "Multiplexed Files"

[Dan Cross]

The subject of Communication Files on DTSS came up recently, and Doug
linked to this wonderful note:
https://www.cs.dartmouth.edu/~doug/DTSS/commfiles.pdf

Over on the Multicians list, I raised a question about the history of the
DTSS emulator on Multics in response to that, which sadly broke down into
antagonism, the details of which aren't terribly interesting. But Barry
Margolin suggested that the closest modern Unix analogue of Communication
Files were pseudo-TTYs, that had generated a dustup here. Doug's note
suggests that Plan 9's userspace filesystems, aided and abetted by mutable
namespaces and 9P as a common sharing mechanism, were a closer analogy.

But I wonder if multiplexed files were perhaps an earlier analogue; my
cursory examination of mpx(2) shows some similarities to the description of
the DTSS mechanism.

But I confess that I know very little about these, other than that they
seem to be an evolutionary dead end (they don't survive in any modern Unix
that I'm aware of, at any rate). I don't see much about them in my
archives; Paul Ruizendaal mentioned them tangentially in 2020 in relation
to non-blocking IO: they are, apparently, due to Chessen?

Does anyone have the story here?

        - Dan C.
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