[TUHS] Unix "Multiplexed Files"

Eric Allman tuhs at eric.allman.name
Thu Feb 9 09:52:34 AEST 2023

As it turns out, the first implementation of syslog used mpx files,
albeit arguably abusing them. I don't remember the details. mpx files
have long since been replaced by modern alternatives.


On 2023-02-08 14:39 , Dan Cross wrote:
> 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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