[TUHS] mmap, was SCCS, TeamWare, BitKeeper, and Git

[Steffen Nurpmeso]

Clem Cole wrote in
 <CAC20D2PfMx=tox0NmLYCi_7345BSr8yoCQZTRVspS8YxAawYdg at mail.gmail.com>:
 |On Mon, Dec 16, 2024 at 10:40 AM Douglas McIlroy <
 |douglas.mcilroy at dartmouth.edu> wrote:
 |
 |>>> Does anyone know whether there are implementations of mmap that
 |>>> do transparent file sharing? It seems to me that should be possible by
 |>>> making the buffer cache share pages with mmapping processes.
 |>
 |>> These days they all do.  The POSIX rationale says:
 |>
 |>> ... When multiple processes map the same memory object, they can
 |>> share access to the underlying data.
 |>
 |> Notice the weasel word "can". It is not guaranteed that they will do so
 |> automatically without delay. Apparently each process may have a physical\
 |> ly
 |> distinct copy of the data, not shared access to a single location.
 |>
 |Hmmm — how did that make it through the IEEE editing process? However, that
 |is not part of the standard if it's in the Rationale - just an explanation
 |of the standard - which I want want look at more carefully to see what it
 |says about what is required of mmap(2).
 |
 |That said, as Heinz and the rest of us who worked on the original version
 |of what would come to POSIX can tell you, the word "can" was always a
 |no-no. The word "SHALL" is the the official IEEE word, and if you use
 |"SHOULD," even in the rationale and it was very much frowned upon.   I'll

They are all lowercase nowadays.

 |have try dig up the troff sources to an early draft of the original and do
 |a "grep -i should * | wc -l"  through it.  I bet the number is very small
 |and "can" is zero.

They offer lots of hints like "can be implemented efficiently" or
"conforming implementations cannot count on", which turns "can"
into a "dying butterfly".  (In the rationale's.)

 |That said, it has been a few years since I have read a current draft, but
 |less the rationale section (which is not the standard itself).

The 2024 version is 4057 pages, all in all.  It would -- in my
opinion -- make a nice discussion whether it would be better for
anybody, including engineers, to take the time to read that amount
of cultural foundation, real literature that is, instead of
a technical standard.  (I admit i personally perform standard
hopping and have at times some spare time for other things,
because if i fail it currently endangers noone for one, and when
i fall i stand up anyway, unless i do no more, but then having
lived in mindsets++ of fantastic literats.)

 |Clem

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)
|
|And in Fall, feel "The Dropbear Bard"s ball(s).
|
|The banded bear
|without a care,
|Banged on himself for e'er and e'er
|
|Farewell, dear collar bear