[TUHS] non-blocking IO

Heinz Lycklama heinz at osta.com
Tue Jun 2 02:58:47 AEST 2020

I did  add a few new features to LSX to deal with contiguous files
and to handle asynchronous read/write's for real time applications.
They are described in the LSX paper in the 1978 BSTJ on the
UNIX Time-Sharing System.


On 5/31/2020 9:46 AM, Warner Losh wrote:
> Sorry to top post, but LSX or Miniunix had non blocking I/O as well. 
> It was in one of the documents that Clem scanned in the last year. It 
> specifically was an experiment into how to do it.
> Warner
> On Sun, May 31, 2020, 10:07 AM Clem Cole <clemc at ccc.com 
> <mailto:clemc at ccc.com>> wrote:
>     On Sun, May 31, 2020 at 7:10 AM Paul Ruizendaal <pnr at planet.nl
>     <mailto:pnr at planet.nl>> wrote:
>          This behaviour seems to have continued into SysVR1, I’m not
>         sure when EAGAIN came into use as a return value for this use
>         case in the SysV lineage. Maybe with SysVR3 networking?
>     Actually, I'm pretty sure that was a product of the POSIX
>     discussions.  BSD already had networking an EWOULDBLOCK.   We had
>     argued about EWOULDBLOCK a great deal, we also were arguing about
>     signal semantics. I've forgotten many of the details, Heinz may
>     remember more than I do.  EAGAIN was created as a compromise --
>     IIRC neither system had it yet.   SVR3 networking was where it
>     went into System V, although some of the AT&T representatives were
>     none too happy about it.
>         In the Research lineage, the above SysIII approach does not
>         seem to exist, although the V8 manual page for open() says
>         under BUGS "It should be possible [...] to optionally call
>         open without the possibility of hanging waiting for carrier on
>         communication lines.” In the same location for V10 it reads
>         "It should be possible to call open without waiting for
>         carrier on communication lines.”
>         The July 1981 design proposals for 4.2BSD note that SysIII
>         non-blocking files are a useful feature and should be included
>         in the new system. In Jan/Feb 1982 this appears to be coded
>         up, although not all affected files are under SCCS tracking at
>         that point in time. Non-blocking behaviour is changed from the
>         SysIII semantics, in that EWOULDBLOCK is returned instead of 0
>         when progress is not possible. The non-blocking behaviour is
>         extended beyond TTY’s and pipes to sockets, with additional
>         errors (such as EINPROGRESS). At this time EWOULDBLOCK is not
>         the same error number as EGAIN.
>     My memory is that Keith was the BSD (CSRG) person at the POSIX
>     meeting (he, Jim McGinness of DEC, and I created PAX at one point
>     as a compromise).   I wish I could remember all of the details,
>     but this was all argued at the POSIX meetings.
>     As I said before the folks from AT&T just wanted to take the SVID
>     and rubber stamp it at the specification.  Part of it the problem
>     was they wanted to be free to do what things/make choices that the
>     rest of us might or might not like (for instance, they did not
>     want the sockets interface).
>         It would seem that the differences between the BSD and SysV
>         lineages in this area persisted until around 2000 or so.
>     Yep - cause around then POSIX started to settle out and both
>     systems began to follow it.

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