[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.
Heinz
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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