[TUHS] non-blocking IO

Warner Losh imp at bsdimp.com
Mon Jun 1 02:46:37 AEST 2020

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.


On Sun, May 31, 2020, 10:07 AM Clem Cole <clemc at ccc.com> wrote:

> On Sun, May 31, 2020 at 7:10 AM Paul Ruizendaal <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
> 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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