Problem with socketpair , AF_UNIX and select call - can anybody through any light on this!
Alan Cox
alan at lxorguk.ukuu.org.uk
Fri Oct 12 13:01:00 UTC 2007
On Fri, 12 Oct 2007 12:02:41 +0100
Andy Green <andy at warmcat.com> wrote:
> Somebody in the thread at some point said:
> > Andy Green wrote,
> >> Why don't you try removing the O_NONBLOCK since you test with
> >> select...
> >
> > Never, ever, do that ... select only returns a hint that an IO operation
> > might succeed, not a guarantee (google for "spurious wakeup"), hence a
> > subsequent read/write might block if the descriptor isn't set to
> > O_NONBLOCK, and that's almost always not what's wanted or expected.
>
> Wah, man select says it too
>
> Under Linux, select() may report a socket file descriptor
> as "ready for reading", while nevertheless a subsequent read
> blocks. This could for example happen when data has arrived but
> upon examination has wrong checksum and is discarded. There
> may be other circumstances in which a file descriptor is
> spuriously reported as ready. Thus it may be safer to use O_NON-
> BLOCK on sockets that should not block.
>
> Pretty freakish, thanks for the tip.
Its the documented behaviour of select. select tells you "at the moment
in time I checked the next whatever operation would not block". It makes
no further guarantees, thus select should almost always be used with non
blocking I/O.
Alan
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