Re: Problem with socketpair , AF_UNIX and select call - can anybody through any light on this!

On Fri, 12 Oct 2007 12:02:41 +0100
Andy Green <andy@xxxxxxxxxxx> 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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