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Date: Sun, 19 Jun 2011 15:45:51 -0700
Message-ID: <BANLkTi=Uz5nZ5gFTQ3NxCSDYTY03v3an9Q@mail.gmail.com>
Subject: Re: Does Linux select() violate POSIX?
From: Nemo Publius <nemo@self-evident.org>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Bernd Petrovitsch <bernd@petrovitsch.priv.at>,
        Eric Dumazet <eric.dumazet@gmail.com>, linux-kernel@vger.kernel.org
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On Sun, Jun 19, 2011 at 3:32 PM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
>
> It's worth noting that the POSIX semantics are actually unimplementable
> for some network protocols anyway particularly on send. TCP is a fine
> example. A remote TCP isn't *supposed* to shrink its window but they can
> do, and that that point the space select() saw for a send is closed down
> again by the remote host.

Which makes me wonder what *BSD does for such a situation.  Although
not enough to check the source.  :-)

> All sorts of similar issues appear all over the place. There are also
> interesting API corner cases such as the behaviour of
>
> ? ? ? ?listen()
> ? ? ? ?select
> ? ? ? ? ? ? ? ? ? ? ? ?connection made
> ? ? ? ?select returns
> ? ? ? ? ? ? ? ? ? ? ? ?remote closes connection
> ? ? ? ?accept
> ? ? ? ? ? ? ? ? ? ? ? ?behaviour is not determinate

Hm, I thought this was what ECONNABORTED was for?

That is, accept() might return ECONNABORTED, or it might return a
descriptor and then a later operation on that descriptor would fail
with ECONNRESET...  But either way, select() followed by accept() need
not block.

> (and in general POSIX doens't address sockets well)

Well, no argument there.

> So for portable code always mix select and poll with non blocking I/O. It
> doesn't matter what the specs say, the real world says drive defensively
> 8)

No argument here, either.  This was mostly for a barroom bet (well,
StackOverflow...  same thing), but also because I was curious.  There
are not a lot of ways in which Linux chooses to violate POSIX.  Which
might make a fun list to put together, come to think of it.

Thanks again, Alan.
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