Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752150Ab1FRSba (ORCPT ); Sat, 18 Jun 2011 14:31:30 -0400 Received: from earthlight.etchedpixels.co.uk ([81.2.110.250]:60410 "EHLO www.etchedpixels.co.uk" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752026Ab1FRSb3 (ORCPT ); Sat, 18 Jun 2011 14:31:29 -0400 Date: Sat, 18 Jun 2011 19:33:40 +0100 From: Alan Cox To: Nemo Publius Cc: Eric Dumazet , linux-kernel@vger.kernel.org Subject: Re: Does Linux select() violate POSIX? Message-ID: <20110618193340.53811cbe@lxorguk.ukuu.org.uk> In-Reply-To: References: <1308418985.3539.58.camel@edumazet-laptop> X-Mailer: Claws Mail 3.7.9 (GTK+ 2.22.0; x86_64-redhat-linux-gnu) Face: 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 Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1721 Lines: 38 > > We dont care, since every sane application using select() should also > > use socket in non blocking mode. > > This is simply not true for any POSIX-compliant operating system. > Which in this case happens to include every Unix ever written since > the beginning of time, apart from Linux. Actually no - there are lots of device cases where instantaneously it is true that a read would not block but the condition then changes again. An obvious simple example beyond that is a socket with two readers. > Put another way... The whole point of the POSIX spec is to allow me > to write portable code. If every random Unix implementation makes up > its own mind about what is "sane" and violates the spec in arbitrary > and unpredictable ways, what is the point of having a spec? Linux follows Posix generally, but nobody writes portable code that does blocking reads on a poll/select interface because there are a bazillion ways it can then block - events read by other tasks, discards due to memory exhaustion, events that are cleared the other end, etc. > > Between time select()/poll() says 'OK you can go', and time you enter > > kernel, conditions might have changed. For example, maybe kernel memory > > is not available and a send() would _block_, even if socket queue is > > empty. > > Sounds like a kernel bug. It's a design decision and a huge performance win. It's one of the areas where POSIX read in its strictest form cripples your performance. Alan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/