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Date: Fri, 12 May 2006 08:32:37 -0600
From: Robert Hancock <hancockr@shaw.ca>
Subject: Re: Linux poll() <sigh> again
In-reply-to: <Pine.LNX.4.61.0605120745050.8670@chaos.analogic.com>
To: "linux-os (Dick Johnson)" <linux-os@analogic.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>
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linux-os (Dick Johnson) wrote:
>> POLLHUP means "The device has been disconnected." This would obviously
>> be appropriate for a device such as a serial line or TTY, etc. but for a
>> socket it is less obvious that this return value is appropriate.
>>
> 
> Hardly "less obvious". SunOs has returned POLLHUP as has other
> Unixes like Interactive, from which the software was ported. It
> went from Interactive, to SunOs, to Linux. Linux was the first
> OS that required the hack. This was reported several years ago
> and I was simply excoriated for having the audacity to report
> such a thing. So, I just implemented a hack. Now the hack is
> biting me. It's about time for poll() to return the correct
> stuff.

The standard doesn't require that a close on a socket should report 
POLLHUP. Thus this behavior may differ between UNIX implementations. If 
your software is requiring a POLLHUP to indicate the socket is closed I 
think it is being unnecessarily picky since read returning 0 universally 
indicates that the connection has been closed. Such are the compromises 
that are sometimes required to write portable software.

> 
>>> I have used the subsequent read() with a returned
>>> value of zero, to indicate that the client disconnected
>>> (as a work around). However, on recent versions of
>>> Linux, this is not reliable and the read() may
>>> wait forever instead of immediately returning.
>> If you want nonblocking behavior, you should set the socket to
>> nonblocking. This is a bit strange though, unless the data was stolen by
>> another thread or something. Are you sure you've seen this?
> 
> I don't use threads. The hang under the specified conditions was first
> observed on 2.6.16.4 (that I'm running on this system). The hack, previously
> used, i.e., the read of zero was used since 2.4.x with success except it's
> a hack and shouldn't be required. It was not ever required on SunOs from
> which the software was ported.

This may be a bug somewhere.. however, once again if you don't want read 
to block under any circumstances, set your sockets to non-blocking!

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

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