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Date: Fri, 12 May 2006 20:16:03 +0530
From: jimmy <jimmyb@huawei.com>
Subject: Re: Linux poll() <sigh> again
In-reply-to: <44649C85.5000704@shaw.ca>
To: Robert Hancock <hancockr@shaw.ca>
Cc: "linux-os (Dick Johnson)" <linux-os@analogic.com>,
       linux-kernel <linux-kernel@vger.kernel.org>
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Robert Hancock wrote:
> 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!
> 
But that's another hack. AFAICS why ppl (mostly) use select/poll wud be 
to know if their send/recv/read/write would go thru rather than getting 
blocked!


-jb
-- 
Only two things are infinite, the universe and human stupidity, and I'm 
not sure about the former. - Albert Einstein
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