Re: Linux poll() <sigh> again

From: linux-os (Dick Johnson)
Date: Fri May 12 2006 - 10:56:55 EST



On Fri, 12 May 2006, jimmy wrote:

> 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.

This is from the Linux man-page shipped with recent distributions


SOCKET(7) Linux ProgrammerâEUR(tm)s Manual SOCKET(7)



+--------------------------------------------------------------------+
| I/O events |
+-----------+-----------+--------------------------------------------+
|Event | Poll flag | Occurrence |
+-----------+-----------+--------------------------------------------+
|Read | POLLIN | New data arrived. |
+-----------+-----------+--------------------------------------------+
|Read | POLLIN | A connection setup has been completed (for |
| | | connection-oriented sockets) |
+-----------+-----------+--------------------------------------------+
|Read | POLLHUP | A disconnection request has been initiated |
| | | by the other end. |
+-----------+-----------+--------------------------------------------+
|Read | POLLHUP | A connection is broken (only for connec- |
| | | tion-oriented protocols). When the socket |
| | | is written SIGPIPE is also sent. |
+-----------+-----------+--------------------------------------------+
|Write | POLLOUT | Socket has enough send buffer space for |
| | | writing new data. |
+-----------+-----------+--------------------------------------------+
|Read/Write | POLLIN| | An outgoing connect(2) finished. |
| | POLLOUT | |
+-----------+-----------+--------------------------------------------+
|Read/Write | POLLERR | An asynchronous error occurred. |
+-----------+-----------+--------------------------------------------+
|Read/Write | POLLHUP | The other end has shut down one direction. |
+-----------+-----------+--------------------------------------------+
|Exception | POLLPRI | Urgent data arrived. SIGURG is sent then. |
+-----------+-----------+--------------------------------------------+


If linux doesn't support POLLHUP, then it shouldn't be documented.
I got the same king of crap^M^M^M^Mresponse the last time I reported
this __very__ __obvious__ defect! The information is available
in the kernel. It should certainly report it, just like other
operating systems do, including <shudder> wsock32.

>>
>>>
>>>>> 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!
>

Yes. You need to know if something has changed. This could mean
many things such as new data available or a disconnection. This
is a communications link for crysake, one needs to handle
communications events.

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

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.4 on an i686 machine (5592.89 BogoMips).
New book: http://www.lymanschool.com
_


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