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Content-class: urn:content-classes:message
Subject: Re: Linux poll() <sigh> again
Date: Fri, 12 May 2006 07:42:07 -0400
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Thread-Topic: Linux poll() <sigh> again
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From: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
To: "Nishanth Aravamudan" <nacc@us.ibm.com>
Cc: "Linux kernel" <linux-kernel@vger.kernel.org>, <staubach@redhat.com>
Reply-To: "linux-os \(Dick Johnson\)" <linux-os@analogic.com>
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On Thu, 11 May 2006, Nishanth Aravamudan wrote:

> On 11.05.2006 [17:04:46 -0400], linux-os (Dick Johnson) wrote:
>>
>> On Thu, 11 May 2006, Nishanth Aravamudan wrote:
>>
>>> On 11.05.2006 [10:25:29 -0400], linux-os (Dick Johnson) wrote:
>>>>
>>>>
>>>> Hello,
>>>>
>>>> I'm trying to fix a long-standing bug which has a
>>>> work-around that has been working for a year or
>>>> so.
>>>
>>> <snip valiant efforts>
>>>
>>>> Here is relevent code:
>>>>
>>>>              for(;;) {
>>>>                  mem->pfd.fd = fd;
>>>>                  mem->pfd.events = POLLIN|POLLERR|POLLHUP|POLLNVAL;
>>>>                  mem->pfd.revents = 0x00;
>>>
>>> Hrm, in looking at the craziness that is sys_poll() for a bit, I think
>>> it's the underlying f_ops that are responsible for not setting POLLHUP,
>>> that is:
>>>
>>>                        if (file != NULL) {
>>>                                mask = DEFAULT_POLLMASK;
>>>                                if (file->f_op && file->f_op->poll)
>>>                                        mask = file->f_op->poll(file, *pwait);
>>>                                mask &= fdp->events | POLLERR | POLLHUP;
>>>                                fput_light(file, fput_needed);
>>>                        }
>>>
>>> and file->f_op->poll(file, *pwait) is not setting POLLHUP on the
>>> disconnect. What filesystem is this?
>>
>> I think that's the problem. A socket isn't a file-system and the
>> code won't set either bits if it isn't. Perhaps, the kernel code
>> needs to consider a socket as a virtual file of some kind? Surely
>> one needs to use poll() on sockets, no?
>
> Duh, I'm not reading well today -- for sockets, we do
>
> file->f_op->poll() -> (socket_file_ops) sock_poll() -> sock->ops->poll()
>
> So, now I need to know what kind of socket is this to go from there ..
>
> Thanks,
> Nish

A stream socket can be "connected". Anything that can be connected
needs to know when the connection is broken.

     socket(AF_INET, SOCK_STREAM, IPPROTO_IP);
     ip_sock.sin_family = AF_INET;

Such a socket is bound to an address and port using bind(), listen()
is established, the accept() is called to accept connections. Accept
returns a socket (fd) of the connected host. It's this fd that needs
to "know" if/when the host disconnects.


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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