Description of POLLHUP in poll(2)

Description of POLLHUP in poll(2)

[Sam Varshavchik]

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I'd like to propose adding the following to the current, somewhat terse, 
description of POLLHUP:

==

For stream sockets, this indicates that the peer closed its side of the 
connection. This does not necessarily imply that there's no more data to be 
read from the socket. POLLHUP may be set even if some unread data remains in 
the socket. Applications that need to process all data sent from their 
socket peer should use read(2) to check for unread data if POLLHUP is set.

==

That's something I didn't know -- I still learn something new every day -- 
When I get a POLLHUP I've assumed that it indicates an end-of-file 
condition, but that's not apparently the case, and having this documented 
would be helpful.


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Re: Description of POLLHUP in poll(2)

[Michael Kerrisk (man-pages)]

Hello Sam,

On 07/25/2010 03:12 AM, Sam Varshavchik wrote:
> I'd like to propose adding the following to the current, somewhat terse, 
> description of POLLHUP:
> 
> ==
> 
> For stream sockets, this indicates that the peer closed its side of the 
> connection. This does not necessarily imply that there's no more data to be 
> read from the socket. POLLHUP may be set even if some unread data remains in 
> the socket. Applications that need to process all data sent from their 
> socket peer should use read(2) to check for unread data if POLLHUP is set.
> 
> ==
> 
> That's something I didn't know -- I still learn something new every day -- 
> When I get a POLLHUP I've assumed that it indicates an end-of-file 
> condition, but that's not apparently the case, and having this documented 
> would be helpful.

Long after the fact...

I added this text:

[[
Note that when reading from a channel such as a pipe or a stream socket,
this event merely indicates that the peer closed its end of the channel.
Subsequent reads from the channel will return 0 (end of file)
only after all outstanding data in the channel has been consumed.
]]

And also made a similar change in epoll_ctl(2).

Thanks for the report.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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