Re: Linux poll() <sigh> again

[Robert Hancock]

linux-os (Dick Johnson) wrote:
> The bug relates to Linux implementation of poll()
> on a connected socket. If poll() is set to detect
> changes on a connected socket, with an infinite
> timeout (-1), and the client disconnects, it returns
> with a positive value (correct). The returned
> events (revents member), shows only POLLIN bit
> set. This, according to all known documentation
> including man pages on the web, is supposed to
> mean that there are data to be read. In fact,
> there are no data and a read will return 0.

According to the Single UNIX Specification:

http://www.opengroup.org/onlinepubs/007908799/xsh/poll.html

POLLIN means "Data other than high-priority data may be read without 
blocking. For STREAMS, this flag is set in revents even if the message 
is of zero length." The way I read it, all this is telling you is that a 
read on that file descriptor will not block at that particular moment. 
It doesn't mean there is actually any data to be read. On a device like 
a socket, read returning 0 tells you that the connection's been closed.

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.

> 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?

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

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