Re: Linux poll() <sigh> again

[Robert Hancock]

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!

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

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