Urgent need for advice: POLLHUP and sockets

Martin Wilck Martin.Wilck at fujitsu-siemens.com
Tue Apr 16 13:40:25 CDT 2002

Hi all,

please consider the following problem:

On sockets, when POLLHUP is received, it is perfectly legitimate to
continue reading data until a read() or recv() call returns 0 bytes.

The current wine implementation calls set_select_events( &sock->obj, -1 )
when POLLHUP or POLLERR is received, disabling any future events from the

However, if an application uses asynchronous IO (or only asynchronous
notification via WSAAsyncSelect() or WSAEventSelect()), this also inhibits
reception of POLLIN events which can perfectly well occur if there are
still unread data in the kernel buffers. Thus the app will never notice
that there is still data to be read.

My first reaction to this was to comment out the above call to
set_select_events() and continue listening to events from the socket.
However, due to the semantics of poll(), this will cause a POLLHUP event
on this socket for each and every iteration of the wineserver's main select
loop, putting the server basically into a busy-wait state.

How can this be resolved? We seem to have the choice between

a) busy loop in the server (bad)
b) discarding data which could be legitimately read by the application
   (very bad).

Any hints/comments/ideas?

Please note that it is insufficient to queue all pending APCs on the
socket when POLLHUP is received, because the app may very well queue
some _after_ the close event arrives.

A minor point is that sock_poll_event() also sets a FD_CLOSE event
(if present) everytime it is called with POLLHUP set, effectively
preventing any APC from being called, at least if the app waits for
FD_CLOSE and asynchronous send/recv operations at the same time (likely).
This can be worked around by setting FD_CLOSE only once and then blocking
it, although I am not sure if that is correct behaviour (permanently
signalling FD_CLOSE is obviously incorrect).

I hope someone has a good idea how to resolve this,

Martin Wilck                Phone: +49 5251 8 15113
Fujitsu Siemens Computers   Fax:   +49 5251 8 20409
Heinz-Nixdorf-Ring 1	    mailto:Martin.Wilck at Fujitsu-Siemens.com
D-33106 Paderborn           http://www.fujitsu-siemens.com/primergy

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