On Wed, 2015-10-21 at 11:49 +0100, Alan Burlison wrote:
> On 21/10/2015 11:25, David Laight wrote:
>
> >> The problem with poll() is that it returns immediately when passed a FD
> >> that is in the listening state. rather than waiting until there's an
> >> incoming connection to handle. As I said, that means you can't use
> >> poll() to multiplex between read/write/accept sockets.
> >
> > That seems to work for me...
>
> In my test case I was setting all the available event bits in
> pollfd.events to see what came back. With poll() on a listen() socket
> you get an immediate return with bits set in revents indicating the
> socket is available for output, which of course it isn't. Indeed an
> attempt to write to it fails. If you remove the output event bits in
> pollfd.events then the poll() waits as expected until there's an
> incoming connection on the socket.
>
> I suppose one answer is "Well, don't do that then" but returning an
> output indication on a socket that's in listen() seems rather odd.
>
> With POLLOUT|POLLWRNORM|POLLWRBAND:
>
> main: polling #1
> [returns immediately]
> main: poll #1: Success
> poll fd: 0 revents: POLLOUT POLLWRBAND
> main: write #1: Transport endpoint is not connected
>
> Without POLLOUT|POLLWRNORM|POLLWRBAND:
>
> main: polling #1
> [waits for connection]
> main: poll #1: Success
> poll fd: 0 revents: POLLIN POLLRDNORM
>
This works for me. Please double check your programs
242046 poll([{fd=3, events=POLLIN|POLLOUT|POLLWRNORM|POLLWRBAND}], 1,
4294967295 <unfinished ...>
242046 <... poll resumed> ) = 1 ([{fd=3, revents=POLLIN}])
242046 accept(3, {sa_family=AF_INET6, sin6_port=htons(35888),
inet_pton(AF_INET6, "::ffff:10.246.7.151", &sin6_addr), sin6_flowinfo=0,
sin6_scope_id=0}, [28]) = 4
242046 close(4) = 0
242046 poll([{fd=3, events=POLLIN|POLLOUT|POLLWRNORM|POLLWRBAND}], 1,
4294967295) = 1 ([{fd=3, revents=POLLIN}])
242046 accept(3, {sa_family=AF_INET6, sin6_port=htons(35890),
inet_pton(AF_INET6, "::ffff:10.246.7.151", &sin6_addr), sin6_flowinfo=0,
sin6_scope_id=0}, [28]) = 4
242046 close(4) = 0
242046 poll([{fd=3, events=POLLIN|POLLOUT|POLLWRNORM|POLLWRBAND}], 1,
4294967295) = 1 ([{fd=3, revents=POLLIN}])
242046 accept(3, {sa_family=AF_INET6, sin6_port=htons(35892),
inet_pton(AF_INET6, "::ffff:10.246.7.151", &sin6_addr), sin6_flowinfo=0,
sin6_scope_id=0}, [28]) = 4
242046 close(4) = 0
242046 poll([{fd=3, events=POLLIN|POLLOUT|POLLWRNORM|POLLWRBAND}], 1,
4294967295) = 1 ([{fd=3, revents=POLLIN}])
242046 accept(3, {sa_family=AF_INET6, sin6_port=htons(35894),
inet_pton(AF_INET6, "::ffff:10.246.7.151", &sin6_addr), sin6_flowinfo=0,
sin6_scope_id=0}, [28]) = 4
242046 close(4) = 0
poll() only cares about POLLIN | POLLRDNORM for a listener.
static inline unsigned int inet_csk_listen_poll(const struct sock *sk)
{
return !reqsk_queue_empty(&inet_csk(sk)->icsk_accept_queue) ?
(POLLIN | POLLRDNORM) : 0;
}
if (sk->sk_state == TCP_LISTEN)
return inet_csk_listen_poll(sk);
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