On Wed, 2015-10-21 at 11:49 +0100, Alan Burlison wrote:

On 21/10/2015 11:25, David Laight wrote:

quoted

quoted

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);