Epoll events for connecting sockets

Question

I create epoll and register some non-blocking sockets which try connect to closed ports on localhost. Why epoll tells me, that i can write to this socket (it give event for one of created socket with eventmask contain EPOLLOUT)? But this socket doesn't open and if i try send something to it i get an error Connection refused.

Another question - what does mean even EPOLLHUP? I thought that this is event for refused connection. But how in this case event can have simultaneously EPOLLHUP and EPOLLOUT events?

Sample code on Python:

import socket
import select

poll = select.epoll()
fd_to_sock = {}
for i in range(1, 3):
    s = socket.socket()
    s.setblocking(0)
    s.connect_ex(('localhost', i))
    poll.register(s, select.EPOLLOUT)
    fd_to_sock[s.fileno()] = s

print(poll.poll(0.1))
# prints '[(4, 28), (5, 28)]'

SergeyA SergeyA · Accepted Answer · 2016-05-06T18:59:17

All that poll guarantees is that your application won't block after calling corresponding function. So you are getting what you've paid for - you can now rest assured writing to this socket won't block - and it didn't block, did it?

Poll never guarantees that corresponding operation will succeed.

Epoll events for connecting sockets

2 Answers