Async connect and disconnect with epoll (Linux)

Question

I need async connect and disconnect for tcp client using epoll for Linux. There are ext. functions in Windows, such as ConnectEx, DisconnectEx, AcceptEx, etc... In tcp server standard accept function is working, but in tcp client doesn't working connect and disconnect... All sockets are nonblocking.

How can I do this?

Thanks!

Answer 1

To do a non-blocking connect(), assuming the socket has already been made non-blocking:

int res = connect(fd, ...);
if (res < 0 && errno != EINPROGRESS) {
    // error, fail somehow, close socket
    return;
}

if (res == 0) {
    // connection has succeeded immediately
} else {
    // connection attempt is in progress
}

For the second case, where connect() failed with EINPROGRESS (and only in this case), you have to wait for the socket to be writable, e.g. for epoll specify that you're waiting for EPOLLOUT on this socket. Once you get notified that it's writable (with epoll, also expect to get an EPOLLERR or EPOLLHUP event), check the result of the connection attempt:

int result;
socklen_t result_len = sizeof(result);
if (getsockopt(fd, SOL_SOCKET, SO_ERROR, &result, &result_len) < 0) {
    // error, fail somehow, close socket
    return;
}

if (result != 0) {
    // connection failed; error code is in 'result'
    return;
}

// socket is ready for read()/write()

In my experience, on Linux, connect() never immediately succeeds and you always have to wait for writability. However, for example, on FreeBSD, I've seen non-blocking connect() to localhost succeeding right away.

Answer 2

From experience, when detect non-blocking connection , epoll is a little different from select and poll.

with epoll:

After connect() call is made, check return code.

If the connection can not be completed immediately, then register EPOLLOUT event with epoll.

Call epoll_wait().

if the connection failed, your events will be fill with EPOLLERR or EPOLLHUP, otherwise EPOLLOUT will be triggered.