Linux, sockets, non-blocking connect - c

Question

I want to create a non-blocking connect.
Like this:
socket.connect(); // returns immediately
For this, I use another thread, an infinite loop and Linux epoll. Like this(pseudocode):
// in another thread
{
create_non_block_socket();
connect();
epoll_create();
epoll_ctl(); // subscribe socket to all events
while (true)
{
epoll_wait(); // wait a small time(~100 ms)
check_socket(); // check on EPOLLOUT event
}
}
If I run a server and then a client, all it works. If I first run a client, wait a some small time, run a server, then the client doesn't connect.
What am I doing wrong? Maybe it can be done differently?

Answer 1

You should use the following steps for an async connect:
create socket with socket(..., SOCK_NONBLOCK, ...)
start connection with connect(fd, ...)
if return value is neither 0 nor EINPROGRESS, then abort with error
wait until fd is signalled as ready for output
check status of socket with getsockopt(fd, SOL_SOCKET, SO_ERROR, ...)
done
No loops - unless you want to handle EINTR.
If the client is started first, you should see the error ECONNREFUSED in the last step. If this happens, close the socket and start from the beginning.
It is difficult to tell what's wrong with your code, without seeing more details. I suppose, that you do not abort on errors in your check_socket operation.

Answer 2

There are a few ways to test if a nonblocking connect succeeds.
call getpeername() first, if it failed with error ENOTCONN, the connection failed. then call getsockopt with SO_ERROR to get the pending error on the socket
call read with a length of 0. if the read failed, the connection failed, and the errno for read indicates why the connection failed; read returns 0 if connection succeeds
call connect again; if the errno is EISCONN, the connection is already connected and the first connect succeeded.
Ref: UNIX Network Programming V1

Answer 3

D. J. Bernstein gathered together various methods how to check if an asynchronous connect() call succeeded or not. Many of these methods do have drawbacks on certain systems, so writing portable code for that is unexpected hard. If anyone want to read all the possible methods and their drawbacks, check out this document.
For those who just want the tl;dr version, the most portable way is the following:
Once the system signals the socket as writable, first call getpeername() to see if it connected or not. If that call succeeded, the socket connected and you can start using it. If that call fails with ENOTCONN, the connection failed. To find out why it failed, try to read one byte from the socket read(fd, &ch, 1), which will fail as well but the error you get is the error you would have gotten from connect() if it wasn't non-blocking.