Non blocking socket - how to check if a connection was successful?

问题

After setting up a non blocking socket correctly I do the following to connect:

Call connect on the socket.
If it returns 0, I have already connected, if not, check errno.
If errno is not EINPROGRESS there is an error.
if errno is EINPROGRESS I can poll the connect status by: select_status = sock_select(FD_SETSIZE, NULL, &file_descriptor_set, NULL, &timeout); if select_status > 0 then check with FD_ISSET if the file descriptor is set.

Is that correct? And should I check for fd_write not fd_read? Should I call getsockopt after select? With what arguments? I cannot find a clear explanation of what needs to be done.

I do connect but my program is not reporting it correctly so I am using select wrong or it is because I don't check getsockopt.

WHat telles me that the connection was made?

回答1:

Call connect on the socket.

If it returns 0, I have already connected, if not, check errno.

If errno is not EINPROGRESS there is an error.

All of the above is correct (except under Windows you'd need to check for WSAEWOULDBLOCK instead of EINPROGRESS).

if errno is EINPROGRESS I can poll the connect status by: select_status = sock_select(FD_SETSIZE, NULL, &file_descriptor_set, NULL, &timeout); if select_status > 0 then check with FD_ISSET if the file descriptor is set.

Correct (assuming sock_select() is the same thing as select()), except that FD_SETSIZE should be the maximum value of all the file-descriptor values you are watching, plus 1.

Is that correct? And should I check for fd_write not fd_read?

Right.

What tells me that the connection was made?

When the async-connection-operation has completed, select() will return and FD_ISSET(theSocket, &writeSet) will return true.

When that happens, you need to find out if the connection attempt succeeded or failed. Here's a function I use for that:

// Returns true if the async connection on (fd) is connected; false if it failed
bool DidTheAsyncTCPConnectionSucceed(int fd)
{
   struct sockaddr_in junk;
   socklen_t length = sizeof(junk);
   memset(&junk, 0, sizeof(junk));
   return (getpeername(fd, (struct sockaddr *)&junk, &length) == 0);
}

If that returns true, your socket is connected and ready to use in the normal ways. If it returns false, the async-connection failed and you should close() the socket and handle that failure somehow.

(Again there is a slightly different behavior under Windows; under Windows, getting notified about the connection succeeding works as described above, but if you want to be notified about the connection failing, you'll need to watch the exception-fd-set as well. Under Windows, FD_ISSET(fd, &exceptionSet) is what will notify you when the asynchronous TCP connection failed).

来源：https://stackoverflow.com/questions/28481993/non-blocking-socket-how-to-check-if-a-connection-was-successful

标签

sockets

select

nonblocking