Socket: non-blocking connect and getsockopt SO_ERROR

Brian Ford
Tue May 17 14:20:00 GMT 2011

On Mon, 16 May 2011, Jacob Eiler wrote:

> I am experiencing an issue with getsockopt when running an application
> under CygWin.
> The application (Kannel sms gateway) creates a new socket, attempt to
> connect non-blocking to the other host and later calls getsockopt to
> check for errors:
> s = socket(PF_INET, SOCK_STREAM, 0);
> ...
> flags = fcntl(s, F_GETFL, 0);
> fcntl(s, F_SETFL, flags | O_NONBLOCK);
> ...
> connect(s, &addr, sizeof(addr))
> connect returns EINPROGRESS and the application has a polling thread
> setup to handle timeout and check for changes. In both cases a callback
> function is invoked.
> The callback function checks the connection by calling
> getsockopt(s, SOL_SOCKET, SO_ERROR, &err, &len)
> but it returns 0 and no error - even if the connection was never
> established. Subsequently the application tries to write data to the
> socket - which fails with a "transport endpoint is not connected" error.
> The code works just fine on Linux with getsockopt returning an error.
> Any pointer on how to get this working will be greatly appreciated.

The following pseudo code works portably for me on Solaris and Cygwin:

fds.fd     = s; = POLL_WRITE;

if (Poll(&fds, 1, 0) > 0)
   len = sizeof(errno);

   getsockopt(fd, SOL_SOCKET, SO_ERROR, &errno, &len)

   if (errno = 0)
	; //connected

I think the key is to make sure select/poll returns (as either writable or
with error) before calling getsockopt(SO_ERROR).  HTH.

Brian Ford
Staff Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
the best safety device in any aircraft is a well-trained crew...

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