Dup'd sockets lose error information
Thu Apr 24 14:14:00 GMT 2014
On Apr 23 17:38, Corinna Vinschen wrote:
> On Apr 23 16:25, qq qq wrote:
> > The following code is a simplified app that was used to test-connect
> > to local ports 55000+ (none of which were actually listening) and
> > received false-positive "connected" results because Cygwin's dup()
> > for socket causes SO_ERROR to be lost. Since FD_SETSIZE is only 64 on
> > Cygwin, the app uses dup()'s to lower the descriptors as it checks
> > them for completion. There is no such problem on Linux.
> > Also, strangely that Cygwin does not accept sin_addr as 0 to connect
> > locally (and either localhost or local host IP must be stuffed in
> > there, otherwise resulting in the "Cannot assign requested address"
> > error).
> This is Winsock at work. Cygwin doesn't check the AF_INET address
> when calling Winsock's connect, so Winsock's connect itself seems to
> have this issue.
> As for the SO_ERROR value, I have to check. Thanks for the testcase.
I've applied a patch which should solve both problems.
Cygwin is using socket events to implement select. It turned out that,
when using the original socket, the socket event as well as the socket's
SO_ERROR value were set correctly. However, when calling select with
the dup'ed socket handle, the socket event contained the correct error
code, but the SO_ERROR value wasn't set. I added code to write back the
error code from the socket event entry to the SO_ERROR socket option and
that seems to do the trick.
As for connecting or sending to INADDR_ANY or in6addr_any, this simply
doesn't work with Winsock. It just returns WSAEADDRNOTAVAIL. I now
added a conversion from INADDR_ANY and in6addr_any to INADDR_LOOPBACK
and in6addr_loopback in calls to connect, sendto, and sendmsg to emulate
the Linux behaviour.
Apart from that it turned out that 64 bit Cygwin was suffering from a
definition problem of a couple of Winsock types used in the Cygwin
socket code. As the (hopefully) only user-visible effect, this led
to a broken address output in /proc/net/if_inet6. This is fixed now
As for FD_SETSIZE, it's defined as 64 by default, but the user code
can override the value by defining FD_SETSIZE to another value prior
to including the standard headers. The underlying code does not
restrict the file descriptor values to < FD_SETSIZE.
Please give the today's developer snapshot from
http://cygwin.com/snapshots/ a try.
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Maintainer cygwin AT cygwin DOT com
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