[PATCH] Fix poll/select signal socket as write ready on connect failure

Corinna Vinschen corinna-cygwin@cygwin.com
Thu Jul 16 09:25:53 GMT 2020

Hi Marc,

On Jul 15 20:54, Marc Hoersken via Cygwin wrote:
> Hello everyone,
> I identified an issue related to the way the events FD_CONNECT and FD_CLOSE
> returned by WSAEnumNetworkEvents are currently handled in
> winsup/cygwin/fhandler_socket_inet.cc.
> It seems like the code does not handle the fact that those events are
> returned only once for a socket and if not acted upon by the calling program
> may not be received again. This means poll and select are currently not
> consistend about the socket still being writable after a connect failure.
> The first call to poll or select would signal the socket as writable, but
> not any following call. The first call consumes the FD_CONNECT and FD_CLOSE
> events, regardless of the event mask supplied by the calling program. So
> even if the calling program does not care about writability in the first
> call, the events are consumed and following calls checking for writability
> will not be able to detect a connection failure.
> [...]
> As far as I understand calling poll and/or select should not change/reset
> the socket readyness state, therefore I created a simple fix which could be
> used to solve this issue. Attached you will find a suggested patch to make
> sure poll and select always signal writability of a connection failed
> socket. With this patch applied the above example command failed with a
> "Connection refused" as expected.
> This patch only fixes the behaviour regarding connection failure (during
> FD_CONNECT), I am not sure if connection closure (during FD_CLOSE) is also
> affected, but I was not able to find code handling the fact that FD_CLOSE is
> only signalled once.
> Please take a look and thanks in advance!

Thanks for the patch.  I pushed it.  But then I got second thoughts in
terms of how to fix the issue.  The reason is that the FD_CLOSE problem
shouldn't exist, simply for the fact that we never remove FD_CLOSE from
the events mask, see


So, rather than setting FD_WRITE at some later point in the code, what
about handling this where the other FD_CONNECT stuff is handled, by
not erasing the FD_CONNECT bit, just like with FD_CLOSE?

diff --git a/winsup/cygwin/fhandler_socket_inet.cc b/winsup/cygwin/fhandler_socket_inet.cc
index e5b0d2d1443e..b64d96225db1 100644
--- a/winsup/cygwin/fhandler_socket_inet.cc
+++ b/winsup/cygwin/fhandler_socket_inet.cc
@@ -354,7 +354,12 @@ fhandler_socket_wsock::evaluate_events (const long event_mask, long &events,
 	    wsock_events->events |= FD_WRITE;
-	  wsock_events->events &= ~FD_CONNECT;
+	  /* Since FD_CONNECT is only given once, we have to keep FD_CONNECT
+	     for connection failed sockets to have consistent behaviour in
+	     programs calling poll/select multiple times.  Example test to
+	     non-listening port: curl -v */
+	  if (connect_state () != connect_failed)
+	    wsock_events->events &= ~FD_CONNECT;
 	  wsock_events->connect_errorcode = 0;
       /* This test makes accept/connect behave as on Linux when accept/connect
@@ -376,12 +381,6 @@ fhandler_socket_wsock::evaluate_events (const long event_mask, long &events,
       if (erase)
 	wsock_events->events &= ~(events & ~(FD_WRITE | FD_CLOSE));
-  /* Since FD_CONNECT is only given once, we manually need to set
-     FD_WRITE for connection failed sockets to have consistent
-     behaviour in programs calling poll/select multiple times.
-     Example test to non-listening port: curl -v */
-  if ((connect_state () == connect_failed) && (event_mask & FD_WRITE))
-    wsock_events->events |= FD_WRITE;
   return ret;

What do you think?


Corinna Vinschen
Cygwin Maintainer

More information about the Cygwin mailing list