> AFAIK, WSAEWOULDBLOCK means that the socket is in nonblocking mode and
> the attempted operation could wait indefinitely to complete.

Rather pessimistic way to see it :) I've always read it as "operation
cannot be completed without blocking".

> But I
> don't understand how that could happen since a previous select()
> confirmed the socket is writeable...

Richard Stevens, MSDN
(http://msdn.microsoft.com/en-us/library/ms741540(VS.85).aspx) and
possibly several other authors and places will tell it is possible to
happen. I've always seen this being handled as "try again later".