All, The documentation states that Boost.Asio requires __USE_W32_SOCKETS must be defined when using Cygwin. That doesn't seem to be true any longer. When I define __USE_W32_SOCKETS compilation of several components (e.g. ./boost/test/impl/debug.ipp) fail with "error: '::select' has not been declared". Without that definition compilation works apparently fine. Regards, -- Bernd