The socket_types.hpp defines _WIN32_WINNT to 0x0501 if neither _WIN32_WINNT nor _WIN32_WINDOWS are defined. However, under Windows Mobile, _WIN32_WINNT should not be defined. It appears this is only being used to determine the value for max_iov_len: # if defined(_WIN32_WINNT) const int max_iov_len = 64; # else const int max_iov_len = 16; # endif So I think that the original test should also test for _WIN32_WCE, so that _WIN32_WINNT is not defined under Windows Mobile: # if !defined(_WIN32_WINNT) && !defined(_WIN32_WINDOWS) && !defined(_WIN32_WCE) Does anyone know what max_iov_len should be set to under Windows Mobile 5, or how I might be able to find out? I assume it's safer to leave it at the lower value (16) for now, but it would be nice to have it set correctly. Thanks, Thomas Aylesworth Senior Software Engineer RealityMobile http://www.realitymobile.com/ 703.636.7202 office 703.209.1081 mobile