Hallo, in socket_types.hpp instead of ... # if defined(__CYGWIN__) # if !defined(__USE_W32_SOCKETS) # error You must add -D__USE_W32_SOCKETS to your compiler options. # endif // !defined(__USE_W32_SOCKETS) # if !defined(NOMINMAX) # define NOMINMAX 1 # endif // !defined(NOMINMAX) # endif // defined(__CYGWIN__) # include <winsock2.h> ... I suggest something like ... # if defined(__CYGWIN__) # if !defined(__USE_W32_SOCKETS) # error You must add -D__USE_W32_SOCKETS to your compiler options. # endif // !defined(__USE_W32_SOCKETS) # endif // defined(__CYGWIN__) # if !defined(NOMINMAX) # define NOMINMAX 1 # define BOOST_NOMINMAX_DEFINED 1 # endif // !defined(NOMINMAX) # include <winsock2.h> # if defined(BOOST_NOMINMAX_DEFINED) # undef NOMINMAX # undef BOOST_NOMINMAX_DEFINED # endif // defined(BOOST_NOMINMAX_DEFINED) ... Thanks. Bjoern Thiel