Hi, I downloaded the "chat_client.cpp" from the boost website (http://www.boost.org/doc/libs/1_37_0/doc/html/boost_asio/examples.html) and try to compile it on Cygwin (see my compile statement below..), but it failed. It's worked on Linux (without the -D__USE_W32_SOCKETS -D_WIN32_WINNT=0x0501) , however. Thanks in advance for your help. Compile statement: g++ -o chat_client chat_client.cpp -D__USE_W32_SOCKETS -D_WIN32_WINNT=0x0501 -Wall -I/opt/boost/include -L/opt/boost/lib -lboost_thread -lboost_system -lboost_t hread Output message: /usr/lib/gcc/i686-pc-cygwin/3.4.4/../../../../i686-pc-cygwin/bin/ld: warning: au to-importing has been activated without --enable-auto-import specified on the co mmand line. This should work unless it involves constant data structures referencing symbols  from auto-imported DLLs./cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:ch at_client.cpp:(.text$_ZN5boost4asio6detail12winsock_initILi2ELi0EE7do_initC1Ev[b oost::asio::detail::winsock_init<2, 0>::do_init::do_init()]+0x1f): undefined ref erence to `_WSAStartup@8' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops5closeEjRNS_6system10error_codeE[boost::asio::detai l::socket_ops::close(unsigned int, boost::system::error_code&)]+0x18): undefined  reference to `_closesocket@4' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops13error_wrapperIiEET_S4_RNS_6system10error_codeE[in t boost::asio::detail::socket_ops::error_wrapper<int>(int, boost::system::error_ code&)]+0x13): undefined reference to `_WSAGetLastError@0' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops11clear_errorERNS_6system10error_codeE[boost::asio: :detail::socket_ops::clear_error(boost::system::error_code&)]+0xf): undefined re ference to `_WSASetLastError@4' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops6socketEiiiRNS_6system10error_codeE[boost::asio::de tail::socket_ops::socket(int, int, int, boost::system::error_code&)]+0x3e): unde fined reference to `_WSASocketA@24' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops6socketEiiiRNS_6system10error_codeE[boost::asio::de tail::socket_ops::socket(int, int, int, boost::system::error_code&)]+0x9a): unde fined reference to `_setsockopt@20' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops13error_wrapperIjEET_S4_RNS_6system10error_codeE[un signed int boost::asio::detail::socket_ops::error_wrapper<unsigned int>(unsigned  int, boost::system::error_code&)]+0x13): undefined reference to `_WSAGetLastErr or@0' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops12call_connectIiEEiMNS2_6msghdrET_jPK8sockaddrj[int  boost::asio::detail::socket_ops::call_connect<int>(int boost::asio::detail::soc ket_ops::msghdr::*, unsigned int, sockaddr const*, unsigned int)]+0x1b): undefin ed reference to `_connect@12' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops5ioctlEjlPmRNS_6system10error_codeE[boost::asio::de tail::socket_ops::ioctl(unsigned int, long, unsigned long*, boost::system::error _code&)]+0x26): undefined reference to `_ioctlsocket@12' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops11getaddrinfoEPKcS4_PK8addrinfoPPS5_RNS_6system10er ror_codeE[boost::asio::detail::socket_ops::getaddrinfo(char const*, char const*,  addrinfo const*, addrinfo**, boost::system::error_code&)]+0x2e): undefined refe rence to `_getaddrinfo@16' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops24translate_addrinfo_errorEi[boost::asio::detail::s ocket_ops::translate_addrinfo_error(int)]+0x228): undefined reference to `_WSAGe tLastError@0' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops4sendEjPK7_WSABUFjiRNS_6system10error_codeE[boost:: asio::detail::socket_ops::send(unsigned int, _WSABUF const*, unsigned int, int, boost::system::error_code&)]+0x57): undefined reference to `_WSASend@28' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops12freeaddrinfoEP8addrinfo[boost::asio::detail::sock et_ops::freeaddrinfo(addrinfo*)]+0xd): undefined reference to `_freeaddrinfo@4' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops15call_setsockoptIiEEiMNS2_6msghdrET_jiiPKvj[int bo ost::asio::detail::socket_ops::call_setsockopt<int>(int boost::asio::detail::soc ket_ops::msghdr::*, unsigned int, int, int, void const*, unsigned int)]+0x29): u ndefined reference to `_setsockopt@20' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail23win_iocp_socket_serviceINS0_2ip3tcpEE13async_receiveINS1_17co nsuming_buffersINS0_14mutable_bufferENS0_17mutable_buffers_1EEENS1_12read_handle rINS0_19basic_stream_socketIS4_NS0_21stream_socket_serviceIS4_EEEES9_NS1_14trans fer_all_tENS_3_bi6bind_tIvNS_4_mfi3mf1Iv11chat_clientRKNS_6system10error_codeEEE NSH_5list2INSH_5valueIPSL_EEPFNS_3argILi1EEEvEEEEEEEEEvRNS5_19implementation_typ eERKT_iT0_[void boost::asio::detail::win_iocp_socket_service<boost::asio::ip::tc p>::async_receive<boost::asio::detail::consuming_buffers<boost::asio::mutable_bu ffer, boost::asio::mutable_buffers_1>, boost::asio::detail::read_handler<boost:: asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_servi ce<boost::asio::ip::tcp> >, boost::asio::mutable_buffers_1, boost::asio::detail: :transfer_all_t, boost::_bi::bind_t<void, boost::_mfi::mf1<void, chat_client, bo ost::system::error_code const&>, boost::_bi::list2<boost::_bi::value<chat_client *>, boost::arg<1> (*)()> > > >(boost::asio::detail::win_iocp_socket_service<boos t::asio::ip::tcp>::implementation_type&, boost::asio::detail::consuming_buffers< boost::asio::mutable_buffer, boost::asio::mutable_buffers_1> const&, int, boost: :asio::detail::read_handler<boost::asio::basic_stream_socket<boost::asio::ip::tc p, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, boost::asio::muta ble_buffers_1, boost::asio::detail::transfer_all_t, boost::_bi::bind_t<void, boo st::_mfi::mf1<void, chat_client, boost::system::error_code const&>, boost::_bi:: list2<boost::_bi::value<chat_client*>, boost::arg<1> (*)()> > >)]+0x41a): undefi ned reference to `_WSARecv@28' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail23win_iocp_socket_serviceINS0_2ip3tcpEE13async_receiveINS1_17co nsuming_buffersINS0_14mutable_bufferENS0_17mutable_buffers_1EEENS1_12read_handle rINS0_19basic_stream_socketIS4_NS0_21stream_socket_serviceIS4_EEEES9_NS1_14trans fer_all_tENS_3_bi6bind_tIvNS_4_mfi3mf1Iv11chat_clientRKNS_6system10error_codeEEE NSH_5list2INSH_5valueIPSL_EEPFNS_3argILi1EEEvEEEEEEEEEvRNS5_19implementation_typ eERKT_iT0_[void boost::asio::detail::win_iocp_socket_service<boost::asio::ip::tc p>::async_receive<boost::asio::detail::consuming_buffers<boost::asio::mutable_bu ffer, boost::asio::mutable_buffers_1>, boost::asio::detail::read_handler<boost:: asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_servi ce<boost::asio::ip::tcp> >, boost::asio::mutable_buffers_1, boost::asio::detail: :transfer_all_t, boost::_bi::bind_t<void, boost::_mfi::mf1<void, chat_client, bo ost::system::error_code const&>, boost::_bi::list2<boost::_bi::value<chat_client *>, boost::arg<1> (*)()> > > >(boost::asio::detail::win_iocp_socket_service<boos t::asio::ip::tcp>::implementation_type&, boost::asio::detail::consuming_buffers< boost::asio::mutable_buffer, boost::asio::mutable_buffers_1> const&, int, boost: :asio::detail::read_handler<boost::asio::basic_stream_socket<boost::asio::ip::tc p, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, boost::asio::muta ble_buffers_1, boost::asio::detail::transfer_all_t, boost::_bi::bind_t<void, boo st::_mfi::mf1<void, chat_client, boost::system::error_code const&>, boost::_bi:: list2<boost::_bi::value<chat_client*>, boost::arg<1> (*)()> > >)]+0x428): undefi ned reference to `_WSAGetLastError@0' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail23win_iocp_socket_serviceINS0_2ip3tcpEE10async_sendINS1_17consu ming_buffersINS0_12const_bufferENS0_17mutable_buffers_1EEENS1_13write_handlerINS 0_19basic_stream_socketIS4_NS0_21stream_socket_serviceIS4_EEEES9_NS1_14transfer_ all_tENS_3_bi6bind_tIvNS_4_mfi3mf1Iv11chat_clientRKNS_6system10error_codeEEENSH_ 5list2INSH_5valueIPSL_EEPFNS_3argILi1EEEvEEEEEEEEEvRNS5_19implementation_typeERK T_iT0_[void boost::asio::detail::win_iocp_socket_service<boost::asio::ip::tcp>:: async_send<boost::asio::detail::consuming_buffers<boost::asio::const_buffer, boo st::asio::mutable_buffers_1>, boost::asio::detail::write_handler<boost::asio::ba sic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost ::asio::ip::tcp> >, boost::asio::mutable_buffers_1, boost::asio::detail::transfe r_all_t, boost::_bi::bind_t<void, boost::_mfi::mf1<void, chat_client, boost::sys tem::error_code const&>, boost::_bi::list2<boost::_bi::value<chat_client*>, boos t::arg<1> (*)()> > > >(boost::asio::detail::win_iocp_socket_service<boost::asio: :ip::tcp>::implementation_type&, boost::asio::detail::consuming_buffers<boost::a sio::const_buffer, boost::asio::mutable_buffers_1> const&, int, boost::asio::det ail::write_handler<boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost: :asio::stream_socket_service<boost::asio::ip::tcp> >, boost::asio::mutable_buffe rs_1, boost::asio::detail::transfer_all_t, boost::_bi::bind_t<void, boost::_mfi: :mf1<void, chat_client, boost::system::error_code const&>, boost::_bi::list2<boo st::_bi::value<chat_client*>, boost::arg<1> (*)()> > >)]+0x3f4): undefined refer ence to `_WSASend@28' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail23win_iocp_socket_serviceINS0_2ip3tcpEE10async_sendINS1_17consu ming_buffersINS0_12const_bufferENS0_17mutable_buffers_1EEENS1_13write_handlerINS 0_19basic_stream_socketIS4_NS0_21stream_socket_serviceIS4_EEEES9_NS1_14transfer_ all_tENS_3_bi6bind_tIvNS_4_mfi3mf1Iv11chat_clientRKNS_6system10error_codeEEENSH_ 5list2INSH_5valueIPSL_EEPFNS_3argILi1EEEvEEEEEEEEEvRNS5_19implementation_typeERK T_iT0_[void boost::asio::detail::win_iocp_socket_service<boost::asio::ip::tcp>:: async_send<boost::asio::detail::consuming_buffers<boost::asio::const_buffer, boo st::asio::mutable_buffers_1>, boost::asio::detail::write_handler<boost::asio::ba sic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost ::asio::ip::tcp> >, boost::asio::mutable_buffers_1, boost::asio::detail::transfe r_all_t, boost::_bi::bind_t<void, boost::_mfi::mf1<void, chat_client, boost::sys tem::error_code const&>, boost::_bi::list2<boost::_bi::value<chat_client*>, boos t::arg<1> (*)()> > > >(boost::asio::detail::win_iocp_socket_service<boost::asio: :ip::tcp>::implementation_type&, boost::asio::detail::consuming_buffers<boost::a sio::const_buffer, boost::asio::mutable_buffers_1> const&, int, boost::asio::det ail::write_handler<boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost: :asio::stream_socket_service<boost::asio::ip::tcp> >, boost::asio::mutable_buffe rs_1, boost::asio::detail::transfer_all_t, boost::_bi::bind_t<void, boost::_mfi: :mf1<void, chat_client, boost::system::error_code const&>, boost::_bi::list2<boo st::_bi::value<chat_client*>, boost::arg<1> (*)()> > >)]+0x402): undefined refer ence to `_WSAGetLastError@0' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail12winsock_initILi2ELi0EE7do_initD1Ev[boost::asio::detail::winso ck_init<2, 0>::do_init::~do_init()]+0x7): undefined reference to `_WSACleanup@0' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail25socket_select_interrupterC1Ev[boost::asio::detail::socket_sel ect_interrupter::socket_select_interrupter()]+0x17f): undefined reference to `_i net_addr@4' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops11call_acceptIiEEjMNS2_6msghdrET_jP8sockaddrPj[unsi gned int boost::asio::detail::socket_ops::call_accept<int>(int boost::asio::deta il::socket_ops::msghdr::*, unsigned int, sockaddr*, unsigned int*)]+0x4d): undef ined reference to `_accept@12' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops6listenEjiRNS_6system10error_codeE[boost::asio::det ail::socket_ops::listen(unsigned int, int, boost::system::error_code&)]+0x1f): u ndefined reference to `_listen@8' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops16call_getsocknameIiEEiMNS2_6msghdrET_jP8sockaddrPj [int boost::asio::detail::socket_ops::call_getsockname<int>(int boost::asio::det ail::socket_ops::msghdr::*, unsigned int, sockaddr*, unsigned int*)]+0x23): unde fined reference to `_getsockname@12' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops9call_bindIiEEiMNS2_6msghdrET_jPK8sockaddrj[int boo st::asio::detail::socket_ops::call_bind<int>(int boost::asio::detail::socket_ops ::msghdr::*, unsigned int, sockaddr const*, unsigned int)]+0x1b): undefined refe rence to `_bind@12' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops4recvEjP7_WSABUFjiRNS_6system10error_codeE[boost::a sio::detail::socket_ops::recv(unsigned int, _WSABUF*, unsigned int, int, boost:: system::error_code&)]+0x57): undefined reference to `_WSARecv@28' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZNK 5boost4asio6detail18win_fd_set_adapter6is_setEj[boost::asio::detail::win_fd_set_ adapter::is_set(unsigned int) const]+0x14): undefined reference to `___WSAFDIsSe t@8' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops6selectEiP6fd_setS4_S4_P7timevalRNS_6system10error_ codeE[boost::asio::detail::socket_ops::select(int, fd_set*, fd_set*, fd_set*, ti meval*, boost::system::error_code&)]+0xe4): undefined reference to `_select@20' /cygdrive/c/DOCUME~1/steven/LOCALS~1/Temp/ccKpZSDF.o:chat_client.cpp:(.text$_ZN5 boost4asio6detail10socket_ops15call_getsockoptIiEEiMNS2_6msghdrET_jiiPvPj[int bo ost::asio::detail::socket_ops::call_getsockopt<int>(int boost::asio::detail::soc ket_ops::msghdr::*, unsigned int, int, int, void*, unsigned int*)]+0x31): undefi ned reference to `_getsockopt@20' Info: resolving vtable for boost::detail::thread_data_baseby linking to __imp___ ZTVN5boost6detail16thread_data_baseE (auto-import) collect2: ld returned 1 exit status make: *** [chat_client] Error 1