Winsock Programming Blocking and Asynchronous Sockets for Windows

History MS-DOS/Windows not designed for networking Early Microsoft operating systems ignored the TCP/IP stack “Visionary” Bill Gates supports NETBIOS over TCP/IP Several packages offered by 3 rd parties “Trumpet Winsock” None work particularly well

What is Winsock? API, SPI and ABI –API for application developers –SPI for network software vendors –ABI to ensure consistent interoperability between applications and various implementations of Winsock Less important now that Microsoft has released a quality Winsock –No significant alternatives

The API Based on BSD sockets API –First of many Microsoft “hacks” Complications due to differences between Unix and Windows –errno (Unix) vs. WSAGetLastError() –BSD sockets based on C and Unix fork() File descriptors

Example: The Unix Way while (1) { newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen); if (newsockfd < 0) error("ERROR on accept"); pid = fork(); if (pid < 0) error("ERROR on fork"); if (pid == 0) { close(sockfd); dostuff(newsockfd); exit(0); } else close(newsockfd); }

Steps to a Listen Socket Initialize Winsock Fill out SOCKADDR_IN (define the socket) Create the socket bind() the socket Make the socket listen() Wait on accept() Handle clients via socket returned by accept()

Example: Listening (simplified) // INITIALIZE sockVersion = MAKEWORD(1, 1); // Winsock version 1.1 WSAStartup(sockVersion, &wsaData); // CREATE SOCKET SOCKET listeningSocket; listeningSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); // DEFINE SOCKET SOCKADDR_IN serverInfo; serverInfo.sin_family = AF_INET; serverInfo.sin_addr.s_addr = INADDR_ANY; serverInfo.sin_port = htons(8888); // BIND bind(listeningSocket, (LPSOCKADDR)&serverInfo, sizeof(struct sockaddr)); // LISTEN listen(listeningSocket, 10); // ACCEPT SOCKET theClient; theClient = accept(listeningSocket, NULL, NULL);

Steps to Connecting Initialize Winsock Fill out HOSTENT Create socket Fill out SOCKADDR_IN Connect Send/recv

Example: Connecting (simplified) // INITIALIZE sockVersion = MAKEWORD(1, 1); WSAStartup(sockVersion, &wsaData); // FILL OUT HOSTENT LPHOSTENT hostEntry = gethostbyname(" // CREATE SOCKET SOCKET theSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); // FILL OUT SOCKADDR_IN SOCKADDR_IN serverInfo; serverInfo.sin_family = AF_INET; serverInfo.sin_addr = *((LPIN_ADDR)*hostEntry->h_addr_list); serverInfo.sin_port = htons(80); // CONNECT connect(theSocket, (LPSOCKADDR)&serverInfo, sizeof(struct sockaddr));

Sending and Receiving int send(SOCKET s, char * buf, int len, int flags); –Returns number of bytes sent or SOCKET_ERROR int recv(SOCKET s, char * buf, int len, int flags); –Returns number of bytes received or SOCKET_ERROR Send and recv must be done in loops to ensure all data is sent/received

Problem: blocking Many of these functions “block” –accept() –connect() –send() –recv() One solution: poll sockets using select() Another solution: threads Coolest solution: asynchronous sockets

Why Asynchronous? Windows handles polling for you Familiar Windows message paradigm Easy to read code

Why NOT Asynchronous? Non-portable Microsoft is evil Use them anyways, they are cool

Windows Messages Message queue Message pump –“first chance” message handling Message Handler –Callback function

Sample Message Pump HRESULT hRet; UINT Msg; while( (hRet = GetMessage( &Msg, NULL, 0, 0 )) != 0) { if (hRet == -1) { PostQuitMessage(1); } else { TranslateMessage(&Msg); DispatchMessage(&Msg); } }

Sample Message Handler LRESULT CALLBACK WinProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam) { switch(uMsg) { case WM_DESTROY: PostQuitMessage(WM_QUIT); break; default: return FALSE; } return TRUE; }

How Does This Apply? Windows sends messages when sockets are waiting –No need to poll –No need for many threads –Create sockets and go about other tasks –Handle sockets just like any other Windows control

Making a socket asynchronous int PASCAL WSAAsyncSelect(SOCKET s, HWND hwnd, unsigned int Msg, long event ); Simple call to WSAAsyncSelect:

Message and Events Five events: –FD_READ –FD_WRITE –FD_CONNECT –FD_ACCEPT –FD_CLOSE –OR these together to tell Windows what notifications you need User defined messages –WM_USER + n

Handling the message wParam: –socket instance lParam: –HIWORD: 0 (success) Error code –LOWORD: FD_READ, FD_WRITE, etc.

Example: Handling FD_ACCEPT SOCKET s; switch (msg) { case WM_USER + 5: switch(WSAGETSELECTEVENT(lParam) { case FD_ACCEPT: s = accept(wParam, NULL, NULL); break; } break; }

Summary Why this is important: –Current Winsock responsible for increase in quantity and popularity of Internet applications –Almost all applications use the Internet somehow, or could be improved by using the Internet

Example: Version Checking DWORD WINAPI CGlowdotChatServer::DoVersionCheckThreadProc(LPVOID pvoid) { struct sockaddr_in sa; struct hostent *hp; hp = gethostbyname(“ memset(&sa,0,sizeof(sa)); // set address and port memcpy((char *)&sa.sin_addr, hp->h_addr, hp->h_length); sa.sin_family = hp->h_addrtype; sa.sin_port = htons((u_short)VERSION_CHECK_PORT); // create socket SOCKET s = socket(hp->h_addrtype, SOCK_STREAM, 0); std::string request ("GET /glowdotchat/server/checkversion.php?getcurrentversion=yes HTTP/1.1\nUser- Agent:Mozilla\nHost: send(s, request.c_str(), request.length(),0); char buf[1000]; std::string response(""); recv(s, buf, 1000, 0); response += std::string(buf); std::string::size_type pos1 = response.find(std::string(" ")); std::string::size_type pos2 = response.find(std::string(" ")); response = response.substr(pos1 + 17, pos2 - pos1 - 17); if (response == std::string(VERSION)) { // server up to date } else { // new version available } return 0; }