Windows Socket常见异步IO模型——代码攻略

Windows Socket常见异步IO模型——代码攻略

  如果你想在Windows平台上构建服务器应用，那么I/O模型是你必须考虑的。Windows操作系统提供了选择（Select）、异步选择（WSAAsyncSelect）、事件选择（WSAEventSelect）、重叠I/O（Overlapped I/O）和完成端口（Completion Port)共五种I/O模型。每一种模型均适用于一种特定的应用场景。程序员应该对自己的应用需求非常明确，而且综合考虑到程序的扩展性和可移植性等因素，作出自己的选择。 

        我会以一个回应反射式服务器（与《Windows网络编程》第八章一样）来介绍这五种I/O模型。
        我们假设客户端的代码如下（为代码直观，省去所有错误检查，以下同）：

        #include <WINSOCK2.H>
        #include <stdio.h>

        #define SERVER_ADDRESS "137.117.2.148"
        #define PORT           5150
        #define MSGSIZE        1024

        #pragma comment(lib, "ws2_32.lib")

        int main()
        {
                WSADATA     wsaData;
                SOCKET      sClient;
                SOCKADDR_IN server;
                char        szMessage[MSGSIZE];
                int         ret;
  
                // Initialize Windows socket library
                WSAStartup(0x0202, &wsaData);

                // Create client socket
                sClient = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

                // Connect to server
                memset(&server, 0, sizeof(SOCKADDR_IN));
                server.sin_family = AF_INET;
                server.sin_addr.S_un.S_addr = inet_addr(SERVER_ADDRESS);
                server.sin_port = htons(PORT);

                connect(sClient, (struct sockaddr *)&server, sizeof(SOCKADDR_IN));

                while (TRUE)
                {
                        printf("Send:");
                        gets(szMessage);

                        // Send message
                        send(sClient, szMessage, strlen(szMessage), 0);

                        // Receive message
                        ret = recv(sClient, szMessage, MSGSIZE, 0);
                        szMessage[ret] = '\0';

                        printf("Received [%d bytes]: '%s'\n", ret, szMessage);
                }

                // Clean up
                closesocket(sClient);
                WSACleanup();
                return 0;
        }

        客户端所做的事情相当简单，创建套接字，连接服务器，然后不停的发送和接收数据。

        比较容易想到的一种服务器模型就是采用一个主线程，负责监听客户端的连接请求，当接收到某个客户端的连接请求后，创建一个专门用于和该客户端通信的套接字和一个辅助线程。以后该客户端和服务器的交互都在这个辅助线程内完成。这种方法比较直观，程序非常简单而且可移植性好，但是不能利用平台相关的特性。例如，如果连接数增多的时候（成千上万的连接），那么线程数成倍增长，操作系统忙于频繁的线程间切换，而且大部分线程在其生命周期内都是处于非活动状态的，这大大浪费了系统的资源。所以，如果你已经知道你的代码只会运行在Windows平台上，建议采用Winsock I/O模型。

        一.选择模型
        Select（选择）模型是Winsock中最常见的I/O模型。之所以称其为“Select模型”，是由于它的“中心思想”便是利用select函数，实现对I/O的管理。最初设计该模型时，主要面向的是某些使用UNIX操作系统的计算机，它们采用的是Berkeley套接字方案。Select模型已集成到Winsock 1.1中，它使那些想避免在套接字调用过程中被无辜“锁定”的应用程序，采取一种有序的方式，同时进行对多个套接字的管理。由于Winsock 1.1向后兼容于Berkeley套接字实施方案，所以假如有一个Berkeley套接字应用使用了select函数，那么从理论角度讲，毋需对其进行任何修改，便可正常运行。（节选自《Windows网络编程》第八章)
下面的这段程序就是利用选择模型实现的Echo服务器的代码（已经不能再精简了）：

        #include <winsock.h>
        #include <stdio.h>

        #define PORT       5150
        #define MSGSIZE    1024

        #pragma comment(lib, "ws2_32.lib")

        int    g_iTotalConn = 0;
        SOCKET g_CliSocketArr[FD_SETSIZE];

        DWORD WINAPI WorkerThread(LPVOID lpParameter);

        int main()
        {
                WSADATA     wsaData;
                SOCKET      sListen, sClient;
                SOCKADDR_IN local, client;
                int         iaddrSize = sizeof(SOCKADDR_IN);
                DWORD       dwThreadId;

                // Initialize Windows socket library
                WSAStartup(0x0202, &wsaData);

                // Create listening socket
                sListen = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

                // Bind
                local.sin_addr.S_un.S_addr = htonl(INADDR_ANY);
                local.sin_family = AF_INET;
                local.sin_port = htons(PORT);
                bind(sListen, (struct sockaddr *)&local, sizeof(SOCKADDR_IN));

                // Listen
                listen(sListen, 3);

                // Create worker thread
                CreateThread(NULL, 0, WorkerThread, NULL, 0, &dwThreadId); 

                while (TRUE)
                {
                        // Accept a connection
                        sClient = accept(sListen, (struct sockaddr *)&client, &iaddrSize);
                        printf("Accepted client:%s:%d\n", inet_ntoa(client.sin_addr), ntohs(client.sin_port));

                        // Add socket to g_CliSocketArr
                        g_CliSocketArr[g_iTotalConn++] = sClient;
                }
  
                return 0;
        }

        DWORD WINAPI WorkerThread(LPVOID lpParam)
        {
                int            i;
                fd_set         fdread;
                int            ret;
                struct timeval tv = {1, 0};
                char           szMessage[MSGSIZE];
  
                while (TRUE)
                {
                        FD_ZERO(&fdread);
                        for (i = 0; i < g_iTotalConn; i++)
                        {
                                FD_SET(g_CliSocketArr[i], &fdread);
                        }

                        // We only care read event
                        ret = select(0, &fdread, NULL, NULL, &tv);

                        if (ret == 0)
                        {
                                // Time expired
                                continue;
                        }

                        for (i = 0; i < g_iTotalConn; i++)
                        {
                                if (FD_ISSET(g_CliSocketArr[i], &fdread))
                                {
                                        // A read event happened on g_CliSocketArr[i]
                                        ret = recv(g_CliSocketArr[i], szMessage, MSGSIZE, 0);
                                        if (ret == 0 || (ret == SOCKET_ERROR && WSAGetLastError() == WSAECONNRESET))
                                        {
                                                // Client socket closed
                                                printf("Client socket %d closed.\n", g_CliSocketArr[i]);
                                                closesocket(g_CliSocketArr[i]);
                                                if (i < g_iTotalConn - 1)
                                                {            
                                                        g_CliSocketArr[i--] = g_CliSocketArr[--g_iTotalConn];
                                                }
                                        }
                                        else
                                        {
                                                // We received a message from client
                                                szMessage[ret] = '\0';
                                                send(g_CliSocketArr[i], szMessage, strlen(szMessage), 0);
                                        }
                                }
                        }
                }
  
                return 0;
        }

        服务器的几个主要动作如下：
        1.创建监听套接字，绑定，监听；
        2.创建工作者线程；
        3.创建一个套接字数组，用来存放当前所有活动的客户端套接字，每accept一个连接就更新一次数组；
        4.接受客户端的连接。这里有一点需要注意的，就是我没有重新定义FD_SETSIZE宏，所以服务器最多支持的并发连接数为64。而且，这里决不能无条件的accept,服务器应该根据当前的连接数来决定是否接受来自某个客户端的连接。一种比较好的实现方案就是采用WSAAccept函数，而且让WSAAccept回调自己实现的Condition Function。如下所示：

        int CALLBACK ConditionFunc(LPWSABUF lpCallerId,LPWSABUF lpCallerData, LPQOS lpSQOS,LPQOS lpGQOS,LPWSABUF lpCalleeId, LPWSABUF lpCalleeData,GROUP FAR * g,DWORD dwCallbackData)
        {
                if (当前连接数 < FD_SETSIZE)
                        return CF_ACCEPT;
                else
                        return CF_REJECT;
        }

        工作者线程里面是一个死循环，一次循环完成的动作是：
        1.将当前所有的客户端套接字加入到读集fdread中；
        2.调用select函数；
        3.查看某个套接字是否仍然处于读集中，如果是，则接收数据。如果接收的数据长度为0，或者发生WSAECONNRESET错误，则表示客户端套接字主动关闭，这时需要将服务器中对应的套接字所绑定的资源释放掉，然后调整我们的套接字数组（将数组中最后一个套接字挪到当前的位置上）

        除了需要有条件接受客户端的连接外，还需要在连接数为0的情形下做特殊处理，因为如果读集中没有任何套接字，select函数会立刻返回，这将导致工作者线程成为一个毫无停顿的死循环，CPU的占用率马上达到100%。

        二.异步选择
        Winsock提供了一个有用的异步I/O模型。利用这个模型，应用程序可在一个套接字上，接收以Windows消息为基础的网络事件通知。具体的做法是在建好一个套接字后，调用WSAAsyncSelect函数。该模型最早出现于Winsock的1.1版本中，用于帮助应用程序开发者面向一些早期的16位Windows平台（如Windows for Workgroups），适应其“落后”的多任务消息环境。应用程序仍可从这种模型中得到好处，特别是它们用一个标准的Windows例程（常称为"WndProc"），对窗口消息进行管理的时候。该模型亦得到了Microsoft Foundation Class（微软基本类，MFC）对象CSocket的采纳。（节选自《Windows网络编程》第八章)
        我还是先贴出代码，然后做详细解释：
        #include <winsock.h>
        #include <tchar.h>

        #define PORT      5150
        #define MSGSIZE   1024
        #define WM_SOCKET WM_USER+0

        #pragma comment(lib, "ws2_32.lib")

        LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

        int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int iCmdShow)
        {
                static TCHAR szAppName[] = _T("AsyncSelect Model");
                HWND         hwnd ;
                MSG          msg ;
                WNDCLASS     wndclass ;

                wndclass.style         = CS_HREDRAW | CS_VREDRAW ;
                wndclass.lpfnWndProc   = WndProc ;
                wndclass.cbClsExtra    = 0 ;
                wndclass.cbWndExtra    = 0 ;
                wndclass.hInstance     = hInstance ;
                wndclass.hIcon         = LoadIcon (NULL, IDI_APPLICATION) ;
                wndclass.hCursor       = LoadCursor (NULL, IDC_ARROW) ;
                wndclass.hbrBackground = (HBRUSH) GetStockObject (WHITE_BRUSH) ;
                wndclass.lpszMenuName  = NULL ;
                wndclass.lpszClassName = szAppName ;

                if (!RegisterClass(&wndclass))
                {
                        MessageBox (NULL, TEXT ("This program requires Windows NT!"), szAppName, MB_ICONERROR) ;
                        return 0 ;
                }

                hwnd = CreateWindow (szAppName,                  // window class name
                        TEXT ("AsyncSelect Model"), // window caption
                        WS_OVERLAPPEDWINDOW,        // window style
                        CW_USEDEFAULT,              // initial x position
                        CW_USEDEFAULT,              // initial y position
                        CW_USEDEFAULT,              // initial x size
                        CW_USEDEFAULT,              // initial y size
                        NULL,                       // parent window handle
                        NULL,                       // window menu handle
                        hInstance,                  // program instance handle
                        NULL) ;                     // creation parameters

                ShowWindow(hwnd, iCmdShow);
                UpdateWindow(hwnd);

                while (GetMessage(&msg, NULL, 0, 0))
                {
                        TranslateMessage(&msg) ;
                        DispatchMessage(&msg) ;
                }
  
                return msg.wParam;
        }

        LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
        {
                WSADATA       wsd;
                static SOCKET sListen;
                SOCKET        sClient;
                SOCKADDR_IN   local, client;
                int           ret, iAddrSize = sizeof(client);
                char          szMessage[MSGSIZE];

                switch (message)
                {
                         case WM_CREATE:
                                // Initialize Windows Socket library
                                WSAStartup(0x0202, &wsd);
  
                                // Create listening socket
                                sListen = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    
                                // Bind
                                local.sin_addr.S_un.S_addr = htonl(INADDR_ANY);
                                local.sin_family = AF_INET;
                                local.sin_port = htons(PORT);
                                bind(sListen, (struct sockaddr *)&local, sizeof(local));
  
                                // Listen
                                listen(sListen, 3);

                                // Associate listening socket with FD_ACCEPT event
                                WSAAsyncSelect(sListen, hwnd, WM_SOCKET, FD_ACCEPT);
                                return 0;

                        case WM_DESTROY:
                                closesocket(sListen);
                                WSACleanup();
                                PostQuitMessage(0);
                                return 0;
  
                        case WM_SOCKET:
                                if (WSAGETSELECTERROR(lParam))
                                {
                                        closesocket(wParam);
                                        break;
                                }
    
                                switch (WSAGETSELECTEVENT(lParam))
                                {
                                        case FD_ACCEPT:
                                                // Accept a connection from client
                                                sClient = accept(wParam, (struct sockaddr *)&client, &iAddrSize);
      
                                                // Associate client socket with FD_READ and FD_CLOSE event
                                                WSAAsyncSelect(sClient, hwnd, WM_SOCKET, FD_READ | FD_CLOSE);
                                                break;

                                        case FD_READ:
                                                ret = recv(wParam, szMessage, MSGSIZE, 0);

                                                if (ret == 0 || ret == SOCKET_ERROR && WSAGetLastError() == WSAECONNRESET)
                                                {
                                                        closesocket(wParam);
                                                }
                                                else
                                                {
                                                        szMessage[ret] = '\0';
                                                        send(wParam, szMessage, strlen(szMessage), 0);
                                                }
                                                break;
      
                                        case FD_CLOSE:
                                                closesocket(wParam);      
                                                break;
                                }
                                return 0;
                }
  
                return DefWindowProc(hwnd, message, wParam, lParam);
        }

        在我看来，WSAAsyncSelect是最简单的一种Winsock I/O模型（之所以说它简单是因为一个主线程就搞定了）。使用Raw Windows API写过窗口类应用程序的人应该都能看得懂。这里，我们需要做的仅仅是：
        1.在WM_CREATE消息处理函数中，初始化Windows Socket library，创建监听套接字，绑定，监听，并且调用WSAAsyncSelect函数表示我们关心在监听套接字上发生的FD_ACCEPT事件；
        2.自定义一个消息WM_SOCKET，一旦在我们所关心的套接字（监听套接字和客户端套接字）上发生了某个事件，系统就会调用WndProc并且message参数被设置为WM_SOCKET；
        3.在WM_SOCKET的消息处理函数中，分别对FD_ACCEPT、FD_READ和FD_CLOSE事件进行处理；
        4.在窗口销毁消息(WM_DESTROY)的处理函数中，我们关闭监听套接字，清除Windows Socket library

        下面这张用于WSAAsyncSelect函数的网络事件类型表可以让你对各个网络事件有更清楚的认识：
        表1

        FD_READ         应用程序想要接收有关是否可读的通知，以便读入数据 
        FD_WRITE         应用程序想要接收有关是否可写的通知，以便写入数据 
        FD_OOB         应用程序想接收是否有带外（OOB）数据抵达的通知 
        FD_ACCEPT         应用程序想接收与进入连接有关的通知 
        FD_CONNECT         应用程序想接收与一次连接或者多点join操作完成的通知 
        FD_CLOSE         应用程序想接收与套接字关闭有关的通知 
        FD_QOS         应用程序想接收套接字“服务质量”（QoS）发生更改的通知 
        FD_GROUP_QOS          应用程序想接收套接字组“服务质量”发生更改的通知（现在没什么用处，为未来套接字组的使用保留） 
        FD_ROUTING_INTERFACE_CHANGE         应用程序想接收在指定的方向上，与路由接口发生变化的通知 
        FD_ADDRESS_LIST_CHANGE          应用程序想接收针对套接字的协议家族，本地地址列表发生变化的通知

        三.事件选择
        Winsock提供了另一个有用的异步I/O模型。和WSAAsyncSelect模型类似的是，它也允许应用程序在一个或多个套接字上，接收以事件为基础的网络事件通知。对于表1总结的、由WSAAsyncSelect模型采用的网络事件来说，它们均可原封不动地移植到新模型。在用新模型开发的应用程序中，也能接收和处理所有那些事件。该模型最主要的差别在于网络事件会投递至一个事件对象句柄，而非投递至一个窗口例程。（节选自《Windows网络编程》第八章)
        还是让我们先看代码然后进行分析：
        #include <winsock2.h>
        #include <stdio.h>

        #define PORT    5150
        #define MSGSIZE 1024

        #pragma comment(lib, "ws2_32.lib")

        int      g_iTotalConn = 0;
        SOCKET   g_CliSocketArr[MAXIMUM_WAIT_OBJECTS];
        WSAEVENT g_CliEventArr[MAXIMUM_WAIT_OBJECTS];

        DWORD WINAPI WorkerThread(LPVOID);
        void Cleanup(int index);

        int main()
        {
                WSADATA     wsaData;
                SOCKET      sListen, sClient;
                SOCKADDR_IN local, client;
                DWORD       dwThreadId;
                int         iaddrSize = sizeof(SOCKADDR_IN);

                // Initialize Windows Socket library
                WSAStartup(0x0202, &wsaData);

                // Create listening socket
                sListen = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

                // Bind
                local.sin_addr.S_un.S_addr = htonl(INADDR_ANY);
                local.sin_family = AF_INET;
                local.sin_port = htons(PORT);
                bind(sListen, (struct sockaddr *)&local, sizeof(SOCKADDR_IN));

                // Listen
                listen(sListen, 3);

                // Create worker thread
                CreateThread(NULL, 0, WorkerThread, NULL, 0, &dwThreadId);

                while (TRUE)
                {
                        // Accept a connection
                        sClient = accept(sListen, (struct sockaddr *)&client, &iaddrSize);
                        printf("Accepted client:%s:%d\n", inet_ntoa(client.sin_addr), ntohs(client.sin_port));

                        // Associate socket with network event
                        g_CliSocketArr[g_iTotalConn] = sClient;
                        g_CliEventArr[g_iTotalConn] = WSACreateEvent();
                        WSAEventSelect(g_CliSocketArr[g_iTotalConn], g_CliEventArr[g_iTotalConn], FD_READ | FD_CLOSE);
                        g_iTotalConn++;
                }
        }

        DWORD WINAPI WorkerThread(LPVOID lpParam)
        {
                int              ret, index;
                WSANETWORKEVENTS NetworkEvents;
                char             szMessage[MSGSIZE];

                while (TRUE)
                {
                        ret = WSAWaitForMultipleEvents(g_iTotalConn, g_CliEventArr, FALSE, 1000, FALSE);
                        if (ret == WSA_WAIT_FAILED || ret == WSA_WAIT_TIMEOUT)
                        {
                                continue;
                        }

                        index = ret - WSA_WAIT_EVENT_0;
                        WSAEnumNetworkEvents(g_CliSocketArr[index], g_CliEventArr[index], &NetworkEvents);

                        if (NetworkEvents.lNetworkEvents & FD_READ)
                        {
                                // Receive message from client
                                ret = recv(g_CliSocketArr[index], szMessage, MSGSIZE, 0);
                                if (ret == 0 || (ret == SOCKET_ERROR && WSAGetLastError() == WSAECONNRESET))
                                {
                                        Cleanup(index);
                                }
                                else
                                {
                                        szMessage[ret] = '\0';
                                        send(g_CliSocketArr[index], szMessage, strlen(szMessage), 0);
                                }
                        }

                        if (NetworkEvents.lNetworkEvents & FD_CLOSE)
                        {
                                Cleanup(index);
                        }
                }
                return 0;
        }

        void Cleanup(int index)
        {
                closesocket(g_CliSocketArr[index]);
                WSACloseEvent(g_CliEventArr[index]);

                if (index < g_iTotalConn - 1)
                {
                        g_CliSocketArr[index] = g_CliSocketArr[g_iTotalConn - 1];
                        g_CliEventArr[index] = g_CliEventArr[g_iTotalConn - 1];
                }
  
                g_iTotalConn--;
        }

        事件选择模型也比较简单，实现起来也不是太复杂，它的基本思想是将每个套接字都和一个WSAEVENT对象对应起来，并且在关联的时候指定需要关注的哪些网络事件。一旦在某个套接字上发生了我们关注的事件（FD_READ和FD_CLOSE），与之相关联的WSAEVENT对象被Signaled。程序定义了两个全局数组，一个套接字数组，一个WSAEVENT对象数组，其大小都是MAXIMUM_WAIT_OBJECTS（64），两个数组中的元素一一对应。
        同样的，这里的程序没有考虑两个问题，一是不能无条件的调用accept，因为我们支持的并发连接数有限。解决方法是将套接字按MAXIMUM_WAIT_OBJECTS分组，每MAXIMUM_WAIT_OBJECTS个套接字一组，每一组分配一个工作者线程；或者采用WSAAccept代替accept，并回调自己定义的Condition Function。第二个问题是没有对连接数为0的情形做特殊处理，程序在连接数为0的时候CPU占用率为100%。