WinSock I/O 模型 -- Select 模型

简介

Select 模型是 WinSock 中最常见的 I/O 模型，这篇文章我们就来看看如何使用 Select api 来实现一个简单的 TCP 服务器.

API 基础

Select 模型依赖 WinSock API Select 来检查当前 Socket 是否可写或者可读。

使用这个 API 的优点是我们不需要使用阻塞的 Socket API (recv, send) 来等待 Socket 状态准备就绪，我们可以异步的检查 Socket 的状态来进行读数据或者写数据.

Select 方法的声明如下：

int WSAAPI select(int           nfds,fd_set        *readfds,fd_set        *writefds,fd_set        *exceptfds,const timeval *timeout
);

其中：
nfds：直接忽略即可，该参数的设计是为了兼容 Berkeley Socket 的实现
redfds：返回值，当前可读的 socket 的集合
writefds：返回值，当前可写的 socket 的集合
exceptfds：返回值，当前发生错误的 socket 的集合
返回值：表示当前准备就绪的 socket 的数量。这里的准备就绪包含可读，可写，或者储出错的socket。如果返回 SOCKET_ERROR，表示发生错误，可以使用 WSAGetLastError 来获取具体的错误码。

fd_set

fd_set 是一个 socket 的集合，作为 select 方法的输入输出参数.

这里使用到的操作包括：

FD_ZERO : 重置 fd_set
FD_SET: 将 socket handle 添加到当前 fd_set
FD_ISSET: 检查某个 socket handle 是否处于当前 fd_set

实现思路

创建一个 socket 作为监听 socket，并将该 socket 设置为非阻塞模式.
使用 select api 来非阻塞的简单该监听socket 是否有新连接进来。如果有，则调用 accept 来接收该 client socket
对于已经与客户段建立的连接，同样的设置为非阻塞模式，使用 select api 来检查该 socket 上是否有数据可读，或者该 socket 是否可写，以便往客户端发送数据。还需要检查socket 是否出错，本文的例子里忽略这点，思路是一样的。
注意，这里所有的操作都是非阻塞的。

解析来我们通过一个例子看看如何使用 Select.

实例

本文的例子可以直接拷贝运行。读者如果不需要运行，直接注意加注释的代码段即可.

服务器实现

#include <WinSock2.h>
#include <Windows.h>
#include <stdio.h>#pragma comment(lib, "ws2_32")#define DEFALT_PORT 8080
#define DATA_BUFFER 8192typedef struct _SOCKET_CONTEXT {SOCKET     Socket;WSABUF     DataBuf;OVERLAPPED Overlapped;CHAR       Buffer[DATA_BUFFER];DWORD      BytesSEND;DWORD      BytesRECV;
} SOCKET_CONTEXT, * LPSOCKET_CONTEXT;BOOL CreateSocketContext(SOCKET s);
void FreeSocketContext(DWORD Index);DWORD TotalSockets = 0;
LPSOCKET_CONTEXT SocketArray[FD_SETSIZE];int main() {INT Ret;WSADATA wsaData;SOCKET ListenSocket;SOCKET AcceptSocket;SOCKADDR_IN Addr;ULONG NonBlock = 1;FD_SET ReadSet;FD_SET WriteSet;DWORD Total;DWORD Flags;DWORD RecvBytes;DWORD SentBytes;DWORD i;if ((Ret = WSAStartup(0x0202, &wsaData)) != 0) {printf("WSAStartup failed with error %d\n", Ret);WSACleanup();return 1;}if ((ListenSocket = WSASocketW(AF_INET, SOCK_STREAM, 0, NULL, 0, WSA_FLAG_OVERLAPPED)) == INVALID_SOCKET) {printf("WSASocket failed with error %d\n", WSAGetLastError());return 1;}Addr.sin_family = AF_INET;Addr.sin_addr.s_addr= htonl(INADDR_ANY);Addr.sin_port = htons(DEFALT_PORT);if (bind(ListenSocket, (PSOCKADDR) &Addr, sizeof(Addr)) == SOCKET_ERROR) {printf("bind failed with error %d\n", WSAGetLastError());return 1;}if (listen(ListenSocket, 10)) {printf("listen failed with eror %d\n", WSAGetLastError());return 1;}// 设置监听socket为异步模式if (ioctlsocket(ListenSocket, FIONBIO, &NonBlock) == SOCKET_ERROR) {printf("ioctlsocket failed with error %d\n", WSAGetLastError());return 1;}while (TRUE) {// 清空 ReadSet 和 WriteSet，我们将该集合中放入我们关心的 socket handleFD_ZERO(&ReadSet);FD_ZERO(&WriteSet);// 将监听socket 放入 ReadSet， 以便当有新连接到来的时候，我们可以检查到该事件FD_SET(ListenSocket, &ReadSet);// 我们同时也关心已经建立的客户段连接的可读可写状态，以便我们从客户端接收数据或者写数据// 这里一些小逻辑，直接忽略for (i = 0; i < TotalSockets; i++) {if (SocketArray[i]->BytesRECV > SocketArray[i]->BytesSEND) {FD_SET(SocketArray[i]->Socket, &WriteSet);} else {FD_SET(SocketArray[i]->Socket, &ReadSet);}}// 使用 select 检查当前 ReadSet 和 WriteSet 中的socket 是否有新的事件到来if ((Total = select(0, &ReadSet, &WriteSet, NULL, NULL)) == SOCKET_ERROR) {printf("select failed with error %d\n", WSAGetLastError());return 1;}// 使用 FD_ISSET 判断监听 socket 是否可以读，也就是说有新的连接到来// 如果有，调用 accept 来接收该新连接if (FD_ISSET(ListenSocket, &ReadSet)) {Total--;if ((AcceptSocket = accept(ListenSocket, NULL, NULL)) != INVALID_SOCKET) {NonBlock = 1;if (ioctlsocket(AcceptSocket, FIONBIO, &NonBlock) == SOCKET_ERROR) {printf("ioctlsocket failed with error %d\n", WSAGetLastError());return 1;}if (CreateSocketContext(AcceptSocket) == FALSE) {printf("CreateSocketContext failed");return 1;}} else {if (WSAGetLastError() != WSAEWOULDBLOCK) {printf("accept failed with error %d\n", WSAGetLastError());return 1;} else {printf("accept returns WSAEWOULDBLOCK\n");}}}// 接下来检查可读的客户段连接for (i = 0; Total > 0 && i < TotalSockets; i++) {LPSOCKET_CONTEXT Ctx = SocketArray[i];if (FD_ISSET(Ctx->Socket, &ReadSet)) {Total--;Ctx->DataBuf.buf = Ctx->Buffer;Ctx->DataBuf.len = DATA_BUFFER;//当前 socket 可读，那么调用 WSARecv 从该 socket 读取数据// 如果 WSARecv 返回 0， 是说该连接已经断开Flags = 0;if (WSARecv(Ctx->Socket, &(Ctx->DataBuf), 1, &RecvBytes, &Flags, NULL, NULL) == SOCKET_ERROR) {if (WSAGetLastError() != WSAEWOULDBLOCK) {printf("WSARecv failed with error %d\n", WSAGetLastError());FreeSocketContext(i);} else {printf("WSARecv returns WSAEWOULDBLOCK");}continue;} else {Ctx->BytesRECV = RecvBytes;// If zero bytes are received, this indicates the peer closed the connection.if (RecvBytes == 0) {FreeSocketContext(i);continue;} else {printf("Recv %d bytes data from the socket %d\n", RecvBytes, Ctx->Socket);}}}// 接下来检查可写的客户段连接if (FD_ISSET(Ctx->Socket, &WriteSet)) {Total--;Ctx->DataBuf.buf = Ctx->Buffer + Ctx->BytesSEND;Ctx->DataBuf.len = Ctx->BytesRECV - Ctx->BytesSEND;if (WSASend(Ctx->Socket, &(Ctx->DataBuf), 1, &SentBytes, 0, NULL, NULL) == SOCKET_ERROR) {if (WSAGetLastError() != WSAEWOULDBLOCK) {printf("WSASend failed with error %d\n", WSAGetLastError());FreeSocketContext(i);} else {printf("WSASend returns WSAEWOULDBLOCK");}continue;} else {Ctx->BytesSEND += SentBytes;if (Ctx->BytesSEND == Ctx->BytesRECV) {Ctx->BytesSEND = 0;Ctx->BytesRECV = 0;}}}}}
}BOOL CreateSocketContext(SOCKET s) {LPSOCKET_CONTEXT Ctx;printf("Accepted a new socket %d\n", s);if ((Ctx = (LPSOCKET_CONTEXT) GlobalAlloc(GPTR, sizeof(SOCKET_CONTEXT))) == NULL) {printf("GlobalAlloc() failed with error %d\n", GetLastError());return FALSE;}Ctx->Socket = s;Ctx->BytesSEND = 0;Ctx->BytesRECV = 0;SocketArray[TotalSockets] = Ctx;TotalSockets++;return TRUE;
}void FreeSocketContext(DWORD Index) {DWORD i;LPSOCKET_CONTEXT Ctx = SocketArray[Index];printf("Closing socket %d\n", Ctx->Socket);closesocket(Ctx->Socket);GlobalFree(Ctx);for (i = Index; i < TotalSockets; i++) {SocketArray[i] = SocketArray[i + 1];}TotalSockets--;
}

客户端实现

搭配该服务器，使用下面 client 实现进行测试。这里仅仅做测试用，忽略了大部分的错误检查.

#include <winsock2.h>
#include <stdio.h>
#include <stdlib.h>#define DEFAULT_COUNT       20
#define DEFAULT_PORT        8080
#define DEFAULT_BUFFER      2048
#define DEFAULT_MESSAGE     "\'A test message from client\'"#pragma warning(disable:4996)
#pragma comment(lib, "ws2_32")char szMessage[1024];
char szServer[128];int main(int argc, char **argv) {WSADATA       wsaData;SOCKET        ClientSocket;char          szBuffer[DEFAULT_BUFFER];int           ret, i;SOCKADDR_IN   ServerAddr;struct hostent    *host = NULL;WSAStartup(0x0202, &wsaData);strcpy_s(szMessage, sizeof(szMessage), DEFAULT_MESSAGE);strcpy_s(szServer, sizeof(szServer), "127.0.0.1");ClientSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);ServerAddr.sin_family = AF_INET;ServerAddr.sin_port = htons(DEFAULT_PORT);ServerAddr.sin_addr.s_addr = inet_addr(szServer);if (connect(ClientSocket, (struct sockaddr *) &ServerAddr, sizeof(ServerAddr)) == SOCKET_ERROR) {printf("connect failed with error %d\n", WSAGetLastError());return 1;}printf("Sending and receiving data if any...\n");for(i = 0; i < DEFAULT_COUNT; i++) {if ((ret = send(ClientSocket, szMessage, strlen(szMessage), 0)) == SOCKET_ERROR) {printf("send() failed with error %d\n", WSAGetLastError());break;}printf("send() is OK. Send %d bytes: %s\n", ret, szBuffer);if ((ret = recv(ClientSocket, szBuffer, DEFAULT_BUFFER, 0)) == SOCKET_ERROR) {printf("recv() failed with error %d\n", WSAGetLastError());break;}if (ret == 0) {printf("It is a graceful close!\n");break;}szBuffer[ret] = '\0';printf("recv() is OK. Received %d bytes: %s\n", ret, szBuffer);}if(closesocket(ClientSocket) == 0) {printf("closesocket() is OK!\n");} else {printf("closesocket() failed with error %d\n", WSAGetLastError());}WSACleanup();return 0;
}

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