簡(jiǎn)介

Select 模型是 WinSock 中最常見(jiàn)的 I/O 模型，這篇文章我們就來(lái)看看如何使用 Select api 來(lái)實(shí)現(xiàn)一個(gè)簡(jiǎn)單的 TCP 服務(wù)器.

API 基礎(chǔ)

Select 模型依賴 WinSock API Select 來(lái)檢查當(dāng)前 Socket 是否可寫或者可讀。

使用這個(gè) API 的優(yōu)點(diǎn)是我們不需要使用阻塞的 Socket API (recv, send) 來(lái)等待 Socket 狀態(tài)準(zhǔn)備就緒，我們可以異步的檢查 Socket 的狀態(tài)來(lái)進(jìn)行讀數(shù)據(jù)或者寫數(shù)據(jù).

Select 方法的聲明如下：

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，該參數(shù)的設(shè)計(jì)是為了兼容 Berkeley Socket 的實(shí)現(xiàn)
redfds： 返回值，當(dāng)前可讀的 socket 的集合
writefds： 返回值，當(dāng)前可寫的 socket 的集合
exceptfds：返回值，當(dāng)前發(fā)生錯(cuò)誤的 socket 的集合
返回值： 表示當(dāng)前準(zhǔn)備就緒的 socket 的數(shù)量。 這里的準(zhǔn)備就緒包含 可讀，可寫，或者儲(chǔ)出錯(cuò)的socket。如果返回 SOCKET_ERROR，表示發(fā)生錯(cuò)誤，可以使用 WSAGetLastError 來(lái)獲取具體的錯(cuò)誤碼。

fd_set

fd_set 是一個(gè) socket 的集合，作為 select 方法的輸入輸出參數(shù).

這里使用到的操作包括：

• FD_ZERO : 重置 fd_set
• FD_SET: 將 socket handle 添加到當(dāng)前 fd_set
• FD_ISSET: 檢查某個(gè) socket handle 是否處于當(dāng)前 fd_set

實(shí)現(xiàn)思路

創(chuàng)建一個(gè) socket 作為監(jiān)聽(tīng) socket，并將該 socket 設(shè)置為非阻塞模式.

使用 select api 來(lái)非阻塞的簡(jiǎn)單該監(jiān)聽(tīng)socket 是否有新連接進(jìn)來(lái)。如果有，則調(diào)用 accept 來(lái)接收該 client socket

對(duì)于已經(jīng)與客戶段建立的連接，同樣的設(shè)置為非阻塞模式，使用 select api 來(lái)檢查該 socket 上是否有數(shù)據(jù)可讀，或者該 socket 是否可寫，以便往客戶端發(fā)送數(shù)據(jù)。還需要檢查socket 是否出錯(cuò)，本文的例子里忽略這點(diǎn)，思路是一樣的。

注意，這里所有的操作都是非阻塞的。

解析來(lái)我們通過(guò)一個(gè)例子看看如何使用 Select.

實(shí)例

本文的例子可以直接拷貝運(yùn)行。 讀者如果不需要運(yùn)行，直接注意加注釋的代碼段即可.

服務(wù)器實(shí)現(xiàn)

#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;}// 設(shè)置監(jiān)聽(tīng)socket為異步模式if (ioctlsocket(ListenSocket, FIONBIO, &NonBlock) == SOCKET_ERROR) {printf("ioctlsocket failed with error %d\n", WSAGetLastError());return 1;}while (TRUE) {// 清空 ReadSet 和 WriteSet，我們將該集合中放入我們關(guān)心的 socket handleFD_ZERO(&ReadSet);FD_ZERO(&WriteSet);// 將監(jiān)聽(tīng)socket 放入 ReadSet， 以便當(dāng)有新連接到來(lái)的時(shí)候，我們可以檢查到該事件FD_SET(ListenSocket, &ReadSet);// 我們同時(shí)也關(guān)心已經(jīng)建立的客戶段連接的可讀可寫狀態(tài)，以便我們從客戶端接收數(shù)據(jù)或者寫數(shù)據(jù)// 這里一些小邏輯，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 檢查當(dāng)前 ReadSet 和 WriteSet 中的socket 是否有新的事件到來(lái)if ((Total = select(0, &ReadSet, &WriteSet, NULL, NULL)) == SOCKET_ERROR) {printf("select failed with error %d\n", WSAGetLastError());return 1;}// 使用 FD_ISSET 判斷監(jiān)聽(tīng) socket 是否可以讀，也就是說(shuō)有新的連接到來(lái)// 如果有，調(diào)用 accept 來(lái)接收該新連接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");}}}// 接下來(lái)檢查可讀的客戶段連接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;//當(dāng)前 socket 可讀，那么調(diào)用 WSARecv 從該 socket 讀取數(shù)據(jù)// 如果 WSARecv 返回 0， 是說(shuō)該連接已經(jīng)斷開(kāi)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);}}}// 接下來(lái)檢查可寫的客戶段連接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--; }

客戶端實(shí)現(xiàn)

搭配該服務(wù)器，使用下面 client 實(shí)現(xiàn)進(jìn)行測(cè)試。 這里僅僅做測(cè)試用，忽略了大部分的錯(cuò)誤檢查.

#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; }

END！！！

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