io复用select理解

USHORT nPort = 4567; //此服务器监听端口号
//创建监听套接字
SOCKET sListen = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
  
sockaddr_in sin;
sin.sin_family = AF_INET;
sin.sin_port = htons(nPort);
sin.sin_addr.S_un.S_addr = INADDR_ANY;
//绑定套接字到本地机器
if(bind(sListen,(sockaddr*)&sin,sizeof(sin))==SOCKET_ERROR)
{
   printf("Failed bind()\n");
   return -1;
}
//进入监听模式
listen(sListen,5);
  
//select模型处理过程
//1)初始化一个套接字集合fdSocket，添加监听套接字句柄到这个集合
fd_set fdSocket; //所有可用的套接字集合
FD_ZERO(&fdSocket);
FD_SET(sListen,&fdSocket);
while(TRUE)
{
   //2)将fdSocket集合的一个拷贝fdRead传递给select函数
   //当有事件发生时，select函数将移除fdRead集合中没有未决I/O操作的套接字句柄，然后返回。
   fd_set fdRead = fdSocket;
   int nRet = select(0,&fdRead,NULL,NULL,NULL);
   if(nRet>0)
   {
    //3)通过将原来的fdSocket集合与select处理过的fdRead集合比较
    //确定都有哪些套接字有未决I/O，并进一步处理
    for(int i=0; i<(int)fdSocket.fd_count; i++)
    {
     if(FD_ISSET(fdSocket.fd_array[i],&fdRead))
     {
      if(fdSocket.fd_array[i]==sListen) //(1)监听套接字接收到新连接
      {
       if(fdSocket.fd_count<FD_SETSIZE)
       {
        sockaddr_in addrRemote;
        int nAddrLen = sizeof(addrRemote);
        SOCKET sNew = accept(sListen,(sockaddr*)&addrRemote,&nAddrLen);
        FD_SET(sNew,&fdSocket);
  
        printf("接收到连接(%s)\n",inet_ntoa(addrRemote.sin_addr));
       }
       else
       {
        printf("Too much connections!\n");
        continue;
       }
      }
      else
      {
       char szText[256];
       int nRecv = recv(fdSocket.fd_array[i],szText,strlen(szText),0);
       if(nRecv>0)     //(2) 可读
       {
        szText[nRecv] = '\0';
        printf("接收到数据：%s\n",szText);
        char str[] = "One World,One Dream!\r\n";
        send(fdSocket.fd_array[i],str,strlen(str),0);
       }
       else      //(3) 连接关闭、重启或者中断
       {
        closesocket(fdSocket.fd_array[i]);
        FD_CLR(fdSocket.fd_array[i],&fdSocket);
       }
      }
     }
    }
   }
   else
   {
    printf("Failed select()\n");
    break;
   }
}
  
return 0;
}