select模型的应用

     for ( ;; )
    {
        fd_set readfds;
        fd_set writefds;

        FD_ZERO( &readfds );
        FD_ZERO( &writefds );

        FD_SET( sock, &readfds );
        int maxfd = sock;

        timeval to;
        to.tv_sec = 2;
        to.tv_usec = 0;

        int n = select( maxfd + 1, &readfds, &writefds, NULL, &to );

        if ( n > 0 )
        {
            if ( FD_ISSET( sock, &readfds ) )
   {
    int iread = recvfrom(sock, (char *)&usercount, sizeof(int), 0, (sockaddr *)&remote, &fromlen);
    if(iread<=0)
    {
     throw Exception("Login error/n");
    }

    break;
   }
        }
        else if ( n < 0 )
        {
   throw Exception("Login error/n");
        }

  sendto(sock, (const char*)&sendbuf, sizeof(sendbuf), 0, (const sockaddr*)&remote,sizeof(remote));
    }