刚接触c/s架构，新手求教~TX

Question

int bytesSent, left, idx=0;
int bytesRecv = SOCKET_ERROR;
char sendbuf[DEFAULT_BUFFER] = "";
char recvbuf[DEFAULT_BUFFER] = "";

while(1)
{
    printf("[The message you want to send to the server]:");
    fgets(sendbuf, DEFAULT_BUFFER, stdin);
    left = strlen(sendbuf);
    while(left > 0)
    {
        bytesSent = send( m_socket, &sendbuf[idx], left, 0 );

//printf("%s was sent\n", sendbuf);
if(bytesSent == 0)
return ;
if(bytesSent == SOCKET_ERROR)
{
printf("send failed:%d", WSAGetLastError());
return ;
}
left -= bytesSent;
idx += bytesSent;

    }

    idx = 0;

    while( bytesRecv == SOCKET_ERROR ) {
        bytesRecv = recv( m_socket, recvbuf, DEFAULT_BUFFER, 0 );

//printf("%s is received\n", recvbuf);
if ( bytesRecv == 0 || bytesRecv == WSAECONNRESET ) {
printf( "Connection Closed.\n");
return;
}
if (bytesRecv < 0)
return;
printf( "[The returned info from server]: %s\n", recvbuf);
}
bytesRecv = SOCKET_ERROR;
memset(sendbuf, 0, DEFAULT_BUFFER);
memset(recvbuf,0, DEFAULT_BUFFER);

}

发送数据的地方使用了一个while(left > 0)的循环，但是我想知道当left>0成立的时候，服务器端有消息发送过来，那么客户端是怎么运作的~谢谢