c++ - 如何在 TCP 监听器中处理异步发送和接收

Question

我是套接字编程新手。我正在尝试制作一个可以处理多个连接的 TCP 监听器。

我找到了this看起来非常有用的例子。

此代码的问题在于，它仅在收到数据时才将数据发送到连接的客户端。我想异步发送数据到连接的客户端。

我看到 select() 函数永远阻塞代码，直到事件到达套接字。

我正在考虑在 select() 函数中添加一个延迟(而不是 NULL)，以便它每隔几微秒超时一次，并且程序将能够发送数据，如果有的话。在 if (buffer[0] > 0)

行

问题是:有没有更好的方法来做我想做的事？我可以用其他方式强制 select() 超时吗？

char buffer[1025]; 是一个从另一个线程填充的全局数组。下面的 while(TRUE) 正在另一个线程上运行。

while(TRUE)   
    {   
        //clear the socket set  
        FD_ZERO(&readfds);   

    //add master socket to set  
    FD_SET(master_socket, &readfds);   
    max_sd = master_socket;   

    //add child sockets to set  
    for ( i = 0 ; i < max_clients ; i++)   
    {   
        //socket descriptor  
        sd = client_socket[i];   

        //if valid socket descriptor then add to read list  
        if(sd > 0)   
            FD_SET( sd , &readfds);   

        //highest file descriptor number, need it for the select function  
        if(sd > max_sd)   
            max_sd = sd;   
    }   

    //wait for an activity on one of the sockets , timeout is NULL ,  
    //so wait indefinitely  
    struct timeval timeout;
    timeout.tv_usec = 10000;
    activity = select( max_sd + 1 , &readfds , NULL , NULL , &timeout);   

    if ((activity < 0) && (errno!=EINTR))   
    {   
        cout << "select error" << endl;
    }   

    //If something happened on the master socket ,  
    //then its an incoming connection  
    if (FD_ISSET(master_socket, &readfds))   
    {   
        if ((new_socket = accept(master_socket,  
                (struct sockaddr *)&address, (socklen_t*)&addrlen))<0)   
        {   
            perror("accept");   
            exit(EXIT_FAILURE);   
        }   

        //inform user of socket number - used in send and receive commands  
        printf("New connection , socket fd is %d , ip is : %s , port : %d  \n" , new_socket , inet_ntoa(address.sin_addr) , ntohs(address.sin_port));   

        //send new connection greeting message  
        if( send(new_socket, message, strlen(message), 0) != strlen(message) )   
        {   
            perror("send");   
        }   

        puts("Welcome message sent successfully");   

        //add new socket to array of sockets  
        for (i = 0; i < max_clients; i++)   
        {   
            //if position is empty  
            if( client_socket[i] == 0 )   
            {   
                client_socket[i] = new_socket;   
                printf("Adding to list of sockets as %d\n" , i);   

                break;   
            }   
        }   
    }   

    if (buffer[0] > 0)
    {
        sd = client_socket[0];    
        send(sd , buffer , strlen(buffer) , 0 );        
    }  

    //else its some IO operation on some other socket 
    for (i = 0; i < max_clients; i++)   
    {   
        sd = client_socket[i];  


        if (FD_ISSET( sd , &readfds))   
        {   
            //Check if it was for closing , and also read the  
            //incoming message  
            if ((valread = read( sd , buffer, 1024)) == 0)   
            {   
                //Somebody disconnected , get his details and print  
                getpeername(sd , (struct sockaddr*)&address , \ 
                    (socklen_t*)&addrlen);   
                printf("Host disconnected , ip %s , port %d \n" ,  
                      inet_ntoa(address.sin_addr) , ntohs(address.sin_port));   

                //Close the socket and mark as 0 in list for reuse  
                close( sd );   
                client_socket[i] = 0;   
            }   

            //Echo back the message that came in  
            else 
            {   
                //set the string terminating NULL byte on the end  
                //of the data read  
                buffer[valread] = '\0';   
                send(sd , buffer , strlen(buffer) , 0 );                     
            }   
        }   
    }   
}

Answer 1

The problem with this code is that it sends data to the connected clients only when data received.

这是因为原始代码实现了请求/响应服务器 - 在本例中，它具体是一个回显服务器。

I want to send data to the connected clients asynchonously.

您可能需要首先枚举服务器开始异步发送数据的条件/触发器。我即兴编造的一个例子是多回波服务器。一旦这样的服务器从客户端接收到一个字符串，它就会回显 3 次，每次回显间隔(比如说)1 秒。

在这种情况下，可以使用选择超时来中断等待，并且可以修改“if buffer[0] > 0”检查以发送任何挂起的回显。

The question is: Is there any better way to do what I want? Can I force select() to timeout with other way?

man-page表示还有第三种方法可以打破选择，即安排信号的传递。但是，我认为，这应该用于处理传递到服务器程序的信号，并且仅用于处理异步 IO 的选择超时。更准确地说，我见过生产级服务器在异步循环中使用 epoll/kqueue(更好的选择替代方案)超时。所以，你的做法看起来很合理。