套接字选择方法不会导致我期望的行为

问题描述

我写了一些服务器代码，其中包含一种处理来自客户端的数据的方法.它具有一个select方法，用于检查要从套接字读取的数据，并且具有超时值.客户端每秒向我的服务器发送一条消息.如果我将代码设置为运行，然后停止客户端，则我希望select方法返回值-1.但这似乎没有发生.如果客户端关闭，返回值似乎是> 0，但是当调用recv方法时，接收到的字节为零.

我将代码放在下面，并附有评论和问题.可以请确切解释发生了什么.我可以让我的代码执行我想做的事情，但不能达到我的期望，所以我真的不明白为什么我的代码行得通！非常感谢.

Hi,

I have written some server code which has a method which handles the receiving of data from the client. It has a select method with checks for data to read from the socket and has a timeout value. The client sends a message to my server every second. If I set my code running, then stop the client, I would have expected the select method to return a value of -1. But this doesn''t appear to happen. If the client closes, the return value appears to be > 0 but then when the recv method is called, the bytes received is zero.

I have put the code below, with comments and questions. Could somebody please explain exactly what is going on please. I can get my code to do what i want, but not how I expected, so I really don''t understand why my code works! Many, many thanks.

//initialise the fd_set rxSet (file descriptor set) values
FD_ZERO(&rxSet);
//add m_clientSocket to the set of file descriptors in rxSet
FD_SET(m_clientSocket, &rxSet);
maxFD = m_clientSocket;//not needed, just for comaptibility with linux select()

//wait for the specified timeOutVal to see if there is any data to read
//on the m_clientSocket
sockVal = select(maxFD, &rxSet, NULL, NULL, &timeOutVal);

//see if there is some data to read on the socket
if(sockVal > 0)
{
	//read the data from the socket
	int bytesReceived = recv(m_clientSocket, recvBuf, recvBufLen - 1, 0);
	//add a null terminator at the end of the data to avoid storing an garbage
	recvBuf[bytesReceived] = '\0';
	//check if there was an error on the socket
	if(bytesReceived == -1)
	{
                //this does not get reached if the client disconnects
                //what would cause this code to be reached?
		printf("server socket error\n");
		break;
	}
	if(bytesReceived > 0)
	{
		//bytes were received
		//read the message, form the response and send response to controller
		RespondToAllMessagesFromController(recvBuf);

		noDataReceivedCount = 0;//reset the count
	}
	if(bytesReceived == 0)
	{
	        //this is reached if the client disconnects - but I don't understand 
                //why because i would have thought that sockVal would not be > 0 in
                //this case

		break;//leave while loop, leave Digitiser running		
	}
}
else if(sockVal < 0)
{
	//there was an error on the socket - but this doesn't get reached if the
        //controller disconnects. What would cause this to be reached?
	//even if the if(bytesReceived == 0){} above is removed.
	int test = 0;
}
else //if(sockVal == 0) because select timed out
{
       //this gets reached when the socket hasn't disconnected but no messages arrive,
       //as I would expect
}

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