blocks|key|3960938|text|你可以像下面这样调用getsockopt：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3960939|int+error+=+0;
socklen_t+len+=+sizeof+(error);
int+retval+=+getsockopt+(socket_fd,+SOL_SOCKET,+SO_ERROR,+&error,+&len);|code-block|syntax|javascript|3960940|要测试套接字是否已启动，请执行以下操作：|3960941|if+(retval+!=+0)+{
++++/*+there+was+a+problem+getting+the+error+code+*/
++++fprintf(stderr,+"error+getting+socket+error+code:+%25s\n",+strerror(retval));
++++return;
}

if+(error+!=+0)+{
++++/*+socket+has+a+non+zero+error+status+*/
++++fprintf(stderr,+"socket+error:+%25s\n",+strerror(error));
}|3960942|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

You could call getsockopt just like the following:

<pre><code>int error = 0;
socklen_t len = sizeof (error);
int retval = getsockopt (socket_fd, SOL_SOCKET, SO_ERROR, &amp;error, &amp;len);
</code></pre>

To test if the socket is up:

<pre><code>if (retval != 0) {
 /* there was a problem getting the error code */
 fprintf(stderr, "error getting socket error code: %s\n", strerror(retval));
 return;
}

if (error != 0) {
 /* socket has a non zero error status */
 fprintf(stderr, "socket error: %s\n", strerror(error));
}
</code></pre>

blocks|key|5378545|text|可靠地检测套接字是否仍在连接的唯一方法是定期尝试发送数据。定义一个客户端忽略的应用层'ping‘数据包通常更方便，但是如果协议已经被指定为没有这样的功能，你应该能够通过设置SO_KEEPALIVE套接字选项来配置tcp套接字来做到这一点。我已经链接到winsock文档，但是所有类似于BSD的套接字堆栈都应该提供相同的功能。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|5378546|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/ee470551(v=VS.85).aspx^0|2E|C|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

The only way to reliably detect if a socket is still connected is to periodically try to send data. Its usually more convenient to define an application level 'ping' packet that the clients ignore, but if the protocol is already specced out without such a capability you should be able to configure tcp sockets to do this by setting the <a href="http://msdn.microsoft.com/en-us/library/ee470551(v=VS.85).aspx" rel="noreferrer">SO_KEEPALIVE</a> socket option. I've linked to the winsock documentation, but the same functionality should be available on all BSD-like socket stacks.

blocks|key|5378513|text|TCP保持连接套接字选项(SO_KEEPALIVE)将在这种情况下提供帮助，并在连接丢失的情况下关闭服务器套接字。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5378514|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

TCP keepalive socket option (SO_KEEPALIVE) would help in this scenario and close server socket in case of connection loss.

blocks|key|5378692|text|有一种简单的方法可以通过poll调用检查套接字连接状态。首先，您需要轮询套接字，看它是否有POLLIN事件。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5378693|5378694|如果套接字未关闭并且有数据要读取，则read将返回大于零的值。|ordered-list-item|5378695|如果套接字上没有新数据，则在revents|5378696|If中POLLIN将设置为0如果套接字已关闭，则POLLIN标志将设置为1，读取将返回0。|5378697|5378698|下面是一小段代码：|5378699|int+client_socket_1,+client_socket_2;
if+((client_socket_1+=+accept(listen_socket,+NULL,+NULL))+<+0)
{
++++perror("Unable+to+accept+s1");
++++abort();
}
if+((client_socket_2+=+accept(listen_socket,+NULL,+NULL))+<+0)
{
++++perror("Unable+to+accept+s2");
++++abort();
}
pollfd+pfd[]={{client_socket_1,POLLIN,0},{client_socket_2,POLLIN,0}};
char+sock_buf[1024];+
while+(true)
{
++++poll(pfd,2,5);
++++if+(pfd[0].revents+&+POLLIN)
++++{
++++++++int+sock_readden+=+read(client_socket_1,+sock_buf,+sizeof(sock_buf));
++++++++if+(sock_readden+==+0)
++++++++++++break;
++++++++if+(sock_readden+>+0)
++++++++++++write(client_socket_2,+sock_buf,+sock_readden);
++++}
++++if+(pfd[1].revents+&+POLLIN)
++++{
++++++++int+sock_readden+=+read(client_socket_2,+sock_buf,+sizeof(sock_buf));
++++++++if+(sock_readden+==+0)
++++++++++++break;
++++++++if+(sock_readden+>+0)
++++++++++++write(client_socket_1,+sock_buf,+sock_readden);
++++}
}|code-block|syntax|javascript|5378700|entityMap^0|C|4|19|6|0|0|I|4|0|E|7|0|3|6|O|6|0|0|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|12|8|@]|D|@]|E|$]]|$1|G|3|H|5|I|7|13|8|@$9|14|A|15|B|C]]|D|@]|E|$]]|$1|J|3|K|5|I|7|16|8|@$9|17|A|18|B|C]]|D|@]|E|$]]|$1|L|3|M|5|I|7|19|8|@$9|1A|A|1B|B|C]|$9|1C|A|1D|B|C]]|D|@]|E|$]]|$1|N|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]|$1|O|3|P|5|6|7|1F|8|@]|D|@]|E|$]]|$1|Q|3|R|5|S|7|1G|8|@]|D|@]|E|$T|U]]|$1|V|3|-4|5|6|7|1H|8|@]|D|@]|E|$]]]|W|$]]

There is an easy way to check socket connection state via <code>poll</code> call. First, you need to poll socket, whether it has <code>POLLIN</code> event.

<ol>
<li>If socket is not closed and there is data to read then <code>read</code> will return more than zero.</li>
<li>If there is no new data on socket, then <code>POLLIN</code> will be set to 0 in <code>revents</code></li>
<li>If socket is closed then <code>POLLIN</code> flag will be set to one and read will return 0.</li>
</ol>

Here is small code snippet:

<pre><code>int client_socket_1, client_socket_2;
if ((client_socket_1 = accept(listen_socket, NULL, NULL)) &lt; 0)
{
 perror("Unable to accept s1");
 abort();
}
if ((client_socket_2 = accept(listen_socket, NULL, NULL)) &lt; 0)
{
 perror("Unable to accept s2");
 abort();
}
pollfd pfd[]={{client_socket_1,POLLIN,0},{client_socket_2,POLLIN,0}};
char sock_buf[1024]; 
while (true)
{
 poll(pfd,2,5);
 if (pfd[0].revents &amp; POLLIN)
 {
 int sock_readden = read(client_socket_1, sock_buf, sizeof(sock_buf));
 if (sock_readden == 0)
 break;
 if (sock_readden &gt; 0)
 write(client_socket_2, sock_buf, sock_readden);
 }
 if (pfd[1].revents &amp; POLLIN)
 {
 int sock_readden = read(client_socket_2, sock_buf, sizeof(sock_buf));
 if (sock_readden == 0)
 break;
 if (sock_readden &gt; 0)
 write(client_socket_1, sock_buf, sock_readden);
 }
}
</code></pre>

blocks|key|3961055|text|我也遇到过类似的问题。我想知道是服务器连接到客户端还是客户端连接到服务器。在这种情况下，recv函数的返回值可以派上用场。如果套接字未连接，它将返回0字节。因此，使用这个函数，我打破了循环，不必使用任何额外的函数线程。如果专家认为这是正确的方法，您也可以使用相同的方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3961056|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

I had a similar problem. I wanted to know whether the server is connected to client or the client is connected to server. In such circumstances the return value of the recv function can come in handy. If the socket is not connected it will return 0 bytes. Thus using this I broke the loop and did not have to use any extra threads of functions. You might also use this same if experts feel this is the correct method.

blocks|key|1547329|text|get+sock+opt可能有点用，但是，另一种方法是为SIGPIPE安装一个信号处理程序。基本上，每当套接字连接中断时，内核都会向进程发送SIGPIPE信号，然后您就可以执行所需的操作。但这仍然不能提供了解连接状态的解决方案。希望这能有所帮助。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1547330|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

get sock opt may be somewhat useful, however, another way would to have a signal handler installed for SIGPIPE. Basically whenever you the socket connection breaks, the kernel will send a SIGPIPE signal to the process and then you can do the needful. But this still does not provide the solution for knowing the status of the connection. hope this helps.

blocks|key|1547334|text|您应该尝试使用:+getpeername函数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1547335|现在，当连接断开时，您将得到errno:+ENOTCONN+-套接字未连接。这对你来说意味着失败。|1547336|否则(如果没有其他故障)，返回代码将为0+-->，这意味着向上。|1547337|资源：手册页：http://man7.org/linux/man-pages/man2/getpeername.2.html|offset|length|style|BOLD|1547338|entityMap|0|LINK|mutability|MUTABLE|url|http://man7.org/linux/man-pages/man2/getpeername.2.html^0|0|0|0|0|3|7|1J|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|U|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|V|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|W|8|@$H|X|I|Y|J|K]]|9|@$H|Z|I|10|1|11]]|A|$]]|$1|L|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|M|$N|$5|O|P|Q|A|$R|S]]]]

You should try to use: getpeername function.

now when the connection is down you will get in errno:
ENOTCONN - The socket is not connected.
which means for you DOWN.

else (if no other failures) there the return code will 0 --> which means UP.

resources: 
man page: <a href="http://man7.org/linux/man-pages/man2/getpeername.2.html" rel="nofollow">http://man7.org/linux/man-pages/man2/getpeername.2.html</a>

blocks|key|3961202|text|非常简单，如recv中所示。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3961203|为了检查您是否想要使用MSG_PEEK和MSG_DONT_WAIT从套接字读取1个字节。这不会使数据出队(PEEK)，并且操作是非阻塞的(DONT_WAIT)|style|CODE|3961204|while+(recv(client->socket,NULL,1,+MSG_PEEK+%7C+MSG_DONTWAIT)+!=+0)+{
++++sleep(rand()+%25+2);+//+Sleep+for+a+bit+to+avoid+spam
++++fflush(stdin);
++++printf("I+am+alive:+%25d\n",+socket);
}

//+When+the+client+has+disconnected,+this+line+will+execute
printf("Client+%25d+went+away+:(\n",+client->socket);|code-block|syntax|javascript|3961205|找到示例here。|3961206|entityMap|0|LINK|mutability|MUTABLE|url|http://man7.org/linux/man-pages/man2/recv.2.html|1|http://stefan.buettcher.org/cs/conn_closed.html^0|6|4|0|0|B|8|K|D|1H|4|1X|9|0|0|4|4|1|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@$A|Z|B|10|1|11]]|C|$]]|$1|D|3|E|5|6|7|12|8|@$A|13|B|14|F|G]|$A|15|B|16|F|G]|$A|17|B|18|F|G]|$A|19|B|1A|F|G]]|9|@]|C|$]]|$1|H|3|I|5|J|7|1B|8|@]|9|@]|C|$K|L]]|$1|M|3|N|5|6|7|1C|8|@]|9|@$A|1D|B|1E|1|1F]]|C|$]]|$1|O|3|-4|5|6|7|1G|8|@]|9|@]|C|$]]]|P|$Q|$5|R|S|T|C|$U|V]]|W|$5|R|S|T|C|$U|X]]]]

Very simple, as pictured in the <a href="http://man7.org/linux/man-pages/man2/recv.2.html" rel="nofollow noreferrer">recv</a>.

To check that you will want to read 1 byte from the socket with <code>MSG_PEEK</code> and <code>MSG_DONT_WAIT</code>. This will not dequeue data (<code>PEEK</code>) and the operation is nonblocking (<code>DONT_WAIT</code>) 

<pre><code>while (recv(client-&gt;socket,NULL,1, MSG_PEEK | MSG_DONTWAIT) != 0) {
 sleep(rand() % 2); // Sleep for a bit to avoid spam
 fflush(stdin);
 printf("I am alive: %d\n", socket);
}

// When the client has disconnected, this line will execute
printf("Client %d went away :(\n", client-&gt;socket);
</code></pre>

Found the example <a href="http://stefan.buettcher.org/cs/conn_closed.html" rel="nofollow noreferrer">here</a>.

blocks|key|1547377|text|在Windows上，您可以使用以下命令查询任何网络适配器上任何端口的精确状态：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1547378|您可以将其过滤为仅与您的流程等相关的内容，并根据需要定期进行监控。这是“另一种”方法。|1547379|您还可以复制套接字句柄，在套接字上设置IOCP/重叠i/o等待，并以这种方式监视它。|1547380|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

On Windows you can query the precise state of any port on any network-adapter using:
<a href="https://docs.microsoft.com/en-us/windows/win32/api/iphlpapi/nf-iphlpapi-getextendedtcptable" rel="nofollow noreferrer">GetExtendedTcpTable</a> 

You can filter it to only those related to your process, etc and do as you wish periodically monitoring as needed. This is "an alternative" approach.

You could also duplicate the socket handle and set up an IOCP/Overlapped i/o wait on the socket and monitor it that way as well.

blocks|key|1543141|text|您可以在getsockopt()函数中使用SS_ISCONNECTED宏。在socketvar.h中定义了SS_ISCONNECTED。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1543142|entityMap^0|4|C|L|E|12|B|1H|E|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|G|$]]

you can use <code>SS_ISCONNECTED</code> macro in <code>getsockopt()</code> function.
<code>SS_ISCONNECTED</code> is define in <code>socketvar.h</code>.

blocks|key|1851036|text|对于BSD套接字，我会去看看Beej's+guide。当recv返回0时，你知道另一端是断开的。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1851037|现在您可能会问，检测另一端断开连接的最简单方法是什么？一种方法是让一个线程总是执行recv。当客户端断开连接时，该线程将能够立即判断。|1851038|entityMap|0|LINK|mutability|MUTABLE|url|http://beej.us/guide/bgnet/output/html/singlepage/bgnet.html#sendrecv^0|E|C|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]]|C|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]]]

For BSD sockets I'd check out <a href="http://beej.us/guide/bgnet/output/html/singlepage/bgnet.html#sendrecv" rel="nofollow">Beej's guide</a>. When recv returns 0 you know the other side disconnected. 

Now you might actually be asking, what is the easiest way to detect the other side disconnecting? One way of doing it is to have a thread always doing a recv. That thread will be able to instantly tell when the client disconnects.

I have a TCP connection. Server just reads data from the client. Now, if the connection is lost, the client will get an error while writing the data to the pipe (broken pipe), but the server still listens on that pipe. Is there any way I can find if the connection is UP or NOT?

How to find the socket connection state in C?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我有一个TCP连接。服务器只是从客户端读取数据。现在，如果连接丢失，客户端将在将数据写入管道(断开的管道)时出现错误，但服务器仍会侦听该管道。有什么方法可以找到连接是否正常吗？

问如何在C中找到套接字的连接状态？
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问如何在C中找到套接字的连接状态？
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