一般情况下,当TCP连接主动关闭时,会向对端发送一个FIN,对端会获得一个读事件,调用read时返回0,表示读到一个EOF,读结束。然而,在有的时候却不是这样的,接下来将讨论一下。
首先是一个简单的服务器程序,accept()后睡眠5s钟,然后关闭连接。
- int main(void)
- {
- int fd;
- fd = socket(AF_INET, SOCK_STREAM, 0);
- struct sockaddr_in servaddr;
- memset(&servaddr, 0, sizeof servaddr);
- servaddr.sin_family = AF_INET;
- servaddr.sin_port = htons(9001);
- servaddr.sin_addr.s_addr = 0;
- const int on = 1;
- setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof on);
- bind(fd, (struct sockaddr *) &servaddr, sizeof servaddr);
- listen(fd, 10);
- int clifd = accept(fd, NULL, NULL);
- sleep(5);
- // char buf[1024];
- // read(clifd, buf, sizeof buf);
- close(clifd);
- close(fd);
- return 0;
- }
下面是一个简单的客户端程序,连接成功后发送1024字节的数据,然后调用read()
- int main(void)
- {
- int fd, ret;
-
- fd = socket(AF_INET, SOCK_STREAM, 0);
- struct sockaddr_in servaddr;
- memset(&servaddr, 0, sizeof servaddr);
- servaddr.sin_family = AF_INET;
- servaddr.sin_port = htons(9001);
- servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
- connect(fd, (SA *) &servaddr, sizeof servaddr);
- char buf[1024];
- if (write(fd, buf, sizeof buf) != sizeof buf)
- err_sys("write error");
- if ((ret = read(fd, buf, sizeof buf)) < 0)
- err_sys("read error");
- fprintf(stderr, "Read %d Bytes\n", ret);
- if (close(fd) < 0)
- err_sys("close error");
- return 0;
- }
运行结果如下:
- read error : Connection reset by peer
可见server在close时向client发送的不是FIN,而是RST,为什么会这样呢?我们从内核中找答案。
见 net/ipv4/tcp.c 中的 tcp_close() 函数,
- /* As outlined in RFC 2525, section 2.17, we send a RST here because
- * data was lost. To witness the awful effects of the old behavior of
- * always doing a FIN, run an older 2.1.x kernel or 2.0.x, start a bulk
- * GET in an FTP client, suspend the process, wait for the client to
- * advertise a zero window, then kill -9 the FTP client, wheee...
- * Note: timeout is always zero in such a case.
- */
- if (data_was_unread) {
- /* Unread data was tossed, zap the connection. */
- NET_INC_STATS_USER(sock_net(sk), LINUX_MIB_TCPABORTONCLOSE);
- tcp_set_state(sk, TCP_CLOSE);
- tcp_send_active_reset(sk, sk->sk_allocation);
- }
代码里面写得很清楚,如果你的接收缓冲区中还有数据,协议栈就会发送RST而不是FIN。
我们再来验证一下,在server中先调用read()清空读缓冲区后再close(),此时发现client会收到FIN了。
可见,学习内核的协议栈是多么的重要啊!
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