TCP通信的两方，其中一方正常关闭（调用close函数或程序正常退出）正处于连接状态的TCP套接字，这时双方的TCP将完成协商关闭连接的四次握手，握手结束以后，另一方的应用层将检测到TCP连接对应的套接字可读，执行read函数的时候倘若返回0，则说明对方已经主动断开了TCP连接，这边只需要调用close函数关闭对应套接字，回收相关资源即可。整个过程就是正常网络连接断开的处理过程。
监控这种正常网络连接断开的情况，使用的最普遍的办法是采用select机制，将所有建立好TCP连接的套接字交给select函数去维护他们的可读状态，select函数维护一组套接字的可读属性，一旦该组集合中有套接字可读，select将返回当前可读的套接字数量，然后调用FD_ISSET宏判断找出当前可读的所有套接字，调用read函数读取这些套接字中的数据，倘若read返回0，即没有读到任何数据，我们则可以断定该连接已经被对端关闭。
下面两个C语言程序分别模拟了服务端跟客户端采用select监控正常网络断开的实际处理方式，先启动server，监听1987端口，再启动client，指定服务器IP，建立连接。可以同时启动多个client连接server，分别结束（Ctrl+C或kill）服务端或客户端，观察客户端或服务端是否能够检测到连接断开。
服务端源代码：server.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/select.h>
#include <netinet/in.h>
#include <errno.h>
#define MAX_SESSION 10
#define MAX_BUF_LEN 1024
#define PORT 1987
#define SLEEP_TIME 10
int main(void)
{
int listen_fd;
int accept_fd[MAX_SESSION] = {0};
struct sockaddr_in addr;
struct timeval select_timeval;
fd_set readfds;
int max_fd;
int fd_num;
int new_fd;
int i;
int recv_len;
char recvBuf[MAX_BUF_LEN] = {0};
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_port = htons(PORT);
addr.sin_addr.s_addr = htonl(INADDR_ANY);
listen_fd = socket(AF_INET, SOCK_STREAM, 0);
if (listen_fd < 0)
{
perror("socket");
return -1;
}
if (bind(listen_fd, (struct sockaddr *)&addr, sizeof(addr)) < 0)
{
perror("bind");
return -1;
}
if (listen(listen_fd, 3) < 0)
{
perror("listen");
return -1;
}
select_timeval.tv_sec = 0;
select_timeval.tv_usec = 0;
while (1)
{
FD_ZERO(&readfds);
max_fd = listen_fd;
FD_SET(listen_fd, &readfds);
for (i = 0; i < MAX_SESSION; i++)
{
if (accept_fd[i] > max_fd)
max_fd = accept_fd[i];
if (accept_fd[i] > 0)
FD_SET(accept_fd[i], &readfds);
}
fd_num = select(max_fd+1, &readfds, NULL, NULL, &select_timeval);
if (fd_num < 0)
{
perror("select");
return -1;
}
else if (0 == fd_num)
{
select_timeval.tv_sec = SLEEP_TIME;
select_timeval.tv_usec = 0;
printf("select timeout...\n");
continue;
}
if (FD_ISSET(listen_fd, &readfds))
{
new_fd = accept(listen_fd, NULL, NULL);
printf("new accept sock: %d\n", new_fd);
if (new_fd < 0)
{
perror("accept");
continue;
}
for (i=0; i<MAX_SESSION; i++)
{
if (0 == accept_fd[i])
{
accept_fd[i] = new_fd;
break;
}
}
if (MAX_SESSION == i)
{
close(new_fd);
printf("beyoned the MAX_SESSION, so closed the new accept sock: %d\n", new_fd);
}
continue;
}
for (i=0; i<MAX_SESSION; i++)
{
if (FD_ISSET(accept_fd[i], &readfds))
{
memset(recvBuf, 0, sizeof(recvBuf));
recv_len = read(accept_fd[i], recvBuf, sizeof(recvBuf));
if (recv_len > 0)
{
printf("Recv msg from sock %d : %s\n", accept_fd[i], recvBuf);
}
else if (0 == recv_len)
{
printf("Disconnected with the sock %d\n", accept_fd[i]);
close(accept_fd[i]);
accept_fd[i] = 0;
}
else
{
perror("read");
close(accept_fd[i]);
accept_fd[i] = 0;
}
}
}
}
return 0;
}
客户端源代码：client.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/select.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <errno.h>
#define MAX_BUF_LEN 1024
#define PORT 1987
#define SLEEP_TIME 10
int main(int argc, char *argv[])
{
int connect_fd;
struct sockaddr_in addr;
struct timeval select_timeval;
fd_set readfds;
int fd_num;
int recv_len;
char recvBuf[MAX_BUF_LEN] = {0};
if (argc != 2)
{
printf("Please set the arguments as follow:\n");
printf("%s <serverIP>\n", argv[0]);
return -1;
}
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_port = htons(PORT);
addr.sin_addr.s_addr = inet_addr(argv[1]);
connect_fd = socket(AF_INET, SOCK_STREAM, 0);
if (connect_fd < 0)
{
perror("socket");
return -1;
}
if (connect(connect_fd, (struct sockaddr *)&addr, sizeof(addr)) < 0)
{
perror("connect");
close(connect_fd);
return -1;
}
select_timeval.tv_sec = 0;
select_timeval.tv_usec = 0;
while (1)
{
FD_ZERO(&readfds);
FD_SET(connect_fd, &readfds);
fd_num = select(connect_fd+1, &readfds, NULL, NULL, &select_timeval);
if (fd_num < 0)
{
perror("select");
return -1;
}
else if (0 == fd_num)
{
select_timeval.tv_sec = SLEEP_TIME;
select_timeval.tv_usec = 0;
printf("select timeout...\n");
continue;
}
if (FD_ISSET(connect_fd, &readfds))
{
memset(recvBuf, 0, sizeof(recvBuf));
recv_len = read(connect_fd, recvBuf, sizeof(recvBuf));
if (recv_len > 0)
{
printf("Recv msg from sock %d : %s\n", connect_fd, recvBuf);
}
else if (0 == recv_len)
{
printf("Disconnected with the sock %d\n", connect_fd);
close(connect_fd);
break;
}
else
{
perror("read");
close(connect_fd);
break;
}
}
}
return 0;
}
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