socket连接过程中，异常断开（服务关闭和网线断开）的处理

╰+攻爆jí腚メ 2023-02-14 06:45 28阅读 0赞

# 场景 #

嵌入式设备经常用到tcp通信，设备端内部负责维护一个长连接的socket，来与服务端通信，但是有时候网络环境会发生波动导致网络异常，这时候就需要设备有一个断线重连机制。有时候客户端和服务端之间未做应用层心跳保活，无法修改服务端内容，这时候就需要设备自己检测网络是否正常了。

# 方案 #

当服务端关闭服务或者服务端异常退出时，我们通过recv和send的返回值能判断链接已经关闭，这时候就可以进入重连流程了。但是如果连接过程中网络断开（不一定是设备端网络断了，可能是路由器，可能是远端服务器），我们recv和send会在很久的一段时间内依旧正常返回（因为没收到相应的关闭握手的数据，依旧发包，重传多次后会进行wait），这不能忍受。  
于是我们可是建立一个独立的监控线程确保服务正常。有两种方法：

1.  客服端对原来的服务创建测试套接字。
    
     *  优点：方法简单，可控探测间隔，服务不可用可迅速检测到
     *  缺点：比较频繁的创建关闭套接字，加大了服务端的压力
2.  通过popen调用ping指令测试服务器是否可用  
    \-优点： 对服务器影响较小，ping指令发送icmp报文，造成的流量不大  
    \-缺点：超时时间设置不灵敏，会通过管道调用ping，引入额外的进程。

#include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <errno.h>
    #include <time.h>
    #include <pthread.h>
    #include <fcntl.h>
    #include <sys/socket.h>
    #include <sys/prctl.h>
    #include <netinet/in.h>
    #include <arpa/inet.h>
    
    
    #define RECONNECT_INTERVAL 5
    #define PROBE_INTERVAL 5
    #define PING_TIMEOUT 10
    #define RECV_BUFFER_SIZE 512
    
    
    char ip[32] = "192.168.6.127";
    char port[8] = "777";
    
    int sok_fd = -1;
    int status = 0;
    char buffer[RECV_BUFFER_SIZE];
    int buf_useable_len = RECV_BUFFER_SIZE;
    
    
    int open_tcp_client_socket_fd(char* serveraddr, char* rport)
    { 
        int nvar;
    	short int nCloseConFlag =  0;
    	struct sockaddr_in stSrvAddr;
    	int	nSendBuf    = 32*1024;
        int netmon_fd;
    	int iKeepAlive = 1;
        struct timeval tv_out;
        fd_set fdsWrite;
        
        //printf("Connecting to server %s port %s \n",serveraddr, rport);
    
    	if((netmon_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    	{ 
    		return  -1;
    	}
    
        //启用tcp协议自带的保活协议，效果比较差，因为为了防止keep-alive流量过大，系统设置的保活间隔很长很长
    	setsockopt(netmon_fd, SOL_SOCKET, SO_KEEPALIVE, (void *)&iKeepAlive, sizeof(iKeepAlive));    
    	
    	memset(&stSrvAddr, 0, sizeof(struct sockaddr_in));
    	stSrvAddr.sin_family = AF_INET;
    	stSrvAddr.sin_addr.s_addr =  inet_addr(serveraddr);
    	stSrvAddr.sin_port = htons(atoi(rport));
    
    	/* 获取当前socket的属性， 并设置 noblocking 属性(connect noblock)*/
    	if((nvar = fcntl(netmon_fd,F_GETFL,0 )) < 0)
    	{ 
    		close(netmon_fd);
    		return  -1;
    	}
    	nvar |= O_NONBLOCK;
    	if(fcntl(netmon_fd,F_SETFL,nvar) < 0)
    	{ 
    		close(netmon_fd);
    		return  -1;
    	}
    
    	/*请求连接*/
    	nvar = connect(netmon_fd,(struct sockaddr*)&stSrvAddr ,sizeof(stSrvAddr));
    	if(nvar != 0)
    	{ 
    		/* it is in the connect process*/
    		if (EINPROGRESS == errno)
    		{ 
                tv_out.tv_sec = 0;
    			tv_out.tv_usec = 300000;
                FD_ZERO(&fdsWrite);
                FD_SET(netmon_fd, &fdsWrite);
                /*确定socket可读写数据*/
                if(select(netmon_fd+1, NULL, &fdsWrite,NULL, &tv_out) > 0)
                { 
                    socklen_t nLen; 
                    nLen=sizeof(nvar);
                    /*下面的一句一定要，主要针对防火墙*/
                    getsockopt(netmon_fd, SOL_SOCKET, SO_ERROR, &nvar, &nLen);
                    /*connect failed*/
                    if(nvar != 0)
                    { 
                        nCloseConFlag = 1;
                    }
                }
                else
                { 
                    nCloseConFlag = 1;
                }
            }
        	/* #define EISCONN 106 : Transport endpoint is already connected */
            else if(EISCONN != errno)
            { 
                nCloseConFlag = 1;
            }
            
            if(nCloseConFlag)
            { 
                printf("errno:%d\n", errno);
                shutdown(netmon_fd, SHUT_RDWR);
                close(netmon_fd);
                netmon_fd = -1;
            }
        }
    	
    	return netmon_fd;
    }
    
    int get_socket_link_status(void)
    { 
        return status;
    }
    
    int set_socket_link_status(int sta)
    { 
        status = sta;
        return 0;
    }
    
    void *net_cable_disconnected_monitor1(void *arg)
    { 
        int ret;
    
        while(1)
        { 
            sleep(PROBE_INTERVAL);
            //if socket not linked we don't do probe, because we are try to reconnected
            if(get_socket_link_status == 0)
            { 
                continue;
            }
    
            //create a new socket. If failed, it indicate link is down, but we haven't received close handshake 
            ret = open_tcp_client_socket_fd(ip, port);
            if(ret == -1)
            { 
                set_socket_link_status(0);
            }
            else
            { 
                close(ret);
            }
        }
        return NULL;
    }
    
    void *net_cable_disconnected_monitor2(void *arg)
    { 
        FILE *fp;
        char buffer[128] = { 0};
        char needle[32] = "Destination Host Unreachable";
        int flag;
    
        sprintf(buffer, "ping -i %d -W %d %s", PROBE_INTERVAL, PING_TIMEOUT, ip);
        fp = popen(buffer, "r");
        while(1)
        { 
            memset(buffer, 0, 128);
            fgets(buffer, 127, fp);
            if(strstr(buffer, needle) != NULL)
            { 
                set_socket_link_status(0);
            }
            //now = time(NULL);
            //printf("buffer:[%s], time: %s, flag: %d\n", buffer, ctime(&now), flag);
            usleep(500000);
        }
        pclose(fp);
    
        return NULL;
    }
    
    
    void *connect_to_server_loop(void *arg)
    { 
        int ret;
        pthread_t tid;
    
        pthread_create(&tid, NULL, net_cable_disconnected_monitor1, NULL);
        pthread_detach(tid);
    
    RE_CONNECT:
        do{ 
            sok_fd = open_tcp_client_socket_fd(ip, port);
            if(sok_fd == -1)
            { 
                sleep(RECONNECT_INTERVAL);
                printf("connected error, sleep %d second try to reconnected!\n", RECONNECT_INTERVAL);
            }
            else
            { 
                set_socket_link_status(1);
                printf("connected success!\n");
            }
        }while(sok_fd == -1);
    
        while(1)
        { 
            ret = recv(sok_fd, buffer, RECV_BUFFER_SIZE, MSG_DONTWAIT);
        	if(ret == -1)
        	{ 
                // socket nonblocking normal returned EAGAIN error
                if(errno != EAGAIN)
                { 
                    printf("error occured, errno is %d/n", errno);
                }
        	}
        	else if (ret == 0)
        	{ 
                set_socket_link_status(0);
                printf("connectecd stream shutdown or received zero byte request\n!");
        	}
        	else
        	{ 
            	printf("get buffer:\n[%s]\n", buffer);
                memset(buffer, 0, RECV_BUFFER_SIZE);
        	}
    
            if(status == 0)
            { 
                printf("link down, try to reconnect!\n");
                close(sok_fd);
                goto RE_CONNECT;
            }
    
            sleep(1);
        }
    
        printf("can't get here!\n");
    	return NULL;
    }
    
    int send_data()
    { 
        int ret;
        char data[128] = "Hello!";
        
        //link is down, we don't send data
        if(get_socket_link_status() == 0)
        { 
            //printf("not connceted to server, don't send data\n");
            return -1;
        }
    
        ret = send(sok_fd, data, strlen(data), 0);
        if(ret == -1)
        { 
            set_socket_link_status(0);
            printf("send data failed, set connect down!\n");
        }
    
        return ret;
    }
    
    
    int main(int argc, char *argv[])
    { 
        pthread_t tid;
        int ret;
        time_t now;
    
        ret = pthread_create(&tid, NULL, connect_to_server_loop, NULL);
        if(ret != 0)
        { 
            printf("thread create failed!\n");
        }
    
        while(1)
        { 
            ret = send_data();
            if(ret != -1)
            { 
                now = time(NULL);
                printf("send data len %d, %s", ret, ctime(&now));
            }
            sleep(1);
        }
    
        printf("man function exit!\n");
        return 0;
    }