socket连接过程中，异常断开（服务关闭和网线断开）的处理

2023-11-14 03:13| 来源: 网络整理| 查看: 265

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：

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 #include #include #include #include #include #include #include #include #include #include #include #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)) close(netmon_fd); return -1; } nvar |= O_NONBLOCK; if(fcntl(netmon_fd,F_SETFL,nvar) /* 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; }

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