linux下C语言判断网络是否连接

本文改写自网上的一个程序，原始程序中为阻塞式调用，而且有进程创建的过程，非常不利于集成到自己程序中，因此对原始程序进行改造，使其可以完成发送一个imcp包的方式来判断网络连通，只需要调用改进后的

 bool NetIsOK()

函数即可，该函数返回true即表示网络状态良好，否则表示网络状态不连同，本程序中只发送了一个icmp包，在实际应用中可以根据需要改进为发送多个imcp包。

修改之后的程序为：只需要调用函数NetIsOK()即可。源码如下所示：

http://blog.csdn.net/houjixin/article/details/8843702

 #include
 #include
 #include
 #include
 #include
 #include
 #include
 #include
 #include
 #include
 #include
 #include
 #include
 #define MAX_WAIT_TIME   1
 #define MAX_NO_PACKETS  1
 #define ICMP_HEADSIZE 8
 #define PACKET_SIZE     4096
 struct timeval tvsend,tvrecv;
 struct sockaddr_in dest_addr,recv_addr;
 int sockfd;
 pid_t pid;
 char sendpacket[PACKET_SIZE];
 char recvpacket[PACKET_SIZE];

 //函数定义
 void timeout(int signo);
 unsigned short cal_chksum(unsigned short *addr,int len);
 int pack(int pkt_no,char *sendpacket);
 int send_packet(int pkt_no,char *sendpacket);
 int recv_packet(int pkt_no,char *recvpacket);
 int unpack(int cur_seq,char *buf,int len);
 void tv_sub(struct timeval *out,struct timeval *in);
 void _CloseSocket();

 bool NetIsOk()
 {

     double rtt;
     struct hostent *host;
     struct protoent *protocol;
     int i,recv_status;

 #ifdef _USE_DNS //如果定义该宏，则可以使用域名进行判断网络连接，例如www.baidu.com
     /* 设置目的地址信息 */
     char hostname[32];
     sprintf(hostname,"%s","www.baidu.com")
     bzero(&dest_addr, sizeof(dest_addr));
     dest_addr.sin_family = AF_INET;

     if((host=gethostbyname(hostname))==NULL)
     {
         printf("[NetStatus]  error : Can't get serverhost info!\n");
         return false;
     }

     bcopy((char*)host->h_addr,(char*)&dest_addr.sin_addr,host->h_length);
 #else //如果不使用域名，则只能用ip地址直接发送icmp包，例如谷歌的地址：8.8.8.8
     dest_addr.sin_addr.s_addr = inet_addr("8.8.8.8");
 #endif


     if ((sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP)) < 0)
     {   /* 创建原始ICMP套接字 */
         printf("[NetStatus]  error : socket");
         return false;
     }

     int iFlag;
     if(iFlag = fcntl(sockfd,F_GETFL,0)<0)
     {
         printf("[NetStatus]  error : fcntl(sockfd,F_GETFL,0)");
         _CloseSocket();
         return false;
     }
     iFlag |= O_NONBLOCK;
     if(iFlag = fcntl(sockfd,F_SETFL,iFlag)<0)
     {
         printf("[NetStatus]  error : fcntl(sockfd,F_SETFL,iFlag )");
         _CloseSocket();
         return false;
     }

     pid=getpid();
     for(i=0;i
     {

         if(send_packet(i,sendpacket)<0)
         {
             printf("[NetStatus]  error : send_packet");
             _CloseSocket();
             return false;
         }

         if(recv_packet(i,recvpacket)>0)
         {
             _CloseSocket();
             return true;
         }

     }
     _CloseSocket();
     return false;
 }



 int send_packet(int pkt_no,char *sendpacket)
 {
     int packetsize;
     packetsize=pack(pkt_no,sendpacket);
     gettimeofday(&tvsend,NULL);
     if(sendto(sockfd,sendpacket,packetsize,0,(struct sockaddr *)&dest_addr,sizeof(dest_addr) )<0)
     {
         printf("[NetStatus]  error : sendto error");
         return -1;
     }
     return 1;
 }


 int pack(int pkt_no,char*sendpacket)
 {
     int i,packsize;
     struct icmp *icmp;
     struct timeval *tval;
     icmp=(struct icmp*)sendpacket;
     icmp->icmp_type=ICMP_ECHO;   //设置类型为ICMP请求报文
     icmp->icmp_code=0;
     icmp->icmp_cksum=0;
     icmp->icmp_seq=pkt_no;
     icmp->icmp_id=pid;           //设置当前进程ID为ICMP标示符
     packsize=ICMP_HEADSIZE+sizeof(struct timeval);
     tval= (struct timeval *)icmp->icmp_data;
     gettimeofday(tval,NULL);
     icmp->icmp_cksum=cal_chksum( (unsigned short *)icmp,packsize);
     return packsize;
 }


 unsigned short cal_chksum(unsigned short *addr,int len)
 {
     int nleft=len;
     int sum=0;
     unsigned short *w=addr;
     unsigned short answer=0;
     while(nleft>1)       //把ICMP报头二进制数据以2字节为单位累加起来
     {
         sum+=*w++;
         nleft-=2;
     }
     if( nleft==1)       //若ICMP报头为奇数个字节,会剩下最后一字节.把最后一个字节视为一个2字节数据的高字节,这个2字节数据的低字节为0,继续累加
     {
         *(unsigned char *)(&answer)=*(unsigned char *)w;
         sum+=answer;
     }
     sum=(sum>>16)+(sum&0xffff);
     sum+=(sum>>16);
     answer=~sum;
     return answer;
 }


 int recv_packet(int pkt_no,char *recvpacket)
 {
     int n,fromlen;
     fd_set rfds;
     FD_ZERO(&rfds);
     FD_SET(sockfd,&rfds);
     signal(SIGALRM,timeout);
     fromlen=sizeof(recv_addr);
     alarm(MAX_WAIT_TIME);
     while(1)
     {
         select(sockfd+1, &rfds, NULL, NULL, NULL);
         if (FD_ISSET(sockfd,&rfds))
         {
             if( (n=recvfrom(sockfd,recvpacket,PACKET_SIZE,0,(struct sockaddr *)&recv_addr,&fromlen)) <0)
             {
             if(errno==EINTR)
                 return -1;
                 perror("recvfrom error");
                 return -2;
             }
         }
         gettimeofday(&tvrecv,NULL);
         if(unpack(pkt_no,recvpacket,n)==-1)
             continue;
         return 1;
     }
 }

 int unpack(int cur_seq,char *buf,int len)
 {
     int iphdrlen;
     struct ip *ip;
     struct icmp *icmp;
     ip=(struct ip *)buf;
     iphdrlen=ip->ip_hl<<2;     //求ip报头长度,即ip报头的长度标志乘4
     icmp=(struct icmp *)(buf+iphdrlen);     //越过ip报头,指向ICMP报头
     len-=iphdrlen;      //ICMP报头及ICMP数据报的总长度
     if( len<8)
         return -1;
     if( (icmp->icmp_type==ICMP_ECHOREPLY) && (icmp->icmp_id==pid) && (icmp->icmp_seq==cur_seq))
         return 0;
     else return -1;
 }


 void timeout(int signo)
 {
     printf("Request Timed Out\n");
 }

 void tv_sub(struct timeval *out,struct timeval *in)
 {
     if( (out->tv_usec-=in->tv_usec)<0)
     {
         --out->tv_sec;
         out->tv_usec+=1000000;
     }
     out->tv_sec-=in->tv_sec;
 }

 void _CloseSocket()
 {
     close(sockfd);
     sockfd = 0;
 }