ping命令的实现
#include    <stdio.h>
#include    <sys/time.h>
#include    <signal.h>
#include    <arpa/inet.h>
#include    <sys/types.h>
#include    <sys/socket.h>
#include    <unistd.h>
#include    <netinet/in.h>
#include    <netinet/ip.h>
#include    <netdb.h>
#include    <setjmp.h>
#include    <errno.h>
#include    <netinet/ip_icmp.h>
#include    <string.h>
#include    <stdlib.h>
#define    PACKET_SIZE 4096            /*  */
#define MAX_WAIT_TIME 5
#define MAX_NO_PACKETS 3

char sendpacket[PACKET_SIZE];
char recvpacket[PACKET_SIZE];
int sockfd,datalen=56;
int nsend=0,nreceived=0;
struct sockaddr_in dest_addr;
pid_t pid;
struct sockaddr_in from;
struct timeval tvrecv;
void statistics(int signo);
unsigned short cal_chksum(unsigned short *addr,int len);
int pack(int pack_no);
void send_packet(void);
void recv_packet(void);
int unpack(char* buf,int len);
void tv_sub(struct timeval *out,struct timeval* in);

void statistics(int signo)
{
   printf("\n-----------------PING statistics----------------\n");
   printf("%d packet transmitted,%d received,%%%d lost \n",nsend,nreceived,(nsend-nreceived)/nsend*100);
   close(sockfd);
   exit(1);
}
/*校验和算法*/
unsigned short cal_chksum(unsigned short *addr,int len)
{
   int nleft=len;
   int sum=0;
   unsigned short *w=addr;
   unsigned short answer=0;
   /*把ICMP报头二进制数据以2字节为单位累加起来*/
   while(nleft>1)
   {
       sum+=*w++;
       nleft-=2;
   }
   /*若ICMP报头为奇数个字节，会剩下最后一个字节。把最后一个字节视为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*/
   if(nleft==1)
   {
       *(unsigned char *)(&answer)=*(unsigned char *)w;
       sum+=answer;
   }
   sum=(sum>>16)+(sum&0xffff);
   sum+=(sum>>16);
   answer=~sum;
   return answer;
}
/*设置ICMP报头*/
int pack(int pack_no)
{
   int packsize;
   struct icmp *icmpp;
   struct timeval *tval;
   icmpp=(struct icmp *)sendpacket;
   icmpp->icmp_type=ICMP_ECHO;
   icmpp->icmp_code=0;
   icmpp->icmp_cksum=0;
   icmpp->icmp_seq=pack_no;
   icmpp->icmp_id=pid;
   packsize=8+datalen;
   tval=(struct timeval *)icmpp->icmp_data;
   gettimeofday(tval,NULL);/*记录发送时间*/
   icmpp->icmp_cksum=cal_chksum((unsigned short*)icmpp,packsize);/*校验算法*/
   return packsize;
}
/*发送3个ICMP报文*/
void send_packet()
{
   int packetsize;
   while(nsend<MAX_NO_PACKETS)
   {
       nsend++;
       packetsize=pack(nsend);/*设置ICMP报文*/
       if(sendto(sockfd,sendpacket,packetsize,0,(struct sockaddr *)&dest_addr,sizeof(dest_addr))<0)
       {
           perror("sendto error");
           continue;
       }
       sleep(1);/*每隔一秒发送一个ICMP报文*/
   }
}
/*接收所有ICMP报文*/
void recv_packet()
{
   int n,fromlen;
   extern int errno;

   signal(SIGALRM,statistics);
   fromlen=sizeof(from);
   while(nreceived<10)
   {
       alarm(MAX_WAIT_TIME);
       if((n=recvfrom(sockfd,recvpacket,sizeof(recvpacket),0,(struct sockaddr *)&from,&fromlen))<0)
       {
           if(errno==EINTR)
           {
               continue;
           }
           perror("recvfrom error\n");
           continue;
       }
       gettimeofday(&tvrecv,NULL); /*记录接收时间*/
       if(unpack(recvpacket,n)==-1)
           continue;
       nreceived++;
   }
}
/*剥去ICMP报头*/
int unpack(char *buf,int len)
{
   int iphdrlen;
   struct ip *ipp;
   struct icmp *icmpp;
   struct timeval *tvsend;
   int rtt;
   ipp=(struct ip *)buf;
   iphdrlen=(ipp->ip_hl)*4;/*求IP报头长度，即IP报头的长度标志乘4*/
   icmpp=(struct icmp*)(buf+iphdrlen);/*超过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)/*小于ICMP报头长度则不合理*/
   {
       printf("ICMP packets\'s length is less than 8\n");
       return -1;
   }
   /*确保所接收的是用户所发的ICMP的回应*/
   if((icmpp->icmp_type==ICMP_ECHOREPLY)&&(icmpp->icmp_id==pid))
   {
       tvsend=(struct timeval *)icmpp->icmp_data;
       tv_sub(&tvrecv,tvsend); /*接收和发送的时间差*/
       rtt=tvrecv.tv_sec*1000+tvrecv.tv_usec/1000;/*以毫秒为单位计算rtt*/
       /*显示相关信息*/
       printf("%d byte from %s:icmp_seq=%u ttl=%d rtt=%d  ms\n",
               len,inet_ntoa(from.sin_addr),icmpp->icmp_seq,ipp->ip_ttl,rtt);
   }
   else
       return -1;
}
void tv_sub(struct timeval *out,struct timeval *in)
{
   if((out->tv_usec-=in->tv_usec)<0)
   {
       out->tv_sec-=1;
       out->tv_usec+=1000000;
   }
   out->tv_sec-=in->tv_sec;
}
int main(int argc,char **argv)
{
   struct hostent *host;
   struct protoent *protocol;
   int size=50*1024;
   if(argc<2)
   {
       printf("usage : %s hostname/IP address \n",argv[0]);
       exit(1);
   }
   if((protocol=getprotobyname("icmp"))==NULL)
   {
       perror("getprotobyname\n");
       exit(1);
   }
   /*生成使用ICMP的原始套接字，这种套接字只是root才能生成*/
   if((sockfd=socket(AF_INET,SOCK_RAW,protocol->p_proto))<0)
   {
       perror("socket error\n");
       exit(1);
   }
   /*回收root权限，设置当前用户权限*/
   setuid(getuid());
   /*扩大套接字接收缓冲区到50K，这样做主要为了减少接收缓冲区溢出的可能性，若无意中ping一个广播地址或多播地址，将会引来大量应答*/
   setsockopt(sockfd,SOL_SOCKET,SO_RCVBUF,&size,sizeof(size));
   bzero(&dest_addr,sizeof(dest_addr));
   dest_addr.sin_family=AF_INET;
   if((host=gethostbyname(argv[1]))==NULL)
   {
       perror("gethostbyname error\n");
       exit(1);
   }
   dest_addr.sin_addr=*((struct in_addr *)host->h_addr);
   pid=getpid();
   printf("PING %s(%s): %d bytes data in ICMP packets.\n",argv[1],inet_ntoa(dest_addr.sin_addr),datalen);
   send_packet();/*发送所有ICMP报文*/
   recv_packet();/*接收所有ICMP报文*/
   statistics(SIGALRM);/*进行统计*/

   return 0;
}