實現ping命令的代碼

#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);
}
/*校驗和算法*/
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報頭二進制數據以2字節為單位累加起來*/
   while(nleft>1)
   {
       sum+=*w++;
       nleft-=2;
   }
   /*若ICMP報頭為奇數個字節，會剩下最後一個字節。把最後一個字節視為2字節數據的高字節，這個2字節數據的低字節為0，繼續累加*/
   if(nleft==1)
   {
       *(unsigned char *)(&answer)=*(unsigned char *)w;
       sum+=answer;
   }
   sum=(sum>>16)+(sum&0xffff);
   sum+=(sum>>16);
   answer=~sum;
   return answer;
}
/*設置ICMP報頭*/
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);/*記錄發送時間*/
   icmpp->icmp_cksum=cal_chksum((unsigned short*)icmpp,packsize);/*校驗算法*/
   return packsize;
}
/*發送3個ICMP報文*/
void send_packet()
{
   int packetsize;
   while(nsend<MAX_NO_PACKETS)
   {
       nsend++;
       packetsize=pack(nsend);/*設置ICMP報文*/
       if(sendto(sockfd,sendpacket,packetsize,0,(struct sockaddr *)&dest_addr,sizeof(dest_addr))<0)
       {
           perror("sendto error");
           continue;
       }
       sleep(1);/*每隔一秒發送一個ICMP報文*/
   }
}
/*接收所有ICMP報文*/
void recv_packet()
{
   int n,fromlen;
   extern int errno;
  /*