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（1）主体代码 
ping代码的主体部分可以四部分，首先是一些头函数及宏定义：  
#include  
#include  
#include  
#include  
#include  
 
#include  
#include  
#include  
#include  
#include  
#include  
#include  
#include  
#include  
#include  
#include  
#include  
#include  
#define	F_FLOOD	0x001 
#define	F_INTERVAL	0x002 
#define	F_NUMERIC	0x004 
#define	F_PINGFILLED	0x008 
#define	F_QUIET	0x010 
#define	F_RROUTE	0x020 
#define	F_SO_DEBUG	0x040 
#define	F_SO_DONTROUTE	0x080 
#define	F_VERBOSE	0x100 
 
/* 多播选项 */ 
int moptions; 
#define MULTICAST_NOLOOP	0x001 
#define MULTICAST_TTL		0x002 
#define MULTICAST_IF		0x004 
… 
接下来的第二部分是建立socket并处理选项： 
Int main(int argc, char *argv[]) 
{ 
	struct timeval timeout; 
	struct hostent *hp; 
	struct sockaddr_in *to; 
	struct protoent *proto; 
	struct in_addr ifaddr; 
	int i; 
	int ch, fdmask, hold, packlen, preload; 
	u_char *datap, *packet; 
	char *target, hnamebuf[MAXHOSTNAMELEN]; 
	u_char ttl, loop; 
	int am_i_root; 
… 
	static char *null = NULL; 
 
	/*__environ = &null;*/ 
	am_i_root = (getuid()==0); 
 
	/* 
	 *建立socket连接，并且测试是否是root用户 
	 */ 
	if ((s = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP)) < 0) { 
		if (errno==EPERM) { 
			fprintf(stderr, "ping: ping must run as root\n"); 
		} 
		else perror("ping: socket"); 
		exit(2); 
	} 
… 
	preload = 0; 
	datap = &outpack[8 + sizeof(struct timeval)]; 
	while ((ch = getopt(argc, argv, "I:LRc:dfh:i:l:np:qrs:t:v")) != EOF) 
		switch(ch) { 
		case 'c': 
			npackets = atoi(optarg); 
			if (npackets <= 0) { 
				(void)fprintf(stderr, 
				    "ping: bad number of packets to transmit.\n"); 
				exit(2); 
			} 
			break; 
/*调用选项*/ 
		case 'd': 
			options |= F_SO_DEBUG; 
			break; 
/*flood选项*/ 
		case 'f': 
			if (!am_i_root) { 
				(void)fprintf(stderr, 
				    "ping: %s\n", strerror(EPERM)); 
				exit(2); 
			} 
			options |= F_FLOOD; 
			setbuf(stdout, NULL); 
			break; 
/*等待选项*/ 
		case 'i':		/* wait between sending packets */ 
			interval = atoi(optarg); 
			if (interval <= 0) { 
				(void)fprintf(stderr, 
				    "ping: bad timing interval.\n"); 
				exit(2); 
			} 
			options |= F_INTERVAL; 
			break; 
		case 'l': 
			if (!am_i_root) { 
				(void)fprintf(stderr, 
				    "ping: %s\n", strerror(EPERM)); 
				exit(2); 
			} 
			preload = atoi(optarg); 
			if (preload < 0) { 
				(void)fprintf(stderr, 
				    "ping: bad preload value.\n"); 
				exit(2); 
			} 
			break; 
	… 
		default: 
			usage(); 
		} 
	argc -= optind; 
	argv += optind; 
	 
	if (argc != 1) 
		usage(); 
	target = *argv; 
 
接下来的第三部分是用于获取地址，这里主要使用了inet_aton函数，将点分十进制地址转化为二进制地址。当然，作为完整的ping程序有较完善的出错处理： 
	memset(&whereto, 0, sizeof(struct sockaddr)); 
	to = (struct sockaddr_in *)&whereto; 
	to->sin_family = AF_INET; 
/*地址转换函数*/ 
	if (inet_aton(target, &to->sin_addr)) { 
		hostname = target; 
	} 
	else { 
#if 0 
		char * addr = resolve_name(target, 0); 
		if (!addr) { 
			(void)fprintf(stderr, 
			    "ping: unknown host %s\n", target); 
			exit(2); 
		} 
		to->sin_addr.s_addr = inet_addr(addr); 
		hostname = target; 
#else 
/*调用gethostbyname识别主机名*/ 
		hp = gethostbyname(target); 
		if (!hp) { 
			(void)fprintf(stderr, 
			    "ping: unknown host %s\n", target); 
			exit(2); 
		} 
		to->sin_family = hp->h_addrtype; 
		if (hp->h_length > (int)sizeof(to->sin_addr)) { 
			hp->h_length = sizeof(to->sin_addr); 
		} 
		memcpy(&to->sin_addr, hp->h_addr, hp->h_length); 
		(void)strncpy(hnamebuf, hp->h_name, sizeof(hnamebuf) - 1); 
		hostname = hnamebuf; 
#endif 
	} 
接下来的一部分主要是对各个选项（如路由、多播）选项的处理，这里就不做介绍。再接下来是ping函数的最主要部分，就是接收无限循环接收回应信息，这里主要用到了函数recvfrom。另外，对用户中断信息也有相应的处理，如下所示： 
	if (to->sin_family == AF_INET) 
		(void)printf("PING %s (%s): %d data bytes\n", hostname, 
		    inet_ntoa(*(struct in_addr *)&to->sin_addr.s_addr), 
		    datalen); 
	else 
		(void)printf("PING %s: %d data bytes\n", hostname, datalen); 
/*若程序接收到SIGINT或SIGALRM信号，调用相关的函数*/ 
	(void)signal(SIGINT, finish); 
	(void)signal(SIGALRM, catcher); 
… 
/*循环等待客户端的回应信息*/ 
	for (;;) { 
		struct sockaddr_in from; 
		register int cc; 
		int fromlen; 
 
		if (options & F_FLOOD) { 
/*形成ICMP回应数据包，在后面会有讲解*/ 
			pinger(); 
/*设定等待实践*/ 
			timeout.tv_sec = 0; 
			timeout.tv_usec = 10000; 
			fdmask = 1 << s; 
/*调用select函数*/ 
			if (select(s + 1, (fd_set *)&fdmask, (fd_set *)NULL, 
			    (fd_set *)NULL, &timeout) < 1) 
				continue; 
		} 
		fromlen = sizeof(from); 
/*接收客户端信息*/ 
		if ((cc = recvfrom(s, (char *)packet, packlen, 0, 
		    (struct sockaddr *)&from, &fromlen)) < 0) { 
			if (errno == EINTR) 
				continue; 
			perror("ping: recvfrom"); 
			continue; 
		} 
		pr_pack((char *)packet, cc, &from); 
		if (npackets && nreceived >= npackets) 
			break; 
	} 
	finish(0); 
	/* NOTREACHED */ 
	return 0; 
} 
（2）其他函数 
下面的函数也是ping程序中用到的重要函数。首先catcher函数是用户在发送SIGINT时调用的函数，在该函数中又调用了SIGALARM信号的处理来结束程序。 
static void 
catcher(int ignore) 
{ 
	int waittime; 
 
	(void)ignore; 
	pinger(); 
/*调用catcher函数*/ 
	(void)signal(SIGALRM, catcher); 
	if (!npackets || ntransmitted < npackets) 
		alarm((u_int)interval); 
	else { 
		if (nreceived) { 
			waittime = 2 * tmax / 1000; 
			if (!waittime) 
				waittime = 1; 
			if (waittime > MAXWAIT) 
				waittime = MAXWAIT; 
		} else 
			waittime = MAXWAIT; 
/*调用finish函数，并设定一定的等待实践*/ 
		(void)signal(SIGALRM, finish); 
		(void)alarm((u_int)waittime); 
	} 
} 
Pinger函数也是一个非常重要的函数，用于形成ICMP回应数据包，其中ID是该进程的ID，数据段中的前8字节用于存放时间间隔，从而可以计算ping程序从对端返回的往返时延差，这里的数据校验用到了后面定义的in_cksum函数。其代码如下所示： 
static void 
pinger(void) 
{ 
	register struct icmphdr *icp; 
	register int cc; 
	int i; 
 
/*形成icmp信息包，填写icmphdr结构体中的各项数据*/ 
	icp = (struct icmphdr *)outpack; 
	icp->icmp_type = ICMP_ECHO; 
	icp->icmp_code = 0; 
	icp->icmp_cksum = 0; 
	icp->icmp_seq = ntransmitted++; 
	icp->icmp_id = ident;			/* ID */ 
 
	CLR(icp->icmp_seq % mx_dup_ck); 
 
/*设定等待实践*/ 
	if (timing) 
		(void)gettimeofday((struct timeval *)&outpack[8], 
		    (struct timezone *)NULL); 
 
	cc = datalen + 8;			/* skips ICMP portion */ 
 
	/* compute ICMP checksum here */ 
	icp->icmp_cksum = in_cksum((u_short *)icp, cc); 
 
	i = sendto(s, (char *)outpack, cc, 0, &whereto, 
	    sizeof(struct sockaddr)); 
 
	if (i < 0 || i != cc)  { 
		if (i < 0) 
			perror("ping: sendto"); 
		(void)printf("ping: wrote %s %d chars, ret=%d\n", 
		    hostname, cc, i); 
	} 
	if (!(options & F_QUIET) && options & F_FLOOD) 
		(void)write(STDOUT_FILENO, &DOT, 1); 
} 
pr_pack是数据包显示函数，分别打印出IP数据包部分和ICMP回应信息。在规范的程序中通常将数据的显示部分独立出来，这样可以很好地加强程序的逻辑性和结构性。 
void 
pr_pack(char *buf, int cc, struct sockaddr_in *from) 
{ 
	register struct icmphdr *icp; 
	register int i; 
	register u_char *cp,*dp; 
/*#if 0*/ 
	register u_long l; 
	register int j; 
	static int old_rrlen; 
	static char old_rr[MAX_IPOPTLEN]; 
/*#endif*/ 
	struct iphdr *ip; 
	struct timeval tv, *tp; 
	long triptime = 0; 
	int hlen, dupflag; 
 
	(void)gettimeofday(&tv, (struct timezone *)NULL); 
 
	/* 检查IP数据包头信息 */ 
	ip = (struct iphdr *)buf; 
	hlen = ip->ip_hl << 2; 
	if (cc < datalen + ICMP_MINLEN) { 
		if (options & F_VERBOSE) 
			(void)fprintf(stderr, 
			  "ping: packet too short (%d bytes) from %s\n", cc, 
			  inet_ntoa(*(struct in_addr *)&from->sin_addr.s_addr)); 
		return; 
	} 
 
	/* ICMP部分显示 */ 
	cc -= hlen; 
	icp = (struct icmphdr *)(buf + hlen); 
	if (icp->icmp_type == ICMP_ECHOREPLY) { 
		if (icp->icmp_id != ident) 
			return;			/* 'Twas not our ECHO */ 
		++nreceived; 
		if (timing) { 
#ifndef icmp_data 
			tp = (struct timeval *)(icp + 1); 
#else 
			tp = (struct timeval *)icp->icmp_data; 
#endif 
			tvsub(&tv, tp); 
			triptime = tv.tv_sec * 10000 + (tv.tv_usec / 100); 
			tsum += triptime; 
			if (triptime < tmin) 
				tmin = triptime; 
			if (triptime > tmax) 
				tmax = triptime; 
		} 
 
		if (TST(icp->icmp_seq % mx_dup_ck)) { 
			++nrepeats; 
			--nreceived; 
			dupflag = 1; 
		} else { 
			SET(icp->icmp_seq % mx_dup_ck); 
			dupflag = 0; 
		} 
 
		if (options & F_QUIET) 
			return; 
 
		if (options & F_FLOOD) 
			(void)write(STDOUT_FILENO, &BSPACE, 1); 
		else { 
			(void)printf("%d bytes from %s: icmp_seq=%u", cc, 
			   inet_ntoa(*(struct in_addr *)&from->sin_addr.s_addr), 
			   icp->icmp_seq); 
			(void)printf(" ttl=%d", ip->ip_ttl); 
			if (timing) 
				(void)printf(" time=%ld.%ld ms", triptime/10, 
						triptime%10); 
			if (dupflag) 
				(void)printf(" (DUP!)"); 
			/* check the data */ 
#ifndef icmp_data 
			cp = ((u_char*)(icp + 1) + 8); 
#else 
			cp = (u_char*)icp->icmp_data + 8; 
#endif 
			dp = &outpack[8 + sizeof(struct timeval)]; 
			for (i = 8; i < datalen; ++i, ++cp, ++dp) { 
				if (*cp != *dp) { 
	(void)printf("\nwrong data byte #%d should be 0x%x but was 0x%x", 
	    i, *dp, *cp); 
					cp = (u_char*)(icp + 1); 
					for (i = 8; i < datalen; ++i, ++cp) { 
						if ((i % 32) == 8) 
							(void)printf("\n\t"); 
						(void)printf("%x ", *cp); 
					} 
					break; 
				} 
			} 
		} 
	} else { 
		/* We've got something other than an ECHOREPLY */ 
		if (!(options & F_VERBOSE)) 
			return; 
		(void)printf("%d bytes from %s: ", cc, 
		    pr_addr(from->sin_addr.s_addr)); 
		pr_icmph(icp); 
	} 
 
/*#if 0*/ 
	/*显示其他IP选项 */ 
	cp = (u_char *)buf + sizeof(struct iphdr); 
 
	for (; hlen > (int)sizeof(struct iphdr); --hlen, ++cp) 
		switch (*cp) { 
		case IPOPT_EOL: 
			hlen = 0; 
			break; 
		case IPOPT_LSRR: 
			(void)printf("\nLSRR: "); 
			hlen -= 2; 
			j = *++cp; 
			++cp; 
			if (j > IPOPT_MINOFF) 
				for (;;) { 
					l = *++cp; 
					l = (l<<8) + *++cp; 
					l = (l<<8) + *++cp; 
					l = (l<<8) + *++cp; 
					if (l == 0) 
						(void)printf("\t0.0.0.0"); 
				else 
					(void)printf("\t%s", pr_addr(ntohl(l))); 
				hlen -= 4; 
				j -= 4; 
				if (j <= IPOPT_MINOFF) 
					break; 
				(void)putchar('\n'); 
			} 
			break; 
		case IPOPT_RR: 
			j = *++cp;		/* get length */ 
			i = *++cp;		/* and pointer */ 
			hlen -= 2; 
			if (i > j) 
				i = j; 
			i -= IPOPT_MINOFF; 
			if (i <= 0) 
				continue; 
			if (i == old_rrlen 
			    && cp == (u_char *)buf + sizeof(struct iphdr) + 2 
			    && !memcmp((char *)cp, old_rr, i) 
			    && !(options & F_FLOOD)) { 
				(void)printf("\t(same route)"); 
				i = ((i + 3) / 4) * 4; 
				hlen -= i; 
				cp += i; 
				break; 
			} 
			old_rrlen = i; 
			memcpy(old_rr, cp, i); 
			(void)printf("\nRR: "); 
			for (;;) { 
				l = *++cp; 
				l = (l<<8) + *++cp; 
				l = (l<<8) + *++cp; 
				l = (l<<8) + *++cp; 
				if (l == 0) 
					(void)printf("\t0.0.0.0"); 
				else 
					(void)printf("\t%s", pr_addr(ntohl(l))); 
				hlen -= 4; 
				i -= 4; 
				if (i <= 0) 
					break; 
				(void)putchar('\n'); 
			} 
			break; 
		case IPOPT_NOP: 
			(void)printf("\nNOP"); 
			break; 
		default: 
			(void)printf("\nunknown option %x", *cp); 
			break; 
		} 
/*#endif*/ 
	if (!(options & F_FLOOD)) { 
		(void)putchar('\n'); 
		(void)fflush(stdout); 
	} 
} 
in_cksum是数据校验程序，如下所示： 
static int 
in_cksum(u_short *addr, int len) 
{ 
	register int nleft = len; 
	register u_short *w = addr; 
	register int sum = 0; 
	u_short answer = 0; 
 
	/*这里的算法很简单，就采用32bit的加法*/ 
	while (nleft > 1)  { 
		sum += *w++; 
		nleft -= 2; 
	} 
 
	if (nleft == 1) { 
		*(u_char *)(&answer) = *(u_char *)w ; 
		sum += answer; 
	} 
 
	/*把高16bit加到低16bit上去*/ 
	sum = (sum >> 16) + (sum & 0xffff); 
	sum += (sum >> 16); 
	answer = ~sum; 
	return(answer); 
} 
Finish程序是ping程序的结束程序，主要是打印出来一些统计信息，如下所示： 
static void 
finish(int ignore) 
{ 
	(void)ignore; 
 
	(void)signal(SIGINT, SIG_IGN); 
	(void)putchar('\n'); 
	(void)fflush(stdout); 
	(void)printf("--- %s ping statistics ---\n", hostname); 
	(void)printf("%ld packets transmitted, ", ntransmitted); 
	(void)printf("%ld packets received, ", nreceived); 
	if (nrepeats) 
		(void)printf("+%ld duplicates, ", nrepeats); 
	if (ntransmitted) 
		if (nreceived > ntransmitted) 
			(void)printf("-- somebody's printing up packets!"); 
		else 
			(void)printf("%d%% packet loss", 
			    (int) (((ntransmitted - nreceived) * 100) / 
			    ntransmitted)); 
	(void)putchar('\n'); 
	if (nreceived && timing) 
		(void)printf("round-trip min/avg/max = %ld.%ld/%lu.%ld/%ld.%ld ms\n", 
			tmin/10, tmin%10, 
			(tsum / (nreceived + nrepeats))/10, 
			(tsum / (nreceived + nrepeats))%10, 
			tmax/10, tmax%10); 
 
	if (nreceived==0) exit(1); 
	exit(0); 
} 
 
#ifdef notdef 
static char *ttab[] = { 
	"Echo Reply",		/* ip + seq + udata */ 
	"Dest Unreachable",	/* net, host, proto, port, frag, sr + IP */ 
	"Source Quench",	/* IP */ 
	"Redirect",		/* redirect 类型, gateway, + IP  */ 
	"Echo", 
	"Time Exceeded",	/*传输超时*/ 
	"Parameter Problem",	/* IP参数问题 */ 
	"Timestamp",		/* id + seq + three timestamps */ 
	"Timestamp Reply",	/* " */ 
	"Info Request",		/* id + sq */ 
	"Info Reply"		/* " */ 
}; 
#endif 
pr_icmph函数是用于打印出ICMP回应信息，如下所示： 
static void 
pr_icmph(struct icmphdr *icp) 
{ 
	switch(icp->icmp_type) { 
/*ICMP回应*/ 
	case ICMP_ECHOREPLY: 
		(void)printf("Echo Reply\n"); 
		/* XXX ID + Seq + Data */ 
		break; 
/*ICMP终点不可达*/ 
	case ICMP_DEST_UNREACH: 
		switch(icp->icmp_code) { 
		case ICMP_NET_UNREACH: 
			(void)printf("Destination Net Unreachable\n"); 
			break; 
		case ICMP_HOST_UNREACH: 
			(void)printf("Destination Host Unreachable\n"); 
			break; 
		case ICMP_PROT_UNREACH: 
			(void)printf("Destination Protocol Unreachable\n"); 
			break; 
		… 
		default: 
			(void)printf("Dest Unreachable, Unknown Code: %d\n", 
			    icp->icmp_code); 
			break; 
		} 
		/* Print returned IP header information */ 
#ifndef icmp_data 
		pr_retip((struct iphdr *)(icp + 1)); 
#else 
		pr_retip((struct iphdr *)icp->icmp_data); 
#endif 
		break; 
		… 
		default: 
			(void)printf("Redirect, Bad Code: %d", icp->icmp_code); 
			break; 
		} 
		(void)printf("(New addr: %s)\n",  
			     inet_ntoa(icp->icmp_gwaddr)); 
#ifndef icmp_data 
		pr_retip((struct iphdr *)(icp + 1)); 
#else 
		pr_retip((struct iphdr *)icp->icmp_data); 
#endif 
		break; 
	case ICMP_ECHO: 
		(void)printf("Echo Request\n"); 
		/* XXX ID + Seq + Data */ 
		break; 
	case ICMP_TIME_EXCEEDED: 
		switch(icp->icmp_code) { 
		case ICMP_EXC_TTL: 
			(void)printf("Time to live exceeded\n"); 
			break; 
		case ICMP_EXC_FRAGTIME: 
			(void)printf("Frag reassembly time exceeded\n"); 
			break; 
		default: 
			(void)printf("Time exceeded, Bad Code: %d\n", 
			    icp->icmp_code); 
			break; 
		} 
	… 
	default: 
		(void)printf("Bad ICMP type: %d\n", icp->icmp_type); 
	} 
} 
pr_iph函数是用于打印IP数据包头选项，如下所示： 
static void 
pr_iph(struct iphdr *ip) 
{ 
	int hlen; 
	u_char *cp; 
 
	hlen = ip->ip_hl << 2; 
	cp = (u_char *)ip + 20;		/* point to options */ 
	(void)printf("Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst Data\n"); 
	(void)printf(" %1x  %1x  %02x %04x %04x", 
	    ip->ip_v, ip->ip_hl, ip->ip_tos, ip->ip_len, ip->ip_id); 
	(void)printf("   %1x %04x", ((ip->ip_off) & 0xe000) >> 13, 
	    (ip->ip_off) & 0x1fff); 
	(void)printf("  %02x  %02x %04x", ip->ip_ttl, ip->ip_p, ip->ip_sum); 
	(void)printf(" %s ", inet_ntoa(*((struct in_addr *) &ip->ip_src))); 
	(void)printf(" %s ", inet_ntoa(*((struct in_addr *) &ip->ip_dst))); 
	/* dump and option bytes */ 
	while (hlen-- > 20) { 
		(void)printf("%02x", *cp++); 
	} 
	(void)putchar('\n'); 
} 
pr_addr是用于将ascii主机地址转换为十进制点分形式并打印出来，这里使用的函数是inet_ntoa，如下所示： 
static char * 
pr_addr(u_long l) 
{ 
	struct hostent *hp; 
	static char buf[256]; 
 
	if ((options & F_NUMERIC) || 
	    !(hp = gethostbyaddr((char *)&l, 4, AF_INET))) 
		(void)sprintf(buf, /*sizeof(buf),*/ "%s",inet_ntoa(*(struct in_addr *)&l)); 
	else 
		(void)sprintf(buf, /*sizeof(buf),*/ "%s (%s)", hp->h_name,inet_ntoa(*(struct in_addr *)&l)); 
	return(buf); 
} 
Usage函数是用于显示帮助信息，如下所示： 
static void 
usage(void) 
{ 
	(void)fprintf(stderr, 
	    "usage: ping [-LRdfnqrv] [-c count] [-i wait] [-l preload]\n\t[-p pattern] [-s packetsize] [-t ttl] [-I interface address] host\n"); 
	exit(2); 
}