ping.c

socket编程的一个例子

（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 <sys/param.h>
#include <sys/socket.h>
#include <sys/file.h>
#include <sys/time.h>
#include <sys/signal.h>

#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <ctype.h>
#include <errno.h>
#include <getopt.h>
#include <resolv.h>
#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;
		}