如果目的主机在工 输出在工状态 如果5妙内无相应 用SIGALRM信号中断进程

#include "unp.h"

void send_echo_req(int sockfd, struct sockaddr_in *dstaddr);

uint16_t in_cksum(uint16_t *addr, int len);

void recv_echo_reply(int sockfd);

int main(int argc, char **argv)

{

int sockfd;

struct sockaddr_in dstaddr;

if ((sockfd = socket(PF_INET, SOCK_RAW, IPPROTO_ICMP)) == -1)

err_sys("socket");

bzero(&dstaddr, sizeof(dstaddr));

dstaddr.sin_family = AF_INET;

dstaddr.sin_port = htons(0);

if (inet_pton(AF_INET, argv[1], &dstaddr.sin_addr) <= 0)

err_sys("inet_pton");

send_echo_req(sockfd, &dstaddr);

recv_echo_reply(sockfd);

exit(0);

}

void send_echo_req(int sockfd, struct sockaddr_in *dstaddr)

{

char buf[100];

size_t len = sizeof(struct icmp);

struct icmp *icmp;

socklen_t dstlen = sizeof(struct sockaddr_in);

bzero(buf, sizeof(buf));

icmp = (struct icmp *)buf;

icmp->icmp_type = ICMP_ECHO;

icmp->icmp_code = 0;

icmp->icmp_id = getpid();

icmp->icmp_seq = 1;

icmp->icmp_cksum = in_cksum((uint16_t *) icmp, sizeof(struct icmp));

if (sendto(sockfd, buf, len, 0, (SA *)dstaddr, dstlen) == -1)

err_sys("sendto");

}

void recv_echo_reply(int sockfd)

{

char buf[100];

ssize_t n;

struct ip *ip;

struct icmp *icmp;

while (1) {

alarm(5); /* set timeout */

if ((n = read(sockfd, buf, sizeof(buf))) == -1)

err_sys("read");

ip = (struct ip *)buf;

if (ip->ip_p != IPPROTO_ICMP) {

fprintf(stderr, "protocol error.\r\n");

exit(1);

}

icmp = (struct icmp *)(buf + sizeof(struct ip));

if (icmp->icmp_type == ICMP_ECHOREPLY) {

if (icmp->icmp_id != getpid()) {

fprintf(stderr, "not this process.\r\n");

exit(1);

} else {

printf("destination host is alive.\r\n");

break;

}

}

}

}

uint16_t in_cksum(uint16_t *addr, int len)

{

int nleft = len;

uint32_t sum = 0;

uint16_t *w = addr;

uint16_t answer = 0;

while (nleft > 1) {

sum += *w++;

nleft -= 2;

}

if (nleft == 1) {

*(unsigned char *)(&answer) = *(unsigned char *)w ;

sum += answer;

}

sum = (sum >> 16) + (sum & 0xffff);

sum += (sum >> 16);

answer = ~sum;

return(answer);

}

void err_sys(const char *errmsg)

{

perror(errmsg);

exit(1);

}