头歌icmp ping实现第三关代码答案

以下是一个使用C语言实现icmp ping的示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/ip_icmp.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <time.h>
#include <sys/time.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;

void send_packet();
void recv_packet();
void calc_rtt(struct timeval tvrecv);
void tv_sub(struct timeval *out, struct timeval *in);

unsigned short cal_chksum(unsigned short *addr, int len) {
    int nleft = len;
    int sum = 0;
    unsigned short *w = addr;
    unsigned short 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 send_packet() {
    memset(sendpacket, 0, sizeof(sendpacket));
    struct icmp *icmp = (struct icmp*)sendpacket;
    icmp->icmp_type = ICMP_ECHO;
    icmp->icmp_code = 0;
    icmp->icmp_id = pid;
    icmp->icmp_seq = nsend++;
    memset(icmp->icmp_data, 'a', datalen);
    gettimeofday((struct timeval*)icmp->icmp_data, NULL);
    icmp->icmp_cksum = cal_chksum((unsigned short*)icmp, sizeof(struct icmp) + datalen);
    sendto(sockfd, sendpacket, sizeof(struct icmp) + datalen, 0, (struct sockaddr *)&dest_addr, sizeof(dest_addr));
}

void recv_packet() {
    int n;
    socklen_t fromlen;
    struct timeval tvrecv;
    struct ip *ip;
    struct icmp *icmp;
    fromlen = sizeof(dest_addr);
    if ((n = recvfrom(sockfd, recvpacket, sizeof(recvpacket), 0, (struct sockaddr *)&dest_addr, &fromlen)) < 0) {
        perror("recvfrom error");
        return;
    }
    gettimeofday(&tvrecv, NULL);
    ip = (struct ip*)recvpacket;
    icmp = (struct icmp*)(recvpacket + (ip->ip_hl << 2));
    if (icmp->icmp_type == ICMP_ECHOREPLY && icmp->icmp_id == pid) {
        calc_rtt(tvrecv);
    }
}

void calc_rtt(struct timeval tvrecv) {
    struct timeval *tvsend;
    double rtt;
    tvsend = (struct timeval*)recvpacket;
    tv_sub(&tvrecv, tvsend);
    rtt = tvrecv.tv_sec * 1000.0 + tvrecv.tv_usec / 1000.0;
    printf("%d bytes from %s: icmp_seq=%u ttl=%d time=%.3f ms\n", datalen, inet_ntoa(dest_addr.sin_addr), nreceived + 1, ip->ip_ttl, rtt);
    nreceived++;
}

void tv_sub(struct timeval *out, struct timeval *in) {
    if ((out->tv_usec -= in->tv_usec) < 0) {
        --out->tv_sec;
        out->tv_usec += 1000000;
    }
    out->tv_sec -= in->tv_sec;
}

int main(int argc, char *argv[]) {
    if (argc != 2) {
        printf("usage: %s <hostname/IP address>\n", argv[0]);
        exit(1);
    }

    struct hostent *host;
    host = gethostbyname(argv[1]);
    if (host == NULL) {
        printf("unknown host %s\n", argv[1]);
        exit(1);
    }

    pid = getpid();
    sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
    if (sockfd < 0) {
        perror("socket error");
        exit(1);
    }

    dest_addr.sin_family = AF_INET;
    dest_addr.sin_addr = *((struct in_addr *)host->h_addr);
    printf("PING %s (%s): %d data bytes\n", argv[1], inet_ntoa(dest_addr.sin_addr), datalen);

    while (nsend < MAX_NO_PACKETS) {
        send_packet();
        recv_packet();
        sleep(1);
    }

    printf("--- %s ping statistics ---\n", argv[1]);
    printf("%d packets transmitted, %d received, %.0f%% packet loss\n", nsend, nreceived, (nsend - nreceived) * 100.0 / nsend);
    close(sockfd);
    return 0;
}

注意在编译时需要加上-lm选项，即：

gcc icmp_ping.c -o icmp_ping -lm

其中-lm选项是为了链接数学库，因为代码中使用了数学库中的函数。