用c语言实现ping命令

实现ping命令需要使用原始套接字，因此需要root权限。以下是一个简单的ping程序实现：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <signal.h>
#include <time.h>

#define PACKET_SIZE  64
#define MAX_WAIT_TIME 5
#define MAX_NO_PACKETS 3

int sockfd, datalen = 56; 
char sendpacket[PACKET_SIZE];
char recvpacket[PACKET_SIZE];
struct sockaddr_in dest_addr;
pid_t pid;
int nsend = 0, nreceived = 0;
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 main(int argc, char *argv[])
{
    struct hostent *host;
    struct protoent *protocol;
    unsigned long inaddr = 0l;
    int waittime = MAX_WAIT_TIME;
    int size = 50 * 1024;

    if(argc < 2) {
        printf("Usage: %s hostname/IP address\n", argv[0]);
        exit(1);
    }

    if((protocol = getprotobyname("icmp")) == NULL) {
        perror("getprotobyname");
        exit(1);
    }

    if((sockfd = socket(AF_INET, SOCK_RAW, protocol->p_proto)) < 0) {
        perror("socket error");
        exit(1);
    }

    setuid(getuid());

    setsockopt(sockfd, SOL_SOCKET, SO_RCVBUF, &size, sizeof(size));

    bzero(&dest_addr, sizeof(dest_addr));
    dest_addr.sin_family = AF_INET;

    if(inaddr = inet_addr(argv[1]) == INADDR_NONE) {
        if((host = gethostbyname(argv[1])) == NULL) {
            perror("gethostbyname error");
            exit(1);
        }
        memcpy((char *)&dest_addr.sin_addr, host->h_addr, host->h_length);
    } else {
        dest_addr.sin_addr.s_addr = inaddr;
    }

    pid = getpid();
    printf("PING %s (%s): %d bytes data in ICMP packets.\n", argv[1], inet_ntoa(dest_addr.sin_addr), datalen);

    signal(SIGALRM, statistics);

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

    return 0;
}

int pack(int pack_no)
{
    int packsize;
    struct icmp *icmp;
    struct timeval *tval;

    icmp = (struct icmp*)sendpacket;
    icmp->icmp_type = ICMP_ECHO;
    icmp->icmp_code = 0;
    icmp->icmp_cksum = 0;
    icmp->icmp_seq = pack_no;
    icmp->icmp_id = pid;
    packsize = 8 + datalen;
    tval = (struct timeval*)icmp->icmp_data;
    gettimeofday(tval, NULL);
    icmp->icmp_cksum = cal_chksum((unsigned short*)icmp, packsize);

    return packsize;
}

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(void)
{
    int packetsize;

    packetsize = pack(nsend);
    if(sendto(sockfd, sendpacket, packetsize, 0, (struct sockaddr *)&dest_addr, sizeof(dest_addr)) < 0) {
        perror("sendto error");
    }
}

void recv_packet(void)
{
    int n;
    socklen_t fromlen;
    extern int errno;
    fromlen = sizeof(dest_addr);

    alarm(MAX_WAIT_TIME);
    while(1) {
        if((n = recvfrom(sockfd, recvpacket, sizeof(recvpacket), 0, (struct sockaddr *)&dest_addr, &fromlen)) < 0) {
            if(errno == EINTR) {
                continue;
            }
            perror("recvfrom error");
            continue;
        }
        gettimeofday(&tvrecv, NULL);

        struct ip *ip = (struct ip *)recvpacket;
        struct icmp *icmp = (struct icmp *)(recvpacket + (ip->ip_hl << 2));
        if(icmp->icmp_type == ICMP_ECHOREPLY) {
            nreceived++;
            printf("%d bytes from %s: icmp_seq=%u ttl=%d time=%.2f ms\n",
                    n, inet_ntoa(dest_addr.sin_addr), icmp->icmp_seq, ip->ip_ttl, 
                    (tvrecv.tv_sec - ((struct timeval *)icmp->icmp_data)->tv_sec) * 1000.0 + 
                    (tvrecv.tv_usec - ((struct timeval *)icmp->icmp_data)->tv_usec) / 1000.0);
        }
        else if (icmp->icmp_type == ICMP_TIME_EXCEEDED) {
            printf(" From %s icmp_seq=%u Time to live exceeded\n", inet_ntoa(dest_addr.sin_addr), icmp->icmp_seq);
        }
        else {
            printf("received packet from %s type %d\n", inet_ntoa(dest_addr.sin_addr), icmp->icmp_type);
        }

        break;
    }
}

void statistics(int signo)
{
    printf("\n%d packets transmitted, %d received, %%%d lost\n", nsend, nreceived, (nsend - nreceived) / nsend * 100);
    close(sockfd);
    exit(1);
}

该程序使用了原始套接字实现ICMP协议的包发送和接收。在发送数据包时，程序填充ICMP数据包结构体，计算校验和并发送数据包；在接收数据包时，程序解析IP和ICMP头部，判断接收到的数据包是不是对自己发送的数据包的回应。

注意：由于该程序需要访问网络层，因此需要root权限运行。