Simple PING implementation in C

#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/signal.h>
#include <string.h>

#define DEFDATALEN      56
#define MAXIPLEN        60
#define MAXICMPLEN      76

static char *hostname = NULL;

static int in_cksum(unsigned short *buf, int sz)
{
  int nleft = sz;
  int sum = 0;
  unsigned short *w = buf;
  unsigned short ans = 0;
  
  while (nleft > 1) {
    sum += *w++;
    nleft -= 2;
  }
  
  if (nleft == 1) {
    *(unsigned char *) (&ans) = *(unsigned char *) w;
    sum += ans;
  }
  
  sum = (sum >> 16) + (sum & 0xFFFF);
  sum += (sum >> 16);
  ans = ˜sum;
  return (ans);
}

static void noresp(int ign)
{
  printf("No response from %s\n", hostname);
  exit(0);
}

static void ping(const char *host)
{
  struct hostent *h;
  struct sockaddr_in pingaddr;
  struct icmp *pkt;
  int pingsock, c;
  char packet[DEFDATALEN + MAXIPLEN + MAXICMPLEN];
  
  if ((pingsock = socket(AF_INET, SOCK_RAW, 1)) < 0) {       /* 1 == ICMP */
    perror("ping: creating a raw socket");
    exit(1);
  }
  
  /* drop root privs if running setuid */
  setuid(getuid());
  
  memset(&pingaddr, 0, sizeof(struct sockaddr_in));
  
  pingaddr.sin_family = AF_INET;
  if (!(h = gethostbyname(host))) {
    fprintf(stderr, "ping: unknown host %s\n", host);
    exit(1);
  }
  memcpy(&pingaddr.sin_addr, h->h_addr, sizeof(pingaddr.sin_addr));
  hostname = h->h_name;
  
  pkt = (struct icmp *) packet;
  memset(pkt, 0, sizeof(packet));
  pkt->icmp_type = ICMP_ECHO;
  pkt->icmp_cksum = in_cksum((unsigned short *) pkt, sizeof(packet));
  
  c = sendto(pingsock, packet, sizeof(packet), 0,
             (struct sockaddr *) &pingaddr, sizeof(struct sockaddr_in));
  
  if (c < 0 || c != sizeof(packet)) {
    if (c < 0)
      perror("ping: sendto");
    fprintf(stderr, "ping: write incomplete\n");
    exit(1);
  }
  
  signal(SIGALRM, noresp);
  alarm(2);                                     /* give the host 5000ms to respond */
  /* listen for replies */
  while (1) {
    struct sockaddr_in from;
    size_t fromlen = sizeof(from);
    
    if ((c = recvfrom(pingsock, packet, sizeof(packet), 0,
                      (struct sockaddr *) &from, &fromlen)) < 0) {
      if (errno == EINTR)
        continue;
      perror("ping: recvfrom");
      continue;
    }
    if (c >= 76) {                   /* ip + icmp */
      struct iphdr *iphdr = (struct iphdr *) packet;
      
      pkt = (struct icmp *) (packet + (iphdr->ihl << 2));      /* skip ip hdr */
      if (pkt->icmp_type == ICMP_ECHOREPLY)
        break;
    }
  }
  printf("%s is alive!\n", hostname);
  return;
}

int main ()
{
  ping ("192.168.1.2");

}

Reference: http://www.koders.com/c/fid30EA22902AFF5800481BBC6F65DADCDAF92D6E37.aspx