#include	"udpcksum.h"struct udpiphdr *udp_check (char *, int);/* * Read from the network until a UDP datagram is read that matches * the arguments. *//* include udp_read */struct udpiphdr *udp_read (void){  int len;  char *ptr;  struct ether_header *eptr;  for (;;)    {      ptr = next_pcap (&len);      switch (datalink)	{	case DLT_NULL:		/* loopback header = 4 bytes */	  return (udp_check (ptr + 4, len - 4));	case DLT_EN10MB:	  eptr = (struct ether_header *) ptr;	  if (ntohs (eptr->ether_type) != ETHERTYPE_IP)	    err_quit ("Ethernet type %x not IP", ntohs (eptr->ether_type));	  return (udp_check (ptr + 14, len - 14));	case DLT_SLIP:		/* SLIP header = 24 bytes */	  return (udp_check (ptr + 24, len - 24));	case DLT_PPP:		/* PPP header = 24 bytes */	  return (udp_check (ptr + 24, len - 24));	default:	  err_quit ("unsupported data link (%d)", datalink);	}    }}/* end udp_read *//* * Check the received packet. * If UDP and OK, return pointer to packet. * If ICMP error, return NULL. * We assume the filter picks out desired UDP datagrams. *//* include udp_check */struct udpiphdr *udp_check (char *ptr, int len){  int hlen;  struct ip *ip;  struct udpiphdr *ui;  if (len < sizeof (struct ip) + sizeof (struct udphdr))      err_quit ("len = %d", len);  /* minimal verification of IP header */  ip = (struct ip *) ptr;  if (ip->ip_v != IPVERSION)    err_quit ("ip_v = %d", ip->ip_v);  hlen = ip->ip_hl << 2;  if (hlen < sizeof (struct ip))      err_quit ("ip_hl = %d", ip->ip_hl);  if (len < hlen + sizeof (struct udphdr))      err_quit ("len = %d, hlen = %d", len, hlen);  if ((ip->ip_sum = in_cksum ((u_short *) ip, hlen)) != 0)    err_quit ("ip checksum error");  if (ip->ip_p == IPPROTO_UDP)    {      ui = (struct udpiphdr *) ip;      return (ui);    }  else    err_quit ("not a UDP packet");}/* end udp_check */