uint8 * res_arp(
???????????????? struct iface *iface,??/* Pointer to interface block */
???????????????? enum?? arp_hwtype hardware,?/* Hardware type */
???????????????? int32? target,???/* Target IP address */
???????????????? struct mbuf **bpp)??/* IP datagram to be queued if unresolved */???????????????
{
??? register struct arp_tab *arp;
??? struct ip ip;
???
??? if((arp = arp_lookup(hardware,target)) != NULL && arp->state == ARP_VALID)
??? ?return arp->hw_addr;
??? if(arp != NULL){
??? ?/* Earlier packets are already pending, kick this one back
??? ? * as a source quench
??? ? */
??? ?ntohip(&ip,bpp);
??? ?icmp_output(&ip,*bpp,ICMP_QUENCH,0,NULL);
??? ?free_p(bpp);
??? } else {
??? ?/* Create an entry and put the datagram on the
??? ? * queue pending an answer
??? ? */
??? ?arp = arp_add(target,hardware,NULL,0);
??? ?enqueue(&arp->pending,bpp);
??? ?arp_output(iface,hardware,target);
??? }
??? return NULL;
}