TCP IP 源代码

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;
}