obtain ARP header from 'sk_buff'

Hello,

kernel 2.4.20 in embedded system. In my driver I need to access ARP header
fields, including MAC addresses; unfortunately in include/linux/if_arp.h
those specific fields are commented (I gueess this is right, as the hardware
addresses might have different lengths depending on network type). So I've
tried this trick -- defined my own 'arp_hdr' structure in my code with
necessary fields:

struct arp_hdr {
    /* all the fields taken from include/linux/if_arp.h */
    /* and  additionaly these*/
   unsigned char       ar_sha[ETH_ALEN];   /* sender hardware address  */
   unsigned char       ar_sip[4];      /* sender IP address        */
   unsigned char       ar_tha[ETH_ALEN];   /* target hardware address  */
   unsigned char       ar_tip[4];
}

int my_func(struct sk_buff *skb)
{
    struct arp_hdr  *arph;

   arph = (struct arp_hdr *)skb->nh.raw;

  /* now print out the contents of the ARP header, but I get garbage */
}

Why do I get garbage and waht is the right way to do what I want?

Thanks in advance!