Thanks for all your help,

So others can benefit, the magic incantation I was looking for (refering to
the original question about how to get an ethernet MAC address)

is 

    unsigned char buf[IEEE802II_NWID_LEN];
    ifr.ifr_data=buf;
    ioctl(s,SIOCG802IINWID,&ifr);

which is defined in net/if_ieee80211.h

Thanks,

Roger