space of ath_node

[jiani1012]

Hi,all    
        A problem puzzled me in ath code. The following code in function  ath_node_alloc( ) shows the space allocated to an,

>struct ath_ratectrl {
>size_t arc_space;    /* space required for per-node state */
>}; 
 
 >const size_t space = sizeof(struct ath_node) + sc->sc_rc->arc_space;
 >struct ath_node *an;
 >an = malloc(space, M_80211_NODE, M_NOWAIT|M_ZERO);
 
but in the two lines below ,what is (an)[1]?  Where is the initialization of (an)[1]? What does it contain? Can you show me? and how to get the content of sn?
 
>struct sample_node *sn = ATH_NODE_SAMPLE(an);       /*in sample.c*/
>#define ATH_NODE_SAMPLE(an) ((struct sample_node *)&(an)[1])      /*in sample.h*/
 
Otherwise, the struct ath_ratectrl has been used in structures ath_softc, sample_softc and so on. Is there any relations between them?
 
Thank you!
Jeny