why does symbol & in #define QO_PTR(q_node, q_type, q_ptr) needed?

[ooze]

typedef unsigned long PARAM;

typedef PARAM SAP;

typedef struct qnode
{
struct qnode *next;
struct qnode *prev;
}
QNODE;

typedef struct qhead
{
struct qnode *head;
struct qnode *tail;
}
QHEAD;

typedef struct
{
QHEAD qhead;
int (*srv_req) (SAP sys_sap);
SAP sys_sap;
}
SSRV_S;

typedef struct sig_s
{
struct sig_s *next;
struct sig_s *prev;
int waiting;
SAP ssrv_sap;
SIG_FUNC func;
SAP user_sap;
PARAM ident;
}
SIG_S;
/*************** *************** *************** *************** ************
* slib_init
*************** *************** *************** *************** *************
*
*/
PUBLIC int slib_init
(
int (*func) (SAP SysSap),
SAP sys_sap,
SAP *ssrv_sap
)
{
SSRV_S *ssrv;

/* Self initialize, if needed.
*/
if (! s_initialized)
s_init ();

/* Try to allocate a sap
*/
#if MAX_SIG_SRV
for (ssrv = ssrv_pool; ssrv < ssrv_pool + MAX_SIG_SRV; ++ssrv)
if (ssrv->srv_req == NULL)
break;
if (ssrv >= ssrv_pool + MAX_SIG_SRV)
return -1;
#else
ssrv = (SSRV_S *) MEM_ALLOC (sizeof (SSRV_S));
if (ssrv == NULL)
return -1;
#endif

/* Initialize it.
*/
Q_INIT (&ssrv->qhead);
ssrv->srv_req = func;
ssrv->sys_sap = sys_sap;

/* Tell the caller the address of the SSRV_S and return success.
*/
*ssrv_sap = (SAP) ssrv;
return 0;
}
/*************** *************** *************** *************** ************
* slib_serv
*************** *************** *************** *************** *************
*
*/
PUBLIC void slib_serv
(
SAP ssrv_sap
)
{
SSRV_S *ssrv = (SSRV_S *) ssrv_sap;
SIG_S *sig;
IPL_VALUE level;
int limit = SIG_SERV_LIMIT;

/* Call the functions for each signaled event.
*/
while (((sig = (SIG_S *) ssrv->qhead.head) != NULL) && (limit-- > 0))
{
level = IPL_SET_NORMAL ();
QO_REMOVE (sig, &ssrv->qhead, SIG_S, next);
sig->waiting = 0;
IPL_RESTORE (level);
(*sig->func) (sig->user_sap, sig->ident);
}
}


#define QO_REMOVE(q_nod e, q_head, q_type, q_ptr) { \
register QNODE * node = (QNODE *) (void *) q_node; \
register QHEAD * head = (QHEAD *) (void *) q_head; \
if (QO_PTR(node, q_type, q_ptr)->prev == NULL) \
head->head = QO_PTR(node, q_type, q_ptr)->next; \
else \
QO_PTR(QO_PTR(n ode, q_type, q_ptr)->prev, q_type, q_ptr)->next = \
QO_PTR(node, q_type, q_ptr)->next; \
if (QO_PTR(node, q_type, q_ptr)->next == NULL) \
head->tail = QO_PTR(node, q_type, q_ptr)->prev; \
else \
QO_PTR(QO_PTR(n ode, q_type, q_ptr)->next, q_type, q_ptr)->prev = \
QO_PTR(node, q_type, q_ptr)->prev; \
QO_PTR(node, q_type, q_ptr)->next = NULL; \
QO_PTR(node, q_type, q_ptr)->prev = NULL; \
}

#define QO_PTR(q_node, q_type, q_ptr) \
((QNODE *) (void *) (&((q_type *) (void *) q_node)->q_ptr))