The zone allocator stores state information inside the items proper,

While an item is not in use, the first sizeof(void *) bytes are used

so structures that will be managed by the zone allocator must reserve

for a pointer to another available item.  When allocated, all of the 

two pointers at the very beginning for internal use by the zone

item's space is available, except with

allocator, as follows:

.Dv INVARIANTS

.Bd -literal

defined, where the second sizeof(void *) bytes are used for sanity

struct my_item {

checks, and we would get a panic trying to free the item if those bytes

        struct my_item  *z_rsvd1;

were somehow changed to

        struct my_item  *z_rsvd2;

.Dv ZENTRY_FREE .

        /* rest of structure */

.Pp

};

A similar algorithm is further described in Donald Knuth's `The Art of

.Ed

Computer Programming' volume 1, starting at page 254 (section 2.2.3).

--

.An Dag-Erling Co\(:idan Sm\(/orgrav Aq des@FreeBSD.org .

.An Dag-Erling Co\(:idan Sm\(/orgrav Aq des@FreeBSD.org

--

and modified by

.An Robert Drehmel .