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.\" Copyright (c) 2001 Dag-Erling Coïdan Smørgrav
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.Dd January 27, 2001
.Dt ZONE 9
.Os
.Sh NAME
.Nm uma_zalloc ,
.Nm uma_zfree ,
.Nm uma_zdestroy
.Nd zone allocator
.Sh SYNOPSIS
.In sys/param.h
.In sys/queue.h
.In vm/uma.h
.Ft void *
.Fn uma_zalloc "vm_zone_t z"
.Ft void
.Fn uma_zfree "vm_zone_t z" "void *item"
.Ft void
.Fn uma_zdestroy "vm_zone_t z"
.Sh DESCRIPTION
The zone allocator provides an efficient interface for managing
dynamically-sized collections of items of similar size.
The zone allocator can work with preallocated zones as well as with
runtime-allocated ones, and is therefore available much earlier in the
boot process than other memory management routines.
.Pp
A zone is an extensible collection of items of identical size.
The zone allocator keeps track of which items are in use and which
are not, and provides functions for allocating items from the zone and
for releasing them back (which makes them available for later use).
.Pp
The zone allocator stores state information inside the items proper
while they are not allocated,
so structures that will be managed by the zone allocator
and wish to use the type stable property of zones by leaving some fields
pre-filled between allocations, must reserve
two pointers at the very beginning for internal use by the zone
allocator, as follows:
.Bd -literal
struct my_item {
        struct my_item  *z_rsvd1;
        struct my_item  *z_rsvd2;
        /* rest of structure */
};
.Ed
.Pp
Alternatively they should assume those entries corrupted
after each allocation.
After the first allocation of an item,
it will have been cleared to zeroes, however subsequent allocations
will retain the contents as of the last free, with the exception of the
fields mentioned above.
.Pp
To allocate an item from a zone, simply call
.Fn uma_zalloc
with a pointer to that zone; it will return a pointer to an item, or
.Dv NULL
in the rare case where all items in the zone are in use and the
allocator is unable to grow the zone.
.Pp
Items are released back to the zone from which they were allocated by
calling
.Fn uma_zfree
with a pointer to the zone and a pointer to the item.
.Pp
Zones created with
.Fn zinit
can be destroyed using
.Fn uma_zdestroy ,
freeing all memory that was allocated for the zone.
All items allocated from the zone with
.Fn uma_zalloc
must have been freed with
.Fn uma_zfree
before.
.Sh RETURN VALUES
.Pp
The
.Fn zinit
function returns a pointer to a fully initialized
.Vt "struct vm_zone" ,
or
.Dv NULL
if it was unable to
.Fn malloc
a
.Vt "struct vm_zone" .
.Pp
The
.Fn uma_zalloc
function returns a pointer to an item, or
.Dv NULL
if the zone ran out of unused items and the allocator was unable to
enlarge it.
.Sh SEE ALSO
.Xr malloc 9
.Sh HISTORY
The zone allocator first appeared in
.Fx 3.0 .
.Sh AUTHORS
.An -nosplit
The zone allocator was written by
.An John S. Dyson .
.Pp
This manual page was written by
.An Dag-Erling Co\(:idan Sm\(/orgrav Aq des@FreeBSD.org .