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.Dd January 20, 2026
.Dt UNR 9
.Os
.Sh NAME
.Nm new_unrhdr ,
.Nm delete_unrhdr ,
.Nm alloc_unr ,
.Nm alloc_unr1 ,
.Nm free_unr
.Nd "kernel unit number allocator"
.Sh SYNOPSIS
.In sys/systm.h
.Ft int
.Fn alloc_unr "struct unrhdr *uh"
.Ft int
.Fn alloc_unrl "struct unrhdr *uh"
.Ft void
.Fn delete_unrhdr "struct unrhdr *uh"
.Ft void
.Fn free_unr "struct unrhdr *uh" "u_int item"
.Ft "struct unrhdr *"
.Fn new_unrhdr "int low" "int high" "struct lock *lock"
.Sh DESCRIPTION
The kernel unit number allocator is a generic facility, which allows to allocate
unit numbers within a specified range.
.Bl -tag -width indent
.It Fn new_unrhdr low high lock
Initialize a new unit number allocator entity.
The
.Fa low
and
.Fa high
arguments
specify minimum and maximum number of unit numbers.
There is no cost associated with the range of unit numbers, so unless the resource
really is finite,
.Dv INT_MAX
can be used.
If
.Fa lock
is not
.Dv NULL ,
it is used for locking when allocating and freeing units.
.It Fn delete_unrhdr uh
Delete specified unit number allocator entity.
This function frees the memory associated with the entity, it does not free
any units.
.It Fn alloc_unr uh
Return a new unit number.
The lowest free number is always allocated.
This function does not allocate memory and never sleeps, however it may
block on a lock.
If no free unit numbers are left,
.Li \-1
is returned.
.It Fn alloc_unrl uh
Same as
.Fn alloc_unr
except that lock is assumed to be already locked and thus is not used.
.It Fn free_unr uh item
Free a previously allocated unit number.
This function may require allocating memory, and thus it can sleep.
There is no pre-locked variant.
.El
.Sh FILES
The above functions are implemented in
.Pa sys/kern/subr_unit.c .
.Sh SEE ALSO
.Xr idr 9
.Sh HISTORY
Kernel unit number allocator first appeared in
.Fx 6.0 ,
and was imported into
.Dx 3.5 .
.Sh AUTHORS
.An -nosplit
Kernel unit number allocator was written by
.An Poul-Henning Kamp .
This manpage was written by
.An Gleb Smirnoff .