NAME

     new_unrhdr, clear_unrhdr, delete_unrhdr, alloc_unr, alloc_unr_specific, free_unr — kernel
     unit number allocator

SYNOPSIS

     #include <sys/systm.h>

     struct unrhdr *
     new_unrhdr(int low, int high, struct mtx *mutex);

     void
     clear_unrhdr(struct unrhdr *uh);

     void
     delete_unrhdr(struct unrhdr *uh);

     int
     alloc_unr(struct unrhdr *uh);

     int
     alloc_unrl(struct unrhdr *uh);

     int
     alloc_unr_specific(struct unrhdr *uh, u_int item);

     void
     free_unr(struct unrhdr *uh, u_int item);

DESCRIPTION

     The kernel unit number allocator is a generic facility, which allows to allocate unit
     numbers within a specified range.

     new_unrhdr(low, high, mutex)
             Initialize a new unit number allocator entity.  The low and 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, INT_MAX can be used.
             If mutex is not NULL, it is used for locking when allocating and freeing units.
             Otherwise, internal mutex is used.

     clear_unrhdr(uh)
             Clear all units from the specified unit number allocator entity.  This function
             resets the entity as if it were just initialized with new_unrhdr().

     delete_unrhdr(uh)
             Delete specified unit number allocator entity.  This function frees the memory
             associated with the entity, it does not free any units.  To free all units use
             clear_unrhdr().

     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 mutex.
             If no free unit numbers are left, -1 is returned.

     alloc_unrl(uh)
             Same as alloc_unr() except that mutex is assumed to be already locked and thus is
             not used.

     alloc_unr_specific(uh, item)
             Allocate a specific unit number.  This function allocates memory and thus may sleep.
             The allocated unit number is returned on success.  If the specified number is
             already allocated or out of the range, -1 is returned.

     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.

CODE REFERENCES

     The above functions are implemented in sys/kern/subr_unit.c.

HISTORY

     Kernel unit number allocator first appeared in FreeBSD 6.0.

AUTHORS

     Kernel unit number allocator was written by Poul-Henning Kamp.  This manpage was written by
     Gleb Smirnoff.