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NAME

     hashinit, hashdestroy, phashinit - manage kernel hash tables

SYNOPSIS

     #include <sys/malloc.h>
     #include <sys/systm.h>
     #include <sys/queue.h>

     void *
     hashinit(int nelements, struct malloc_type *type, u_long *hashmask);

     void
     hashdestroy(void *hashtbl, struct malloc_type *type, u_long hashmask);

     void *
     phashinit(int nelements, struct malloc_type *type, u_long *nentries);

DESCRIPTION

     The hashinit() and phashinit() functions allocate space for hash tables
     of size given by the argument nelements.

     The hashinit() function allocates hash tables that are sized to largest
     power of two less than or equal to argument nelements.  The phashinit()
     function allocates hash tables that are sized to the largest prime number
     less than or equal to argument nelements.  Allocated hash tables are
     contiguous arrays of LIST_HEAD(3) entries, allocated using malloc(9), and
     initialized using LIST_INIT(3).  The malloc arena to be used for
     allocation is pointed to by argument type.

     The hashdestroy() function frees the space occupied by the hash table
     pointed to by argument hashtbl.  Argument type determines the malloc
     arena to use when freeing space.  The argument hashmask should be the bit
     mask returned by the call to hashinit() that allocated the hash table.

IMPLEMENTATION NOTES

     The largest prime hash value chosen by phashinit() is 32749.

RETURN VALUES

     The hashinit() function returns a pointer to an allocated hash table and
     sets the location pointed to by hashmask to the bit mask to be used for
     computing the correct slot in the hash table.

     The phashinit() function returns a pointer to an allocated hash table and
     sets the location pointed to by nentries to the number of rows in the
     hash table.

EXAMPLES

     A typical example is shown below:

           ...
           static LIST_HEAD(foo, foo) *footable;
           static u_long foomask;
           ...
           footable = hashinit(32, M_FOO, &foomask);

     Here we allocate a hash table with 32 entries from the malloc arena
     pointed to by M_FOO.  The mask for the allocated hash table is returned
     in foomask.  A subsequent call to hashdestroy() uses the value in
     foomask:

           ...
           hashdestroy(footable, M_FOO, foomask);

DIAGNOSTICS

     The hashinit() and phashinit() functions will panic if argument nelements
     is less than or equal to zero.

     The hashdestroy() function will panic if the hash table pointed to by
     hashtbl is not empty.

SEE ALSO

     LIST_HEAD(3), malloc(9)

BUGS

     There is no phashdestroy() function, and using hashdestroy() to free a
     hash table allocated by phashinit() usually has grave consequences.