hashinit(9)

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 mal

loc(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 mal

loc 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 argu

ment 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 conse

quences.

BSD October 10, 2004

BSD