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Man Pages


Manual Reference Pages  - HASHINIT (9)

NAME

hashinit, hashdestroy, phashinit - manage kernel hash tables

CONTENTS

Synopsis
Description
Implementation Notes
Return Values
Examples
Diagnostics
See Also
Bugs

SYNOPSIS


.In sys/malloc.h
.In sys/systm.h
.In 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.
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October 10, 2004 HASHINIT (9)

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