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HASHINIT(9) Kernel Developer's Manual HASHINIT(9)

NAME

hashinit, hashinit_flags, hashdestroy, phashinit, phashinit_flags —
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
hashinit_flags(int nelements, struct malloc_type *type, u_long *hashmask, int flags);

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

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

phashinit_flags(int nelements, struct malloc_type *type, u_long *nentries, int flags);

DESCRIPTION

The hashinit(), hashinit_flags(), phashinit() and phashinit_flags() 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. The hashinit_flags() function operates like hashinit() but also accepts an additional argument flags which control various options during allocation. phashinit_flags() function operates like phashinit() but also accepts an additional argument flags which control various options during allocation. 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. The argument flags must be used with one of the following values.

Any malloc performed by the hashinit_flags() and phashinit_flags() function will not be allowed to wait, and therefore may fail.
Any malloc performed by hashinit_flags() and phashinit_flags() function is allowed to wait for memory. This is also the behavior of hashinit() and phashinit().

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.
April 29, 2016 Linux 4.9.0-9-amd64