hash(9)

NAME

hash - general kernel hashing functions

SYNOPSIS

#include <sys/hash.h>
uint32_t
hash32_buf(void *buf, size_t len, uint32_t hash);
uint32_t
hash32_str(void *buf, uint32_t hash);
uint32_t
hash32_strn(void *buf, size_t len, uint32_t hash);
uint32_t
hash32_stre(void *buf, int end, char **ep, uint32_t hash);
uint32_t
hash32_strne(void *buf, size_t  len,  int  end,  char  **ep,
uint32_t hash);

DESCRIPTION

The hash32() functions are used to give a consistent and

general interface to a decent hashing algorithm within the kernel. These

functions
can be used to hash ASCII NUL terminated strings, as well as

blocks of
memory.

The hash32_buf() function is used as a general buffer hash

ing function.
The argument buf is used to pass in the location, and len is

the length
of the buffer. The argument hash is used to extend an ex

isting hash, or
is passed the initial value HASHINIT to start a new hash.

The hash32_str() function is used to hash a NUL terminated

string passed
in buf with initial hash value given in hash.

The hash32_strn() function is like the hash32_str() func

tion, except it
also takes a len argument, which is the maximal length of

the expected
string.

The hash32_stre() and hash32_strne() functions are helper

functions used
by the kernel to hash pathname components. These functions

have the
additional termination condition of terminating when they

find a character given by end in the string to be hashed. If the argu

ment ep is not
NULL, it is set to the point in the buffer at which the hash

function
terminated hashing.

RETURN VALUES

The hash32() functions return a 32 bit hash value of the

buffer or
string.

EXAMPLES

LIST_HEAD(head, cache) *hashtbl = NULL;
u_long mask = 0;

void
sample_init(void)
{

hashtbl = hashinit(numwanted, type, flags,

&mask);

}

void
sample_use(char *str, int len)
{

uint32_t hash;

hash = hash32_str(str, HASHINIT);
hash = hash32_buf(&len, sizeof(len), hash);
hashtbl[hash & mask] = len;

}

SEE ALSO

free(9), hashinit(9), malloc(9)

LIMITATIONS

The hash32() functions are only 32 bit functions. They will

prove to
give poor 64 bit performance, especially for the top 32

bits. At the
current time, this is not seen as a great limitation, as

these hash values are usually used to index into an array. Should these

hash values be
used for other means, this limitation should be revisited.

HISTORY

The hash functions were first committed to NetBSD 1.6. The

OpenBSD versions were written and massaged for OpenBSD 2.3 by Tobias

Weingartner,
and finally committed for OpenBSD 3.2.

BSD December 8, 2001

BSD