hash32_buf (9)

Leading comments

Copyright (c) 2001 Tobias Weingartner
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or...

(The comments found at the beginning of the groff file "man9/hash32_buf.9freebsd".)

NAME

hash hash32 hash32_buf hash32_str hash32_strn hash32_stre hash32_strne jenkins_hash32 jenkins_hash - general kernel hashing functions

SYNOPSIS

In sys/hash.h Ft uint32_t Fn hash32_buf const void *buf size_t len uint32_t hash Ft uint32_t Fn hash32_str const void *buf uint32_t hash Ft uint32_t Fn hash32_strn const void *buf size_t len uint32_t hash Ft uint32_t Fn hash32_stre const void *buf int end const char **ep uint32_t hash Ft uint32_t Fn hash32_strne const void *buf size_t len int end const char **ep uint32_t hash Ft uint32_t Fn jenkins_hash const void *buf size_t len uint32_t hash Ft uint32_t Fn jenkins_hash32 const uint32_t *buf size_t count uint32_t hash

DESCRIPTION

The Fn 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 Fn hash32_buf function is used as a general buffer hashing function. The argument Fa buf is used to pass in the location, and Fa len is the length of the buffer. The argument Fa hash is used to extend an existing hash, or is passed the initial value HASHINIT to start a new hash.

The Fn hash32_str function is used to hash a NUL terminated string passed in Fa buf with initial hash value given in Fa hash .

The Fn hash32_strn function is like the Fn hash32_str function, except it also takes a Fa len argument, which is the maximal length of the expected string.

The Fn hash32_stre and Fn 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 Fa end in the string to be hashed. If the argument Fa ep is not NULL it is set to the point in the buffer at which the hash function terminated hashing.

The Fn jenkins_hash function has same semantics as the Fn hash32_buf , but provides more advanced hashing algorithm with better distribution.

The Fn jenkins_hash32 uses same hashing algorithm as the Fn jenkins_hash function, but works only on Ft uint32_t sized arrays, thus is simplier and faster. It accepts an array of Ft uint32_t values in its first argument and size of this array in the second argument.

RETURN VALUES

The Fn 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 Fn 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 ifconfig functions first appeared in Nx 1.6 . The current implementation of hash32 functions was first committed to Ox 3.2 , and later imported to Fx 6.1 . The jenkins_hash functions were added in Fx 10.0 .

AUTHORS

The hash32 functions were written by An Tobias Weingartner . The jenkins_hash functions was written by Bob Jenkins .