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hash.9freebsd

Langue: en

Version: 306948 (debian - 07/07/09)

Section: 9 (Appels noyau Linux)


BSD mandoc

NAME

hash hash32 hash32_buf hash32_str hash32_strn hash32_stre hash32_strne - 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

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.

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 functions were first committed to Nx 1.6 . The Ox versions were written and massaged for Ox 2.3 by Tobias Weingartner, and finally committed for Ox 3.2 .
Lorsque l'hiver commence à mordre
et que les pierres craquent dans la nuit glaciale,
lorsque les étangs sont noirs et les arbres dénudés,
il est mauvais dans les Terres Sauvages de voyager.
-- J.R.R. Tolkien, "La Communauté de l'Anneau"