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hash32_stre (9)

>> hash32_stre (9) ( FreeBSD man: Ядро )

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NAME



hash

 
hash32

 
hash32_buf

 
hash32_str

 
hash32_strn

 
hash32_stre

 
hash32_strne

 - general kernel hashing functions

SYNOPSIS

#include <sys/hash.h>
uint32_t hash32_buf (const void *buf size_t len uint32_t hash);
uint32_t hash32_str (const void *buf uint32_t hash);
uint32_t hash32_strn (const void *buf size_t len uint32_t hash);
uint32_t hash32_stre (const void *buf int end const char **ep uint32_t hash);
uint32_t hash32_strne (const void *buf size_t len int end const 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 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 hash32_str ();
function is used to hash a NUL terminated string passed in Fa buf with initial hash value given in Fa hash .

The hash32_strn ();
function is like the hash32_str ();
function, except it also takes a Fa 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 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 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;
}

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 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 .

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Created 1996-2024 by Maxim Chirkov
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