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HASH(9) | Kernel Developer's Manual | HASH(9) |
NAME¶
hash, hash32, hash32_buf, hash32_str, hash32_strn, hash32_stre, hash32_strne — general kernel hashing functionsSYNOPSIS¶
#include <sys/hash.h> uint32_thash32_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 ASCIINUL
terminated
strings, as well as blocks of memory.
The hash32_buf() function is used as a general buffer hashing
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 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
buf with initial hash value given in
hash.
The hash32_strn() function is like the
hash32_str() function, 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 argument 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.April 3, 2007 | Debian |