NAME

       hash,   hash32,   hash32_buf,   hash32_str,   hash32_strn,   hash32_stre,  hash32_strne,  jenkins_hash32,
       jenkins_hash — 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);

       uint32_t
       jenkins_hash(const void *buf, size_t len, uint32_t hash);

       uint32_t
       jenkins_hash32(const uint32_t *buf, size_t count, 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 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.

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

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

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 first appeared in NetBSD 1.6.  The current  implementation  of  hash32  functions  was
       first committed to OpenBSD 3.2, and later imported to FreeBSD 6.1.  The jenkins_hash functions were added
       in FreeBSD 10.0.

AUTHORS

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

Debian                                          September 4, 2012                                        HASH(9)