HASH(9)                FreeBSD Kernel Developer's Manual               HASH(9)

NAME
     hash, hash32, hash32_buf, hash32_str, hash32_strn, hash32_stre,
     hash32_strne, jenkins_hash, jenkins_hash32, murmur3_32_hash,
     murmur3_32_hash32 - 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);

     uint32_t
     murmur3_32_hash(const void *buf, size_t len, uint32_t hash);

     uint32_t
     murmur3_32_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.

     A len argument is the length of the buffer in bytes.  A count argument is
     the length of the buffer in 32-bit words.

     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 in bytes.  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 simpler and
     faster.  It accepts an array of uint32_t values in its first argument and
     size of this array in the second argument.

     The murmur3_32_hash() and murmur3_32_hash32() functions are similar to
     jenkins_hash() and jenkins_hash32(), but implement the 32-bit version of
     MurmurHash3.

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.  The murmur3_32_hash functions were added in
     FreeBSD 10.1.

AUTHORS
     The hash32 functions were written by Tobias Weingartner.  The
     jenkins_hash functions were written by
     Bob Jenkins.  The murmur3_32_hash functions were written by
     Dag-Erling Sm/rgrav <des@FreeBSD.org>.

FreeBSD 13.1-RELEASE-p6          June 30, 2015         FreeBSD 13.1-RELEASE-p6