MirOS Manual: hash(9)

HASH(9)                       BSD Kernel Manual                        HASH(9)

NAME

     hash - general kernel hashing functions

SYNOPSIS

     #include <sys/hash.h>

     uint32_t
     hash32_buf(void *buf, size_t len, uint32_t hash);

     uint32_t
     hash32_str(void *buf, uint32_t hash);

     uint32_t
     hash32_strn(void *buf, size_t len, uint32_t hash);

     uint32_t
     hash32_stre(void *buf, int end, char **ep, uint32_t hash);

     uint32_t
     hash32_strne(void *buf, size_t len, int end, char **ep, uint32_t hash);

DESCRIPTION

     The hash32() functions are used to give a consistent and general inter-
     face 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 addi-
     tional 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 terminat-
     ed 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 ver-
     sions were written and massaged for OpenBSD 2.3 by Tobias Weingartner,
     and finally committed for OpenBSD 3.2.

MirOS BSD #10-current          December 8, 2001                              1

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