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HASH(3)                FreeBSD Library Functions Manual                HASH(3)

NAME
     hash - hash database access method

SYNOPSIS
     #include <sys/types.h>
     #include <db.h>

DESCRIPTION
     The routine dbopen() is the library interface to database files.  One of
     the supported file formats is hash files.  The general description of the
     database access methods is in dbopen(3), this manual page describes only
     the hash specific information.

     The hash data structure is an extensible, dynamic hashing scheme.

     The access method specific data structure provided to dbopen() is defined
     in the <db.h> include file as follows:

     typedef struct {
             u_int bsize;
             u_int ffactor;
             u_int nelem;
             u_int cachesize;
             u_int32_t (*hash)(const void *, size_t);
             int lorder;
     } HASHINFO;

     The elements of this structure are as follows:

     bsize   The bsize element defines the hash table bucket size, and is, by
             default, 256 bytes.  It may be preferable to increase the page
             size for disk-resident tables and tables with large data items.

     ffactor
             The ffactor element indicates a desired density within the hash
             table.  It is an approximation of the number of keys allowed to
             accumulate in any one bucket, determining when the hash table
             grows or shrinks.  The default value is 8.

     nelem   The nelem element is an estimate of the final size of the hash
             table.  If not set or set too low, hash tables will expand
             gracefully as keys are entered, although a slight performance
             degradation may be noticed.  The default value is 1.

     cachesize
             A suggested maximum size, in bytes, of the memory cache.  This
             value is only advisory, and the access method will allocate more
             memory rather than fail.

     hash    The hash element is a user defined hash function.  Since no hash
             function performs equally well on all possible data, the user may
             find that the built-in hash function does poorly on a particular
             data set.  User specified hash functions must take two arguments
             (a pointer to a byte string and a length) and return a 32-bit
             quantity to be used as the hash value.

     lorder  The byte order for integers in the stored database metadata.  The
             number should represent the order as an integer; for example, big
             endian order would be the number 4,321.  If lorder is 0 (no order
             is specified) the current host order is used.  If the file
             already exists, the specified value is ignored and the value
             specified when the tree was created is used.

     If the file already exists (and the O_TRUNC flag is not specified), the
     values specified for the bsize, ffactor, lorder and nelem arguments are
     ignored and the values specified when the tree was created are used.

     If a hash function is specified, hash_open() will attempt to determine if
     the hash function specified is the same as the one with which the
     database was created, and will fail if it is not.

     Backward compatible interfaces to the older dbm and ndbm routines are
     provided, however these interfaces are not compatible with previous file
     formats.

ERRORS
     The hash access method routines may fail and set errno for any of the
     errors specified for the library routine dbopen(3).

SEE ALSO
     btree(3), dbopen(3), mpool(3), recno(3)

     Per-Ake Larson, Dynamic Hash Tables, Communications of the ACM, April
     1988.

     Margo Seltzer, A New Hash Package for UNIX, USENIX Proceedings, Winter
     1991.

BUGS
     Only big and little endian byte order is supported.

FreeBSD 11.0-PRERELEASE         August 18, 1994        FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | ERRORS | SEE ALSO | BUGS

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