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       hash - hash database access method


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


       The  routine dbopen(3) 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(3)  is
       defined in the <db.h> include file as follows:

           typedef struct {
               unsigned int       bsize;
               unsigned int       ffactor;
               unsigned int       nelem;
               unsigned int       cachesize;
               uint32_t         (*hash)(const void *, size_t);
               int         lorder;
           } HASHINFO;

       The elements of this structure are as follows:

       bsize     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   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     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 is the 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      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.  A 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    is 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 bsize, ffactor, lorder, and nelem 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 routines described in dbm(3), and
       ndbm(3)  are provided, however these interfaces are not compatible with
       previous file formats.


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


       Only big and little endian byte order are supported.


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

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

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


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