NAME

       hash - hash database access method

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

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

DESCRIPTION

       Note  well: This page documents interfaces provided up until glibc 2.1.  Since glibc 2.2, glibc no longer
       provides these interfaces.  Probably, you are looking for the APIs provided by the libdb library instead.

       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 attempts to determine if the hash function specified is the
       same as the one with which the database was created, and fails 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.

ERRORS

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

BUGS

       Only big and little endian byte order are supported.

SEE ALSO

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

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

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

4.4 Berkeley Distribution                          2023-10-31                                            hash(3)