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man:hash

HASH(3) Linux Programmer's Manual HASH(3)

NAME

     hash - hash database access method

SYNOPSIS

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

DESCRIPTION

     Note  well:  This  page documents interfaces provided in glibc up until
     version 2.1.  Since version 2.2, glibc no longer provides these  inter-
     faces.   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 allo-
               cate more memory rather than fail.
     hash      is a user-defined hash function.  Since no hash function per-
               forms  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 meta-
               data.  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.

COLOPHON

     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at
     https://www.kernel.org/doc/man-pages/.

4.4 Berkeley Distribution 2017-09-15 HASH(3)

/data/webs/external/dokuwiki/data/pages/man/hash.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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