# hash

## 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.
    


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