4.4BSD/usr/share/man/cat3/hash.0

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


NNAAMMEE
       hash - hash database access method

SSYYNNOOPPSSIISS
       ##iinncclluuddee <<ssyyss//ttyyppeess..hh>>
       ##iinncclluuddee <<ddbb..hh>>

DDEESSCCRRIIPPTTIIOONN
       The  routine  _d_b_o_p_e_n  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 _d_b_o_p_e_n(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
       _d_b_o_p_e_n is defined in the <db.h> include file as follows:

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

       The elements of this structure are as follows:

       bsize  _B_s_i_z_e  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.

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

       ffactor
              _F_f_a_c_t_o_r 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.

       hash   _H_a_s_h  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



4.4 Berkeley Distribution July 19, 1993                         1








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


              function does poorly  on  a  particular  data  set.
              User  specified  hash functions must take two argu-
              ments (a pointer to a byte string and a length) and
              return an u_long 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 _l_o_r_d_e_r  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.

       nelem  _N_e_l_e_m 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.

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

       If a hash function is specified, _h_a_s_h___o_p_e_n 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 _d_b_m(3), and _n_d_b_m(3) are provided, however, these inter-
       faces are not compatible with previous file formats.

SSEEEE AALLSSOO
       _b_t_r_e_e(3), _d_b_o_p_e_n(3), _m_p_o_o_l(3), _r_e_c_n_o(3)
       _D_y_n_a_m_i_c _H_a_s_h _T_a_b_l_e_s, Per-Ake Larson, Communications of the
       ACM, April 1988.
       _A  _N_e_w  _H_a_s_h  _P_a_c_k_a_g_e _f_o_r _U_N_I_X, Margo Seltzer, USENIX Pro-
       ceedings, Winter 1991.

BBUUGGSS
       Only big and little endian byte order is supported.











4.4 Berkeley Distribution July 19, 1993                         2