DBM(3X)                                                                DBM(3X)



NAME
       dbminit,  fetch,  store,  delete, firstkey, nextkey - data base subrou‐
       tines

SYNOPSIS
       #include <dbm.h>

       typedef struct {
            char *dptr;
            int dsize;
       } datum;

       dbminit(file)
       char *file;

       datum fetch(key)
       datum key;

       store(key, content)
       datum key, content;

       delete(key)
       datum key;

       datum firstkey()

       datum nextkey(key)
       datum key;

DESCRIPTION
       Note: the dbm library has been superceded by ndbm(3), and is now imple‐‐
       mented  using  ndbm.   These  functions maintain key/content pairs in a
       data base.  The functions will handle very  large  (a  billion  blocks)
       databases  and  will  access  a  keyed  item  in one or two file system
       accesses.  The functions are obtained with the loader option -ldbm.

       _K_e_ys and _c_o_n_t_e_n_ts are described by the _d_a_t_u_m typedef.  A  _d_a_t_u_m  speci‐
       fies  a  string  of  _d_s_i_z_e  bytes pointed to by _d_p_t_r_.  Arbitrary binary
       data, as well as normal ASCII strings, are allowed.  The data  base  is
       stored  in two files.  One file is a directory containing a bit map and
       has ‘.dir’ as its suffix.  The second file contains all  data  and  has
       ‘.pag’ as its suffix.

       Before  a  database  can be accessed, it must be opened by _d_b_m_i_n_i_t_.  At
       the time of this call, the files _f_i_l_e.dir and _f_i_l_e.pag must exist.  (An
       empty  database  is  created  by creating zero-length ‘.dir’ and ‘.pag’
       files.)

       Once open, the data stored under a key is accessed by _f_e_t_c_h and data is
       placed  under  a  key by _s_t_o_r_e.  A key (and its associated contents) is
       deleted by _d_e_l_e_t_e.  A linear pass through all keys in a database may be
       made,  in an (apparently) random order, by use of _f_i_r_s_t_k_e_y and _n_e_x_t_k_e_y.
       _F_i_r_s_t_k_e_y will return the first key in the database.  With any key  _n_e_x_‐
       _t_k_e_y will return the next key in the database.  This code will traverse
       the data base:

              for (key = firstkey(); key.dptr != NULL; key = nextkey(key))

DIAGNOSTICS
       All functions that return an _i_n_t indicate errors with negative  values.
       A  zero  return  indicates  ok.   Routines that return a _d_a_t_u_m indicate
       errors with a null (0) _d_p_t_r_.

SEE ALSO
       ndbm(3)

BUGS
       The ‘.pag’ file will contain holes so that its apparent size  is  about
       four times its actual content.  Older UNIX systems may create real file
       blocks for these holes when touched.  These files cannot be  copied  by
       normal means (cp, cat, tp, tar, ar) without filling in the holes.

       _D_p_t_r  pointers  returned by these subroutines point into static storage
       that is changed by subsequent calls.

       The sum of the sizes of a key/content pair must not exceed the internal
       block size (currently 1024 bytes).  Moreover all key/content pairs that
       hash together must fit on a single block.  _S_t_o_r_e will return  an  error
       in the event that a disk block fills with inseparable data.

       _D_e_l_e_t_e does not physically reclaim file space, although it does make it
       available for reuse.

       The order of keys presented by _f_i_r_s_t_k_e_y and _n_e_x_t_k_e_y depends on a  hash‐
       ing function, not on anything interesting.



4th Berkeley Distribution        May 12, 1986                          DBM(3X)