4.3BSD-UWisc/man/cat3/ndbm.3

NDBM(3)             UNIX Programmer's Manual              NDBM(3)



NAME
     dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete,
     dbm_firstkey, dbm_nextkey, dbm_error, dbm_clearerr - data
     base subroutines

SYNOPSIS
     #include <ndbm.h>

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

     DBM *dbm_open(file, flags, mode)
         char *file;
         int flags, mode;

     void dbm_close(db)
         DBM *db;

     datum dbm_fetch(db, key)
         DBM *db;
         datum key;

     int dbm_store(db, key, content, flags)
         DBM *db;
         datum key, content;
         int flags;

     int dbm_delete(db, key)
         DBM *db;
         datum key;

     datum dbm_firstkey(db)
         DBM *db;

     datum dbm_nextkey(db)
         DBM *db;

     int dbm_error(db)
         DBM *db;

     int dbm_clearerr(db)
         DBM *db;

DESCRIPTION
     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.  This package replaces the earlier _d_b_m(3x)
     library, which managed only a single database.




Printed 12/27/86          May 20, 1986                          1






NDBM(3)             UNIX Programmer's Manual              NDBM(3)



     _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 specifies 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 suf-
     fix.  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__o_p_e_n.  This will open and/or create the files _f_i_l_e.dir
     and _f_i_l_e.pag depending on the flags parameter (see _o_p_e_n(2)).

     Once open, the data stored under a key is accessed by
     _d_b_m__f_e_t_c_h and data is placed under a key by _d_b_m__s_t_o_r_e.  The
     _f_l_a_g_s field can be either DBM_INSERT or DBM_REPLACE.
     DBM_INSERT will only insert new entries into the database
     and will not change an existing entry with the same key.
     DBM_REPLACE will replace an existing entry if it has the
     same key.  A key (and its associated contents) is deleted by
     _d_b_m__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
     _d_b_m__f_i_r_s_t_k_e_y and _d_b_m__n_e_x_t_k_e_y.  _D_b_m__f_i_r_s_t_k_e_y will return the
     first key in the database.  _D_b_m__n_e_x_t_k_e_y will return the next
     key in the database.  This code will traverse the data base:

          for (key = dbm_firstkey(db); key.dptr != NULL; key =
          dbm_nextkey(db))

     _D_b_m__e_r_r_o_r returns non-zero when an error has occurred read-
     ing or writing the database.  _D_b_m__c_l_e_a_r_e_r_r resets the error
     condition on the named database.

DIAGNOSTICS
     All functions that return an _i_n_t indicate errors with nega-
     tive 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. If
     _d_b_m__s_t_o_r_e called with a _f_l_a_g_s value of DBM_INSERT finds an
     existing entry with the same key it returns 1.

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 4096 bytes).  Moreover
     all key/content pairs that hash together must fit on a



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NDBM(3)             UNIX Programmer's Manual              NDBM(3)



     single block.  _D_b_m__s_t_o_r_e will return an error in the event
     that a disk block fills with inseparable data.

     _D_b_m__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 _d_b_m__f_i_r_s_t_k_e_y and _d_b_m__n_e_x_t_k_e_y
     depends on a hashing function, not on anything interesting.

SEE ALSO
     dbm(3X)












































Printed 12/27/86          May 20, 1986                          3