NDBM(3)                              BSD                               NDBM(3)



NAME
     dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete, dbm_firstkey,
     dbm_nextkey, dbm_error, dbm_clearerr - database 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 database.  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 dbm(3x) library, which managed only a single database.

     keys and contents are described by the datum typedef.  A datum specifies
     a string of dsize bytes pointed to by dptr. Arbitrary binary data, as
     well as normal ASCII strings, are allowed.  The database 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 dbm_open.  This
     will open and/or create the files file.dir and file.pag depending on the
     flags parameter (see open(2)).

     Once open, the data stored under a key is accessed by dbm_fetch and data
     is placed under a key by dbm_store.  The flags 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 dbm_delete.  A linear pass
     through all keys in a database may be made, in an (apparently) random
     order, by use of dbm_firstkey and dbm_nextkey.  dbm_firstkey will return
     the first key in the database.  dbm_nextkey will return the next key in
     the database.  This code will traverse the database:

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

     dbm_error returns nonzero when an error has occurred reading or writing
     the database.  dbm_clearerr resets the error condition on the named
     database.

DIAGNOSTICS
     All functions that return an int indicate errors with negative values.  A
     0 return indicates ok.  Routines that return a datum indicate errors with
     a null (0) dptr. If dbm_store, called with a flags 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.

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

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

     The order of keys presented by dbm_firstkey and dbm_nextkey depends on a
     hashing function, not on anything interesting.

SEE ALSO
     dbm(3X)