2.11BSD#431 - /usr/man/cat3/ndbm.0

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 dbm(3x)
     library, which managed only a single database.


Printed 11/26/99	  May 20, 1986				1


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


     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 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
     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 data base:

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

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

DIAGNOSTICS
     All functions that return an int indicate errors with nega-
     tive values.  A zero 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


Printed 11/26/99	  May 20, 1986				2


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


     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)


Printed 11/26/99	  May 20, 1986				3