.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved. The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\" @(#)dbm.3 6.4 (Berkeley) 4/10/89
.TH DBM 3X "April 10, 1989"
dbminit, fetch, store, delete, firstkey, nextkey \- data base subroutines
The dbm library has been obsoleted by ndbm(3),
and is now implemented 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
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
At the time of this call, the files
(An empty database is created by creating zero-length
`.dir' and `.pag' files.)
Once open, the data stored under a key is accessed by
and data is placed under a key by
A key (and its associated contents) is deleted by
A linear pass through all keys in a database may be made,
in an (apparently) random order, by use of
will return the first key in the database. With any key
will return the next key in the database.
This code will traverse the data base:
(key = firstkey(); key.dptr != NULL; key = nextkey(key))
All functions that return an
indicate errors with negative values. A zero return indicates ok.
indicate errors with a null (0)
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.
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.
will return an error in the event that a disk block fills with inseparable data.
does not physically reclaim file space,
although it does make it available for reuse.
The order of keys presented by
depends on a hashing function, not on anything interesting.