ndbm(3X) ndbm(3X)
NAME
dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete, dbm_firstkey, dbm_nextkey, dbm_error, dbm_clearerr - database subroutines
SYNOPSIS
DESCRIPTION
These functions maintain key/content pairs in a database. They handle very large (a billion blocks (block = 1024 bytes)) databases and can
access a keyed item in one or two file system accesses.
key and content parameters are described by the type. A 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 of keys and
has as its suffix. The second file contains all data and has as its suffix.
Before a database can be accessed, it must be opened by This will open and/or create the files and depending on the flags parameter (see
open(2)).
Once open, the data stored under a key is accessed by and data is placed under a key by The flags field can be either or can only insert
new entries into the database, and cannot change an existing entry having the same key. replaces an existing entry if it has the same key.
A key (and its associated contents) is deleted by A linear pass through all keys in a database can be made in (apparently) random order by
use of and returns the first key in the database. returns the next key in the database, The following code can be used to traverse the
database:
returns non-zero when an error has occurred reading or writing the database. resets the error condition on the named database.
DIAGNOSTICS
All functions that return an indicate errors with negative values and success with zero. Functions that return a indicate errors with a
null dptr. If is called with a flags value of and finds an existing entry with the same key, a value of is returned. If a call to results
in an internal block overflow, a value of -2 is returned.
WARNINGS
The ndbm functions provided in this library should not be confused in any way with those of a general-purpose database management system.
These functions provide for multiple search keys per entry, they protect against multi-user access (in other words they do not lock records
or files), and they provide the many other useful database functions that are found in more robust database management systems. Creating
and updating databases by use of these functions is relatively slow because of data copies that occur upon hash collisions. These func-
tions for applications requiring fast lookup of relatively static information that is to be indexed by a single key.
The pointer to data that is returned from these functions are not aligned. This can cause problems if the block contains data that must be
aligned to a specific boundry. If the block contains data that must be aligned, the block should be copied to an appropriately aligned
area.
The file will contain holes so that its apparent size is about four times its actual content. Some older UNIX systems create real file
blocks for these holes when touched. These files cannot be copied by normal means (such as cp(1), cat(1), tar(1), or ar(1)) without expan-
sion.
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 1024 bytes). Moreover, all key/content pairs
that hash together must fit on a single block. returns 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 and depends on a hashing function, not on anything interesting.
A or during a pass through the keys by and may yield unexpected results.
AUTHOR
ndbm(3X) was developed by the University of California, Berkeley.
SEE ALSO
dbm(3C), thread_safety(5).
ndbm(3X)