dbminit, fetch, store, delete, firstkey, nextkey, dbmclose - database
These functions maintain key/content pairs in a database. They handle
very large (a billion blocks (block = 1024 bytes)) databases and can
locate 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
Before a database can be accessed, it must be opened by At the time of
this call, the files and must exist. (An empty database is created by
creating zero-length and files.)
Once open, data stored under a key is accessed by and data is placed
under a key by Storing data on an existing key replaces the existing
data. 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 using and returns the first key in the database. With any
key, returns the next key in the database. The following code can be
used to traverse the database:
A database can be closed by calling A currently open database must be
closed before opening a new one.
All functions that return an indicate errors with negative values and
success with zero. Functions that return a indicate errors with a null
The dbm functions provided in this library should not be confused in
any way with those of a general-purpose database management system such
as ALLBASE/HP-UX SQL. 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
data base 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 functions for applications requiring fast lookup of relatively
static information that is to be indexed by a single key.
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‐
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 if a
disk block fills with inseparable data.
does not physically reclaim file space, although it does make it avail‐
able 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
AUTHORdbm(3C) was developed by the University of California, Berkeley.