DBM(3X) DBM(3X)
NAME
dbminit, fetch, store, delete, firstkey, nextkey - database
subroutines
SYNOPSIS
dbminit(file)
char *file;
datum fetch(key)
datum key;
store(key, value)
datum key, value;
delete(key)
datum key;
datum firstkey()
datum nextkey(key)
datum key;
DESCRIPTION
These functions maintain key/value pairs (each pair is a
datum) in a data base. The functions will handle very large
databases in one or two file system accesses per key. The
functions are loaded with ld(1) option -ldbm. A datum is
defined as
typedef struct {
char *dptr;
int dsize;
} datum;
A datum object 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 suffix. The second file contains all data and has
`.pag' as its suffix.
Before a database can be accessed, it must be opened by
dbminit. At the time of this call, the files file.dir and
file.pag must exist. (An empty database has empty `.dir'
and `.pag' files.)
The value associated with a key is retrieved by fetch and
assigned by store. A key and its associated value are
deleted by delete. A linear pass through all keys in a data-
base may be made, in random order, by use of firstkey and
DBM(3X) DBM(3X)
nextkey. Firstkey will return the first key in the database.
With any key nextkey will return the next key in the data-
base. This code will traverse the data base:
for(key = firstkey(); key.dptr != NULL; key =
nextkey(key))
SEE ALSO
cbt(3)
DIAGNOSTICS
All functions that return integers indicate errors with neg-
ative values. A zero return indicates success. Routines
that return a datum indicate errors with zero dptr.
BUGS
The `.pag' file contains holes; its apparent size is about
four times its actual content. These files cannot be copied
by normal means (cat(1), tar(1), cpio(1), ar(1)) without
filling in the holes.
Pointers returned by these subroutines refer to static data
that is changed by subsequent calls.
The sum of the sizes of a key/value pair must not exceed a
fixed internal block size. Moreover all key/value pairs
that hash together must fit on a single block. Store will
return an error in the event that a disk block fills with
inseparable data.
Delete does not physically reclaim file space, although it
does make it available for reuse.