DBM(3X) DBM(3X)
NAME
dbminit, fetch, store, delete, firstkey, nextkey - data base
subroutines
SYNOPSIS
typedef struct { char *dptr; int dsize; } datum;
dbminit(file)
char *file;
datum fetch(key)
datum key;
store(key, content)
datum key, content;
delete(key)
datum key;
datum firstkey();
datum nextkey(key);
datum key;
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
filesystem accesses. The functions are obtained with the
loader option -ldbm.
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
dbminit. At the time of this call, the files file.dir and
file.pag must exist. (An empty database is created by cre-
ating zero-length `.dir' and `.pag' files.)
Once open, the data stored under a key is accessed by fetch
and data is placed under a key by store. A key (and its
associated contents) is deleted by delete. A linear pass
through all keys in a database may be made, in an (appar-
ently) random order, by use of firstkey and nextkey.
Firstkey will return the first key in the database. With
DBM(3X) DBM(3X)
any key nextkey will return the next key in the database.
This code will traverse the data base:
for(key=firstkey(); key.dptr!=NULL; key=nextkey(key))
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.
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 512 bytes). Moreover all
key/content 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.
The order of keys presented by firstkey and nextkey depends
on a hashing function, not on anything interesting.