term% cat index.txt DBM(3X) DBM(3X)
NAME
dbminit, fetch, store, delete, firstkey, nextkey - data base subrou‐
tines
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 func‐
tions will handle very large (a billion blocks) databases and will ac‐
cess 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 speci‐
fies 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 is created by creating 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 (apparently) random order, by use of firstkey and nextkey.
Firstkey will return the first key in the database. With any key nex‐
tkey 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 negative values.
A zero return indicates ok. Routines that return a datum indicate er‐
rors 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 hash‐
ing function, not on anything interesting.
DBM(3X)