DBM(2) DBM(2)
NAME
Dbm: Dbf, init - data base with hashed indexing
SYNOPSIS
include "dbm.m";
dbm := load Dbm Dbm->PATH;
Datum, Dbf: import dbm;
Datum: type array of byte;
Dbf: adt {
create: fn(file: string, perm: int): ref Dbf;
open: fn(file: string, flags: int): ref Dbf;
fetch: fn(db: self ref Dbf, key: Datum): Datum;
delete: fn(db: self ref Dbf, key: Datum): int;
store: fn(db: self ref Dbf, key: Datum, data: Datum,
replace: int): int;
firstkey: fn(db: self ref Dbf): Datum;
nextkey: fn(db: self ref Dbf, key: Datum): Datum;
flush: fn(db: self ref Dbf);
isrdonly: fn(db: self ref Dbf): int;
};
init: fn();
DESCRIPTION
Dbm maintains key/content pairs in a data base. The func-
tions will handle very large (a billion blocks) databases
and will access a keyed item in one or two filesystem
accesses.
Keys and contents are both represented by arrays of bytes
(with the synonym Datum), allowing arbitrary binary values.
The data base is stored in two files. One file is a direc-
tory containing a bit map and has `.dir' as its suffix. The
second file contains all data and has `.pag' as its suffix.
An application can access several databases at once, but
must avoid concurrent operations on any one database (eg, by
using a monitor process to control access).
Init must be called before any other operation of the mod-
ule.
A database is created by Dbf.create, which accepts a file
permission parameter perm, as described for Sys->create (see
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DBM(2) DBM(2)
sys-open(2)); it creates the two files file.dir and
file.pag. If successful, it returns a Dbf reference
describing the database, which is open for reading and writ-
ing. (It will truncate an existing database.) It returns
nil if it cannot create the database for some reason, and
sets the error string.
Dbf.open accepts a mode parameter as described in sys-
open(2), and opens the existing database in file.dir and
file.pag. If successful, it returns a Dbf reference
describing the database, which is open either for reading
and writing (ie, Sys->ORDWR), or only for reading
(Sys->OREAD) as determined by mode. It returns nil if the
database cannot be opened successfully, and sets the error
string.
The remaining operations apply to an existing Dbf reference
db:
db.fetch(key)
Return the data stored under a key; nil is returned if
the key is not in the database.
db.store(key, data, replace)
Store data under the given key. If replace is non-zero,
store will simply replace the existing value by the new
one if the key is already in the database; if replace
is zero store will return 0 if the new item was
inserted, but 1 if the key already appears in the data-
base, and the new value will not be stored.
db.delete(key)
Key and its associated value is removed from the data-
base.
db.firstkey()
Return the first key in the database; return nil if the
database is empty.
db.nextkey(key)
Return the key following the given key, or nil if there
is none.
db.flush()
Discard any data cached from the file. The cache is
write-through, so it is not necessary to flush the file
before the application exits.
db.isrdonly()
Return true if db was opened only for reading and
writes are not allowed.
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DBM(2) DBM(2)
EXAMPLE
A linear pass through all keys in a database may be made, in
an (apparently) random order, by use of Dbf.firstkey and
Dbf.nextkey. This code will traverse the data base:
for(key := db.firstkey(); key != nil; key = db.nextkey(key)){
d := db.fetch(key);
}
The order of keys presented by Dbf.firstkey and Dbf.nextkey
depends on a hashing function, not on anything interesting.
SOURCE
/appl/lib/dbm.b
DIAGNOSTICS
All functions that return an int indicate errors with nega-
tive values. A zero return indicates success. Routines
that return pointers, including values of Datum, return nil
values on error. Dbf.create and Dbf.open return nil on
failure to access the database, setting the error string to
a more detailed diagnostic.
BUGS
On some systems (notably Plan 9 but also some Unix systems),
the .pag file might contain holes where no data block has
ever been written so that its apparent size is about four
times its actual content. These files cannot be copied by
normal means (cp, cat) without filling in the holes.
Except for firstkey and nextkey, Datum values returned by
these functions point to storage that is changed by subse-
quent 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. Dbf.store will return an error in the event that a
block fills with inseparable data.
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