ref: 02ac617541ca1a7bf82b1615fb5a58235469b5d3
dir: /man/2/dbm/
.TH DBM 2 .SH NAME Dbm: Dbf, init \- data base with hashed indexing .SH SYNOPSIS .EX 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(); .EE .SH DESCRIPTION .B Dbm maintains 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. .PP .IR Key s and .IR content s are both represented by arrays of bytes (with the synonym .BR Datum ), allowing arbitrary binary values. .PP The data base is stored in two files. One file is a directory containing a bit map and has .L .dir as its suffix. The second file contains all data and has .L .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). .PP .B Init must be called before any other operation of the module. .PP A database is created by .IR Dbf.create , which accepts a file permission parameter .IR perm , as described for .B Sys->create (see .IR sys-open (2)); it creates the two files .IB file .dir and .IB file .pag . If successful, it returns a .B Dbf reference describing the database, which is open for reading and writing. (It will truncate an existing database.) It returns nil if it cannot create the database for some reason, and sets the error string. .PP .B Dbf.open accepts a .I mode parameter as described in .IR sys-open (2), and opens the existing database in .IB file .dir and .IB file .pag . If successful, it returns a .B Dbf reference describing the database, which is open either for reading and writing (ie, .BR Sys->ORDWR ), or only for reading .RB ( Sys->OREAD ) as determined by .IR mode . It returns nil if the database cannot be opened successfully, and sets the error string. .PP The remaining operations apply to an existing .B Dbf reference .IR db : .TP .IB db .fetch( key ) Return the data stored under a .IR key ; nil is returned if the key is not in the database. .TP .IB db .store( key,\ data,\ replace ) Store .I data under the given .IR key . If .I replace is non-zero, .B store will simply replace the existing value by the new one if the key is already in the database; if .I replace is zero .B store will return 0 if the new item was inserted, but 1 if the key already appears in the database, and the new value will not be stored. .TP .IB db .delete( key ) .I Key and its associated value is removed from the database. .TP .IB db .firstkey() Return the first key in the database; return nil if the database is empty. .TP .IB db .nextkey( key ) Return the key following the given .IR key , or nil if there is none. .TP .IB 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. .TP .IB db .isrdonly() Return true if .I db was opened only for reading and writes are not allowed. .SH EXAMPLE .PP A linear pass through all keys in a database may be made, in an (apparently) random order, by use of .B Dbf.firstkey and .BR Dbf.nextkey . This code will traverse the data base: .IP .EX for(key := db.firstkey(); key != nil; key = db.nextkey(key)){ d := db.fetch(key); } .EE .PP The order of keys presented by .B Dbf.firstkey and .B Dbf.nextkey depends on a hashing function, not on anything interesting. .SH SOURCE .B /appl/lib/dbm.b .SH DIAGNOSTICS All functions that return an .I int indicate errors with negative values. A zero return indicates success. Routines that return pointers, including values of .BR Datum , return nil values on error. .B Dbf.create and .B Dbf.open return nil on failure to access the database, setting the error string to a more detailed diagnostic. .SH BUGS On some systems (notably Plan 9 but also some Unix systems), the .B .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. .PP Except for .B firstkey and .BR nextkey , .B Datum values returned by these functions point to storage that is changed by subsequent calls. .PP 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. .B Dbf.store will return an error in the event that a block fills with inseparable data.