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Class: GDBM

Relationships & Source Files
Super Chains via Extension / Inclusion / Inheritance
Instance Chain:
self, Enumerable
Inherits: Object
Defined in: ext/gdbm/gdbm.c,
ext/gdbm/gdbm.c

Overview

Summary

Ruby extension for GNU dbm (gdbm) – a simple database engine for storing key-value pairs on disk.

Description

GNU dbm is a library for simple databases. A database is a file that stores key-value pairs. Gdbm allows the user to store, retrieve, and delete data by key. It furthermore allows a non-sorted traversal of all key-value pairs. A gdbm database thus provides the same functionality as a hash. As with objects of the Hash class, elements can be accessed with #[]. Furthermore, GDBM mixes in the Enumerable module, thus providing convenient methods such as #find, #collect, #map, etc.

A process is allowed to open several different databases at the same time. A process can open a database as a “reader” or a “writer”. Whereas a reader has only read-access to the database, a writer has read- and write-access. A database can be accessed either by any number of readers or by exactly one writer at the same time.

Examples

  1. Opening/creating a database, and filling it with some entries:

    require 'gdbm'

gdbm = GDBM.new("fruitstore.db")
gdbm["ananas"]    = "3"
gdbm["banana"]    = "8"
gdbm["cranberry"] = "4909"
gdbm.close

  2. Reading out a database:

    require 'gdbm'

gdbm = GDBM.new("fruitstore.db")
gdbm.each_pair do |key, value|
  print "#{key}: #{value}\n"
end
gdbm.close

    produces

    banana: 8
ananas: 3
cranberry: 4909

Constant Summary

Class Method Summary

Instance Attribute Summary

Instance Method Summary

Constructor Details

.new(filename, mode = 0666, flags = nil)

Creates a new GDBM instance by opening a gdbm file named filename. If the file does not exist, a new file with file mode mode will be created. flags may be one of the following:

  • READER - open as a reader

  • WRITER - open as a writer

  • WRCREAT - open as a writer; if the database does not exist, create a new one

  • NEWDB - open as a writer; overwrite any existing databases

The values WRITER, WRCREAT and NEWDB may be combined with the following values by bitwise or:

  • SYNC - cause all database operations to be synchronized to the disk

  • NOLOCK - do not lock the database file

If no flags are specified, the GDBM object will try to open the database file as a writer and will create it if it does not already exist (cf. flag WRCREAT). If this fails (for instance, if another process has already opened the database as a reader), it will try to open the database file as a reader (cf. flag READER).

Class Method Details

.open(filename, mode = 0666, flags = nil) .open(filename, mode = 0666, flags = nil) {|gdbm| ... }

If called without a block, this is synonymous to .new. If a block is given, the new GDBM instance will be passed to the block as a parameter, and the corresponding database file will be closed after the execution of the block code has been finished.

Example for an open call with a block:

require 'gdbm'
GDBM.open("fruitstore.db") do |gdbm|
  gdbm.each_pair do |key, value|
    print "#{key}: #{value}\n"
  end
end

Instance Attribute Details

#cachesize=(size) ⇒ size (writeonly)

Sets the size of the internal bucket cache to size.

#closed? ⇒ Boolean (readonly)

Returns true if the associated database file has been closed.

#empty? ⇒ Boolean (readonly)

Returns true if the database is empty.

#fastmode=(boolean) ⇒ Boolean (writeonly)

Turns the database's fast mode on or off. If fast mode is turned on, gdbm does not wait for writes to be flushed to the disk before continuing.

This option is obsolete for gdbm >= 1.8 since fast mode is turned on by default. See also: #syncmode=

#syncmode=(boolean) ⇒ Boolean (writeonly)

Turns the database's synchronization mode on or off. If the synchronization mode is turned on, the database's in-memory state will be synchronized to disk after every database modification operation. If the synchronization mode is turned off, GDBM does not wait for writes to be flushed to the disk before continuing.

This option is only available for gdbm >= 1.8 where syncmode is turned off by default. See also: #fastmode=

Instance Method Details

#[](key) ⇒ value

Retrieves the value corresponding to key.

#[]=(key, value) ⇒ value #store(key, value) ⇒ value
Also known as: #store

Associates the value value with the specified key.

#clear ⇒ GDBM

Removes all the key-value pairs within gdbm.

#close

#delete(key) ⇒ value?

Removes the key-value-pair with the specified key from this database and returns the corresponding value. Returns nil if the database is empty.

#delete_if {|key, value| ... } ⇒ GDBM #reject! {|key, value| ... } ⇒ GDBM

Alias for #reject!.

#each_pair {|key, value| ... } ⇒ GDBM Also known as: #each_pair

Executes block for each key in the database, passing the key and the corresponding value as a parameter.

#each_key {|key| ... } ⇒ GDBM

Executes block for each key in the database, passing the key as a parameter.

#each_pair {|key, value| ... } ⇒ GDBM #each_pair {|key, value| ... } ⇒ GDBM

Alias for #each.

#each_value {|value| ... } ⇒ GDBM

Executes block for each key in the database, passing the corresponding value as a parameter.

#fetch(key [, default]) ⇒ value

Retrieves the value corresponding to key. If there is no value associated with key, default will be returned instead.

#include?(k) ⇒ Boolean #has_key?(k) ⇒ Boolean #member?(k) ⇒ Boolean #key?(k) ⇒ Boolean

Alias for #key?.

#has_value?(v) ⇒ Boolean #value?(v) ⇒ Boolean

Alias for #value?.

#include?(k) ⇒ Boolean #has_key?(k) ⇒ Boolean #member?(k) ⇒ Boolean #key?(k) ⇒ Boolean

Alias for #key?.

#invert ⇒ Hash

Returns a hash created by using gdbm's values as keys, and the keys as values.

#key(value) ⇒ key

Returns the key for a given value. If several keys may map to the same value, the key that is found first will be returned.

#include?(k) ⇒ Boolean #has_key?(k) ⇒ Boolean #member?(k) ⇒ Boolean #key?(k) ⇒ Boolean
Also known as: #include?, #has_key?, #member?

Returns true if the given key k exists within the database. Returns false otherwise.

#keys ⇒ Array

Returns an array of all keys of this database.

#length ⇒ Fixnum #size ⇒ Fixnum
Also known as: #size

Returns the number of key-value pairs in this database.

#include?(k) ⇒ Boolean #has_key?(k) ⇒ Boolean #member?(k) ⇒ Boolean #key?(k) ⇒ Boolean

Alias for #key?.

#reject {|key, value| ... } ⇒ Hash

Returns a hash copy of gdbm where all key-value pairs from gdbm for which block evaluates to true are removed. See also: #delete_if

#delete_if {|key, value| ... } ⇒ GDBM #reject! {|key, value| ... } ⇒ GDBM
Also known as: #delete_if

Deletes every key-value pair from gdbm for which block evaluates to true.

#reorganize ⇒ GDBM

Reorganizes the database file. This operation removes reserved space of elements that have already been deleted. It is only useful after a lot of deletions in the database.

#replace(other) ⇒ GDBM

Replaces the content of gdbm with the key-value pairs of other. other must have an each_pair method.

#select {|key, value| ... } ⇒ Array

Returns a new array of all key-value pairs of the database for which block evaluates to true.

#shift ⇒ (key, value)?

Removes a key-value-pair from this database and returns it as a two-item array [ key, value ]. Returns nil if the database is empty.

#length ⇒ Fixnum #size ⇒ Fixnum

Alias for #length.

#[]=(key, value) ⇒ value #store(key, value) ⇒ value

Alias for #[]=.

#sync ⇒ GDBM

Unless the gdbm object has been opened with the SYNC flag, it is not guaranteed that database modification operations are immediately applied to the database file. This method ensures that all recent modifications to the database are written to the file. Blocks until all writing operations to the disk have been finished.

#to_a ⇒ Array

Returns an array of all key-value pairs contained in the database.

#to_hash ⇒ Hash

Returns a hash of all key-value pairs contained in the database.

#update(other) ⇒ GDBM

Adds the key-value pairs of other to gdbm, overwriting entries with duplicate keys with those from other. other must have an each_pair method.

#has_value?(v) ⇒ Boolean #value?(v) ⇒ Boolean
Also known as: #has_value?

Returns true if the given value v exists within the database. Returns false otherwise.

#values ⇒ Array

Returns an array of all values of this database.

#values_at(key, ...) ⇒ Array

Returns an array of the values associated with each specified key.