NSUserDefaults provides an interface to the defaults system, which allows an application access to global and/or application specific defaults set by the user. A particular instance of NSUserDefaults, standardUserDefaults, is provided as a convenience. Most of the information described below pertains to the standardUserDefaults. It is unlikely that you would want to instantiate your own userDefaults object, since it would not be set up in the same way as the standardUserDefaults.

Defaults are managed based on domains. Certain domains, such as NSGlobalDomain, are persistent. These domains have defaults that are stored externally. Other domains are volatile. The defaults in these domains remain in effect only during the existence of the application and may in fact be different for applications running at the same time. When asking for a default value from standardUserDefaults, NSUserDefaults looks through the various domains in a particular order.

GSPrimaryDomain... volatile

Contains values set at runtime and intended to supercede any values set in other domains. This should be used with great care since it overrides values which may have been set explicitly by the user.

NSArgumentDomain... volatile

Contains defaults read from the arguments provided to the application at startup.
Pairs of arguments are used for this, with the first argument in each pair being the name of a default (with a hyphen prepended) and the second argument of the pair being the value of the default.
NB. In GNUstep special arguments of the form --GNU-Debug=... are used to enable debugging. Despite beginning with a hyphen, these are not treated as default keys.

Application (name of the current process)... persistent

Contains application specific defaults, such as window positions. This is the domain used by the -setObject:forKey: method and is the domain normally used when setting preferences for an application.

NSGlobalDomain... persistent

Global defaults applicable to all applications.

Language (name based on users's language)... volatile

Constants that help with localization to the users's language.

GSConfigDomain... volatile

Information retrieved from the GNUstep configuration system. Usually the system wide and user specific GNUstep.conf files, or from information compiled in when the base library was built.
In addition to this standard configuration information, this domain contains all values from property lists store in the GlobalDefaults subdirectory or from the GlobalDefaults.plist file stored in the same directory as the system wide GNUstep.conf file.

NSRegistrationDomain... volatile

Temporary defaults set up by the application.

The NSLanguages default value is used to set up the constants for localization. GNUstep will also look for the LANGUAGES environment variable if it is not set in the defaults system. If it exists, it consists of an array of languages that the user prefers. At least one of the languages should have a corresponding localization file (typically located in the Languages directory of the GNUstep resources).

As a special extension, on systems that support locales (e.g. GNU/Linux and Solaris), GNUstep will use information from the user specified locale, if the NSLanguages default value is not found. Typically the locale is specified in the environment with the LANG environment variable.

The first change to a persistent domain after a -synchronize will cause an NSUserDefaultsDidChangeNotification to be posted (as will any change caused by reading new values from disk), so your application can keep track of changes made to the defaults by other software.

NB. The GNUstep implementation differs from the Apple one in that it is thread-safe while Apple's (as of MacOS-X 10.1) is not.