Predefined Keys

An application must open a key before it can add data to the registry. To open a key, an application must supply the handle of another key in the registry that is already open. The system defines standard handles that are always open. An application can use these predefined handles as entry points to the registry.

The system provides two predefined keys at the root of the registry: HKEY_LOCAL_MACHINE and HKEY_USERS. In addition, the system defines two subkeys: HKEY_CLASSES_ROOT (a subkey of HKEY_LOCAL_MACHINE) and HKEY_CURRENT_USER (a subkey of HKEY_USERS). These registry handles are valid for all Win32 implementations of the registry, although the use of the handles may vary from platform to platform.

Predefined keys help an application navigate in the registry and make it possible to develop tools that allow a system administrator to manipulate categories of data. Applications that add data to the registry should always work within the framework of predefined keys, so administrative tools can find and use the new data.

These predefined keys are used as entry points to the registry.

Entry point
Registry entries subordinate to this key define types (or classes) of documents and the properties associated with those types. Data stored under this key is used by Windows shell applications and by object linking and embedding (OLE) applications.
Registry entries subordinate to this key define the preferences of the current user. These preferences include the settings of environment variables, data about program groups, colors, printers, network connections, and application preferences.
Registry entries subordinate to this key define the physical state of the computer, including data about the bus type, system memory, and installed hardware and software.
Registry entries subordinate to this key define the default user configuration for new users on the local computer and the user configuration for the current user.

The use of HKEY_CURRENT_USER, HKEY_LOCAL_MACHINE, and HKEY_USERS varies depending on the implementation of the registry. In addition, other predefined handles have been defined for specific Windows platforms.

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