An application must open a key before it can add data to the registry. To open a key, an application must supply a handle to 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 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. In addition, other predefined handles have been defined for specific platforms.
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.
The following predefined keys are used as entry points to the registry.
| Entry point | Use |
|---|---|
| HKEY_CLASSES_ROOT | 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 shell applications and by object linking and embedding (OLE) applications. |
| HKEY_CURRENT_USER | 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. |
| HKEY_DYN_DATA | Windows 95 and Windows 98: Registry entries subordinate to this key allow you to collect performance data. |
| HKEY_LOCAL_MACHINE | 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. |
| HKEY_PERFORMANCE_DATA | Windows NT: Registry entries subordinate to this key allow you to access performance data. The data is not actually stored in the registry; the registry functions cause the system to collect the data from its source. |
| HKEY_USERS | 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.