| Platform SDK: Registry |
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_CURRENT_CONFIG (a subkey of HKEY_LOCAL_MACHINE), HKEY_CURRENT_USER (a subkey of HKEY_USERS), and HKEY_CLASSES_ROOT (a subkey that merges information from HKEY_LOCAL_MACHINE and HKEY_CURRENT_USER). 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 RegOverridePredefKey function enables you to map a predefined registry key to a specified key in the registry. For instance, a software installation program could remap a predefined key before installing a DLL component. This enables the installation program to easily examine the information that the DLL's installation procedure writes to the predefined key. 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. Shell and COM applications use the information stored under this key. This key also provides backward compatibility with the Windows 3.1 registration database by storing information for DDE and OLE support. File viewers and user interface extensions store their OLE class identifiers in HKEY_CLASSES_ROOT, and in-process servers are registered in this key. |
| HKEY_CURRENT_CONFIG | Contains information about the current hardware profile of the local computer system. The information under HKEY_CURRENT_CONFIG describes only the differences between the current hardware configuration and the standard configuration. Information about the standard hardware configuration is stored under the Software and System keys of HKEY_LOCAL_MACHINE.
HKEY_CURRENT_CONFIG is an alias for HKEY_LOCAL_MACHINE\System\CurrentControlSet\Hardware Profiles\Current. This key was introduced in Windows 95 and Windows NT version 4.0. |
| 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. This key makes it easier to establish the current user's settings; the key maps to the current user's branch in HKEY_USERS. In HKEY_CURRENT_USER, software vendors store the current user-specific preferences to be used within their applications. Microsoft, for example, creates the HKEY_CURRENT_USER\Software\Microsoft key for its applications to use, with each application creating its own subkey under the Microsoft key. |
| HKEY_DYN_DATA | Windows 95/98: 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. It contains subkeys that hold current configuration data, including Plug and Play information (the Enum branch, which includes a complete list of all hardware that has ever been on the system), network logon preferences, network security information, software-related information (such as server names and the location of the server), and other system information. |
| HKEY_PERFORMANCE_DATA | Windows NT/2000: 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. Because Windows 95/98 also supports HKEY_CURRENT_USER, applications can access the user-specific information the same way they do under Windows NT. |