Windows Registry

Windows Registry is a hierarchical database that stores configuration settings and options on Microsoft Windows operating systems. It contains settings for low-level operating system components as well as the applications running on the platform: the kernel, device drivers, services, SAM, user interface and third party applications all make use of the Registry. The registry also provides a means to access counters for profiling system performance.

When first introduced with Windows 3.1, the Windows registry's primary purpose was to store configuration information for COM-based components. With the introduction of Windows 95 and Windows NT, its use was extended to tidy up the profusion of per-program INI files that had previously been used to store configuration settings for Windows programs.

Rationale
.INI files stored each program's user settings in a separate file. By contrast, the Windows registry stores all application settings in one central repository and in a standardized form. This offers several advantages over INI files. Since accessing the registry does not require parsing, it may be read from or written to more quickly than an INI file. As well, strongly-typed data can be stored in the registry, as opposed to the text information stored in INI files. Because user-based registry settings are loaded from a user-specific path rather than from a read-only system location, the registry allows multiple users to share the same machine, and also allows programs to work for a least-privilege user. Backup and restoration is also simplified as the registry can be accessed over a network connection for remote management/support, including from scripts, using the standard set of APIs, as long as the Remote Registry service is running and firewall rules permit this.

The registry has features that improve system integrity, as the registry is constructed as a database and offers database-like features such as atomic updates. If two processes attempt to update the same registry value at the same time, one process's change will precede the other's and the overall consistency of the data will be maintained. Where changes are made to INI files, such race conditions can result in inconsistent data which doesn't match either attempted update. Windows Vista and Windows 7 provide transactional updates to the registry, extending the atomicity guarantees across multiple key and/or value changes, with traditional commit-abort semantics. (Note however that NTFS provides such support for the file system as well, so the same guarantees could, in theory, be obtained with traditional configuration files.)

Keys and values
The registry contains two basic elements: keys and values.

Registry Keys are similar to folders - in addition to values, each key can contain subkeys, which may contain further subkeys, and so on. Keys are referenced with a syntax similar to Windows' path names, using backslashes to indicate levels of hierarchy. E.g. HKEY_LOCAL_MACHINE\Software\Microsoft\Windows refers to the subkey "Windows" of the subkey "Microsoft" of the subkey "Software" of the HKEY_LOCAL_MACHINE key.

There are six Root Keys:
 * HKEY_CLASSES_ROOT
 * HKEY_CURRENT_USER
 * HKEY_LOCAL_MACHINE
 * HKEY_USERS
 * HKEY_CURRENT_CONFIG
 * HKEY_DYN_DATA

Registry Values are name/data pairs stored within keys. Values are referenced separately from keys. Value names can contain backslashes but doing so makes them difficult to distinguish from their key paths. The Windows API functions that query and manipulate registry values take value names separately from the key path and/or handle that identifies the parent key. The terminology is somewhat misleading, as the values are similar to an associative array, where standard terminology would refer to the name part of the value as a "key". The terms are a holdout from the 16-bit registry in Windows 3, in which keys could not contain arbitrary name/data pairs, but rather contained only one unnamed value (which had to be a string). In this sense, the entire registry was like an associative array where the keys (in both the registry sense and dictionary sense) formed a hierarchy, and the values were all strings. When the 32-bit registry was created, so was the additional capability of creating multiple named values per key, and the meanings of the names were somewhat distorted.

There are a number of different types of values:

Hives
The Registry is split into a number of logical sections, or "hives" (the reason the word hive was used is an in-joke). Hives are generally named by their Windows API definitions, which all begin "HKEY". They are abbreviated to a three- or four-letter short name starting with "HK" (e.g. HKCU and HKLM).

The HKEY_LOCAL_MACHINE (local machine-specific configuration data) and HKEY_CURRENT_USER (user-specific configuration data) nodes have a similar structure to each other; user applications typically look up their settings by first checking for them in "HKEY_CURRENT_USER\Software\Vendor's name\Application's name\Version\Setting name", and if the setting is not found, look instead in the same location under the HKEY_LOCAL_MACHINE key. However, the converse may apply for administrator-enforced policy settings where HKLM may take precedence over HKCU. The Windows Logo Program has specific requirements for where different types of user data may be stored, and that the concept of least privilege be followed so that administrator-level access is not required to use an application.

HKEY_CLASSES_ROOT (HKCR)
Abbreviated HKCR, HKEY_CLASSES_ROOT stores information about registered applications, such as file associations and OLE Object Class IDs, tying them to the applications used to handle these items. On Windows 2000 and above, HKCR is a compilation of user-based HKCU\Software\Classes and machine-based HKLM\Software\Classes. If a given value exists in both of the subkeys above, the one in HKCU\Software\Classes takes precedence. The design allows for either machine- or user-specific registration of COM objects. The user-specific classes hive, unlike the HKCU hive, does not form part of a roaming user profile.

HKEY_CURRENT_USER (HKCU)
Abbreviated HKCU, HKEY_CURRENT_USER stores settings that are specific to the currently logged-in user. The HKCU key is a link to the subkey of HKEY_USERS that corresponds to the user; the same information is accessible in both locations. On Windows-                    NT based systems, each user's settings are stored in their own files called NTUSER.DAT and USRCLASS.DAT inside their own Documents and Settings subfolder (or their own Users subfolder in Windows Vista). Settings in this hive follow users with a roaming profile from machine to machine.

HKEY_LOCAL_MACHINE (HKLM)
Abbreviated HKLM, HKEY_LOCAL_MACHINE stores settings that are specific to the local computer. On NT-based versions of Windows, HKLM contains four subkeys, SAM, SECURITY, SOFTWARE and SYSTEM, that are found within their respective files located in the %SystemRoot%\System32\config folder. A fifth subkey, HARDWARE, is volatile and is created dynamically, and as such is not stored in a file. Information about system hardware drivers and services are located under the SYSTEM subkey, while the SOFTWARE subkey contains software and Windows settings.

HKEY_USERS (HKU)
Abbreviated HKU, HKEY_USERS contains subkeys corresponding to the HKEY_CURRENT_USER keys for each user profile actively loaded on the machine, though user hives are usually only loaded for currently logged-in users.

HKEY_CURRENT_CONFIG (HKCC)
Abbreviated HKCC, HKEY_CURRENT_CONFIG contains information gathered at runtime; information stored in this key is not permanently stored on disk, but rather regenerated at the boot time. It is a link to HKEY_LOCAL_MACHINE\System\CurrentControlSet\Hardware Profiles\Current.

HKEY_PERFORMANCE_DATA
This key provides runtime information into performance data provided by either the NT kernel itself or other programs that provide performance data. This key is not displayed in the Registry Editor, but it is visible through the registry functions in the Windows API.

HKEY_DYN_DATA
This key is used only on Windows 95, Windows 98 and Windows Me. It contains information about hardware devices, including Plug and Play and network performance statistics. The information in this hive is also not stored on the hard drive. The Plug and Play information is gathered and configured at startup and is stored in memory.

Aliases
Aliases in the Windows 9x registry:

Manual editing


The Windows registry can be edited manually using programs such as regedit.exe and regedt32.exe.

As a careless change could cause irreversible damage, a backup of the registry before editing is recommended by Microsoft.

A simple implementation of the current registry tool appeared in Windows 3.x, called the "Registration Info Editor" or "Registration Editor". This was basically just a database of applications used to edit embedded OLE objects in documents.

Windows 9x operating systems include REGEDIT.EXE which can be used in Windows and also in real mode MS-DOS. Windows NT introduced permissions for Registry editing. Windows NT 4.0 and Windows 2000 were distributed with both the Windows 9x REGEDIT.EXE program and Windows NT 3.x's REGEDT32.EXE program. There are several differences between the two editors on these platforms:
 * REGEDIT.EXE has a left-side tree view that begins at "My Computer" and lists all loaded hives. REGEDT32.EXE has a left-side tree view, but each hive has its own window, so the tree displays only keys.
 * REGEDIT.EXE represents the three components of a value (its name, type, and data) as separate columns of a table. REGEDT32.EXE represents them as a list of strings.
 * REGEDIT.EXE supports right-clicking of entries in a tree view to adjust properties and other settings. REGEDT32.EXE requires all actions to be performed from the top menu bar.
 * REGEDIT.EXE supports searching for key names, values, or data throughout the entire registry, whereas REGEDT32.EXE only supports searching for key names in one hive at a time.
 * Because REGEDIT.EXE was directly ported from Windows 95, it does not support editing permissions. (Permissions do not exist on Windows 9x).  Therefore, of the two, only REGEDT32.EXE can access the full functionality of an NT registry.
 * REGEDIT.EXE only supports string (REG_SZ), binary (REG_BINARY), and DWORD (REG_DWORD) values. REGEDT32.EXE supports those, plus expandable string (REG_EXPAND_SZ) and multi-string (REG_MULTI_SZ). Attempting to edit unsupported key types with REGEDIT.EXE on Windows 2000 or Windows NT 4.0 will result in irreversible conversion to a supported type.

Windows XP was the first system to integrate these two programs into one, adopting the old REGEDIT.EXE interface and adding the REGEDT32.EXE functionality. The differences listed above are not applicable on Windows XP and newer systems; REGEDIT.EXE is the improved editor, and REGEDT32.EXE is simply a stub that invokes REGEDIT.EXE.

The Registry Editor allows users to perform the following functions:
 * Creating, manipulating, renaming and deleting registry keys, subkeys, values and value data
 * Importing and exporting .REG files, exporting data in the binary hive format
 * Loading, manipulating and unloading registry hive format files (Windows NT-based systems only)
 * Setting permissions based on ACLs (Windows NT-based systems only)
 * Bookmarking user-selected registry keys as Favorites
 * Finding particular strings in key names, value names and value data
 * Remotely editing the registry on another networked computer

It is also possible to edit the registry under Linux using the open source Offline NT Password & Registry Editor to edit the files.

.REG files
.REG files (also known as Registration entries) are text-based human-readable files for storing portions of the registry. On Windows 2000 and later NT-based operating systems, they contain the string Windows Registry Editor Version 5.00 at the beginning and are Unicode-based. On Windows 9x and NT 4.0 systems, they contain the string REGEDIT4 and are ANSI-based. Windows 9x format .REG files are compatible with Windows 2000 and later NT-based systems. The Registry Editor on Windows on these systems also supports exporting .REG files in Windows 9x/NT format. Data is stored in .REG files in the following syntax: The Default Value of a key can be edited by using @ instead of "Value Name": String values do not require a  (see example).

For example, to add the values "Value A", "Value B", "Value C", "Value D", and "Value E" to the HKLM\SOFTWARE\Microsoft key, Data from .REG</tt> files can be added/merged with the registry by double-clicking these files or using the /s switch in the command line. .REG</tt> files can also be used to remove registry data.

To remove a key (and all subkeys, values and data), the key name must be preceded by a minus sign ("-").

For example, to remove the <Key Name> key (and all subkeys, values and data),

To remove a value (and its data), the values to be removed must have a minus sign ("-") after the equal sign ("=").

For example, to remove only the "Value A" and "Value B" values (and their data) from the <Key Name> key,

To remove only the (Default) value of the key <Key Name> (and its data),

Command line editing
The registry can be manipulated in a number of ways from the command line. The  utility tool is included in Windows XP and later versions of Windows. Alternative locations for legacy versions of Windows include the Resource Kit CDs or the original Installation CD of Windows.

Also, a  file can be imported from the command line with the following command:

regedit.exe /s file

The /s means the file will be silent merged to the Registry. If the  parameter is omitted the user will be asked to confirm the operation. In Windows 98, Windows 95 and at least some configurations of Windows XP the  switch also causes   to ignore the setting in the registry that allows administrators to disable it. When using the  switch Regedit does not return an appropriate return code if the operation fails, unlike reg.exe which does.

The default association for  files in many versions of Microsoft Windows.

Other command line options include a VBScript or JScript together with CScript, WMI or  and Windows PowerShell.

Registry permissions can be manipulated through the command line using the  tool. For example, the permissions on the HKEY_LOCAL_MACHINE\SOFTWARE key can be displayed using:

subinacl /keyreg HKEY_LOCAL_MACHINE\SOFTWARE /display

Programs or scripts
The registry can be edited through the APIs of the Advanced Windows 32 Base API Library (advapi32.dll).

Many programming languages offer built-in runtime library functions or classes that enable programs to store settings in the registry (e.g.  in VB.NET and C#, or   in Delphi). COM-enabled applications like Visual Basic 6 can use the WSH  object. Another way is to use the Windows Resource Kit Tool, Reg.exe by executing it from code, although this is considered poor programming practice.

Similarly, scripting languages such as Perl (with ), Windows PowerShell and Windows Scripting Host also enable registry editing from scripts.

Locations
The Registry is stored in several files; depending upon the version of Windows, there will be different files and different locations for these files, but they are all on the local machine. The user-specific HKEY_CURRENT_USER user registry hive is stored in Ntuser.dat</tt> inside the user profile. There is one of these per user; if a user has a roaming profile, then this file will be copied to and from a server at logout and login respectively.

Windows NT-based operating systems
Windows NT-based systems store the registry in a binary hive format which is the same format that can be exported, loaded and unloaded by the Registry Editor in these operating systems. The following Registry files are stored in %SystemRoot%\System32\Config\</tt>:
 * Sam</tt> – HKEY_LOCAL_MACHINE\SAM
 * Security</tt> – HKEY_LOCAL_MACHINE\SECURITY
 * Software</tt> – HKEY_LOCAL_MACHINE\SOFTWARE
 * System</tt> – HKEY_LOCAL_MACHINE\SYSTEM
 * Default</tt> – HKEY_USERS\.DEFAULT
 * Userdiff</tt> – Not associated with a hive. Used only when upgrading operating systems.

The following file is stored in each user's profile folder:
 * %UserProfile%\Ntuser.dat</tt> – HKEY_USERS\<User SID> (linked to by HKEY_CURRENT_USER)

For NT and XP, you also have the file For Vista you have the file
 * %UserProfile%\Local Settings\Application Data\Microsoft\Windows\Usrclass.dat</tt> (path is localized) – HKEY_USERS\<User SID>_Classes (HKEY_CURRENT_USER\Software\Classes)
 * %UserProfile%\AppData\Local\Microsoft\Windows\Usrclass.dat</tt> (path is localized) – HKEY_USERS\<User SID>_Classes (HKEY_CURRENT_USER\Software\Classes)

Windows 2000 keeps an alternate copy of the registry hives (.ALT) and attempts to switch to it when corruption is detected. Windows XP and Windows Server 2003 do not maintain a System.alt hive because NTLDR on those versions of Windows can process the System.log file to bring up to date a System hive that has become inconsistent during a shutdown or crash. In addition, the %Windir%\Repair</tt> folder contains a copy of the system's registry hives that were created after installation and the first successful startup of Windows.

Windows 95, 98, and Me
The registry files are named USER.DAT</tt> and SYSTEM.DAT</tt> are stored in the %WINDIR%</tt> directory. In Windows Me, <tt>Classes.dat</tt> was added. Also, each user profile (if profiles are enabled) has its own <tt>USER.DAT</tt> in profile's directory.

Windows 3.11
The registry file is <tt>Reg.dat,</tt> <tt>system.dat</tt> and is stored in the <tt>C:\WINDOWS</tt> directory.

These files also contain system registry info: <tt>user.dat</tt> and is stored in the <tt>C:\windows\profiles\ \user.dat</tt> directory.

Backups and recovery
Windows supports several methods to back up and restore the registry:
 * System Restore can back up the registry and restore it as long as Windows is bootable, or from the Windows Recovery Environment starting with Windows Vista.
 * NTBackup can back up the registry as part of the System State and restore it.
 * On Windows NT-based systems, the Last Known Good Configuration option in startup menu relinks the <tt>HKLM\SYSTEM\CurrentControlSet</tt> key, which stores hardware and device driver information.
 * Windows 98 and Windows Me include command line (Scanreg.exe) and GUI (Scanregw.exe) registry checker tools to check and fix the integrity of the registry, create up to five automatic regular backups by default and restore them manually or whenever corruption is detected. The registry checker tool backs up the registry, by default, to <tt>%Windir%\Sysbckup</tt> Scanreg.exe can also run from MS-DOS.
 * The Windows 95 CD-ROM included an Emergency Recovery Utility (ERU.exe) and a Configuration Backup Tool (Cfgback.exe) to back up and restore the registry. Additionally Windows 95 backs up the registry to the files system.da0 and user.da0 on every successful boot.

Group policy
Windows 2000 and later versions of Windows use Group Policy to enforce Registry settings. Policy may be applied locally to a single computer using <tt>GPEdit.msc</tt>, or to multiple computers in a domain using <tt>gpmc.msc</tt>.

Legacy systems
With Windows 95, Windows 98, Windows Me and Windows NT, administrators can use a special file to be merged into the registry, called a policy file (<tt>POLICY.POL</tt>). The policy file allows administrators to prevent non-administrator users from changing registry settings like, for instance, the security level of Internet Explorer and the desktop background wallpaper. The policy file is primarily used in a business with a large number of computers where the business needs to be protected from rogue or careless users.

The default extension for the policy file is <tt>.POL</tt>. The policy file filters the settings it enforces by user and by group (a "group" is a defined set of users). To do that the policy file merges into the registry, preventing users from circumventing it by simply changing back the settings. The policy file is usually distributed through a LAN, but can be placed on the local computer.

The policy file is created by a free tool by Microsoft that goes by the filename <tt>poledit.exe</tt> for Windows 95/Windows 98 and with a computer management module for NT-based systems. The editor requires administrative permissions to be run on systems that uses permissions. The editor can also directly change the current registry settings of the local computer and if the remote registry service is installed and started on another computer it can also change the registry on that computer. The policy editor loads the settings it can change from <tt>.ADM</tt> files, of which one is included, that contains the settings the Windows shell provides. The <tt>.ADM</tt> file is plain text and supports easy localisation by allowing all the strings to be stored in one place.

.INI file virtualization
Windows NT kernels support redirection of INI file-related APIs into a virtual file in a Registry location such as HKEY_CURRENT_USER using a feature called "InifileMapping". This functionality was introduced to allow legacy applications written for 16-bit versions of Windows to be able to run under Windows NT platforms on which the System folder is no longer considered an appropriate location for user-specific data or configuration. Non-compliant 32-bit applications can also be redirected in this manner, even though the feature was originally intended for 16-bit applications.

Registry virtualization
Windows Vista has introduced limited Registry virtualization, whereby poorly written applications that do not respect the principle of least privilege and instead try to write user data to a read-only system location (such as the HKEY_LOCAL_MACHINE hive), can be redirected to a more appropriate location, without changing the application itself. The operation is transparent to the application, as it does not know that its Registry operations have been directed elsewhere.

Similarly, application virtualization redirects all of an application's Registry operations to a non-Registry backed location, such as a file. Used together with file virtualization, this approach allows applications to run without being installed on the location machine.

Low integrity processes may also leverage registry virtualisation. For example as Internet Explorer 7 or 8 running in "Protected Mode" on Windows Vista or Windows 7 will automatically redirect registry writes by ActiveX controls to a sandboxed location in order to frustrate some classes of security exploits.

Lastly, the Application Compatibility Toolkit provides shims that can transparently redirect HKEY_LOCAL_MACHINE or HKEY_CLASSES_ROOT Registry operations to HKEY_CURRENT_USER to address "LUA" bugs that cause applications not to work for limited users.

Equivalents in other operating systems
In contrast to the Windows registry's binary-based database model, some other operating systems use separate plain-text files for daemon and application configuration, but group these configurations together for ease of management.


 * Under Unix-like operating systems e.g. Linux that follow the Filesystem Hierarchy Standard, system-wide configuration files (information similar to what would appear in HKEY_LOCAL_MACHINE on Windows) are traditionally stored in files in <tt>/etc/</tt> and its subdirectories, or sometimes in <tt>/usr/local/etc</tt>. Per-user information (information that would be roughly equivalent to that in HKEY_CURRENT_USER) is stored in hidden directories and files (that start with a period/full stop) within the user's home directory. However XDG-compliant applications should refer to the environment variables defined in the Base Directory specification.
 * Applications running on Apple Inc.'s Mac OS X operating system typically store settings in property list files which are usually stored in each user's Library folder.
 * RISC OS also allows applications to be copied into directories easily, as opposed to the separate installation program that typifies Windows applications. If one wishes to remove the application, it is possible to simply delete the folder belonging to the application. This will often not remove configuration settings which are stored independently from the application, usually within the computer's !Boot structure, in !Boot.Choices, but potentially anywhere on a network fileserver.
 * IBM AIX (a Unix variant) uses a registry component called Object Data Manager (ODM). The ODM is used to store information about system and device configuration. An extensive set of tools and utilities provides users with means of extending, checking, correcting the ODM database. The ODM stores its information in several files, default location is /etc/objrepos.
 * The GNOME desktop environment uses a registry-like interface called GConf for storing configuration settings for the desktop and applications. However, in GConf, all application settings are stored in separate files, thereby eliminating a single point of failure.  Conversely, this also creates multiple points of failure, and the likelihood of one or more files being destroyed is increased.
 * The Elektra Initiative provides an alternative back-end for text configuration files for the Linux operating system, similar to the registry.
 * While not an operating system, the Wine compatibility layer, which allows Windows software to run on a Unix-like system, also employs a Windows-like registry as text files in the WINEPREFIX folder: system.reg (HKEY_LOCAL_MACHINE), user.reg (HKEY_CURRENT_USER) and userdef.reg.

Criticism
While offering improvements over application-specific .INI files, the organization and implementation of the registry also has potential problems. Criticisms include the following:
 * The registry duplicates much of the functionality of the file system.
 * Centralizing configurations makes it difficult to back up and recover individual applications.
 * Because the Registry structure is contained in files which are not human readable, damage to the registry itself is difficult or impossible to repair. Because information required for loading device drivers is stored in the registry, a damaged registry may prevent a Windows system from booting successfully. While damaged configuration files can have the same result to other operating systems, the damage can be more easily repaired by booting to another operating system, and using a text editor.
 * Installers and uninstallers become more complicated, because application configuration settings cannot be transferred by simply copying the files that comprise the application.
 * Applications that make use of the registry to store and retrieve their settings are unsuitable for use on portable devices used to carry applications from one system to another.
 * Since an application's configuration is centralized away from the application itself, it is often not possible to copy installed applications that use the Registry to another computer. This means that software usually has to be reinstalled from original media on a computer upgrade or rebuild, rather than just copying the user and software folder to the new computer.
 * The Windows Registry is said to be a single point of failure.