win32reg provides access to the registry on Windows

win32reg provides read and write access to the registry on Windows. It is essentially a port driver wrapped around the Win32 API calls for accessing the registry.

The registry is a hierarchical database, used to store various system and software information in Windows. It is available in Windows 95 and Windows NT. It contains installation data, and is updated by installers and system programs. The Erlang installer updates the registry by adding data that Erlang needs.

The registry contains keys and values. Keys are like the directories in a file system, they form a hierarchy. Values are like files, they have a name and a value, and also a type.

Paths to keys are left to right, with sub-keys to the right and backslash between keys. (Remember that backslashes must be doubled in Erlang strings.) Case is preserved but not significant. Example: "\\hkey_local_machine\\software\\Ericsson\\Erlang\\5.0" is the key for the installation data for the latest Erlang release.

There are six entry points in the Windows registry, top level keys. They can be abbreviated in the win32reg module as:

Abbrev.          Registry key
=======          ============      
hkcr             HKEY_CLASSES_ROOT
current_user     HKEY_CURRENT_USER
hkcu             HKEY_CURRENT_USER
local_machine    HKEY_LOCAL_MACHINE
hklm             HKEY_LOCAL_MACHINE
users            HKEY_USERS
hku              HKEY_USERS
current_config   HKEY_CURRENT_CONFIG
hkcc             HKEY_CURRENT_CONFIG
dyn_data         HKEY_DYN_DATA
hkdd             HKEY_DYN_DATA

The key above could be written as "\\hklm\\software\\ericsson\\erlang\\5.0".

The win32reg module uses a current key. It works much like the current directory. From the current key, values can be fetched, sub-keys can be listed, and so on.

Under a key, any number of named values can be stored. They have name, and types, and data.

Currently, the win32reg module supports storing only the following types: REG_DWORD, which is an integer, REG_SZ which is a string and REG_BINARY which is a binary. Other types can be read, and will be returned as binaries.

There is also a "default" value, which has the empty string as name. It is read and written with the atom default instead of the name.

Some registry values are stored as strings with references to environment variables, e.g. "%SystemRoot%Windows". SystemRoot is an environment variable, and should be replaced with its value. A function expand/1 is provided, so that environment variables surrounded in % can be expanded to their values.

For additional information on the Windows registry consult the Win32 Programmer's Reference.



As returned by open/1.

name() = string() | default

value() = string() | integer() | binary()


change_key(RegHandle, Key) -> ReturnValue

  • RegHandle = reg_handle()
  • Key = string()
  • ReturnValue = ok | {error, ErrorId :: atom()}

Changes the current key to another key. Works like cd. The key can be specified as a relative path or as an absolute path, starting with \.

change_key_create(RegHandle, Key) -> ReturnValue

  • RegHandle = reg_handle()
  • Key = string()
  • ReturnValue = ok | {error, ErrorId :: atom()}

Creates a key, or just changes to it, if it is already there. Works like a combination of mkdir and cd. Calls the Win32 API function RegCreateKeyEx().

The registry must have been opened in write-mode.

close(RegHandle) -> ok

Closes the registry. After that, the RegHandle cannot be used.

current_key(RegHandle) -> ReturnValue

Returns the path to the current key. This is the equivalent of pwd.

Note that the current key is stored in the driver, and might be invalid (e.g. if the key has been removed).

delete_key(RegHandle) -> ReturnValue

  • RegHandle = reg_handle()
  • ReturnValue = ok | {error, ErrorId :: atom()}

Deletes the current key, if it is valid. Calls the Win32 API function RegDeleteKey(). Note that this call does not change the current key, (unlike change_key_create/2.) This means that after the call, the current key is invalid.

delete_value(RegHandle, Name) -> ReturnValue

Deletes a named value on the current key. The atom default is used for the the default value.

The registry must have been opened in write-mode.

expand(String) -> ExpandedString

  • String = ExpandedString = string()

Expands a string containing environment variables between percent characters. Anything between two % is taken for a environment variable, and is replaced by the value. Two consecutive % is replaced by one %.

A variable name that is not in the environment, will result in an error.

format_error(ErrorId) -> ErrorString

  • ErrorId = atom()
  • ErrorString = string()

Convert an POSIX errorcode to a string (by calling erl_posix_msg:message).

open(OpenModeList) -> ReturnValue

  • OpenModeList = [OpenMode]
  • OpenMode = read | write
  • ReturnValue = {ok, RegHandle} | {error, ErrorId :: enotsup}
  • RegHandle = reg_handle()

Opens the registry for reading or writing. The current key will be the root (HKEY_CLASSES_ROOT). The read flag in the mode list can be omitted.

Use change_key/2 with an absolute path after open.

set_value(RegHandle, Name, Value) -> ReturnValue

Sets the named (or default) value to value. Calls the Win32 API function RegSetValueEx(). The value can be of three types, and the corresponding registry type will be used. Currently the types supported are: REG_DWORD for integers, REG_SZ for strings and REG_BINARY for binaries. Other types cannot currently be added or changed.

The registry must have been opened in write-mode.

sub_keys(RegHandle) -> ReturnValue

  • RegHandle = reg_handle()
  • ReturnValue = {ok, [SubKey]} | {error, ErrorId :: atom()}
  • SubKey = string()

Returns a list of subkeys to the current key. Calls the Win32 API function EnumRegKeysEx().

Avoid calling this on the root keys, it can be slow.

value(RegHandle, Name) -> ReturnValue

Retrieves the named value (or default) on the current key. Registry values of type REG_SZ, are returned as strings. Type REG_DWORD values are returned as integers. All other types are returned as binaries.

values(RegHandle) -> ReturnValue

  • RegHandle = reg_handle()
  • ReturnValue = {ok, [ValuePair]} | {error, ErrorId :: atom()}
  • ValuePair = {Name :: name(), Value :: value()}

Retrieves a list of all values on the current key. The values have types corresponding to the registry types, see value. Calls the Win32 API function EnumRegValuesEx().


Win32 Programmer's Reference (from Microsoft)


The Windows 95 Registry (book from O'Reilly)