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# win32reg (stdlib v7.3)

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Provides access to the registry on Windows.

This module 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 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 subkeys to the right and backslash between keys. (Remember that backslashes must be doubled in Erlang strings.) Case is preserved but not significant.

For 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 this module as follows:

    Abbreviation     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 can be written as `"\\hklm\\software\\ericsson\\erlang\\5.0"`.

This module uses a current key. It works much like the current directory. From the current key, values can be fetched, subkeys can be listed, and so on.

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

`win32reg` supports storing of the following types:

- `REG_DWORD`, which is an integer
- `REG_SZ`, which is a string
- `REG_BINARY`, which is a binary

Other types can be read, and are 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, for example, `%SystemRoot%Windows`. `SystemRoot` is an environment variable, and is to be replaced with its value. Function `expand/1` is provided so that environment variables surrounded by `%` can be expanded to their values.

For more information on the Windows registry, see consult the Win32 Programmer's Reference.

## See Also

`erl_posix_msg`, The Windows 95 Registry (book from O'Reilly), Win32 Programmer's Reference (from Microsoft)

## Summary

### Types

name()
reg_handle()

As returned by `open/1`.

value()

### Functions

change_key(RegHandle, Key)

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)

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()`.

close(RegHandle)

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

current_key(RegHandle)

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

delete_key(RegHandle)

Deletes the current key, if it is valid. Calls the Win32 API function `RegDeleteKey()`. Notice 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)

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

expand(String)

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

format_error(ErrorId)

Converts a POSIX error code to a string (by calling `file:format_error/1`).

open(OpenModeList)

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

set_value(RegHandle, Name, Value)

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 is used. The supported types are the following

sub_keys(RegHandle)

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

value(RegHandle, Name)

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)

Retrieves a list of all values on the current key. The values have types corresponding to the registry types, see `value/2`. Calls the Win32 API function `EnumRegValuesEx()`.

## Types

# name()

(not exported)

    -type name() :: string() | default.

# reg_handle()

    -opaque reg_handle()

As returned by `open/1`.

# value()

(not exported)

    -type value() :: string() | integer() | binary().

## Functions

# change_key(RegHandle, Key)

    -spec change_key(RegHandle, Key) -> ReturnValue
                        when
                            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)

    -spec change_key_create(RegHandle, Key) -> ReturnValue
                               when
                                   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)

    -spec close(RegHandle) -> ok when RegHandle :: reg_handle().

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

# current_key(RegHandle)

    -spec current_key(RegHandle) -> ReturnValue
                         when RegHandle :: reg_handle(), ReturnValue :: {ok, string()}.

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

Notice that the current key is stored in the driver, and can be invalid (for example, if the key has been removed).

# delete_key(RegHandle)

    -spec delete_key(RegHandle) -> ReturnValue
                        when RegHandle :: reg_handle(), ReturnValue :: ok | {error, ErrorId :: atom()}.

Deletes the current key, if it is valid. Calls the Win32 API function `RegDeleteKey()`. Notice 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)

    -spec delete_value(RegHandle, Name) -> ReturnValue
                          when
                              RegHandle :: reg_handle(),
                              Name :: name(),
                              ReturnValue :: ok | {error, ErrorId :: atom()}.

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

The registry must have been opened in write mode.

# expand(String)

    -spec expand(String) -> ExpandedString when String :: string(), ExpandedString :: string().

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

A variable name that is not in the environment results in an error.

# format_error(ErrorId)

    -spec format_error(ErrorId) -> ErrorString when ErrorId :: atom(), ErrorString :: string().

Converts a POSIX error code to a string (by calling `file:format_error/1`).

# open(OpenModeList)

    -spec open(OpenModeList) -> ReturnValue
                  when
                      OpenModeList :: [OpenMode],
                      OpenMode :: read | write,
                      ReturnValue :: {ok, RegHandle} | {error, ErrorId :: enotsup},
                      RegHandle :: reg_handle().

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

Use `change_key/2` with an absolute path after `open`.

# set_value(RegHandle, Name, Value)

    -spec set_value(RegHandle, Name, Value) -> ReturnValue
                       when
                           RegHandle :: reg_handle(),
                           Name :: name(),
                           Value :: value(),
                           ReturnValue :: ok | {error, ErrorId :: atom()}.

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 is used. The supported types are the following:

- `REG_DWORD` for integers
- `REG_SZ` for strings
- `REG_BINARY` for binaries

Other types cannot be added or changed.

The registry must have been opened in write mode.

# sub_keys(RegHandle)

    -spec sub_keys(RegHandle) -> ReturnValue
                      when
                          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, as it can be slow.

# value(RegHandle, Name)

    -spec value(RegHandle, Name) -> ReturnValue
                   when
                       RegHandle :: reg_handle(),
                       Name :: name(),
                       ReturnValue :: {ok, Value :: value()} | {error, ErrorId :: atom()}.

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)

    -spec values(RegHandle) -> ReturnValue
                    when
                        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/2`. Calls the Win32 API function `EnumRegValuesEx()`.