# X Library Controls

The Xkb extension is composed of two parts: a server extension, and a client-side X library extension. Keyboard_Controls discusses functions used to modify controls affecting the behavior of the server portion of the Xkb extension. This chapter discusses functions used to modify controls that affect only the behavior of the client portion of the extension; these controls are known as Library Controls.

All of the Library Controls are boolean flags that may be enabled and disabled. The controls can be divided into several categories:

- Controls affecting general string lookups

- Controls affecting compose processing

- Controls affecting event delivery

There are two types of string lookups performed by `XLookupString`.
The first type involves translating a single keycode into a string; the controls in the first category affect this type of lookup. The second type involves translating a series of keysyms into a string; the controls in the second category affect this type of lookup.

An Xkb implementation is required to support the programming interface for all of the controls. However, an implementation may choose not to support the semantics associated with the controls that deal with compose processing. In this case, a program that accesses these controls should still function normally; however, the feedback that would normally occur with the controls enabled may be missing.

## Controls Affecting Keycode-to-String Translation

The first type of string lookups, which are here called simple string lookups, involves translating a single keycode into a string. Because these simple lookups involve only a single keycode, all of the information needed to do the translation is contained in the keyboard state in a single event. The controls affecting simple string lookups are:

- ForceLatin1Lookup
- ConsumeLookupMods
- LevelOneUsesShiftAndLock

### ForceLatin1Lookup

If the ForceLatin1Lookup control is enabled, `XLookupString` only returns strings using the Latin1 character set. If ForceLatin1Lookup is not enabled, `XLookupString` can return characters that are not in the Latin1 set. By default, this control is disabled, allowing characters outside of the Latin1 set to be returned.

### ConsumeLookupMods

Simple string lookups in `XLookupString` involve two different translation phases. The first phase translates raw device keycodes to individual keysyms. The second phase attempts to map the resulting keysym into a string of one or more characters. In the first phase, some of the modifiers are normally used to determine the appropriate shift level for a key.

The ConsumeLookupMods control determines whether or not `XLookupString` consumes the modifiers it uses during the first phase of processing (mapping a keycode to a keysym). When a modifier is consumed, it is effectively removed from the working copy of the keyboard state information `XLookupString` is using and appears to be unset for the remainder of the processing.

If the ConsumeLookupMods control is enabled, `XLookupString` does not use the modifiers used to translate the keycode of the event to a keysym when it is determining the string associated with a keysym. For example, assume the keymap for the ‘A’ key only contains the shift modifier and the ConsumeLookupMods control is enabled. If a user presses the Shift key and the A key while the Num_Lock key is locked, `XLookupString` uses the Shift modifier when mapping the keycode for the ‘a’ key to the keysym for ‘A’; subsequently, it only uses the NumLock modifier when determining the string associated with the keysym ‘A’.

If the ConsumeLookupMods control is not enabled, `XLookupString` uses all of the event modifiers to determine the string associated with a keysym. This behavior mirrors the behavior of `XLookupString` in the core implementation.

The ConsumeLookupMods control is unset by default. For more information on modifier consumption, refer to Interpreting_Key_Events.

### AlwaysConsumeShiftAndLock

The AlwaysConsumeShiftAndLock control, if enabled, forces `XLookupString` to consume the Shift and Lock modifiers when processing all keys, even if the definition for the key type does not specify these modifiers. The AlwaysConsumeShiftAndLock control is unset by default. See section 15.2 for a discussion of key types.

## Controls Affecting Compose Processing

The second type of string lookup performed by `XLookupString` involves translating a series of keysyms into a string. Because these lookups can involve more than one key event, they require `XLookupString` to retain some state information between successive calls. The process of mapping a series of keysyms to a string is known as compose processing.
The controls affecting compose processing are:

- ConsumeKeysOnComposeFail
- ComposeLED
- BeepOnComposeFail

Because different vendors have historically used different algorithms to implement compose processing, and these algorithms may be incompatible with the semantics required by the Xkb compose processing controls, implementation of the compose processing controls is optional in an Xkb implementation.

### ConsumeKeysOnComposeFail

Some compose processing algorithms signal the start of a compose sequence by a key event meaning start compose.

Another possibility is to have the compose processing simply be the result of a finite state acceptor; a compose sequence would never fail for a properly written finite state acceptor.

The subsequent key events should normally result in a valid composition yielding a valid translation to a string. If the subsequent key events do not have a valid translation, some decision must be made about what to do with the key events that were processed while attempting the compose. The ConsumeKeysOnComposeFail control allows a client to specify what happens with the key events `XLookupString` has been considering when it reaches a dead end in a compose sequence.

If the ConsumeKeysOnComposeFail control is set, all keys associated with a failed compose sequence should be consumed (discarded). If the ConsumeKeysOnComposeFail control is not set, the key events associated with a failed compose sequence should be processed as a normal sequence of key events.

The ConsumeKeysOnComposeFail control is disabled by default.

### ComposeLED

The ComposeLED control allows a client to specify whether or not an indicator should be set and cleared to provide feedback when compose processing is in progress. The control does not specify which indicator should be used; the mapping for this is up to the individual implementation. If the ComposeLED control is enabled, it specifies that an indicator should be set when a compose sequence is in progress and cleared when one is not in progress. The ComposeLED control is disabled by default.

While the Xkb extension does not specify the type of type of indicator to be used when the ComposeLED control is implemented, a consistent convention between implementations is to everyone’s benefit. If a named indicator is used for this purpose, the recommended name is “`Compose`”. Note that some implementations may use an unnamed, custom hardware LED for this purpose.

### BeepOnComposeFail

The BeepOnComposeFail control allows a client to specify whether or not a bell should be activated to provide feedback when a compose sequence fails. The control does not specify the type of bell that should be used; the mapping for this is up to the individual implementation. If the BeepOnComposeFail control is enabled, it specifies that a bell should be activated when a compose sequence fails. The BeepOnComposeFail control is disabled by default. If implemented, the bell should be activated using `XkbBell` or `XkbDeviceBell`.

While the Xkb extension does not specify the type of bell to be used when the BeepOnComposeFail control is implemented, a consistent convention between implementations is to everyone’s benefit. If a named bell is used for this purpose, the recommended name is “`ComposeFail`”.

## Controls Effecting Event Delivery

### IgnoreNewKeyboards

When Xkb is initialized, it implicitly forces requests for NewKeyboardNotify events. These events may be used by the Xkb library extension internally; they are normally translated into core protocol MappingNotify events before being passed to the client. While delivering the event to the client is appropriate in most cases, it is not appropriate for some clients that maintain per-key data structures. This is because once the server has sent a NewKeyboardNotify event, it is free to send the client events for all keys in the new range and that range may be outside of the per-key data structures the client is maintaining.

The IgnoreNewKeyboards control, if enabled, prevents Xkb from mapping NewKeyboardNotify events to core MappingNotify events and passing them to the client. The control is initially disabled.

## Manipulating the Library Controls

The Library Controls are manipulated using functions that deal with bitmasks to indicate which controls to manipulate. The controls are identified by the masks defined in Table 11.1.

## Library Control Masks

Library Control Mask | Value
XkbLC_ForceLatin1Lookup | (1 << 0)
XkbLC_ConsumeLookupMods | (1 << 1)
XkbLC_AlwaysConsumeShiftAndLock | (1 << 2)
XkbLC_IgnoreNewKeyboards | (1 << 3)
XkbLC_ConsumeKeysOnComposeFail | (1 << 29)
XkbLC_ComposeLED | (1 << 30)
XkbLC_BeepOnComposeFail | (1 << 31)
XkbLC_AllControls | (0xc0000007)

### Determining Which Library Controls are Implemented

To determine which Library Controls are actually implemented, use `XkbXlibControlsImplemented`.

        unsigned int XkbXlibControlsImplemented

        Display *display

`display`

- connection to X server

`XkbXlibControlsImplemented` returns a bitmask indicating the controls actually implemented in the Xkb library and is composed of an inclusive OR of bits from Table 11.1.

### Determining the State of the Library Controls

To determine the current state of the Library Controls, use `XkbGetXlibControls`.

        unsigned int XkbGetXlibControls

        Display *display

`display`

- connection to X server

`XkbGetXlibControls` returns the current state of the Library Controls as a bit mask that is an inclusive OR of the control masks from Table 11.1 for the controls that are enabled. For the optional compose processing controls, the fact that a control is enabled does not imply that it is actually implemented.

### Changing the State of the Library Controls

To change the state of the Library Controls, use `XkbSetXlibControls`.

        Bool XkbSetXlibControls

        Display *display
        unsigned long bits_to_change
        unsigned long values_for_bits

`display`

- connection to X server

`bits_to_change`

- selects controls to be modified

`values_for_bits`

- turns selected controls on (1) or off (0)

`XkbSetXlibControls` modifies the state of the controls selected by `bits_to_change`; only the controls selected by `bits_to_change` are modified. If the bit corresponding to a control is on in `bits_to_change` and also on in values_for_bits, the control is enabled. If the bit corresponding to a control is on in `bits_to_change` but off in `values_for_bits`, the control is disabled. `bits_to_change` should be an inclusive OR of bits from Table 11.1.