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The Xkb extension is composed of two parts: a server extension, and a client-side X library extension. Chapter 10 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.
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
LevelOneUsesShiftAndLockIf 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.
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 Chapter 12.
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.
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
BeepOnComposeFailBecause 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.
Some compose processing algorithms signal the start of a compose sequence by a key event meaning "start compose". [5] 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.
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.
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 ".
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.
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.
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) |
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.
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.
To change the state of the Library Controls, use XkbSetXlibControls.
| Bool XkbSetXlibControls ( display, bits_to_change, values_for_bits ) |
| Display * display ; /* connection to X server */ |
| unsigned long bits_to_change ; /* selects controls to be modified */ |
| unsigned long 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.
[5] 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.