my transcription of X11R6.4 Xlib Reference Mannual
mmc: GrabModeAsync is a bad name for the-other-device mode, when it means "no change in behaviour".
Xlib provides functions that you can use to grab or ungrab the keyboard as well as allow events.
For many functions in this section, you pass keymask bits. The valid keymask bits are: ShiftMask, LockMask, ControlMask, Mod1Mask, Mod2Mask, Mod3Mask, Mod4Mask, and Mod5Mask.
To grab the keyboard, use XGrabKeyboard.
int XGrabKeyboard(display, grab_window, owner_events, pointer_mode, keyboard_mode, time) Display *display; Window grab_window; Bool owner_events; int pointer_mode, keyboard_mode; Time time; display Specifies the connection to the X server. grab_window Specifies the grab window. owner_events Specifies a Boolean value that indicates whether the keyboard events are to be reported as usual. pointer_mode Specifies further processing of pointer events. You can pass GrabModeSync or GrabModeAsync. keyboard_mode Specifies further processing of keyboard events. You can pass GrabModeSync or GrabModeAsync. time Specifies the time. You can pass either a timestamp or CurrentTime.
The XGrabKeyboard function actively grabs control of the keyboard and generates FocusIn and FocusOut events. Further key events are reported only to the grabbing client. XGrabKeyboard overrides any active keyboard grab by this client. If owner_events is False, all generated key events are reported with respect to grab_window. If owner_events is True and if a generated key event would normally be reported to this client, it is reported normally; otherwise, the event is reported with respect to the grab_window. Both KeyPress and KeyRelease events are always reported, independent of any event selection made by the client.
If the keyboard_mode argument is GrabModeAsync, keyboard event processing continues as usual. If the keyboard is currently frozen by this client, then processing of keyboard events is resumed. If the keyboard_mode argument is GrabModeSync, the state of the keyboard (as seen by client applications) appears to freeze, and the X server generates no further keyboard events until the grabbing client issues a releasing XAllowEvents call or until the keyboard grab is released. Actual keyboard changes are not lost while the keyboard is frozen; they are simply queued in the server for later processing.
If pointer_mode is GrabModeAsync, pointer event processing is unaffected by activation of the grab. If pointer_mode is GrabModeSync, the state of the pointer (as seen by client applications) appears to freeze, and the X server generates no further pointer events until the grabbing client issues a releasing XAllowEvents call or until the keyboard grab is released. Actual pointer changes are not lost while the pointer is frozen; they are simply queued in the server for later processing.
If the keyboard is actively grabbed by some other client, XGrabKeyboard fails and returns AlreadyGrabbed. If grab_window is not viewable, it fails and returns GrabNotViewable. If the keyboard is frozen by an active grab of another client, it fails and returns GrabFrozen. If the specified time is earlier than the last-keyboard-grab time or later than the current X server time, it fails and returns GrabInvalidTime. Otherwise, the last-keyboard-grab time is set to the specified time (CurrentTime is replaced by the current X server time).
XGrabKeyboard can generate BadValue and BadWindow errors.
To ungrab the keyboard, use XUngrabKeyboard.
XUngrabKeyboard(display, time) Display *display; Time time; display Specifies the connection to the X server. time Specifies the time. You can pass either a timestamp or CurrentTime.
The XUngrabKeyboard function releases the keyboard and any queued events if this client has it actively grabbed from either XGrabKeyboard or XGrabKey. XUngrabKeyboard does not release the keyboard and any queued events if the specified time is earlier than the last-keyboard-grab time or is later than the current X server time. It also generates FocusIn and FocusOut events. The X server automatically performs an UngrabKeyboard request if the event window for an active keyboard grab becomes not viewable.
To passively grab a single key of the keyboard, use XGrabKey.
XGrabKey(display, keycode, modifiers, grab_window, owner_events, pointer_mode, keyboard_mode) Display *display; int keycode; unsigned int modifiers; Window grab_window; Bool owner_events; int pointer_mode, keyboard_mode; display Specifies the connection to the X server. keycode Specifies the KeyCode or AnyKey. modifiers Specifies the set of keymasks or AnyModifier. The mask is the bitwise inclusive OR of the valid keymask bits. grab_window Specifies the grab window. owner_events Specifies a Boolean value that indicates whether the keyboard events are to be reported as usual. pointer_mode Specifies further processing of pointer events. You can pass GrabModeSync or GrabModeAsync. keyboard_mode Specifies further processing of keyboard events. You can pass GrabModeSync or GrabModeAsync.
The XGrabKey function establishes a passive grab on the key board. In the future, the keyboard is actively grabbed (as for XGrabKeyboard), the last-keyboard-grab time is set to the time at which the key was pressed (as transmitted in the KeyPress event), and the KeyPress event is reported if all of the following conditions are true:
The keyboard is not grabbed and the specified key (which can itself be a modifier key) is logically pressed when the specified modifier keys are logically down, and no other modifier keys are logically down.
The interpretation of the remaining arguments is as for XGrabKeyboard. The active grab is terminated automatically when the logical state of the keyboard has the specified key released (independent of the logical state of the modifier keys).
Note that the logical state of a device (as seen by client applications) may lag the physical state if device event processing is frozen.
A modifiers argument of AnyModifier is equivalent to issuing the request for all possible modifier combinations (including the combination of no modifiers). It is not required that all modifiers specified have currently assigned KeyCodes. A keycode argument of AnyKey is equivalent to issuing the request for all possible KeyCodes. Otherwise, the specified keycode must be in the range specified by min_keycode and max_keycode in the connection setup, or a BadValue error results.
If some other client has issued a XGrabKey with the same key combination on the same window, a BadAccess error results. When using AnyModifier or AnyKey, the request fails completely, and a BadAccess error results (no grabs are established) if there is a conflicting grab for any combination.
XGrabKey can generate BadAccess, BadValue, and BadWindow errors.
XUngrabKey(display, keycode, modifiers, grab_window) Display *display; int keycode; unsigned int modifiers; Window grab_window; display Specifies the connection to the X server. keycode Specifies the KeyCode or AnyKey. modifiers Specifies the set of keymasks or AnyModifier. The mask is the bitwise inclusive OR of the valid keymask bits. grab_window Specifies the grab window.
The XUngrabKey function releases the key combination on the specified window if it was grabbed by this client. It has
no effect on an active grab. A modifiers of AnyModifier is equivalent to issuing the request for all possible modifier combinations (including the combination of no modifiers). A keycode argument of AnyKey is equivalent to issuing the request for all possible key codes.
XUngrabKey can generate BadValue and BadWindow errors.
The previous sections discussed grab mechanisms with which processing of events by the server can be temporarily suspended. This section describes the mechanism for resuming event processing.
To allow further events to be processed when the device has been frozen, use XAllowEvents.
XAllowEvents(display, event_mode, time) Display *display; int event_mode; Time time; display Specifies the connection to the X server. xbggppppggg event_mode Specifies the event mode. You can pass AsyncPointer, SyncPointer, AsyncKeyboard, SyncKeyboard, ReplayPointer, ReplayKeyboard, AsyncBoth, or SyncBoth. time Specifies the time. You can pass either a timestamp or CurrentTime.
The XAllowEvents function releases some queued events if the client has caused a device to freeze. It has no effect if the specified time is earlier than the last-grab time of the most recent active grab for the client or if the specified time is later than the current X server time. Depending on the event_mode argument, the following occurs:
AsyncPointer, SyncPointer, and ReplayPointer have no effect on the processing of keyboard events. AsyncKeyboard, SyncKeyboard, and ReplayKeyboard have no effect on the processing of pointer events. It is possible for both a pointer grab and a keyboard grab (by the same or different clients) to be active simultaneously. If a device is frozen on behalf of either grab, no event processing is performed for the device. It is possible for a single device to be frozen because of both grabs. In this case, the freeze must be released on behalf of both grabs before events can again be processed. If a device is frozen twice by a single client, then a single AllowEvents releases both.
XAllowEvents can generate a BadValue error.