XISetFocus man page
XISetFocus, XIGetFocus — set or get the device's focus.
Status XISetFocus( Display *display, int deviceid, Window focus, Time time);
Status XIGetFocus( Display *display, Window *focus_return);
display Specifies the connection to the X server.
deviceid Specifies the device whose focus is to be queried or changed.
focus The new focus window.
focus_return Returns the current focus window.
time A valid timestamp or CurrentTime.
XISetFocus changes the focus of the specified device ans its last-focus-change time. It has no effect if the specified time is earlier than the current last-focus-change time or is later than the current X server time. Otherwise, the last-focus-change time is set to the specified time. CurrentTime is replaced by the current X server time). XISetFocus causes the X server to generate core, XI and XI2 focus events.
If the focus window is None all keyboard events by this device are discarded until a new focus window is set. Otherwise, if focus is a window, it becomes the device's focus window. If a generated device event would normally be reported to this window or one of its inferiors, the event is reported as usual. Otherwise, the event is reported relative to the focus window.
The specified focus window must be viewable at the time XISetFocus is called, or a BadMatch error results. If the focus window later becomes not viewable, the focus reverts to the parent (or the closest viewable ancestor. When the focus reverts, the X server generates core, XI and XI2 focus events but the last-focus-change time is not affected.
Attempting to set the focus on a master pointer device or an attached slave device will result in a BadDevice error.
XISetFocus can generate BadDevice, BadMatch, BadValue, and BadWindow errors.
BadValue A value is outside of the permitted range.
BadDevice An invalid device was specified. The device does not exist or is not a appropriate for the type of change.
BadMatch The window is not viewable.
BadWindow A value for a Window argument does not name a defined Window.
XIGetFocus(3) is an alias of XISetFocus(3).