XChangeKeyboardDevice man page
XChangeKeyboardDevice — change which device is used as the X keyboard
Status XChangeKeyboardDevice( Display *display, XDevice *device);
display Specifies the connection to the X server.
device Specifies the device to be used as the X keyboard.
The XChangeKeyboardDevice request causes the server to use the specified device as the X keyboard. The device must have been previously opened by the requesting client via XOpenDevice or a BadDevice error will result. The device must support input class Keys, or a BadMatch error will result. If the server implementation does not support using the requested device as the X keyboard, a BadDevice error will result.
If the specified device is grabbed by another client, AlreadyGrabbed is returned. If the specified device is frozen by a grab on another device, GrabFrozen is returned. If the request is successful, Success is returned.
If the request succeeds, a ChangeDeviceNotify event is sent to all clients that have selected that event. A MappingNotify event with request = MappingKeyboard is sent to all clients. The specified device becomes the X keyboard and the old X keyboard becomes accessible through the input extension protocol requests.
XChangeKeyboardDevice can generate a BadDevice or a BadMatch error.
BadDevice An invalid device was specified. The specified device does not exist, has not been opened by this client via XOpenInputDevice, or is already one of the core X device (pointer or keyboard). This error may also occur if the server implementation does not support using the specified device as the X keyboard.
BadMatch This error may occur if an XChangeKeyboardDevice request was made specifying a device that has no keys.