XChangePointerDevice man page
XChangePointerDevice — change which device is the X pointer
Status XChangePointerDevice( Display *display, XDevice *device, int xaxis, int yaxis);
display Specifies the connection to the X server.
device Specifies the device to be used as the X pointer.
xaxis Specifies the axis of the device to be used as the X pointer x-axis.
yaxis Specifies the axis of the device to be used as the X pointer y-axis.
The XChangePointerDevice request causes the server to use the specified device as the X pointer. The device must have been opened by the client via XOpenDevice or a BadDevice error will result. The device must support input class Valuators or a BadMatch error will result. If the implementation does not support use of the specified device as the X pointer, 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 = MappingPointer is sent to all clients. The specified device becomes the X pointer, and the old X pointer becomes accessible through the input extension protocol requests.
XChangePointerDevice 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 input devices (pointer or keyboard). This error may also occur if the server implementation does not support using the specified device as the X pointer.
BadMatch This error may occur if an XChangePointerDevice request was made specifying a device that has less than two valuators, or specifying a valuator index beyond the range supported by the device.