In addition to a single pointer and keyboard for user interface input,
GDK contains support for a variety of input devices, including graphics
tablets, touchscreens and multiple pointers/keyboards interacting
simultaneously with the user interface. Under X, the support for multiple
input devices is done through the XInput 2 extension,
which also supports additional features such as sub-pixel positioning
information and additional device-dependent information.
By default, and if the platform supports it, GDK is aware of multiple
keyboard/pointer pairs and multitouch devices, this behavior can be
changed by calling gdk_disable_multidevice() before gdk_display_open(),
although there would rarely be a reason to do that. For a widget or
window to be dealt as multipointer aware,
gdk_window_set_support_multidevice() or
gtk_widget_set_support_multidevice() must have been called on it.
Conceptually, in multidevice mode there are 2 device types. Virtual
devices (or master devices) are represented by the pointer cursors
and keyboard foci that are seen on the screen. Physical devices (or
slave devices) represent the hardware that is controlling the virtual
devices, and thus have no visible cursor on the screen.
Virtual devices are always paired, so there is a keyboard device for every
pointer device. Associations between devices may be inspected through
gdk_device_get_associated_device().
There may be several virtual devices, and several physical devices could
be controlling each of these virtual devices. Physical devices may also
be "floating", which means they are not attached to any virtual device.
By default, GDK will automatically listen for events coming from all
master devices, setting the GdkDevice for all events coming from input
devices,

Events containing device information are GDK_MOTION_NOTIFY,
GDK_BUTTON_PRESS, GDK_2BUTTON_PRESS, GDK_3BUTTON_PRESS,
GDK_BUTTON_RELEASE, GDK_SCROLL, GDK_KEY_PRESS, GDK_KEY_RELEASE,
GDK_ENTER_NOTIFY, GDK_LEAVE_NOTIFY, GDK_FOCUS_CHANGE,
GDK_PROXIMITY_IN, GDK_PROXIMITY_OUT, GDK_DRAG_ENTER, GDK_DRAG_LEAVE,
GDK_DRAG_MOTION, GDK_DRAG_STATUS, GDK_DROP_START, GDK_DROP_FINISHED
and GDK_GRAB_BROKEN.

Although gdk_window_set_support_multidevice() must be called on
GdkWindows in order to support additional features of multiple pointer
interaction, such as multiple per-device enter/leave events, the default
setting will emit just one enter/leave event pair for all devices on the
window. See gdk_window_set_support_multidevice() documentation for more
information.
In order to listen for events coming from other than a virtual device,
gdk_window_set_device_events() must be called. Generally, this function
can be used to modify the event mask for any given device.
Input devices may also provide additional information besides X/Y.
For example, graphics tablets may also provide pressure and X/Y tilt
information. This information is device-dependent, and may be
queried through gdk_device_get_axis(). In multidevice mode, virtual
devices will change axes in order to always represent the physical
device that is routing events through it. Whenever the physical device
changes, the GdkDevice:n-axes property will be notified, and
gdk_device_list_axes() will return the new device axes.
Devices may also have associated keys or
macro buttons. Such keys can be globally set to map into normal X
keyboard events. The mapping is set using gdk_device_set_key().
In order to query the device hierarchy and be aware of changes in the
device hierarchy (such as virtual devices being created or removed, or
physical devices being plugged or unplugged), GDK provides
GdkDeviceManager. On X11, multidevice support is implemented through
XInput 2. Unless gdk_disable_multidevice() is called, the XInput 2.x
GdkDeviceManager implementation will be used as the input source. Otherwise
either the core or XInput 1.x implementations will be used.