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A client selects which types of events it receives from a specific window. The window event-mask attribute, set by the client, determines which event types are selected (see window-event-mask in Window Attributes). Most types of events are received by a client only if they are selected for some window.
In the X protocol, an event-mask is represented as a bit string. CLX also allows an event mask to be defined by a list of event-mask-class keywords. The functions make-event-keys and make-event-mask can be used to convert between these two forms of an event-mask. In general, including an event-mask-class keyword in an event-mask causes one or more related event types to be selected. The following table describes the event types selected by each event-mask-class keyword.
| Event Mask Keyword | Event Types Selected |
| :button-1-motion | :motion-notify when :button-1 is down |
| :button-2-motion | :motion-notify when :button-2 is down |
| :button-3-motion | :motion-notify when :button-3 is down |
| :button-4-motion | :motion-notify when :button-4 is down |
| :button-5-motion | :motion-notify when :button-5 is down |
| :button-motion | :motion-notify when any pointer button is down |
| :button-press | :button-press |
| :button-release | :button-release |
| :colormap-change | :colormap-notify |
| :enter-window | :enter-notify |
| :exposure | :exposure |
| :focus-change | :focus-in :focus-out |
| :key-press | :key-press |
| :key-release | :key-release |
| :keymap-state | :keymap-notify |
| :leave-window | :leave-notify |
| :owner-grab-button | Pointer events while button is grabbed |
| :pointer-motion | :motion-notify |
| :pointer-motion-hint | Single :motion-notify only |
| :property-change | :property-notify |
| :resize-redirect | :resize-request |
| :structure-notify | :circulate-notify :configure-notify :destroy-notify :gravity-notify :map-notify :reparent-notify :unmap-notify |
| :substructure-redirect | :circulate-request :configure-request :map-request |
| :visibility-change | :visibility-notify |
Some types of events do not have to be selected to be received and therefore are not represented in an event-mask. For example, the copy-plane and copy-area functions cause :graphics-exposure and :no-exposure events to be reported, unless exposures are turned :off in the graphics context (see copy-area and copy-plane in Area and Plane Operations, and gcontext-exposures in paragraph 5.4.6, Exposures). Also, :selection-clear, :selection-request, :selection-notify and :client-message events can be received at any time, but they are generally sent only to clients using selections (see Client Communications Events). :mapping-notify is always sent to clients when the keyboard mapping is changed.
Any client can select events for any window. A window maintains a separate event-mask for each interested client. In general, multiple clients can select for the same events on a window. After the X server generates an event, it sends it to all clients which selected it. However, the following restrictions apply to sharing window events among multiple clients. For a given window:
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