The function requests that the X server report the events associated with the specified event mask. Initially, X will not report any of these events. Events are reported relative to a window. If a window is not interested in a device event, it usually propagates to the closest ancestor that is interested, unless the do_not_propagate mask prohibits it.
Setting the event-mask attribute of a window overrides any previous call for the same window but not for other clients. Multiple clients can select for the same events on the same window with the following restrictions:
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Multiple clients can select events on the same window because their event masks are disjoint. When the X server generates an event, it reports it to all interested clients.
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Only one client at a time can select or events, which are associated with the event mask
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Only one client at a time can select a event, which is associated with the event mask
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Only one client at a time can select a event, which is associated with the event mask
The server reports the event to all interested clients.
can generate a error.