XSelectInput(3) Library Functions Manual XSelectInput(3)

XSelectInput - select input events

int XSelectInput(Display *display, Window w, long event_mask);

display
Specifies the connection to the X server.
event_mask
Specifies the event mask.
w
Specifies the window whose events you are interested in.

The XSelectInput 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:

The server reports the event to all interested clients.

XSelectInput can generate a BadWindow error.

BadWindow
A value for a Window argument does not name a defined Window.

Xlib - C Language X Interface
X Version 11 XLIB FUNCTIONS