XNextEvent

XNextEvent is a function in the X Window System that retrieves the next event from a connection's event queue. It is a fundamental part of how X clients interact with the X server. When an X client wants to know if any events have occurred, such as a key press, mouse movement, or window exposure, it calls XNextEvent.

The function blocks until an event is available. Once an event is received, XNextEvent copies it into the event structure provided by the caller and then returns. The type of event can be determined by examining the `type` field of the returned event structure. Different event types correspond to different user interactions or server-generated notifications.

Common event types include KeyPress, KeyRelease, ButtonPress, ButtonRelease, MotionNotify, Expose, ConfigureNotify, and ClientMessage. Applications typically use XNextEvent within a loop to continuously process events from the X server. This event loop is the heart of any graphical application running on X11.

XNextEvent is a synchronous function, meaning the program will halt execution at this point until an event is available. This blocking behavior is efficient as it avoids constant polling of the event queue. Understanding and correctly handling events through functions like XNextEvent is crucial for developing responsive and interactive graphical user interfaces on the X Window System.