These are two different functions. XtAppAddInput adds a external source -- a socket. When there is input, your input callback is called as a normal part of Xt`s event-loop processing, which you`d normally call via XtAppNextEvent(), as an infinite loop. You provide an input callback which handles the data, in the same way that you provide a timer callback that handles Xt timers or work procedure callbacks that handle Xt work procedures.