A.1 Definition of a Callback

  The callback is a concept that has been adopted from the X11 Window System. Its fundamental purpose is to inform the system of an incoming event if it is in the main event waiting loop. The callback function has the following three parameters:

1. The object on which the callback was registered. The event is associated to this object. The type is depending on the callback. This may be an I/O identifier, a process, a timeout identifier or for X11 (as the original) a widget.
2. A user parameter which has been registered together with the callback function. This can be used to pass additional external information from outside to a callback and allows to reuse the same callback code more than once.
3. An event specific parameter to give more information about the event. In case of a terminated process this parameter is the return code of the command, for I/O it holds the new already read input and for timeouts it carries the current system time.

The callbacks are automatically invoked, if the main program waits for some input, e.g., from the user.