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(Courtesy of Donna Converse, converse@x.org; 5/10/92)

The Intrinsics library do not guarantee an order.  This is because
both the widget writer and the application writer have the ability to
modify the entire contents of the callback list.  Neither one
currently knows what the other is doing and so the Intrinsics cannot
guarantee the order of execution.

The application programmer cannot rely on the widget writer; the
widget writer is not required to document when the widget will add and
remove callbacks from the list or what effect this will have;
therefore the functionality contained in a callback should be
independent of the functionality contained in other callbacks on the
list.

Even though the Xt standard in the definition of XtAddCallback
says:

 	"callback_name: Specifies the callback list to which the
 	procedure is to be appended."
 
you may not infer from the word "appended" that the callback routines
are called in the same order as they have been added to the callback
list.

Some parts © 2009 Advameg, Inc.