Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!bbn.com!metasoft!alan
From: alan@metasoft.UUCP (Alan Epstein)
Newsgroups: comp.windows.x
Subject: Bug in _XtFindRemapWidget ?
Message-ID: <1874@metasoft.UUCP>
Date: 5 Oct 90 15:03:36 GMT
Organization: Meta Software Corporation, Cambridge MA
Lines: 31


Recently our application began crashing reliably during a mouse
down event in a small widget hierarchy. We traced the problem
to _XtFindRemapWidget, which makes a preliminary test to
determine whether the pdi->trace array should be refilled (by
calling _XtFillAncestorList). In some cases where it decides
not to refill the array, the existing array contains garbage,
causing a crash later when _XtGetPerWidgetInput tries to
dereference through the invalid widget.

The bug appears to be due to the possibility that Xt reassigns
widget id's, ie: after a widget is destroyed (and presumably
its id is freed), XtCreate might return the same id.
This new id is valid, but the result is that the test in
_XtFindRemapWidget might succeed (ie: widget == pdi->trace[0])
when the other elements in the pdi->trace array are old and obsolete.

The fix I am trying involves merely removing the test at the
head of _XtFindRemapWidget, causing _XtFillAncestorList to
make the array right every time. This seems like little overhead
since the latter routine doesn't need to do much.

I don't profess to be an expert in the Xt code, so I would
appreciate a confirmation by someone who is.

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Alan Epstein
Meta Software Corp                   UUCP:  ...bbn!metasoft!alan
150 Cambridgepark Dr        Internet/ARPA:  alan%metasoft@bbn.com
Cambridge, MA 02140  USA              TEL:  (617) 576.6920
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