Path: utzoo!censor!geac!jtsv16!uunet!samsung!brutus.cs.uiuc.edu!usc!snorkelwacker!bloom-beacon!EXPO.LCS.MIT.EDU!rws
From: rws@EXPO.LCS.MIT.EDU (Bob Scheifler)
Newsgroups: comp.windows.x
Subject: Re: How can I check for a valid Window id?
Message-ID: <8911142110.AA17542@expire.lcs.mit.edu>
Date: 14 Nov 89 21:10:35 GMT
References: <3103@harrier.ukc.ac.uk>
Sender: daemon@bloom-beacon.MIT.EDU
Organization: The Internet
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    From my reading
    of section 4.1 if a bad Window is passed then it should return a Status
    of 0 (failure).  Should this be happening?

Yes, if your error handler returns.

    Do I have to trap the error using an error handler?

Yes.

    If I do have to install an error handler how can I re-install the
    previous error handler if one had been installed (*I* cannot know about
    previous handlers that may have been installed)?

In R4, XSetErrorHandler and XSetIOErrorHandler return the old handler.