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From: rws@EXPO.LCS.MIT.EDU (Bob Scheifler)
Newsgroups: comp.windows.x
Subject: Re: Re^2: How can I check for a valid Window id?
Message-ID: <8911152346.AA19834@expire.lcs.mit.edu>
Date: 15 Nov 89 23:46:43 GMT
References: <127935@sun.Eng.Sun.COM>
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    However, they are unable to reset the value of a non-default
    handler correctly after their operation is complete.

As I said, in the R4 Xlib XSetErrorHandler will return the old handler:

	oldhandler = XSetErrorHandler(myhandler);
	<do your own thing>
	XSetErrorHandler(oldhandler);