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From: schoch@trident.arc.nasa.gov (Steve Schoch)
Newsgroups: comp.windows.x
Subject: Re: Problem with X11R4 server `ghosting' on a 68k box.
Message-ID: <1991Mar13.234049.19438@news.arc.nasa.gov>
Date: 13 Mar 91 23:40:49 GMT
References: <1333@amix.commodore.com>
Sender: usenet@news.arc.nasa.gov (USENET Administration)
Reply-To: schoch@sheba.arc.nasa.gov (Steve Schoch)
Organization: NASA Ames Research Center, Moffett Field, CA
Lines: 37

I have seen this problem with a server I was working on once.  What happens
is that rectangles can get removed from the clip lists of all windows,
including the root.  Once this happens, there is no way to get it back
because no window "owns" that rectangle by having it as part of its
cliplist.

The problem turned out to be in server/ddx/mi/miregion.c.   I don't know if
this was fixed in later versions but I doubt it because the problem was
also a problem with bcopy() from libc on this machine.  The section of code
is:

    if (prepend)
    {
        new = REGION_BOX(dstrgn, numRects);
        if (dnumRects == 1)
            *new = *REGION_BOXPTR(dstrgn);
        else
            bcopy((char *)REGION_BOXPTR(dstrgn), (char *)new,
                  dnumRects * sizeof(BoxRec));
        new = REGION_BOXPTR(dstrgn);
    }

If you look carefully at this piece of code (which only gets called if prepend
is TRUE, which isn't very often), you will see that when bcopy() is called,
both REGION_PTR(dstrgn) and new point to the same buffer, but new is a higher
address (new = REGION_PTR(dstrgn) + numRects).

This is the only piece of code I could find in the MIT X source in which
it is assumed that bcopy can handle overlapping buffers.  On some machines,
perhaps yours, bcopy always copies in the forward direction which will
overwrite one or more rectangles in the region.

If this is your problem, call ovbcopy() instead of bcopy() if you have it,
or write your own ovbcopy().

	Steve
	schoch@trident.arc.nasa.gov