XGetImage returns junk with Xorg/dummy_drv/VTs

Adam Jackson ajax at nwnk.net
Mon Nov 24 07:55:27 PST 2008

On Fri, 2008-11-21 at 15:23 -0700, McDonald, Michael-p7438c wrote:
> I'm trying to run the Xorg server using the dummy driver in the
> "background" in Linux. Everything works fine as long as the dummy Xorg
> has the VT "focus". If it's in the background, then XGetImage returns
> junk data.
>   It seems like the VT code is somehow causing the framebuffer to get
> swapped out. Can someone give me a hint as to what's going on? And how
> to work around it if there is a work around?

Yes, it does that.  When Xorg loses the VT, it empties the clip list for
the root window.  All drawing to the root window gets thrown away, and
all readbacks from it do nothing (which looks like uninitialized data).

I believe some combination of -novtswitch and -sharevts should get
around this by effectively making it so the X server doesn't have a VT
at all, and thus can't lose it.

- ajax
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