XSetBackgroundPixmap question

[Adam Jackson]

On Sat, 2012-05-19 at 23:00 +0100, Glynn Clements wrote:

> I've encountered code which uses a background pixmap to implement a
> persistent screen, and drawing to the pixmap then calling XClearWindow
> (without calling XSetWindowBackgroundPixmap in between) results in the
> updated pixmap contents appearing in the window. So it appears to use
> reference counting or similar.

Any code which does this is being inadvisably friendly with the
implementation.  From the protocol spec:

    Subsequent drawing into the background or border pixmap has an
    undefined effect on the window state. The server might or might
    not make a copy of the pixmap.

The sample implementation (at least at the dix level) has always done
the current behaviour of taking a reference on the pixmap, but the
sample implementation is not the only X server ever written.

- ajax
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