[cairo] Strange values of cairo_xlib_surface_get_width

[Stefan Salewski]

Hello,

I have just started working on my first non trivial GTK+/Cairo program.
(picture of early draft of GUI: http://www.ssalewski.de/tmp/GUI.png )

My code is mainly based on the examples from A.Krauses GTK book, the
http://zetcode.com/tutorials/cairographicstutorial/ and other web
resources.

One of my task was to find the size of the GTK drawing area, which is
used for cairo drawing.

My first attempt was using 

cairo_xlib_surface_get_width (surface)

which I found in an example in the web.
But that gave me sometimes strange results, while
widget->allocation.width seems to work fine.

I was going to just ignore this problem, but now I decided to search for
an explanation -- to understand the problem and to avoid similar
problems in future. So I stripped down my code to this example:

http://www.ssalewski.de/tmp/main.c

Excerpt:
  cairosurfacewidth = widget->allocation.width;
  cairosurfaceheight = widget->allocation.height;

  cr = gdk_cairo_create (widget->window);
  surface = cairo_get_target (cr);
  if (cairo_surface_get_type (surface) != CAIRO_SURFACE_TYPE_XLIB)
    g_print("we have problems\n");
  cairowidthfromsurface = cairo_xlib_surface_get_width (surface);
  cairoheightfromsurface = cairo_xlib_surface_get_height (surface);

  g_print("widget->allocation: %d  %d ",cairosurfacewidth, cairosurfaceheight);
  g_print("cairo_xlib_surface_get: %d  %d\n",cairowidthfromsurface, cairoheightfromsurface);


Output looks like

stefan at AMD64X2 ~/cairobug $ gcc -o test `pkg-config --cflags --libs gtk+-2.0` main.c
stefan at AMD64X2 ~/cairobug $ ./test 
widget->allocation: 100  100 cairo_xlib_surface_get: 182  110
widget->allocation: 101  101 cairo_xlib_surface_get: 183  111
widget->allocation: 101  102 cairo_xlib_surface_get: 183  107

widget->allocation: 108  248 cairo_xlib_surface_get: 197  253
widget->allocation: 109  248 cairo_xlib_surface_get: 114  258
widget->allocation: 109  249 cairo_xlib_surface_get: 200  259
widget->allocation: 110  249 cairo_xlib_surface_get: 115  259
widget->allocation: 111  249 cairo_xlib_surface_get: 116  259
widget->allocation: 111  250 cairo_xlib_surface_get: 204  260
widget->allocation: 112  250 cairo_xlib_surface_get: 117  260
widget->allocation: 111  250 cairo_xlib_surface_get: 112  250

While the height (second value each) seems to be not too wrong, the
width result of  cairo_xlib_surface_get_width makes very strange output,
with jumping values. 

This is on a 64 bit AMD dual core box with Gentoo-Linux, 

stefan at AMD64X2 ~ $ emerge -pv gtk+ cairo

x11-libs/gtk+-2.18.9  USE="cups jpeg jpeg2k tiff (-aqua) -debug -doc -test -vim-syntax -xinerama" 17,769 kB

x11-libs/cairo-1.8.8  USE="X opengl svg (-aqua) -cleartype -debug -directfb -doc -glitz -xcb" 6,491 kB

gcc version 4.4.4

Best regards

Stefan Salewski