Something

#include <stdio.h>
#include <cairo.h>
#include <math.h>

int
main()
{
  cairo_surface_t *img = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, 500, 500);

  cairo_t *cr = cairo_create (img);

  cairo_set_source_rgb (cr, 1.0, 1.0, 1.0);
  cairo_paint (cr);

  cairo_rotate (cr, M_PI/4);
  cairo_rectangle (cr, 0, 0, 10, 10);

  double x1, y1, x2, y2;
  cairo_stroke_extents (cr, &x1, &y1, &x2, &y2);

  printf ("%g %g  %g %g\n", x1, y1, x2, y2);
}