curve rectangle

Unless you know what you are doing, you should probably use rounded rectangle instead.

/* a custom shape that could be wrapped in a function */
double x0      = 25.6,   /* parameters like cairo_rectangle */
       y0      = 25.6,
       rect_width  = 204.8,
       rect_height = 204.8,
       radius = 102.4;   /* and an approximate curvature radius */

double x1,y1;

x1=x0+rect_width;
y1=y0+rect_height;
if (!rect_width || !rect_height)
    return;
if (rect_width/2<radius) {
    if (rect_height/2<radius) {
        cairo_move_to  (cr, x0, (y0 + y1)/2);
        cairo_curve_to (cr, x0 ,y0, x0, y0, (x0 + x1)/2, y0);
        cairo_curve_to (cr, x1, y0, x1, y0, x1, (y0 + y1)/2);
        cairo_curve_to (cr, x1, y1, x1, y1, (x1 + x0)/2, y1);
        cairo_curve_to (cr, x0, y1, x0, y1, x0, (y0 + y1)/2);
    } else {
        cairo_move_to  (cr, x0, y0 + radius);
        cairo_curve_to (cr, x0 ,y0, x0, y0, (x0 + x1)/2, y0);
        cairo_curve_to (cr, x1, y0, x1, y0, x1, y0 + radius);
        cairo_line_to (cr, x1 , y1 - radius);
        cairo_curve_to (cr, x1, y1, x1, y1, (x1 + x0)/2, y1);
        cairo_curve_to (cr, x0, y1, x0, y1, x0, y1- radius);
    }
} else {
    if (rect_height/2<radius) {
        cairo_move_to  (cr, x0, (y0 + y1)/2);
        cairo_curve_to (cr, x0 , y0, x0 , y0, x0 + radius, y0);
        cairo_line_to (cr, x1 - radius, y0);
        cairo_curve_to (cr, x1, y0, x1, y0, x1, (y0 + y1)/2);
        cairo_curve_to (cr, x1, y1, x1, y1, x1 - radius, y1);
        cairo_line_to (cr, x0 + radius, y1);
        cairo_curve_to (cr, x0, y1, x0, y1, x0, (y0 + y1)/2);
    } else {
        cairo_move_to  (cr, x0, y0 + radius);
        cairo_curve_to (cr, x0 , y0, x0 , y0, x0 + radius, y0);
        cairo_line_to (cr, x1 - radius, y0);
        cairo_curve_to (cr, x1, y0, x1, y0, x1, y0 + radius);
        cairo_line_to (cr, x1 , y1 - radius);
        cairo_curve_to (cr, x1, y1, x1, y1, x1 - radius, y1);
        cairo_line_to (cr, x0 + radius, y1);
        cairo_curve_to (cr, x0, y1, x0, y1, x0, y1- radius);
    }
}
cairo_close_path (cr);

cairo_set_source_rgb (cr, 0.5, 0.5, 1);
cairo_fill_preserve (cr);
cairo_set_source_rgba (cr, 0.5, 0, 0, 0.5);
cairo_set_line_width (cr, 10.0);
cairo_stroke (cr);