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);