[cairo] How can I scale a calro surface

[Meryl Silverburgh]

How can I scale a calro surface?

I have the following code, but it does not work (does not scale):
 cairo_t* cr = cairo_create(tmpSurf);

    double scale = 0.5;

    cairo_scale (cr, scale, scale);

    cairo_surface_write_to_png(tmpSurf, "tmpSurf.png");

Can you please tell me why it does not work?
Thank you.