[cairo] slim_hidden_def() in cairo

[ying lcs]

Hi,

Can you please tell me what is the purpose of 'slim_hidden_def'
function calls in cairo source code?

Why some cairo method calls this 'slim_hidden_def({cairo method
name})' and some do not?
For example,

cairo_scaled_font_t *
cairo_scaled_font_reference (cairo_scaled_font_t *scaled_font)
{
    if (scaled_font == NULL ||
	    CAIRO_REFERENCE_COUNT_IS_INVALID (&scaled_font->ref_count))
	return scaled_font;

    assert (CAIRO_REFERENCE_COUNT_HAS_REFERENCE (&scaled_font->ref_count));

    _cairo_reference_count_inc (&scaled_font->ref_count);

    return scaled_font;
}
slim_hidden_def (cairo_scaled_font_reference);

Thank you.