Function lv_buttonmatrix_get_button_text 

pub unsafe extern "C" fn lv_buttonmatrix_get_button_text(
    obj: *const lv_obj_t,
    btn_id: u32,
) -> *const c_char

Get the button’s text @param obj pointer to button matrix object @param btn_id the index a button not counting new line characters. @return text of btn_index` button