Why gtk_bin_get_child() does not work?

["chao yeaj" <yeajchao gmail com>]

Hello ,everyone

   I have a problem  with  "gtk_bin_get_child"

    my code is like this:
/********************************************************************/
GtkWidget * radio_button = NULL;
GtkWidget * label = NULL;

radio_button = gtk_radio_button_new(NULL);
/*   There ,the pointer  radio_button is not NULL*/

label =  gtk_bin_get_child(GTK_BIN(GTK_BUTTON(radio_button)));
/*     But ,there ,the pointer  label is still  NULL, Why ???  */*/

/********************************************************************************************/

I just want get the label of the radio button,
and then ,use gtk_label_set_markup()  to change the text's font and color

I need you help
Any comments would be much appreciated ,thanks in advance!