libnotify (g_signal_connect() 3rd argument help) c++

Hello, this is my code:

#include <glib.h>
#include <unistd.h>
#include <libnotify/notify.h>

void push_notification (gchar* title,
                   gchar* body,
                   gchar* icon)
{
        NotifyNotification* notification;
        gboolean            success;
        GError*             error = NULL;

        /* initial notification */
        notification = notify_notification_new (title, body, icon, NULL);
        notify_notification_set_hint_string (notification,
                                             "x-canonical-append",
                                             "");
        error = NULL;
        success = notify_notification_show (notification, &error);
        if (!success)
        {
                g_print ("That did not work ... \"%s\".\n",
                         error->message);
                g_error_free (error);
        }

        g_signal_connect(G_OBJECT (notification),
                          "closed",
                          G_CALLBACK (NULL),
                          NULL);
        g_object_unref (G_OBJECT (notification));
        notify_uninit ();
        sleep (3); /* simulate a user typing */
}

int main ()
{
        if (!notify_init ("append-hint-example"))
                return 1;


                push_notification ("Program",
                                   "Hello",
                                   "notification-message-im");

                push_notification ("Program",
                                   "how are you?",
                                   "notification-gsm-medium");

                push_notification ("Program",
                                   "Good to hear that",
                                   "notification-message-im");
        return 0;
}

It's the sample code provided by notify-osd's source for the append ability of notifications. In the g_signal_connect() function I have no idea what should I put as 3rd argument (inside the G_CALLBACK()i put NULL and it works just fine, but it outputs error message

 p, li { white-space: pre-wrap; }  GLib-GObject-CRITICAL **: g_signal_connect_data: assertion `c_handler != NULL' failed

Im running Ubuntu 11.04 thanks in advance for any answers!