beginner needs help with "casting" with gtk

[Julien Martin <balteo gmail com>]

Hello,
I have the following callback function:

static void hello(GtkWidget *widget, gpointer data) {
    gchar * _data = (gchar *) data;

    g_print(_data);
    g_print("\n");

}

wich I call as follows:

g_signal_connect(G_OBJECT(button), "clicked", G_CALLBACK(hello), "hello toto");

I get the following warning each time I compile:

warning: the format is not a litteral string and no format argument

Can anyone please help?
I need to "cast" the data variable to gchar *. What am I doing wrong?

Thanks in advance,

Julien.