Someone

void on_clear_clicked(GtkButton *button, gpointer user_data)
{
   // GtkWidget *textview1 = lookup_widget(button, "textview1");
   GtkTextBuffer *buffer;
			
   buffer = gtk_text_view_get_buffer(GTK_TEXT_VIEW(textview1));
   gtk_text_buffer_set_text(buffer, "XXX\n", -1);
}

int main(int argc, char *argv[])
{
    GladeXML *main_window;
    GtkWidget *widget;

    gtk_init(&argc, &argv);

    main_window = glade_xml_new("gdso.glade", NULL, NULL);

    widget = glade_xml_get_widget(main_window, "window1");
    g_signal_connect(G_OBJECT(widget), "delete_event",
            G_CALLBACK(gtk_main_quit), NULL);

    widget = glade_xml_get_widget(main_window, "clear");
    g_signal_connect(G_OBJECT(widget), "clicked",
            G_CALLBACK(on_clear_clicked), GTK_BUTTON(widget));

    gtk_main();

    return 0;
}