Index: src/setup/gnunet-setup.c
===================================================================
--- src/setup/gnunet-setup.c	(revision 19067)
+++ src/setup/gnunet-setup.c	(working copy)
@@ -171,6 +171,8 @@
                 os->widget_name);
     return;
   }
+  if (NULL != os->input_validation_function)
+    os->input_validation_function (os->input_validation_function_cls, widget);
   if ((os->section != NULL) && (os->option != NULL))
     GNUNET_assert (GNUNET_OK ==
                    GNUNET_CONFIGURATION_get_value_string (cfg, os->section,
@@ -246,6 +248,8 @@
         }
       }
     }
+    if (NULL != os->input_validation_function)
+      os->input_validation_function (os->input_validation_function_cls, widget);
     if (os->help_text != NULL)
     {
       g_signal_connect (widget, "button-press-event",
Index: src/setup/gnunet-setup-options.c
===================================================================
--- src/setup/gnunet-setup-options.c	(revision 19067)
+++ src/setup/gnunet-setup-options.c	(working copy)
@@ -38,8 +38,38 @@
  */
 #define REX_NO "^NO$"
 
+/**
+ * Color for marking invalid input
+ */
+#define INVALID_INPUT_COLOR "red"
 
+
 /**
+ * Structs of this type specify connection between widgets storing
+ * port numbers and widgets controlling status of respective transport.
+ */
+struct PortSpecification
+{
+  /**
+   * Name of the GtkSpinButton storing the port value.
+   */
+  const char *spinbutton_name;
+
+  /**
+   * Name of the GtkCheckButton controlling whether the respective
+   * transport/daemon is enabled.
+   */
+  const char *checkbutton_name;
+
+  /**
+   * Name of the transport/daemon which owns this port.
+   */
+  const char *owner_name;
+
+};
+
+
+/**
  * Initialize a toggle button based on an options 'yes/no' value.
  *
  * @param cls closure
@@ -312,6 +342,159 @@
 
 
 /**
+ * NULL-terminated list of port specifications, which should be checked
+ * for collisions.
+ */
+static struct PortSpecification port_specifications[] = {
+  {
+   "GNUNET_setup_hostlist_server_port_spin_button",
+   "GNUNET_setup_hostlist_offer_hostlist_checkbutton",
+   gettext_noop ("the hostlist server"),},
+  {
+   "GNUNET_setup_transport_tcp_port_spinbutton",
+   "GNUNET_setup_transport_tcp_checkbutton",
+   gettext_noop ("the TCP transport plugin"),},
+  {
+   "GNUNET_setup_transport_http_port_spinbutton",
+   "GNUNET_setup_transport_http_checkbutton",
+   gettext_noop ("the HTTP transport plugin"),},
+  {
+   "GNUNET_setup_transport_https_port_spinbutton",
+   "GNUNET_setup_transport_https_checkbutton",
+   gettext_noop ("the HTTPS transport plugin"),},
+  {NULL, NULL}
+};
+
+
+/**
+ * Find spinbutton associated with a port specification.
+ *
+ * @param i index of the respective port specification
+ * @return spinbutton or NULL in case of an error or if respecitve transport is disabled
+ */
+static GtkSpinButton *
+get_port_spinbutton (unsigned int i)
+{
+  const char *sb_name;
+  const char *cb_name;
+  GtkWidget *widget;
+  GtkToggleButton *checkbt;
+  GtkSpinButton *spinbt;
+  gboolean mode;
+
+  if (port_specifications[i].spinbutton_name == NULL ||
+      port_specifications[i].checkbutton_name == NULL)
+    return NULL;
+  cb_name = port_specifications[i].checkbutton_name;
+  widget = GTK_WIDGET (GNUNET_SETUP_get_object (cb_name));
+  if (widget == NULL)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_WARNING, _("Widget `%s' not found\n"),
+                cb_name);
+    return NULL;
+  }
+  checkbt = GTK_TOGGLE_BUTTON (widget);
+  if (checkbt == NULL)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                _("Specified widget `%s' is not a checkbutton\n"),
+                cb_name);
+    return NULL;
+  }
+  mode = gtk_toggle_button_get_active (checkbt);
+  if (TRUE != mode)
+    return NULL;
+
+  sb_name = port_specifications[i].spinbutton_name;
+  widget = GTK_WIDGET (GNUNET_SETUP_get_object (sb_name));
+  if (widget == NULL)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_WARNING, _("Widget `%s' not found\n"),
+                sb_name);
+    return NULL;
+  }
+  spinbt = GTK_SPIN_BUTTON (widget);
+  if (spinbt == NULL)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                _("Specified widget `%s' is not a spinbutton\n"),
+                sb_name);
+    return NULL;
+  }
+  return spinbt;
+}
+
+
+/**
+ * Check port numbers for collisions and highlight conflicting ports, if any.
+ *
+ * @param cls closure (unused)
+ * @param widget widget whose state was changed (unused)
+ */
+static void
+highlight_port_collisions (const void *cls,
+                           GObject *widget)
+{
+  unsigned int i;
+  unsigned int j;
+  unsigned int port;
+  GtkSpinButton *spinbt_i;
+  GtkSpinButton *spinbt_j;
+  GdkColor color;
+  GdkColor *pcolor;
+  int found;
+  char *tooltip;
+
+  gdk_color_parse (INVALID_INPUT_COLOR, &color);
+  for (i = 0;
+       port_specifications[i].spinbutton_name != NULL;
+       i++)
+  {
+    spinbt_i = get_port_spinbutton (i);
+    if (spinbt_i == NULL)
+      continue;
+    else
+      port = gtk_spin_button_get_value_as_int (spinbt_i);
+
+    found = GNUNET_NO;
+    for (j = 0;
+         port_specifications[j].spinbutton_name != NULL;
+         j++)
+    {
+      if (i == j) continue;
+      spinbt_j = get_port_spinbutton (j);
+      if (spinbt_j == NULL)
+        continue;
+      if (port == gtk_spin_button_get_value_as_int (spinbt_j))
+      {
+        found = GNUNET_YES;
+        break;
+      }
+    }
+    if (GNUNET_YES == found)
+    {
+      pcolor = &color;
+      GNUNET_asprintf (&tooltip, _("This port is already occupied by %s."),
+		       port_specifications[j].owner_name);
+    }
+    else
+    {
+      pcolor = NULL;
+      tooltip = GNUNET_strdup ("");
+    }
+    gtk_widget_modify_text (GTK_WIDGET (spinbt_i),
+                            GTK_STATE_NORMAL, pcolor);
+    gtk_widget_modify_text (GTK_WIDGET (spinbt_i),
+                            GTK_STATE_ACTIVE, pcolor);
+    gtk_widget_modify_text (GTK_WIDGET (spinbt_i),
+                            GTK_STATE_SELECTED, pcolor);
+    gtk_widget_set_tooltip_text (GTK_WIDGET (spinbt_i), tooltip);
+    GNUNET_free (tooltip);
+  }
+}
+
+
+/**
  * Initialize a toggle button based on the existence of a word
  * in an option value.
  *
@@ -1465,6 +1648,7 @@
    "https://gnunet.org/configuration-f2f",
    &load_yes_no,
    &save_yes_no, NULL,
+   NULL, NULL,
    hide_min_connected_friends},
 
   {
@@ -1476,6 +1660,7 @@
    "https://gnunet.org/configuration-f2f",
    &load_filename,
    &save_filename, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1487,6 +1672,7 @@
    "https://gnunet.org/configuration-f2f",
    &load_number,
    &save_number, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1498,6 +1684,7 @@
    "https://gnunet.org/configuration-topology",
    &load_option_list,
    &save_option_list, "topology",
+   NULL, NULL,
    NULL},
 
   {
@@ -1510,6 +1697,7 @@
    "https://gnunet.org/configuration-hostlist",
    &load_option_list,
    &save_option_list, "hostlist",
+   NULL, NULL,
    hide_hostlist_options},
 
 
@@ -1522,6 +1710,7 @@
    "https://gnunet.org/configuration-fs",
    &load_option_list,
    &save_option_list, "fs",
+   NULL, NULL,
    hide_fs_tab},
 
   {
@@ -1533,6 +1722,7 @@
    "https://gnunet.org/configuration-vpn",
    &load_option_list,
    &save_option_list, "vpn",
+   NULL, NULL,
    hide_vpn_tab},
 
   {
@@ -1544,6 +1734,7 @@
    "https://gnunet.org/configuration-hostlist",
    &load_option_list,
    &save_option_list, "-b",
+   NULL, NULL,
    NULL},
 
   {
@@ -1555,6 +1746,7 @@
    "https://gnunet.org/configuration-hostlist",
    &load_option_list,
    &save_option_list, "-e",
+   NULL, NULL,
    NULL},
 
   {
@@ -1566,6 +1758,7 @@
    "https://gnunet.org/configuration-hostlist-server",
    &load_option_list,
    &save_option_list, "-p",
+   &highlight_port_collisions, NULL,
    hide_hostlist_server_options},
 
   {
@@ -1577,6 +1770,7 @@
    "https://gnunet.org/configuration-hostlist-server",
    &load_option_list,
    &save_option_list, "-a",
+   NULL, NULL,
    NULL},
 
   {
@@ -1588,6 +1782,7 @@
    "https://gnunet.org/configuration-hostlist-server",
    &load_number,
    &save_number, NULL,
+   &highlight_port_collisions, NULL,
    NULL},
 
   {
@@ -1598,6 +1793,7 @@
    NULL, NULL,
    &load_string_list_store,
    &save_string_list_store, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1606,6 +1802,7 @@
    gettext_noop ("Known hostlist URLs"),
    "https://gnunet.org/configuration-hostlist",
    NULL, NULL, NULL,            /* FIXME */
+   NULL, NULL,
    NULL},
 
   {
@@ -1617,6 +1814,7 @@
    "https://gnunet.org/configuration-bandwidth",
    &load_number,
    &save_number, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1628,6 +1826,7 @@
    "https://gnunet.org/configuration-bandwidth",
    &load_number,
    &save_number, NULL,
+   NULL, NULL,
    NULL},
 
   /* Transport TAB */
@@ -1641,6 +1840,7 @@
    "https://gnunet.org/configuration-tcp",
    &load_option_list,
    &save_option_list, "tcp",
+   &highlight_port_collisions, NULL,
    hide_tcp_tab},
 
   {
@@ -1652,6 +1852,7 @@
    "https://gnunet.org/configuration-udp",
    &load_option_list,
    &save_option_list, "udp",
+   NULL, NULL,
    hide_udp_tab},
 
   {
@@ -1663,6 +1864,7 @@
    "https://gnunet.org/configuration-http",
    &load_option_list,
    &save_option_list, "http",
+   &highlight_port_collisions, NULL,
    hide_http_tab},
 
   {
@@ -1674,6 +1876,7 @@
    "https://gnunet.org/configuration-https",
    &load_option_list,
    &save_option_list, "https",
+   &highlight_port_collisions, NULL,
    hide_https_tab},
 
   {
@@ -1685,6 +1888,7 @@
    "https://gnunet.org/configuration-dv",
    &load_option_list,
    &save_option_list, "dv",
+   NULL, NULL,
    hide_dv_tab},
 
   {
@@ -1696,6 +1900,7 @@
    "https://gnunet.org/configuration-wlan",
    &load_option_list,
    &save_option_list, "wlan",
+   NULL, NULL,
    hide_wlan_tab},
 
   {
@@ -1707,6 +1912,7 @@
    "https://gnunet.org/configuration-tcp",
    &load_number,
    &save_number, NULL,
+   &highlight_port_collisions, NULL,
    hide_all_tcp_options},
 
   {
@@ -1718,6 +1924,7 @@
    "https://gnunet.org/configuration-tcp",
    &load_number,
    &save_number, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1729,6 +1936,7 @@
    "https://gnunet.org/configuration-nat",
    &load_yes_no,
    &save_yes_no, NULL,
+   NULL, NULL,
    toggle_nat_options},
 
   {
@@ -1740,6 +1948,7 @@
    "https://gnunet.org/configuration-nat",
    &load_yes_no,
    &save_yes_no, NULL,
+   NULL, NULL,
    toggle_nat_punched_options},
 
   {
@@ -1751,6 +1960,7 @@
    "https://gnunet.org/configuration-nat",
    &load_yes_no,
    &save_yes_no, NULL,
+   NULL, NULL,
    NULL,
    },
 
@@ -1763,6 +1973,7 @@
    "https://gnunet.org/configuration-nat",
    &load_yes_no,
    &save_yes_no, NULL,
+   NULL, NULL,
    NULL,
    },
 
@@ -1775,6 +1986,7 @@
    "https://gnunet.org/configuration-nat",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1786,6 +1998,7 @@
    "https://gnunet.org/configuration-nat",
    &load_yes_no,
    &save_yes_no, NULL,
+   NULL, NULL,
    toggle_internal_ip},
 
   {
@@ -1797,6 +2010,7 @@
    "https://gnunet.org/configuration-nat",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1808,6 +2022,7 @@
    "https://gnunet.org/configuration-ipv6",
    &load_yes_no,
    &save_yes_no, NULL,
+   NULL, NULL,
    NULL,
    },
 
@@ -1820,6 +2035,7 @@
    "https://gnunet.org/configuration-udp",
    &load_number,
    &save_number, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1831,6 +2047,7 @@
    "https://gnunet.org/configuration-http",
    &load_number,
    &save_number, NULL,
+   &highlight_port_collisions, NULL,
    NULL},
 
   {
@@ -1842,6 +2059,7 @@
    "https://gnunet.org/configuration-https",
    &load_number,
    &save_number, NULL,
+   &highlight_port_collisions, NULL,
    NULL},
 
   /* FS TAB */
@@ -1855,6 +2073,7 @@
    "https://gnunet.org/configuration-datastore",
    &load_option_list /* abuse! */ ,
    &save_option_list /* abuse! */ , "sqlite",
+   NULL, NULL,
    hide_sqlite_datastore_tab},
 
   {
@@ -1866,6 +2085,7 @@
    "https://gnunet.org/configuration-datastore",
    &load_option_list /* abuse! */ ,
    &save_option_list /* abuse! */ , "mysql",
+   NULL, NULL,
    hide_mysql_datastore_tab},
 
   {
@@ -1877,6 +2097,7 @@
    "https://gnunet.org/configuration-datastore",
    &load_option_list /* abuse! */ ,
    &save_option_list /* abuse! */ , "postgres",
+   NULL, NULL,
    hide_postgres_datastore_tab},
 
   {
@@ -1888,6 +2109,7 @@
    "https://gnunet.org/configuration-datastore",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1899,6 +2121,7 @@
    "http://dev.mysql.com/doc/refman/5.5/en/option-files.html",
    &load_filename,
    &save_filename, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1910,6 +2133,7 @@
    "https://gnunet.org/configuration-datastore",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1921,6 +2145,7 @@
    "https://gnunet.org/configuration-datastore",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1932,6 +2157,7 @@
    "https://gnunet.org/configuration-datastore",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1943,6 +2169,7 @@
    "https://gnunet.org/configuration-datastore",
    &load_number,
    &save_number, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1954,6 +2181,7 @@
    "http://www.postgresql.org/docs/8.1/static/libpq.html#LIBPQ-CONNECT",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
 
@@ -1966,6 +2194,7 @@
    "https://gnunet.org/configuration-fs",
    &load_yes_no,
    &save_yes_no, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1977,6 +2206,7 @@
    "https://gnunet.org/configuration-fs",
    &load_yes_no,
    &save_yes_no, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -1988,6 +2218,7 @@
    "https://gnunet.org/configuration-datacache",
    &load_option_list /* abuse! */ ,
    &save_option_list /* abuse! */ , "sqlite",
+   NULL, NULL,
    hide_sqlite_datacache_tab},
 
   {
@@ -1999,6 +2230,7 @@
    "https://gnunet.org/configuration-datacache",
    &load_option_list /* abuse! */ ,
    &save_option_list /* abuse! */ , "mysql",
+   NULL, NULL,
    hide_mysql_datacache_tab},
 
   {
@@ -2010,6 +2242,7 @@
    "https://gnunet.org/configuration-datacache",
    &load_option_list /* abuse! */ ,
    &save_option_list /* abuse! */ , "postgres",
+   NULL, NULL,
    hide_postgres_datacache_tab},
 
   {
@@ -2021,6 +2254,7 @@
    "https://gnunet.org/configuration-datacache",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2032,6 +2266,7 @@
    "http://dev.mysql.com/doc/refman/5.5/en/option-files.html",
    &load_filename,
    &save_filename, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2043,6 +2278,7 @@
    "https://gnunet.org/configuration-datacache",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2054,6 +2290,7 @@
    "https://gnunet.org/configuration-datacache",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2065,6 +2302,7 @@
    "https://gnunet.org/configuration-datacache",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2076,6 +2314,7 @@
    "https://gnunet.org/configuration-wlan",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2087,6 +2326,7 @@
    "https://gnunet.org/configuration-datacache",
    &load_number,
    &save_number, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2098,6 +2338,7 @@
    "http://www.postgresql.org/docs/8.1/static/libpq.html#LIBPQ-CONNECT",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2109,6 +2350,7 @@
    "https://gnunet.org/configuration-vpn",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2120,6 +2362,7 @@
    "https://gnunet.org/configuration-vpn",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2131,6 +2374,7 @@
    "https://gnunet.org/configuration-vpn",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2142,6 +2386,7 @@
    "https://gnunet.org/configuration-vpn",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2153,6 +2398,7 @@
    "https://gnunet.org/configuration-vpn",
    &load_number,
    &save_number, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2165,6 +2411,7 @@
    "https://gnunet.org/configuration-vpn",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2177,6 +2424,7 @@
    "https://gnunet.org/configuration-exit",
    &load_option_list,
    &save_option_list, "exit",
+   NULL, NULL,
    hide_exit_options},
 
   {
@@ -2189,6 +2437,7 @@
    "https://gnunet.org/configuration-dns",
    &load_yes_no,
    &save_yes_no, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2201,6 +2450,7 @@
    "https://gnunet.org/configuration-exit",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2212,6 +2462,7 @@
    "https://gnunet.org/configuration-exit",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2223,6 +2474,7 @@
    "https://gnunet.org/configuration-exit",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2234,6 +2486,7 @@
    "https://gnunet.org/configuration-exit",
    &load_text,
    &save_text, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2246,6 +2499,7 @@
    "https://gnunet.org/configuration-exit",
    &load_number,
    &save_number, NULL,
+   NULL, NULL,
    NULL},
 
 
@@ -2259,6 +2513,7 @@
    "https://gnunet.org/configuration-exit",
    &load_yes_no,
    &save_yes_no, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2271,6 +2526,7 @@
    "https://gnunet.org/configuration-exit",
    &load_yes_no,
    &save_yes_no, NULL,
+   NULL, NULL,
    NULL},
 
   /* DNS treeview */
@@ -2285,6 +2541,7 @@
    "https://gnunet.org/configuration-dns",
    &load_vpn_dns_configuration,
    NULL, NULL,
+   NULL, NULL,
    NULL},
 
   {
@@ -2297,6 +2554,8 @@
    NULL,
    &vpn_dns_service_dnsname_install_edited_handler,
    NULL,
+   NULL,
+   NULL,
    NULL},
 
   {
@@ -2309,6 +2568,8 @@
    NULL,
    &vpn_dns_service_tcpudp_install_edited_handler,
    NULL,
+   NULL,
+   NULL,
    NULL},
 
   {
@@ -2321,6 +2582,8 @@
    NULL,
    &vpn_dns_service_sourceport_install_edited_handler,
    NULL,
+   NULL,
+   NULL,
    NULL},
 
   {
@@ -2333,6 +2596,8 @@
    NULL,
    &vpn_dns_service_targethostname_install_edited_handler,
    NULL,
+   NULL,
+   NULL,
    NULL},
 
   {
@@ -2345,12 +2610,15 @@
    NULL,
    &vpn_dns_service_targetport_install_edited_handler,
    NULL,
+   NULL,
+   NULL,
    NULL},
 
   /* END of list */
 
   {NULL, NULL, NULL, NULL, NULL,
-   NULL, NULL, NULL, NULL}
+   NULL, NULL, NULL, NULL, NULL,
+   NULL}
 };
 
 /* end of gnunet-setup-options.c */
Index: src/setup/gnunet-setup-options.h
===================================================================
--- src/setup/gnunet-setup-options.h	(revision 19067)
+++ src/setup/gnunet-setup-options.h	(working copy)
@@ -65,6 +65,17 @@
 
 
 /**
+ * Function called each time the widget changes its state to validate
+ * its value.
+ *
+ * @param cls closure
+ * @param widget widget whose state was changed
+ */
+typedef void (*GNUNET_SETUP_InputValidationFunction) (const void *cls,
+                                                      GObject *widget);
+
+
+/**
  * Structs of this type specify under which conditions the values of
  * a particular option impact the visibility (or sensitivity) of some
  * other widget.
@@ -143,6 +154,16 @@
   const void *load_save_cls;
 
   /**
+   * Function to call to validate the value of the widget.
+   */
+  GNUNET_SETUP_InputValidationFunction input_validation_function;
+
+  /**
+   * Closure for 'input_validation_function'.
+   */
+  const void *input_validation_function_cls;
+
+  /**
    * Visibility changes to apply if this option changes (NULL, or
    * NULL-terminated).
    */