diff --git a/gtk/deprecated/gtkstatusicon.h b/gtk/deprecated/gtkstatusicon.h
index 6b1518ca9e..a4bca12bb8 100644
--- a/gtk/deprecated/gtkstatusicon.h
+++ b/gtk/deprecated/gtkstatusicon.h
@@ -71,10 +71,10 @@ struct _GtkStatusIconClass
                                      gboolean        keyboard_mode,
                                      GtkTooltip     *tooltip);
 
-  void (*__gtk_reserved1);
-  void (*__gtk_reserved2);
-  void (*__gtk_reserved3);
-  void (*__gtk_reserved4);
+  void (*__gtk_reserved1) (void);
+  void (*__gtk_reserved2) (void);
+  void (*__gtk_reserved3) (void);
+  void (*__gtk_reserved4) (void);
 };
 
 GDK_AVAILABLE_IN_ALL
diff --git a/gtk/deprecated/gtktrayicon.h b/gtk/deprecated/gtktrayicon.h
index 4be745e341..245fc385ad 100644
--- a/gtk/deprecated/gtktrayicon.h
+++ b/gtk/deprecated/gtktrayicon.h
@@ -45,10 +45,10 @@ struct _GtkTrayIconClass
   GtkPlugClass parent_class;
 
   /* Padding for future expansion */
-  void (*_gtk_reserved1);
-  void (*_gtk_reserved2);
-  void (*_gtk_reserved3);
-  void (*_gtk_reserved4);
+  void (*__gtk_reserved1) (void);
+  void (*__gtk_reserved2) (void);
+  void (*__gtk_reserved3) (void);
+  void (*__gtk_reserved4) (void);
 };
 
 GDK_AVAILABLE_IN_ALL
diff --git a/gtk/gtkfilechooserbutton.h b/gtk/gtkfilechooserbutton.h
index bf0fe91aeb..5da2061837 100644
--- a/gtk/gtkfilechooserbutton.h
+++ b/gtk/gtkfilechooserbutton.h
@@ -63,10 +63,10 @@ struct _GtkFileChooserButtonClass
   /*< private >*/
 
   /* Padding for future expansion */
-  void (*__gtk_reserved1);
-  void (*__gtk_reserved2);
-  void (*__gtk_reserved3);
-  void (*__gtk_reserved4);
+  void (*__gtk_reserved1) (void);
+  void (*__gtk_reserved2) (void);
+  void (*__gtk_reserved3) (void);
+  void (*__gtk_reserved4) (void);
 };