diff --git a/glib/glib-private.h b/glib/glib-private.h
index 44cbf3d94..6195ad975 100644
--- a/glib/glib-private.h
+++ b/glib/glib-private.h
@@ -23,6 +23,12 @@
 #include <glib.h>
 #include "gwakeup.h"
 
+#if defined(__GNUC__)
+# define _g_alignof(type) (__alignof__ (type))
+#else
+# define _g_alignof(type) (G_STRUCT_OFFSET (struct { char a; type b; }, b))
+#endif
+
 GMainContext *          g_get_worker_context            (void);
 gboolean                g_check_setuid                  (void);
 GMainContext *          g_main_context_new_with_next_id (guint next_id);