diff --git a/include/lib/libc/aarch32/stddef_.h b/include/lib/libc/aarch32/stddef_.h
index 928481c..1babfad 100644
--- a/include/lib/libc/aarch32/stddef_.h
+++ b/include/lib/libc/aarch32/stddef_.h
@@ -4,6 +4,9 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
+#ifndef STDDEF__H
+#define STDDEF__H
+
 #ifndef SIZET_
 typedef unsigned int size_t;
 #define SIZET_
@@ -13,3 +16,5 @@
 typedef long ptrdiff_t;
 #define _PTRDIFF_T
 #endif
+
+#endif /* STDDEF__H */