diff --git a/src/public_include/decaf/decaf_255.hxx b/src/public_include/decaf/decaf_255.hxx
index e84a792..7350e7a 100644
--- a/src/public_include/decaf/decaf_255.hxx
+++ b/src/public_include/decaf/decaf_255.hxx
@@ -23,7 +23,9 @@
 #define __DECAF_255_HXX__ 1
 
 /** This code uses posix_memalign. */
+#ifndef _XOPEN_SOURCE
 #define _XOPEN_SOURCE 600 
+#endif
 #include <stdlib.h>
 #include <string.h> /* for memcpy */
 
diff --git a/src/public_include/decaf/decaf_448.hxx b/src/public_include/decaf/decaf_448.hxx
index c043fcb..287236b 100644
--- a/src/public_include/decaf/decaf_448.hxx
+++ b/src/public_include/decaf/decaf_448.hxx
@@ -23,7 +23,9 @@
 #define __DECAF_448_HXX__ 1
 
 /** This code uses posix_memalign. */
-#define _XOPEN_SOURCE 600 
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 600
+#endif
 #include <stdlib.h>
 #include <string.h> /* for memcpy */