diff --git a/H/Yap.h b/H/Yap.h
index 630326d71..7821833d1 100644
--- a/H/Yap.h
+++ b/H/Yap.h
@@ -145,8 +145,10 @@
 
 #ifdef THREADS
 #if USE_PTHREAD_LOCKING
+#ifndef _XOPEN_SOURCE
 #define _XOPEN_SOURCE 600
 #endif
+#endif
 #include <pthread.h>
 #endif