diff --git a/fmacros.h b/fmacros.h
index 24c9bac..14fed60 100644
--- a/fmacros.h
+++ b/fmacros.h
@@ -13,8 +13,10 @@
 #if defined(__sun__)
 #define _POSIX_C_SOURCE 200112L
 #else
+#if !(defined(__APPLE__) && defined(__MACH__))
 #define _XOPEN_SOURCE 600
 #endif
+#endif
 
 #if defined(__APPLE__) && defined(__MACH__)
 #define _OSX