diff --git a/libtransmission/utils.c b/libtransmission/utils.c
index 8cbda3e61..ced4e1597 100644
--- a/libtransmission/utils.c
+++ b/libtransmission/utils.c
@@ -21,7 +21,9 @@
 #endif
 
 #if defined(HAVE_POSIX_MEMALIGN) || defined(HAVE_VALLOC)
- #define _XOPEN_SOURCE 600 /* pick posix_memalign and valloc declarations */
+ #if !defined(SYS_DARWIN)
+  #define _XOPEN_SOURCE 600 /* pick posix_memalign and valloc declarations */
+ #endif
 #endif
 
 #include <assert.h>