diff --git a/xC.c b/xC.c
index b4b4784..20845a1 100644
--- a/xC.c
+++ b/xC.c
@@ -49,7 +49,7 @@ enum
 #define PROGRAM_NAME "xC"
 
 // fmemopen
-#define _POSIX_C_SOURCE 200809L
+#define _XOPEN_SOURCE 700
 
 #include "common.c"
 #include "xD-replies.c"