diff --git a/fmacros.h b/fmacros.h
index 83c20cd..21cd9cf 100644
--- a/fmacros.h
+++ b/fmacros.h
@@ -8,7 +8,7 @@
 #if defined(__sun__)
 #define _POSIX_C_SOURCE 200112L
 #elif defined(__linux__)
-#define _XOPEN_SOURCE 700
+#define _XOPEN_SOURCE 600
 #else
 #define _XOPEN_SOURCE
 #endif