diff --git a/fmacros.h b/fmacros.h
index 799c12c..9e5fec0 100644
--- a/fmacros.h
+++ b/fmacros.h
@@ -7,7 +7,7 @@
 
 #if defined(__sun__)
 #define _POSIX_C_SOURCE 200112L
-#elif defined(__linux__)
+#elif defined(__linux__) || defined(__OpenBSD__) || defined(__NetBSD__)
 #define _XOPEN_SOURCE 600
 #else
 #define _XOPEN_SOURCE