diff --git a/man3/ffs.3 b/man3/ffs.3
index 11802074c..35da288ae 100644
--- a/man3/ffs.3
+++ b/man3/ffs.3
@@ -57,7 +57,8 @@ Feature Test Macro Requirements for glibc (see
 .RS 4
 .TP 4
 Since glibc 2.12:
-_POSIX_C_SOURCE\ >=\ 200809L
+    _XOPEN_SOURCE >= 700
+    || ! (_POSIX_C_SOURCE\ >=\ 200809L)
     || /* Glibc since 2.19: */ _DEFAULT_SOURCE
     || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
 .TP