diff --git a/include/strings.h b/include/strings.h
index 345c6517..2b7e086c 100644
--- a/include/strings.h
+++ b/include/strings.h
@@ -10,15 +10,17 @@ extern "C" {
 #define __NEED_locale_t
 #include <bits/alltypes.h>
 
-
+#if defined(_GNU_SOURCE) || defined(_BSD_SOURCE) || defined(_POSIX_SOURCE) \
+ || (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE+0 < 200809L) \
+ || (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE+0 < 700)
 int bcmp (const void *, const void *, size_t);
 void bcopy (const void *, void *, size_t);
 void bzero (void *, size_t);
-
-int ffs (int);
-
 char *index (const char *, int);
 char *rindex (const char *, int);
+#endif
+
+int ffs (int);
 
 int strcasecmp (const char *, const char *);
 int strncasecmp (const char *, const char *, size_t);