diff --git a/include/strings.h b/include/strings.h
index 2b7e086cc8ac46393dabdd8b594019139f906dfa..4d7d69c31ab06564e2fcfa19a7d3cc2f204ede0f 100644
--- a/include/strings.h
+++ b/include/strings.h
@@ -20,7 +20,9 @@ char *index (const char *, int);
 char *rindex (const char *, int);
 #endif
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE)  || defined(_BSD_SOURCE)
 int ffs (int);
+#endif
 
 int strcasecmp (const char *, const char *);
 int strncasecmp (const char *, const char *, size_t);