diff --git a/libc/include/strings.h b/libc/include/strings.h
index 1200e77..ca9f658 100644
--- a/libc/include/strings.h
+++ b/libc/include/strings.h
@@ -54,7 +54,11 @@
 #define bzero(b, len) (void)(__builtin_memset((b), '\0', (len)))
 #endif
 
+#if defined(__i386__)
+int ffs(int) __INTRODUCED_IN(21);
+#else
 int ffs(int);
+#endif
 
 __END_DECLS