diff --git a/lib/libc/string/ffs.3 b/lib/libc/string/ffs.3
index 71e13b2a617..fd4de44b195 100644
--- a/lib/libc/string/ffs.3
+++ b/lib/libc/string/ffs.3
@@ -43,7 +43,7 @@
 .Sh LIBRARY
 .Lb libc
 .Sh SYNOPSIS
-.In string.h
+.In strings.h
 .Ft int
 .Fn ffs "int value"
 .Sh DESCRIPTION
@@ -62,3 +62,10 @@ The
 .Fn ffs
 function appeared in
 .Bx 4.3 .
+Its prototype existed previously in
+.Aq Pa string.h
+before it was moved to
+.Aq Pa strings.h
+for
+.St -p1003.1-2001
+compliance.
diff --git a/lib/libc/string/ffs.c b/lib/libc/string/ffs.c
index 14985a4e7bf..f08e0d124e3 100644
--- a/lib/libc/string/ffs.c
+++ b/lib/libc/string/ffs.c
@@ -37,14 +37,13 @@ static char sccsid[] = "@(#)ffs.c	8.1 (Berkeley) 6/4/93";
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
-#include <string.h>
+#include <strings.h>
 
 /*
  * ffs -- vax ffs instruction
  */
 int
-ffs(mask)
-	int mask;
+ffs(int mask)
 {
 	int bit;