diff --git a/include/stdlib.h b/include/stdlib.h
index c3d9d136aa25..33cd6e4dd7d9 100644
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -120,10 +120,10 @@ unsigned long
 int	 system(const char *);
 
 int	 mblen(const char *, size_t);
-size_t	 mbstowcs(wchar_t *, const char *, size_t);
+size_t	 mbstowcs(wchar_t *__restrict , const char *__restrict, size_t);
 int	 wctomb(char *, wchar_t);
-int	 mbtowc(wchar_t *, const char *, size_t);
-size_t	 wcstombs(char *, const wchar_t *, size_t);
+int	 mbtowc(wchar_t *__restrict, const char *__restrict, size_t);
+size_t	 wcstombs(char *__restrict, const wchar_t *__restrict, size_t);
 
 #if !defined(_ANSI_SOURCE) && !defined(_POSIX_SOURCE)
 extern const char *_malloc_options;
diff --git a/lib/libc/locale/mbstowcs.c b/lib/libc/locale/mbstowcs.c
index b473f89d33b7..f27f8978ba3a 100644
--- a/lib/libc/locale/mbstowcs.c
+++ b/lib/libc/locale/mbstowcs.c
@@ -45,8 +45,8 @@ __FBSDID("$FreeBSD$");
 
 size_t
 mbstowcs(pwcs, s, n)
-	wchar_t *pwcs;
-	const char *s;
+	wchar_t *__restrict pwcs;
+	const char *__restrict s;
 	size_t n;
 {
 	char const *e;
diff --git a/lib/libc/locale/mbtowc.c b/lib/libc/locale/mbtowc.c
index 166ea7991c12..67771a095665 100644
--- a/lib/libc/locale/mbtowc.c
+++ b/lib/libc/locale/mbtowc.c
@@ -43,8 +43,8 @@ __FBSDID("$FreeBSD$");
 
 int
 mbtowc(pwc, s, n)
-	wchar_t *pwc;
-	const char *s;
+	wchar_t *__restrict pwc;
+	const char *__restrict s;
 	size_t n;
 {
 	char const *e;
diff --git a/lib/libc/locale/multibyte.3 b/lib/libc/locale/multibyte.3
index 6cfaa7a6f8fa..7069b7ce9963 100644
--- a/lib/libc/locale/multibyte.3
+++ b/lib/libc/locale/multibyte.3
@@ -52,11 +52,11 @@
 .Ft int
 .Fn mblen "const char *mbchar" "size_t nbytes"
 .Ft size_t
-.Fn mbstowcs "wchar_t *wcstring" "const char *mbstring" "size_t nwchars"
+.Fn mbstowcs "wchar_t *restrict wcstring" "const char *restrict mbstring" "size_t nwchars"
 .Ft int
-.Fn mbtowc "wchar_t *wcharp" "const char *mbchar" "size_t nbytes"
+.Fn mbtowc "wchar_t *restrict wcharp" "const char *restrict mbchar" "size_t nbytes"
 .Ft size_t
-.Fn wcstombs "char *mbstring" "const wchar_t *wcstring" "size_t nbytes"
+.Fn wcstombs "char *restrict mbstring" "const wchar_t *restrict wcstring" "size_t nbytes"
 .Ft int
 .Fn wctomb "char *mbchar" "wchar_t wchar"
 .Sh DESCRIPTION
diff --git a/lib/libc/locale/wcstombs.c b/lib/libc/locale/wcstombs.c
index ed9397ab90c1..208aac868738 100644
--- a/lib/libc/locale/wcstombs.c
+++ b/lib/libc/locale/wcstombs.c
@@ -45,8 +45,8 @@ __FBSDID("$FreeBSD$");
 
 size_t
 wcstombs(s, pwcs, n)
-	char *s;
-	const wchar_t *pwcs;
+	char *__restrict s;
+	const wchar_t *__restrict pwcs;
 	size_t n;
 {
 	char buf[MB_LEN_MAX];