Index: include/err.h
===================================================================
--- include/err.h
+++ include/err.h
@@ -58,7 +58,7 @@
 void	warnx(const char *, ...) __printflike(1, 2);
 void	vwarnx(const char *, __va_list) __printflike(1, 0);
 void	err_set_file(void *);
-void	err_set_exit(void (*)(int));
+void	err_set_exit(void (* _Nullable)(int));
 __END_DECLS
 
 #endif /* !_ERR_H_ */
Index: include/signal.h
===================================================================
--- include/signal.h
+++ include/signal.h
@@ -104,7 +104,7 @@
 int	sigignore(int);
 int	sigpause(int);
 int	sigrelse(int);
-void	(*sigset(int, void (*)(int)))(int);
+void	(* _Nullable sigset(int, void (* _Nullable)(int)))(int);
 int	xsi_sigpause(int);
 #endif
 
Index: include/stdio.h
===================================================================
--- include/stdio.h
+++ include/stdio.h
@@ -123,10 +123,10 @@
 
 	/* operations */
 	void	*_cookie;	/* (*) cookie passed to io functions */
-	int	(*_close)(void *);
-	int	(*_read)(void *, char *, int);
-	fpos_t	(*_seek)(void *, fpos_t, int);
-	int	(*_write)(void *, const char *, int);
+	int	(* _Nullable _close)(void *);
+	int	(* _Nullable _read)(void *, char *, int);
+	fpos_t	(* _Nullable _seek)(void *, fpos_t, int);
+	int	(* _Nullable _write)(void *, const char *, int);
 
 	/* separate buffer for long sequences of ungetc() */
 	struct	__sbuf _ub;	/* ungetc buffer */
@@ -390,10 +390,10 @@
  * Stdio function-access interface.
  */
 FILE	*funopen(const void *,
-	    int (*)(void *, char *, int),
-	    int (*)(void *, const char *, int),
-	    fpos_t (*)(void *, fpos_t, int),
-	    int (*)(void *));
+	    int (* _Nullable)(void *, char *, int),
+	    int (* _Nullable)(void *, const char *, int),
+	    fpos_t (* _Nullable)(void *, fpos_t, int),
+	    int (* _Nullable)(void *));
 #define	fropen(cookie, fn) funopen(cookie, fn, 0, 0, 0)
 #define	fwopen(cookie, fn) funopen(cookie, 0, fn, 0, 0)
 
Index: include/stdlib.h
===================================================================
--- include/stdlib.h
+++ include/stdlib.h
@@ -81,12 +81,12 @@
 
 _Noreturn void	 abort(void);
 int	 abs(int) __pure2;
-int	 atexit(void (*)(void));
+int	 atexit(void (* _Nonnull)(void));
 double	 atof(const char *);
 int	 atoi(const char *);
 long	 atol(const char *);
 void	*bsearch(const void *, const void *, size_t,
-	    size_t, int (*)(const void *, const void *));
+	    size_t, int (*)(const void * _Nonnull, const void *));
 void	*calloc(size_t, size_t) __malloc_like __result_use_check
 	     __alloc_size(1) __alloc_size(2);
 div_t	 div(int, int) __pure2;
@@ -100,7 +100,7 @@
 size_t	 mbstowcs(wchar_t * __restrict , const char * __restrict, size_t);
 int	 mbtowc(wchar_t * __restrict, const char * __restrict, size_t);
 void	 qsort(void *, size_t, size_t,
-	    int (*)(const void *, const void *));
+	    int (* _Nonnull)(const void *, const void *));
 int	 rand(void);
 void	*realloc(void *, size_t) __result_use_check __alloc_size(2);
 void	 srand(unsigned);
@@ -256,9 +256,9 @@
 __uint32_t 
 	 arc4random_uniform(__uint32_t);
 #ifdef __BLOCKS__
-int	 atexit_b(void (^)(void));
+int	 atexit_b(void (^ _Nonnull)(void));
 void	*bsearch_b(const void *, const void *, size_t,
-	    size_t, int (^)(const void *, const void *));
+	    size_t, int (^ _Nonnull)(const void *, const void *));
 #endif
 char	*getbsize(int *, long *);
 					/* getcap(3) functions */
@@ -282,11 +282,13 @@
 const char *
 	 getprogname(void);
 
-int	 heapsort(void *, size_t, size_t, int (*)(const void *, const void *));
+int	 heapsort(void *, size_t, size_t,
+	    int (* _Nonnull)(const void *, const void *));
 #ifdef __BLOCKS__
-int	 heapsort_b(void *, size_t, size_t, int (^)(const void *, const void *));
+int	 heapsort_b(void *, size_t, size_t,
+	    int (^ _Nonnull)(const void *, const void *));
 void	 qsort_b(void *, size_t, size_t,
-	    int (^)(const void *, const void *));
+	    int (^ _Nonnull)(const void *, const void *));
 #endif
 int	 l64a_r(long, char *, int);
 int	 mergesort(void *, size_t, size_t, int (*)(const void *, const void *));
Index: sys/sys/cdefs.h
===================================================================
--- sys/sys/cdefs.h
+++ sys/sys/cdefs.h
@@ -804,6 +804,16 @@
 #endif
 
 /*
+ * Nullability qualifiers: currently only supported by Clang.
+ */
+#if !__has_attribute(_Nonnull)
+#define	_Nonnull
+#endif
+#if !__has_attribute(_Nullable)
+#define	_Nullable
+#endif
+
+/*
  * Type Safety Checking
  *
  * Clang provides additional attributes to enable checking type safety