diff --git a/include/stdlib.h b/include/stdlib.h
index d7e49d028bc4..c19eef055dc0 100644
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -76,7 +76,7 @@ extern int __mb_cur_max;
 extern int ___mb_cur_max(void);
 #define	MB_CUR_MAX	(___mb_cur_max())
 
-__noreturn void	 abort(void);
+_Noreturn void	 abort(void);
 int	 abs(int) __pure2;
 int	 atexit(void (*)(void));
 double	 atof(const char *);
@@ -86,7 +86,7 @@ void	*bsearch(const void *, const void *, size_t,
 	    size_t, int (*)(const void *, const void *));
 void	*calloc(size_t, size_t) __malloc_like;
 div_t	 div(int, int) __pure2;
-__noreturn void	 exit(int);
+_Noreturn void	 exit(int);
 void	 free(void *);
 char	*getenv(const char *);
 long	 labs(long) __pure2;
@@ -145,14 +145,14 @@ unsigned long long
 	 strtoull(const char * __restrict, char ** __restrict, int);
 #endif /* __LONG_LONG_SUPPORTED */
 
-__noreturn void	 _Exit(int);
+_Noreturn void	 _Exit(int);
 #endif /* __ISO_C_VISIBLE >= 1999 */
 
 /*
  * If we're in a mode greater than C99, expose C1x functions.
  */
 #if __ISO_C_VISIBLE > 1999
-__noreturn void quick_exit(int)
+_Noreturn void quick_exit(int)
 int
 at_quick_exit(void (*func)(void));
 #endif /* __ISO_C_VISIBLE > 1999 */
diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
index ca485b3dcb91..e7f447d6d7e9 100644
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -220,13 +220,13 @@
 
 
 #if defined(__cplusplus) && __cplusplus >= 201103L
-#define	__noreturn		[[noreturn]]
+#define	_Noreturn		[[noreturn]]
 #elif defined(__STDC_VERSION__) && __STDC_VERSION__ > 201000L
-#define	__noreturn		_Noreturn
+/* Do nothing - _Noreturn is a keyword */
 #elif defined(__GNUC__)
-#define	__noreturn		__attribute__((__noreturn__))
+#define	_Noreturn		__attribute__((__noreturn__))
 #else
-#define	__noreturn
+#define	_Noreturn
 #endif
 
 #if __GNUC_PREREQ__(2, 96)