Index: include/stdlib.h
===================================================================
--- include/stdlib.h
+++ include/stdlib.h
@@ -87,21 +87,21 @@
 long	 atol(const char *);
 void	*bsearch(const void *, const void *, size_t,
 	    size_t, int (*)(const void *, const void *));
-void	*calloc(size_t, size_t) __malloc_like;
+void	*calloc(size_t, size_t) __malloc_like __wur __alloc_size(1, 2);
 div_t	 div(int, int) __pure2;
 _Noreturn void	 exit(int);
 void	 free(void *);
 char	*getenv(const char *);
 long	 labs(long) __pure2;
 ldiv_t	 ldiv(long, long) __pure2;
-void	*malloc(size_t) __malloc_like;
+void	*malloc(size_t) __malloc_like __wur __alloc_size(1);
 int	 mblen(const char *, size_t);
 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	 rand(void);
-void	*realloc(void *, size_t);
+void	*realloc(void *, size_t) __wur __alloc_size(2);
 void	 srand(unsigned);
 double	 strtod(const char * __restrict, char ** __restrict);
 float	 strtof(const char * __restrict, char ** __restrict);
Index: sys/sys/cdefs.h
===================================================================
--- sys/sys/cdefs.h
+++ sys/sys/cdefs.h
@@ -40,6 +40,9 @@
  * Testing against Clang-specific extensions.
  */
 
+#ifndef	__has_attribute
+#define	__has_attribute(x)	0
+#endif
 #ifndef	__has_extension
 #define	__has_extension		__has_feature
 #endif
@@ -363,8 +366,10 @@
 
 #if __GNUC_PREREQ__(3, 4)
 #define	__fastcall	__attribute__((__fastcall__))
+#define __wur		__attribute__((__warn_unused_result__))
 #else
 #define	__fastcall
+#define __wur
 #endif
 
 #if __GNUC_PREREQ__(4, 1)
@@ -373,6 +378,12 @@
 #define	__returns_twice
 #endif
 
+#if __has_attribute(alloc_size) || __GNUC_PREREQ__(4, 3)
+#define	__alloc_size(...)	__attribute__((alloc_size(__VA_ARGS__)))
+#else
+#define	__alloc_size(...)
+#endif
+
 /* XXX: should use `#if __STDC_VERSION__ < 199901'. */
 #if !__GNUC_PREREQ__(2, 7) && !defined(__INTEL_COMPILER)
 #define	__func__	NULL