diff --git a/include/string.h b/include/string.h
index 4770d101a139..75e85580d21b 100644
--- a/include/string.h
+++ b/include/string.h
@@ -63,7 +63,7 @@ void	*memrchr(const void *, int, size_t) __pure;
 int	 memcmp(const void *, const void *, size_t) __pure;
 void	*memcpy(void * __restrict, const void * __restrict, size_t);
 #if __BSD_VISIBLE
-void	*memmem(const void *, size_t, const void *, size_t);
+void	*memmem(const void *, size_t, const void *, size_t) __pure;
 #endif
 void	*memmove(void *, const void *, size_t);
 void	*memset(void *, int, size_t);