[Tarantool-patches] [PATCH small 1/1] rlist: use built-in offsetof() when possible

[Vladislav Shpilevoy <v.shpilevoy@tarantool.org>]

Rlist implemented its own offsetof() as a classic NULL
pointer cast to the target type with taking address of
the member. So it turned into NULL + member offset.

But appeared undefined behaviour clang sanitizer is
not friendly to this hack.

The patch makes rlist define its own offsetof only as
a last resort.

Part of https://github.com/tarantool/tarantool/issues/4609
---
Branch: http://github.com/tarantool/small/tree/gerold103/rlist-offsetof
Issue: https://github.com/tarantool/tarantool/issues/4609

 small/rlist.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/small/rlist.h b/small/rlist.h
index 2b0bcb4..26edd2e 100644
--- a/small/rlist.h
+++ b/small/rlist.h
@@ -40,6 +40,10 @@ extern "C" {
 #define typeof __typeof__
 #endif
 
+#ifndef offsetof
+#define offsetof(type, member) ((size_t) &((type *)0)->member)
+#endif
+
 /**
  * list entry and head structure
  */
@@ -272,8 +276,9 @@ rlist_cut_before(struct rlist *head1, struct rlist *head2, struct rlist *item)
  * return entry by list item
  */
 #define rlist_entry(item, type, member) ({				\
-	const typeof( ((type *)0)->member ) *__mptr = (item);		\
-	(type *)( (char *)__mptr - ((size_t) &((type *)0)->member) ); })
+	const typeof(((type *)0)->member) *__mptr = (item);		\
+	(type *)( (char *)__mptr - offsetof(type,member));		\
+})
 
 /**
  * return first entry
-- 
2.21.1 (Apple Git-122.3)