[PATCH v2 1/1] list: Use offsetof() and typeof() to determine member offsets within a structure

[Jeremy Huddleston Sequoia]

Some compilers have difficulty with the previous implementation which
relies on undefined behavior according to the C standard.  Using
offsetof() from <stddef.h> (which most likely just uses
__builtin_offsetof on modern compilers) allows us to accomplish this
without ambiguity.

This fix also requires support for typeof().  If your compiler does not
support typeof(), then the old implementation will be used.  If you see
failures in test/list, please try a more modern compiler.

v2: Added fallback if typeof() is not present.

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu at apple.com>
---
 configure.ac            |  1 +
 include/dix-config.h.in |  3 +++
 include/list.h          | 25 +++++++++++++++++++++----
 3 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/configure.ac b/configure.ac
index abfe727..ab89027 100644
--- a/configure.ac
+++ b/configure.ac
@@ -137,6 +137,7 @@ AC_CHECK_HEADERS([fcntl.h stdlib.h string.h unistd.h dlfcn.h stropts.h fnmatch.h
 
 dnl Checks for typedefs, structures, and compiler characteristics.
 AC_C_CONST
+AC_C_TYPEOF
 AC_C_BIGENDIAN([ENDIAN="X_BIG_ENDIAN"], [ENDIAN="X_LITTLE_ENDIAN"])
 
 AC_CHECK_SIZEOF([unsigned long])
diff --git a/include/dix-config.h.in b/include/dix-config.h.in
index 77681a9..578f249 100644
--- a/include/dix-config.h.in
+++ b/include/dix-config.h.in
@@ -423,6 +423,9 @@
 /* Define to 64-bit byteswap macro */
 #undef bswap_64
 
+/* Define to 1 if typeof works with your compiler. */
+#undef HAVE_TYPEOF
+
 /* The compiler supported TLS storage class, prefering initial-exec if tls_model is supported */
 #undef TLS
 
diff --git a/include/list.h b/include/list.h
index d54a207..edf10ba 100644
--- a/include/list.h
+++ b/include/list.h
@@ -26,6 +26,12 @@
 #ifndef _XORG_LIST_H_
 #define _XORG_LIST_H_
 
+#ifdef HAVE_DIX_CONFIG_H
+#include "dix-config.h"
+#endif
+
+#include <stddef.h> /* offsetof() */
+
 /**
  * @file Classic doubly-link circular list implementation.
  * For real usage examples of the linked list, see the file test/list.c
@@ -232,7 +238,7 @@ xorg_list_is_empty(struct xorg_list *head)
  */
 #ifndef container_of
 #define container_of(ptr, type, member) \
-    (type *)((char *)(ptr) - (char *) &((type *)0)->member)
+    (type *)((char *)(ptr) - offsetof(type, member))
 #endif
 
 /**
@@ -271,9 +277,20 @@ xorg_list_is_empty(struct xorg_list *head)
 #define xorg_list_last_entry(ptr, type, member) \
     xorg_list_entry((ptr)->prev, type, member)
 
-#define __container_of(ptr, sample, member)				\
-    (void *)((char *)(ptr)						\
-	     - ((char *)&(sample)->member - (char *)(sample)))
+#ifdef HAVE_TYPEOF
+#define __container_of(ptr, sample, member)			\
+    container_of(ptr, typeof(*sample), member)
+#else
+/* This implementation of __container_of has undefined behavior according
+ * to the C standard, but it works in many cases.  If your compiler doesn't
+ * support typeof() and fails with this implementation, please try a newer
+ * compiler.
+ */
+#define __container_of(ptr, sample, member)                            \
+    (void *)((char *)(ptr)                                             \
+            - ((char *)&(sample)->member - (char *)(sample)))
+#endif
+
 /**
  * Loop through the list given by head and set pos to struct in the list.
  *
-- 
1.7.11.5