X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~mdw/git/mLib/blobdiff_plain/d04c0e00da3a27693bbf9cc4f2d5c88e56d80f20..d93e9a33ee30faed3945b2a7acfc46b84197d432:/utils/macros.h

diff --git a/utils/macros.h b/utils/macros.h
index 2ca5a06..b48e6f6 100644
--- a/utils/macros.h
+++ b/utils/macros.h
@@ -92,7 +92,7 @@
 	IGNORABLE extern char static_assert_failed[1 - 2*!(cond)]
 #endif
 
-/* --- @CHECK_TYPE@ ---
+/* --- @CHECK_TYPE@ --- *
  *
  * Arguments:	@expty@ = expected type of @x@
  *		@expty x@ = some object
@@ -105,7 +105,7 @@
 
 #define CHECK_TYPE(expty, x) (!sizeof(*(expty *)0 = (x)))
 
-/* --- @CONVERT_CAREFULLY@ ---
+/* --- @CONVERT_CAREFULLY@ --- *
  *
  * Arguments:	@newty@ = new type for the result
  *		@expty@ = expected type of @x@
@@ -121,6 +121,19 @@
 #define CONVERT_CAREFULLY(newty, expty, x)				\
 	(CHECK_TYPE(expty, x) + (/*unconst unvolatile*/ newty)(x))
 
+/* --- @CONTAINER@ --- *
+ *
+ * Arguments:	@type@ = the parent type, a structure or union
+ *		@mem@ = the name of the member we have a pointer to
+ *		@memty *p@ = pointer to @member@ within @type@
+ *
+ * Returns:	The address of the containing @ty@ object.
+ */
+
+#define CONTAINER(type, member, p)					\
+  (!sizeof((p) = &((type *)0)->mem) +					\
+   (type *)((unsigned char *)(p) - offsetof(type, mem)))
+
 /* --- @UNCONST@, @UNVOLATILE@, @UNQUALIFY@ --- *
  *
  * Arguments:	@type@ = a type name