X-Git-Url: http://git.ozlabs.org/?p=ccan;a=blobdiff_plain;f=ccan%2Fcontainer_of%2Fcontainer_of.h;h=ea8f1dd36cc191d6d3d3f690d342c0845d61d32e;hp=2a6b1cd54e9dcf97f57b0b282e23bb7de82ae0db;hb=578da7e7b6265153fa0519035fc52a086a711ac0;hpb=da72623aec30213a593bd23dca80c89416598f75

diff --git a/ccan/container_of/container_of.h b/ccan/container_of/container_of.h
index 2a6b1cd5..ea8f1dd3 100644
--- a/ccan/container_of/container_of.h
+++ b/ccan/container_of/container_of.h
@@ -31,14 +31,41 @@
  */
 #define container_of(member_ptr, containing_type, member)		\
 	 ((containing_type *)						\
-	  ((char *)(member_ptr) - offsetof(containing_type, member))	\
-	  - check_types_match(*(member_ptr), ((containing_type *)0)->member))
+	  ((char *)(member_ptr)						\
+	   - container_off(containing_type, member))			\
+	  + check_types_match(*(member_ptr), ((containing_type *)0)->member))
 
+/**
+ * container_off - get offset to enclosing structure
+ * @containing_type: the type this member is within
+ * @member: the name of this member within the structure.
+ *
+ * Given a pointer to a member of a structure, this macro does
+ * typechecking and figures out the offset to the enclosing type.
+ *
+ * Example:
+ *	struct foo {
+ *		int fielda, fieldb;
+ *		// ...
+ *	};
+ *	struct info {
+ *		int some_other_field;
+ *		struct foo my_foo;
+ *	};
+ *
+ *	static struct info *foo_to_info(struct foo *foo)
+ *	{
+ *		size_t off = container_off(struct info, my_foo);
+ *		return (void *)((char *)foo - off);
+ *	}
+ */
+#define container_off(containing_type, member)	\
+	offsetof(containing_type, member)
 
 /**
  * container_of_var - get pointer to enclosing structure using a variable
  * @member_ptr: pointer to the structure member
- * @var: a pointer to a structure of same type as this member is within
+ * @container_var: a pointer of same type as this member's container
  * @member: the name of this member within the structure.
  *
  * Given a pointer to a member of a structure, this macro does pointer
@@ -52,12 +79,13 @@
  *	}
  */
 #if HAVE_TYPEOF
-#define container_of_var(member_ptr, var, member) \
-	container_of(member_ptr, typeof(*var), member)
+#define container_of_var(member_ptr, container_var, member) \
+	container_of(member_ptr, typeof(*container_var), member)
 #else
-#define container_of_var(member_ptr, var, member)		\
-	((void *)((char *)(member_ptr)				\
-		  - ((char *)&(var)->member - (char *)(var))))
+#define container_of_var(member_ptr, container_var, member)		\
+	((void *)((char *)(member_ptr)					\
+		  - ((char *)&(container_var)->member			\
+		     - (char *)(container_var))))
 #endif
 
 #endif /* CCAN_CONTAINER_OF_H */