ccan/container_of: add container_of_or_null

* ccan/container_of/container_of.h (container_of_or_null): added
  [ccan 7ec5b8e06b2fd5fa98b1fcde1158c286d2d429d8] (David Gibson)

It's quite common to have a pointer which could be either a pointer to a
structure member, or NULL.  This needs special casing with container_of(),
or it will convert NULL into something strange.

This patch adds container_of_or_null(), which will return NULL if passed
(an appropriately typed) NULL, or the containining structure as
container_of() otherwise.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47579 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,8 @@
+Sun Sep 14 08:43:37 2014  Eric Wong  <e@80x24.org>
+
+	* ccan/container_of/container_of.h (container_of_or_null): added
+	  [ccan 7ec5b8e06b2fd5fa98b1fcde1158c286d2d429d8] (David Gibson)
+
 Sun Sep 14 08:41:44 2014  Eric Wong  <e@80x24.org>
 
 	* ccan/list/list.h (list_del_init, list_node_init): new functions

ccan/container_of/container_of.h

@@ -33,6 +33,42 @@
 	   - container_off(containing_type, member))			\
 	  + check_types_match(*(member_ptr), ((containing_type *)0)->member))
 
+
+/**
+ * container_of_or_null - get pointer to enclosing structure, or NULL
+ * @member_ptr: pointer to the structure member
+ * @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 pointer
+ * subtraction to return the pointer to the enclosing type, unless it
+ * is given NULL, in which case it also returns NULL.
+ *
+ * Example:
+ *	struct foo {
+ *		int fielda, fieldb;
+ *		// ...
+ *	};
+ *	struct info {
+ *		int some_other_field;
+ *		struct foo my_foo;
+ *	};
+ *
+ *	static struct info *foo_to_info_allowing_null(struct foo *foo)
+ *	{
+ *		return container_of_or_null(foo, struct info, my_foo);
+ *	}
+ */
+static inline char *container_of_or_null_(void *member_ptr, size_t offset)
+{
+	return member_ptr ? (char *)member_ptr - offset : NULL;
+}
+#define container_of_or_null(member_ptr, containing_type, member)	\
+	((containing_type *)						\
+	 container_of_or_null_(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