container_of: don't put member_ptr in container_off.

[ccan] / ccan / container_of / container_of.h

diff --git a/ccan/container_of/container_of.h b/ccan/container_of/container_of.h
index 2a6b1cd54e9dcf97f57b0b282e23bb7de82ae0db..ea8f1dd36cc191d6d3d3f690d342c0845d61d32e 100644 (file)
--- 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 */