container_of: add container_off() helper.

Just like container_of() (with typechecking, etc), but return the offset
to the enclosing structure.

diff --git a/ccan/container_of/container_of.h b/ccan/container_of/container_of.h
index 2a6b1cd54e9dcf97f57b0b282e23bb7de82ae0db..4490184c2529b97ced2b9e3611e0f3671bae19f7 100644 (file)
--- a/ccan/container_of/container_of.h
+++ b/ccan/container_of/container_of.h
@@ -31,9 +31,39 @@
  */
 #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((member_ptr), containing_type, member)))
 
+/**
+ * container_off - get offset to enclosing structure
+ * @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
+ * typechecking and figures out the offset to the enclosing type.
+ *
+ * Note that @member_ptr is not evaluated.
+ *
+ * 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(foo, struct info, my_foo);
+ *             return (void *)((char *)foo - off);
+ *     }
+ */
+#define container_off(member_ptr, containing_type, member)             \
+       (offsetof(containing_type, member)                              \
+        + check_types_match(*(member_ptr), ((containing_type *)0)->member))
 
 /**
  * container_of_var - get pointer to enclosing structure using a variable

diff --git a/ccan/container_of/test/run.c b/ccan/container_of/test/run.c
index f28d9998503dc63e482355927fe66ebfbe445b9d..8f86bc8da3a4db10eb53c32e9d6ee6590b8b9dcb 100644 (file)
--- a/ccan/container_of/test/run.c
+++ b/ccan/container_of/test/run.c
@@ -12,10 +12,13 @@ int main(int argc, char *argv[])
        int *intp = &foo.a;
        char *charp = &foo.b;
 
-       plan_tests(4);
+       plan_tests(6);
        ok1(container_of(intp, struct foo, a) == &foo);
        ok1(container_of(charp, struct foo, b) == &foo);
        ok1(container_of_var(intp, &foo, a) == &foo);
        ok1(container_of_var(charp, &foo, b) == &foo);
+
+       ok1(container_off(intp, struct foo, a) == 0);
+       ok1(container_off(charp, struct foo, b) == offsetof(struct foo, b));
        return exit_status();
 }