Re: [Xen-devel] [PATCH 12/10] libxl: New convenience macro CONTAINING_STRUCT

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Ian Campbell writes ("Re: [Xen-devel] [PATCH 12/10] libxl: New convenience 
macro CONTAINING_STRUCT"):
> On Mon, 2012-01-09 at 17:34 +0000, Ian Jackson wrote:
> > Provide a convenient and type-safe wrapper which does the correct
> > dance to subtract offsetof.
...
> > + *    void GET_CONTAINING_STRUCT(outer_type *outer_var [NB:UPDATED],
> > + *                               some_type *inner_ptr,
> > + *                               member_name);
> > + *    outer_type *CONTAINING_STRUCT(outer_type,
> > + *                                  some_type *inner_ptr,
> > + *                                  member_name);
> > + * The semantics are that after:
> > + *    outer_type outer, *outer_var;
> > + *    member_type *inner_ptr = &outer->member_name;
> > + *    GET_CONTAINING_STRUCT(outer_var, &outer_ptr->member_name, 
> > member_name)
> > + * The following hold:
> > + *    CONTAINING_STRUCT(inner_ptr, outer_type, member_name) == outer_ptr
> 
> There is no outer_ptr in the givens, did you mean outer_var or something
> else?

I meant &outer.  Fixed.

> It's not entirely clear to me what the distinction between the GET_ and
> non GET_ variant is (just that one returns the thing and the other
> updates a variable?) and/or why we would need both.

That's exactly the difference.

> The common operation is to go from inner_ptr to outer_ptr I think
> and CONTAINING_STRUCT seems to fill that niche just fine.

The reason we need GET_CONTAINING_STRUCT is because we want to write
this:

    libxl__ev_devstate *ds;
    GET_CONTAINING_STRUCT(ds, watch, watch);

If we have to use CONTAINING_STRUCT, we have to write:

    libxl__ev_devstate *ds =
        CONTAINING_STRUCT(libxl__ev_devstate, watch, watch);

which is really unnecessarily verbose, because of the repetition of
libxl__ev_devstate.

> BTW, in Linux this is called container_of, which is maybe more familiar
> to people?

I'm not attached to the name.  If we pick Linux's name we should make
sure its semantics are identical, so the argument order should change.

Should it be called "container_of" or "CONTAINER_OF" ?

I still think we need something like the GET_... version, or some
other construct that allows us to avoid writing out the type name
twice.

Hmm, actually, in our version, you can write,

    libxl__ev_devstate *ds = CONTAINING_STRUCT(ds, watch, watch);

since we call typeof on the type argument.

So how about I change everything to use that pattern, rename it
CONTAINER_OF, and remove the GET_ version ?

Ian.

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