Re: [Qemu-devel] [PATCH v5 1/5] qobject: ensure base is at offset 0

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> writes:

> On 04/17/2018 08:35 AM, Marc-André Lureau wrote:
>> All QObject types have the base QObject as first field. This allows to
>
> s/as/as their/
>
>> simplify qobject_to() and will allow further simplification in
>
> s/allows to simplify/allows the simplification of/
> s/in/in the/
>
>> following patch.
>
> Might also be worth mentioning that this explicitly guarantees that
> existing casts work correctly (even though we'd prefer to get rid of
> such casts in any location except the qobject.h macros); Markus pointed out:
>
>>> Uh, there's another reason: existing type casts from QObject * to
>>> subtypes.  I just spotted one in tests/check-qdict.c:
>>> 
>>>     dst = (QDict *)qdict_crumple(src, &error_abort);

As far as I'm concerned, that's the real reason.  The simplification
plus the check to make it safe seems like a wash.

The cast I spotted appears to be the only one, though:

    $ git-grep '(Q[A-Z][a-z]* \*)'
    hmp.c:    qmp_device_add((QDict *)qdict, NULL, &err);
    include/qapi/qmp/qobject.h:        return (QObject *)obj;
    qobject/qobject.c:static void (*qdestroy[QTYPE__MAX])(QObject *) = {
    tests/check-qdict.c:    dst = (QDict *)qdict_crumple(src, &error_abort);

The first two cast away const, the third isn't a type cast.  The fourth
one should use qobject_to() instead, regardless of this patch.

Do we want to force base to come first anyway?

Where does PATCH 2 exploit "base first"?

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