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From: Eric Blake <eblake@redhat.com>
Message-ID: <858d67db-409f-c23a-3f3c-2812ab1cc1b5@redhat.com>
Date: Wed, 21 Mar 2018 12:38:16 -0500
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Subject: Re: [Qemu-devel] [PATCH 2/3] qobject: assume base of a qobject is
 at offset 0
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To: =?UTF-8?Q?Marc-Andr=c3=a9_Lureau?= <marcandre.lureau@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, "Armbruster, Markus" <armbru@redhat.com>

On 03/21/2018 12:23 PM, Marc-Andr=C3=A9 Lureau wrote:

>>>> struct QObjectCommon {
>>>>       QType type;
>>>>       size_t refcnt;
>>>> }
>>>>
>>>> struct QObject {
>>>>       QObjectCommon base;
>>>> }
>>>>
>>>> struct QString {
>>>>       QObjectCommon base;
>>>>       ...
>>>> }

> What is the QOJECT() macro you proposed with that?

Here's what you can do while still leaving all "subtypes" of QObject=20
free to put base at a non-zero offset:

#define QOBJECT(o) ((QObject *)(&(o)->base))

QEMU_BUILD_BUG_MSG(offsetof(QObject, base),
     "base of QObject must be at offset 0");

#define qobject_to(type, obj) ({ \
     QObjectCommon *_tmp =3D qobject_check_type(obj, glue(QTYPE_CAST_TO_,=
=20
type)); \
     _tmp ? container_of(_tmp, type, base) : (type *)NULL; })


static inline QObjectCommon *qobject_check_type(const QObject *obj,=20
QType type)
{
     if (obj && qobject_type(obj) =3D=3D type) {
         return QOBJECT(obj);
     } else {
         return NULL;
     }
}


Or, you keep your idea of requiring ALL subtypes to put base at offset=20
0, so that qobject_to() also becomes simpler.


--=20
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org