Re: [Qemu-devel] [PATCH 2/3] qobject: assume base of a qobject is at offset 0

[Eric Blake]

On 03/21/2018 08:40 AM, Marc-André Lureau wrote:
> Simplify casting to QObject by not requiring the definition of the
> type and assuming base QObject is at offset 0. Add a static check for
> this.
>
> Use the QEMU_GENERIC macro for keeping type safety check and error
> when given an unknown type.
>
> Signed-off-by: Marc-André Lureau <address@hidden>
> ---
>   include/qapi/qmp/qobject.h | 36 ++++++++++++++++++++++++++++++++++--
>   qobject/qobject.c          |  9 +++++++++
>   2 files changed, 43 insertions(+), 2 deletions(-)

> +++ b/qobject/qobject.c
> @@ -16,6 +16,15 @@
>   #include "qapi/qmp/qlist.h"
>   #include "qapi/qmp/qstring.h"
>   
> +QEMU_BUILD_BUG_MSG(
> +    offsetof(QNull, base) != 0 ||
> +    offsetof(QNum, base) != 0 ||
> +    offsetof(QString, base) != 0 ||
> +    offsetof(QDict, base) != 0 ||
> +    offsetof(QList, base) != 0 ||
> +    offsetof(QBool, base) != 0,
> +    "base qobject must be at offset 0");

Interesting.  If we do this, we should also simplify qobject_to() from 
its current:

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

to the simpler:

#define qobject_to(type, obj) \
    ((type *)qobject_check_type(obj, glue(QTYPE_CAST_TO_, type)))


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