[Qemu-devel] qmp_device_add() ret_data argument

[Qemu-devel] qmp_device_add() ret_data argument

[Programmingkid]

What does the function qmp_device_add() have a ret_data argument if it isn't used? This function is located in qdev-monitor.c.

Re: [Qemu-devel] qmp_device_add() ret_data argument

[Eric Blake]

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On 09/09/2015 09:08 PM, Programmingkid wrote:
> What does the function qmp_device_add() have a ret_data argument if it isn't used? This function is located in qdev-monitor.c.

[meta-comment - can you teach your mailer to wrap long lines? It's a
pain to reply to something that scrolls off the screen]

Because the function is installed as a command handler callback, and all
command handler callbacks must have the same signature (QDict *, QObject
**, Error **).

Most qapi-fied commands install a generated handler named
qmp_marshal_*() with the requisite signature, that then parses the QDict
* into a nicer breakdown of C parameters to the real handler, then
collect the returned struct back into the QObject for return.  So if you
want to know how to manipulate QDict/QObject directly, look at the
generated qmp-marshal.c for inspiration.

Meanwhile, device_add has not yet been fully qapi-fied.

I'm about to post a series that completes the qapi-fication of
netdev_add, which is somewhat similar to device_add; if you want to look
at that for ideas of what is involved, be my guest (netdev_add had an
incomplete qapi-fication in qapi-schema.json; device_add isn't even
listed there yet so it is even further behind).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] qmp_device_add() ret_data argument

[Markus Armbruster]

Programmingkid <programmingkidx@gmail.com> writes:

> What does the function qmp_device_add() have a ret_data argument if it
> isn't used? This function is located in qdev-monitor.c.

QMP command handlers all implement the same interface:

        void (*cmd_new)(QDict *params, QObject **ret_data, Error **errp);

Handlers of commands returning a value store the value to *ret_data.
Handlers of commands not returning a value (such as device_add) leave
ret_data alone.

We could require them to store NULL, but that would be easy to forget,
and usually catastrophic, so we don't.