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Re: [Qemu-devel] [PATCH 07/18] qom: add link properties

From: Avi Kivity
Subject: Re: [Qemu-devel] [PATCH 07/18] qom: add link properties
Date: Thu, 01 Dec 2011 15:50:58 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:8.0) Gecko/20111115 Thunderbird/8.0

On 12/01/2011 03:47 PM, Anthony Liguori wrote:
> On 12/01/2011 06:34 AM, Avi Kivity wrote:
>> On 12/01/2011 01:35 PM, Stefan Hajnoczi wrote:
>>>>> +static void qdev_get_link_property(DeviceState *dev, Visitor *v,
>>>>> void *opaque,
>>>>> +                                   const char *name, Error **errp)
>>>>> +{
>>>>> +    DeviceState **child = opaque;
>>>>> +    gchar *path;
>>>>> +
>>>>> +    if (*child) {
>>>>> +        path = qdev_get_canonical_path(*child);
>>>>> +        visit_type_str(v,&path, name, errp);
>>>>> +        g_free(path);
>>>>> +    } else {
>>>>> +        path = (gchar *)"";
>>>> If gchar != char, this is wrong.  Also, you're converting a const
>>>> pointer into a non-const pointer, discarding type safety.
>>> This looked weird to me too but the cast has to do with the fact that
>>> the visitor function works both for input and output visitors.  The
>>> output visitor needs to write to gchar** while the input visitor does
>>> not.
>> So you need to pass a non-const pointer to an array of const char, or
>> const gchar **.  You don't modify the string in place, you allocate a
>> new string and free the old one.
>>> When this function is called with the correct visitor type we are
>>> guaranteed that path will not be modified.
>> What if it's called with the output visitor? (warning: confusing
>> convention).
> The reason there's a single Visitor type that works for both input and
> output visitors is to make it so you can write a single visit function
> that works for input and output.  This works very well for most cases
> (in fact, QAPI makes heavy use of it).
> That said, I'm starting to feel like there should be a separate input
> and output visitor interface.  It would make a number of things (like
> visiting lists) significantly simpler.

Perhaps.  But that's independent of the issue here.  No matter how many
visitors you have, you never change a gchar in a string in place, you
always allocate a new string (or you can't change the string length). 
So the type needs to be const gchar **, not gchar **.

error compiling committee.c: too many arguments to function

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