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

[Avi Kivity]

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