On Wed, Mar 08, 2023 at 11:26:04AM -0500, Michael S. Tsirkin wrote:
> On Wed, Mar 08, 2023 at 10:59:57AM -0500, Feng Liu wrote:
> >
> >
> > On 2023-03-08 a.m.9:13, Michael S. Tsirkin wrote:
> > > External email: Use caution opening links or attachments
> > >
> > >
> > > On Tue, Mar 07, 2023 at 09:17:55PM +0000, Feng Liu wrote:
> > > > On 2023-03-07 04:14, David Edmondson wrote:
> > > > > External email: Use caution opening links or attachments
> > > > >
> > > > >
> > > > > Feng Liu via Virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>
> > > > > writes:
> > > > >
> > > > > > Add const to make the read-only pointer parameters clear, similar to
> > > > > > many existing functions.
> > > > >
> > > > > In many of the modified functions the local variable that is a cast of
> > > > > the argument could also be const. Is there a reason not to do both at
> > > > > the same time?
> > > > >
> > > >
> > > > Hi,David
> > > >
> > > > In order to prevent the content of a pointer parameter from being
> > > > modified and increase the readability of the function, it is recommended
> > > > to add the 'const' keyword to the parameter. This is not necessary for
> > > > local variables and non-pointer parameters, as they are only stored on
> > > > the stack and do not affect the original value or structure member
> > > > passed into the function. Therefore, in this case, the 'const' keyword
> > > > is only added to pointer parameters.
> > >
> > > This makes no sense to me. If ytou cast away the const then it is
> > > pointless.
> > >
> >
> > Hi, Michael
> >
> > I really don't quite understand your point of view.
> > Is a local variable that needs to be add const? Can you help to point
> > out the specific problem/point ?
>
> I just repeated what David said. Basically most of these functions use
> to_vvq which uses container_of which in turn loses const qualifier.
> So your change is poinless since rest of code accesses vq through
> to_vvq.
>
> What to do? I don't like the idea of to_vvq_const.
> So I propose a version of container_of using _Generic
> which preserves the const qualifier.
>
>
> #define container_of(ptr, type, member) \
> ({ \
> const void *__mptr = (ptr); \
> static_assert(__same_type(*(ptr), ((type *)0)->member) || \
> __same_type(*(ptr), void), \
> "pointer type mismatch in container_of()"); \
> _Generic((ptr), \
> typeof(&((const type *)0)->member): \
> (const type *)(__mptr - offsetof(type, member)), \
> default: \
> (type *)(__mptr - offsetof(type, member))); \
> })
>
>
> I'll hack it up in a day or two and post.
>
Oh wait a second. There's already container_of_const.
So just use it in to_vvq.
--
MST
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