Re: [Qemu-devel] [PATCH for-4.1? 2/2] vmstate.h: Type check VMSTATE_STRUCT_VARRAY macros

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Maydell (peter.maydell@linaro.org) wrote:
> On Thu, 25 Jul 2019 at 19:00, Dr. David Alan Gilbert
> <dgilbert@redhat.com> wrote:
> >
> > * Peter Maydell (peter.maydell@linaro.org) wrote:
> > > On Thu, 25 Jul 2019 at 18:27, Dr. David Alan Gilbert
> > > <dgilbert@redhat.com> wrote:
> > > >
> > > > * Peter Maydell (peter.maydell@linaro.org) wrote:
> > > > >  #define type_check_2darray(t1,t2,n,m) ((t1(*)[n][m])0 - (t2*)0)
> > > > > +/* Check that t2 is an array of t1 of size n */
> > > > >  #define type_check_array(t1,t2,n) ((t1(*)[n])0 - (t2*)0)
> > > >
> > > > I'd have to admit I don't understand why that does what you say;
> > > > I'd expected something to index a t2 pointer with [n].
> > >
> > > Note that this is just a comment describing what the existing
> > > macro does, as a way to distinguish its job from that of the
> > > new macro I'm adding.
> > >
> > > What happens here is that t2 is a type like "foo [32]", ie
> > > it is an array type already. t1 is the base 'foo' type; so the macro
> > > is checking that t1[n] matches t2, where n is passed in to us
> > > and must match the declared array size of the field (32 in
> > > my example). (In C the size of the array is carried around as
> > > part of its type, and must match on both sides of the expression;
> > > so if you pass in the name of an array field that's the wrong size the
> > > type check will fail, which is what we want.)
> >
> > Ah, OK that makes sense; what it really needs is that example to make
> > me realise that t2 was already the array.
> 
> Would
> 
> /*
>  * Check that type t2 is an array of type t1 of size n,
>  * eg if t1 is 'foo' and n is 32 then t2 must be 'foo[32]'
>  */
> 
> be clearer ?

Yep.

Dave

> thanks
> -- PMM
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK