Re: [PATCH v2 1/5] qemu/qarray.h: introduce QArray

[Christian Schoenebeck]

On Mittwoch, 29. September 2021 19:48:38 CEST Daniel P. Berrangé wrote:
> On Wed, Sep 29, 2021 at 07:32:39PM +0200, Christian Schoenebeck wrote:
> > On Dienstag, 28. September 2021 18:41:17 CEST Daniel P. Berrangé wrote:
> > > On Tue, Sep 28, 2021 at 06:23:23PM +0200, Christian Schoenebeck wrote:
> > > > On Dienstag, 28. September 2021 15:04:36 CEST Daniel P. Berrangé 
wrote:
> > > > > On Sun, Aug 22, 2021 at 03:16:46PM +0200, Christian Schoenebeck 
wrote:
> > [...]
> > > The GLib automatic memory support is explicitly designed to be extendd
> > > with support for application specific types. We already do exactly that
> > > all over QEMU with many calls to G_DEFINE_AUTOPTR_CLEANUP_FUNC(..) to
> > > register functions for free'ing specific types, such that you can
> > > use 'g_autoptr' with them.
> > 
> > Ok, just to make sure that I am not missing something here, because really
> > if there is already something that does the job that I simply haven't
> > seen, then I happily drop this QArray code.
> 
> I don't believe there is anything that currently addresses this well.
> 
> > But AFAICS this G_DEFINE_AUTOPTR_CLEANUP_FUNC() & g_autoptr concept does
> > not have any notion of "size" or "amount", right?
> 
> Correct, all it knows is that there's a data type and an associated
> free function.

Ok, thanks for the clarification.

> > So let's say you already have the following type and cleanup function in
> > your existing code:
> > 
> > typedef struct MyScalar {
> > 
> >     int a;
> >     char *b;
> > 
> > } MyScalar;
> > 
> > void myscalar_free(MayScalar *s) {
> > 
> >     g_free(s->b);
> > 
> > }
> > 
> > Then if you want to use G_DEFINE_AUTOPTR_CLEANUP_FUNC() for an array on
> > that scalar type, then you still would need to *manually* write
> > additionally a separate type and cleanup function like:
> > 
> > typedef struct MyArray {
> > 
> >     MyScalar *s;
> >     int n;
> > 
> > };
> > 
> > void myarray_free(MyArray *a) {
> > 
> >     for (int i = 0; i < a->n; ++i) {
> >     
> >         myscalar_free(a->s[i]);
> >     
> >     }
> >     g_free(a);
> > 
> > }
> > 
> > Plus you have to manually populate that field 'n' after allocation.
> > 
> > Am I wrong?
> 
> Yes and no.  You can of course manually write all these stuff
> as you describe, but since we expect the array wrappers to be
> needed for more than one type it makes more sense to have
> that all done via macros.
> 
> Your patch contains a DECLARE_QARRAY_TYPE and DEFINE_QARRAY_TYPE
> that provide all this reqiured boilerplate code.  The essential
> difference that I'm suggesting is that the array struct type emitted
> by the macro is explicitly visible as a concept to calling code such
> that it is used directly used with g_autoptr.

I got that, but your preferred user pattern was this:

    DECLARE_QARRAY_TYPE(Foo);
         ...
    g_autoptr(FooArray) foos = foo_array_new(n);

I don't see a portable way to do upper-case to lower-case conversion with the 
C preprocessor. So you would end up like this instead:

    g_autoptr(FooArray) foos = Foo_array_new(n);

Which does not really fit into common QEMU naming conventions either, does it?

And I can help it, I don't see what's wrong in exposing a regular C-array to 
user code. I mean in the Linux kernel for instance it is absolutely normal to 
convert from a compound structure to its parent structure. I don't find 
anything magical about that and it is simply less code and better readable.

Best regards,
Christian Schoenebeck