Re: [Xen-devel] [PATCH v2 03/20] piix4: Add a i8259 Interrupt Controller as specified in datasheet

[Peter Maydell <peter.maydell@xxxxxxxxxx>]

On Tue, 22 Oct 2019 at 09:52, Esteban Bosse <estebanbosse@xxxxxxxxx> wrote:
>
> El vie, 18-10-2019 a las 15:47 +0200, Philippe Mathieu-Daudé escribió:
> > +static void piix4_request_i8259_irq(void *opaque, int irq, int
> > level)
> > +{
> > +    PIIX4State *s = opaque;
> > +    qemu_set_irq(s->cpu_intr, level);
> > +}
> I would like to understand why in `PIIX4State *s = opaque;` its not
> necessary a cast or a object macro magic.
> Something like:
> PIIX4State *s = (PIIX4State*)opaque;
> PIIX4State *s = PIIX4STATE(opaque);

The simple answer to "why don't we need a cast" is
"because the type of 'opaque' is 'void *', and in C there is
no need to explicitly cast a 'void *' as it will be implicitly
converted to the pointer type of the destination". (This is
different from C++, which does require an explicit cast for void*.)

For QOM types, QEMU conventionally uses the QOM casting
macro to convert a pointer-to-instance to
pointer-to-instance-of-parent-class and vice versa.
In some places, like this one, what we have is just a
void* representing opaque data having been passed around.
You could use the QOM cast macro here, which would add
a bit of extra type-safety, but the project doesn't have
a strong convention here on whether to do so or not, so
you'll often see the just-assignment code.

thanks
-- PMM

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