Re: [PATCH 2/4] packet: Offset plus length is not always uint32_t, but it's always size_t

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 30 Nov 2023 11:18:48 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Wed, Nov 29, 2023 at 02:46:08PM +0100, Stefano Brivio wrote:
> > According to gcc, PRIu32 matches the type of the argument we're
> > printing here on both 64 and 32-bits architectures. According to
> > Clang, though, that's not the case, as the result of the sum is an
> > unsigned long on 64-bit.
> > 
> > Use the z modifier, given that we're summing uint32_t to size_t, and
> > the result is at most promoted to size_t.  
> 
> Heh, sorry, obviously hadn't read this patch when I commented on this
> spot in the first one.  The problem here is that the final promoted
> type depends on whether size_t is wider than uint32_t or not, which
> can vary with architecture.

...I'm not sure if it's just a matter of warnings, but gcc is perfectly
happy with PRIu32 for uint32_t + size_t on x86_64, so on top of the
architecture, promotion rules also seem to vary between compilers. Or
maybe it just doesn't complain about the possible format truncation.

> That said, I doubt we're likely to support anything with a size_t
> strictly *less* than 32-bits, so %zu is probably safe.

Ah, yes, I took that for granted. Looking into older architectures
where C would commonly be used, it looks like 16 bits of size_t would
only suffice for *selected versions* of PDP-11 (PDP-11/15 and
PDP-11/20, but not PDP-11/45 already, because the addressing space is
larger than 64 KiB).

Indeed there are 8 and 16 bits processors, but there doesn't appear to
be any other modern architecture where 16 bits suffice for addressable
memory (by design).

-- 
Stefano