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

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Nov 30, 2023 at 10:07:00AM +0100, Stefano Brivio wrote:
> 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.

Huh.. that's pretty surprising.

> > 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).

Right.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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