Re: [PATCH v3 2/4] util: Introduce read_file() and read_file_long() function

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 29 Oct 2025 20:35:08 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Wed, Oct 29, 2025 at 05:43:16AM +0100, Stefano Brivio wrote:
> > On Wed, 29 Oct 2025 11:43:00 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > On Wed, Oct 29, 2025 at 12:12:48AM +0100, Stefano Brivio wrote:  
> [snip]
> > > > By the way, while it doesn't cover intmax_t explicitly, I think this is
> > > > a pretty good resource as it covers most architectures supported by the
> > > > Linux kernel (hence, most architectures we support):
> > > > 
> > > >   https://wiki.debian.org/ArchitectureSpecificsMemo#Summary    
> > > 
> > > Oh, nice, that is a very handy resource.
> > >   
> > > > and judging from intmax_t(3type) I'd say that the sizeof(long double)
> > > > column tells you how big intmax_t is.    
> > >   
> > > > Well, at least, that's the page I use to know which architectures I can
> > > > use to check things when I suspect a type portability bug.
> > > > 
> > > > That's because 'long double' should always be the biggest "native" data
> > > > type, that is, excluding __int128 or vectorised / SIMD types such as
> > > > __m256i.    
> > > 
> > > This part isn't true, alas.  Theoretically speaking there's not
> > > necessarily any relation between the largest native integer type and
> > > the largest native float type.  
> > 
> > Oops, yes, I misread intmax_t(3type), that's *integer* only (of course,
> > the name says it). So probably it has to match sizeof(long long)?
> >   
> > > But more importantly, it's not true in practice: according to the
> > > table sizeof(long double) is 16 for amd64, but sizeof(intmax_t) is 8
> > > empirically.
> > > 
> > > I think sizeof(long long) is more likely to match sizeof(intmax_t),
> > > but I don't love relying on it.  
> > 
> > Right... well, about relying on it, without a change in the C11
> > standard, can it ever differ? I don't think so. We could have a look at
> > C17 / C23 and if long long is still the largest integer type, we know
> > we're fine for quite a few years / pretty much forever.  
> 
> Uh.. maybe?  I'm never clear on what's guaranteed by the C standard
> and what's left to the platform / ABI.  AIUI the reason for intmax_t's
> existence is because an awful lot is not pinned down by the standard.

The standard specifies long long, without a width, and not long long
long, so we know that long long is the longest we can have at the
moment.

> __int128 does appear to be a thing that is longer than long long.

Yes but that's what I meant by "native" type, for lack of a better
name. It looks like they're more commonly called "main" types instead.

__int128 is a GNU extension and specifies a 128-bit width just like
__m256i specifies a 256-bit width. Those can be bigger than intmax_t.

> Maybe there's a rule that doesn't allow intmax_t to be __int128, but
> I'm not sure where we'd find it.

No real rule but BUGS in intmax_t(3type) reports this (even though it
doesn't look like a bug to me).

-- 
Stefano