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

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 15 Oct 2025 15:46:12 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Wed, Oct 15, 2025 at 11:50:53AM +0800, Yumei Huang wrote:
> > On Wed, Oct 15, 2025 at 7:28 AM David Gibson
> > <david@gibson.dropbear.id.au> wrote:  
> > > On Tue, Oct 14, 2025 at 03:38:34PM +0800, Yumei Huang wrote:  
> > > > Signed-off-by: Yumei Huang <yuhuang@redhat.com>  
> [snip]
> > > > + * @path: Path to the sysctl file
> > > > + * @fallback: Default value if file can't be read
> > > > + *
> > > > + * Return: Parameter value, fallback on failure
> > > > +*/
> > > > +long read_file_long(const char *path, long fallback)
> > > > +{
> > > > +        char buf[32];  
> > >
> > > Rather than just using a semi-arbitrary 32 here, I'd suggest defining
> > > a new constant similar to UINT16_STRLEN.  Except that's trickier for a
> > > type that doesn't have a known fixed width.  Pity the C library
> > > doesn't have constants for these AFAICT.  
> > 
> > I will just define a UINTMAX_STRLEN with (sizeof("2147483647")).  
> 
> That's not quite right.
>  - It should be INTMAX_STRLEN (signed), UINTMAX would be for the
>    unsigned version
>  - That assumes intmax_t is 32-bit which is probably not the case (it
>    will be 64-bit, maybe even 128-bit on modern systems)
>  - For signed cases, it's the minimum (negative) value that gives the
>    longest possible string (for 32-bit, "-2147483648")

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

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.

-- 
Stefano