Re: [PATCH RFC v3 1/5] lib: kstrtox: support E notation in kstrtoudec64() and kstrtodec64()

[Rodrigo Alencar]

On 21/10/26 11:45, Andy Shevchenko wrote:
> On Tue, Oct 20, 2026 at 09:26:59AM -0300, Rodrigo Alencar via B4 Relay wrote:
> > 
> > Allow decimal strings to carry an exponent suffix, such as "2.7e10" or
> > "271828e-5", in kstrtoudec64() and kstrtodec64(). The exponent is added
> > to the requested scale to work out how many integer and fractional digits
> > are consumed. A negative effective exponent drops integer digits. Digits
> > beyond the resulting precision are truncated, as before.
> > 
> > The digits are now scanned first and the string termination is checked
> > before any conversion. A missing or malformed exponent ("1e", "1e+")
> > returns -EINVAL. An exponent that overflows when added to the scale, or
> > that makes the result exceed 64 bits, returns -ERANGE.

...

> >  static int _kstrtoudec64(const char *s, unsigned int scale, u64 *res)
> >  {
> > -	unsigned int rv_int, rv_frac;
> > +	const char *s_int, *s_frac;
> > +	int len_int, len_frac;
> > +	unsigned int rv;
> >  	u64 _res = 0;
> > +	int exp = 0;
> > +	int ret;
> >  
> > -	rv_int = _parse_integer(s, 10, &_res);
> > -	if (rv_int & KSTRTOX_OVERFLOW)
> > -		return -ERANGE;
> > -	s += rv_int;
> > +	s_int = s;
> > +	len_int = _skip_digits(&s);
> >  
> >  	if (*s == '.')
> >  		s++; /* skip decimal point */
> 
> So, I know that '.NNN' is valid notation, but is 'NNN.'?

It looks weird, but that is also valid notation.

> 
> > -	rv_frac = _parse_integer(s, 10, &_res, scale, _res);
> > -	if (rv_frac & KSTRTOX_OVERFLOW)
> > +	s_frac = s;
> > +	len_frac = _skip_digits(&s);
> > +
> > +	if (!len_int && !len_frac)
> > +		return -EINVAL;
> > +
> > +	if (*s == 'e' || *s == 'E') { /* E Notation */
> 
> You may use _tolower() helper (yes, the one with a single leading underscore).

ok, so we only check for _tolower(*s) == 'e'

> > +		s++;
> > +		ret = kstrtoint(s, 10, &exp);
> > +		if (ret)
> > +			return ret;
> 
> So, with the above in mind, this may lead to 'NNN.e10' input which I find a bit
> confusing.

might be, but as mentioned, it is considered valid, e.g.:

- python:

	>>> x = 2.e10
	>>> print(x)
	20000000000.0
	>>> y = .5e-3
	>>> print(y)
	0.0005

- c++

	int main()
	{
	    float x = 2.e3;
	    std::cout<<"x = " << x << std::endl;

	    return 0;
	}

	x = 2000

-- 
Kind regards,

Rodrigo Alencar