Re: [PATCH v5 2/7] lib: kstrtox: round to nearest, ties to even in kstrto[u]dec64()

[Andy Shevchenko]

On Wed, Nov 04, 2026 at 06:04:38AM -0300, Rodrigo Alencar wrote:
> On 03/11/26 19:55, Andy Shevchenko wrote:
> > On Mon, Nov 02, 2026 at 07:41:00AM -0300, Rodrigo Alencar via B4 Relay wrote:
> > 
> > > kstrtoudec64() and kstrtodec64() drop the digits beyond the requested
> > > scale, which truncates the result toward zero. The error is up to one
> > > unit in the last place and is always in the same direction.
> > > 
> > > This is easy to hit with values produced by userspace floating point
> > > formatting. Printing 0.3 with "%.17g" gives "0.29999999999999999",
> > > which is parsed as 299999 with a scale of 6 instead of 300000. With E
> > > notation a negative exponent can also drop integer digits, so "19e-1"
> > > with a scale of 0 gives 1 instead of 2.
> > 
> > To avoid unneeded brain work it would be nice to add an example for negative
> > input, like -19e-01. And I hope (and it seems the case) we have test cases
> > for that.
> 
> We do have the tests:
> 
> 	/* E notation: negative exponent rounds integer digits */
> 	{"-27e-4",		3,	-3LL},
> 	{"-125e-1",		0,	-12LL},
> 	{"-135e-1",		0,	-14LL},

Thanks! What I meant is to elaborate in the commit message, that the reader can
easily understand how it works for either of the sign of a value.

> > > Round the result to nearest instead, with ties to even. This is the
> > > IEEE 754 default rounding mode, as used by strtod() and printf(), and it
> > > does not bias results in either direction. The first discarded digit is
> > > known from where parsing stopped, so no extra pass over the string is
> > > needed: the remaining digits are only checked when the first discarded
> > > digit is a 5. Since round-half-even is symmetric, kstrtodec64() keeps
> > > rounding the magnitude and negates it.
> > > 
> > > Inputs that fit within the scale are parsed exactly as before. An input
> > > that rounds up past the type limit, such as "18446744073.7095516155"
> > > with a scale of 9, now returns -ERANGE.
> > > 
> > > Update the existing truncation test cases and add cases for ties and
> > > for rounding at the type limits.

-- 
With Best Regards,
Andy Shevchenko