Re: [PATCH v12 02/11] lib: kstrtox: add kstrtoudec64() and kstrtodec64()

[David Laight]

On Fri, 15 May 2026 17:05:06 +0100
Rodrigo Alencar <455.rodrigo.alencar@xxxxxxxxx> wrote:

> On 26/05/13 10:41AM, Rodrigo Alencar wrote:
> > On 26/05/10 01:42PM, Rodrigo Alencar via B4 Relay wrote:  
> > > From: Rodrigo Alencar <rodrigo.alencar@xxxxxxxxxx>
> > > 
> > > Add helpers that parses decimal numbers into 64-bit number, i.e., decimal
> > > point numbers with pre-defined scale are parsed into a 64-bit value (fixed
> > > precision). After the decimal point, digits beyond the specified scale
> > > are ignored.  
> > 
> > Hi Andy,
> > 
> > I am starting over here, the other conversation is getting hard to follow.
> > This is my new proposal...  
> 
> +cc David

I just wouldn't do it this way :-)

You end up with more code than you would get if you just converted the digits.

-- David

>  
> > ...
> >   
> > > +static int _kstrtoudec64(const char *s, unsigned int scale, u64 *res)
> > > +{
> > > +	u64 _res = 0, _frac = 0;
> > > +	unsigned int rv;
> > > +
> > > +	if (scale > 19) /* log10(2^64) = 19.26 */
> > > +		return -EINVAL;
> > > +
> > > +	if (*s != '.') {
> > > +		rv = _parse_integer(s, 10, &_res);
> > > +		if (rv & KSTRTOX_OVERFLOW)
> > > +			return -ERANGE;
> > > +		if (rv == 0)
> > > +			return -EINVAL;
> > > +		s += rv;
> > > +	}
> > > +
> > > +	if (*s == '.' && scale) {
> > > +		s++; /* skip decimal point */
> > > +		rv = _parse_integer_limit(s, 10, &_frac, scale);
> > > +		if (rv & KSTRTOX_OVERFLOW)
> > > +			return -ERANGE;
> > > +		if (rv == 0)
> > > +			return -EINVAL;
> > > +		s += rv;
> > > +		if (rv < scale)
> > > +			_frac *= int_pow(10, scale - rv);
> > > +		while (isdigit(*s)) /* truncate */
> > > +			s++;
> > > +	}
> > > +
> > > +	if (*s == '\n')
> > > +		s++;
> > > +	if (*s)
> > > +		return -EINVAL;
> > > +
> > > +	if (check_mul_overflow(_res, int_pow(10, scale), &_res) ||
> > > +	    check_add_overflow(_res, _frac, &_res))
> > > +		return -ERANGE;
> > > +
> > > +	*res = _res;
> > > +	return 0;
> > > +}  
> > 
> > This function now becomes:
> > 
> > 	static int _kstrtoudec64(const char *s, unsigned int scale, u64 *res)
> > 	{
> > 		u64 _res = 0;
> > 		unsigned int rv_int, rv_frac;
> > 
> > 		rv_int = _parse_integer(s, 10, &_res);
> > 		if (rv_int & KSTRTOX_OVERFLOW)
> > 			return -ERANGE;
> > 		s += rv_int;
> > 
> > 		if (*s == '.')
> > 			s++; /* skip decimal point */
> > 
> > 		rv_frac = _parse_integer_limit_init(s, 10, _res, &_res, scale);
> > 		if (rv_frac & KSTRTOX_OVERFLOW)
> > 			return -ERANGE;
> > 		s += rv_frac;
> > 
> > 		if (!rv_int && !rv_frac && !isdigit(*s))
> > 			return -EINVAL; /* no digits at all */
> > 
> > 		while (isdigit(*s)) /* truncate digits */
> > 			s++;
> > 
> > 		if (*s == '\n')
> > 			s++;
> > 		if (*s)
> > 			return -EINVAL;
> > 
> > 		if (_res && (scale > (19 + rv_frac) || /* log10(2^64) = 19.26 */
> > 		    check_mul_overflow(_res, int_pow(10, scale - rv_frac), &_res)))
> > 			return -ERANGE;
> > 
> > 		*res = _res;
> > 		return 0;
> > 	}
> > 
> > The new thing here is _parse_integer_limit_init(), which is a local modified
> > helper that accepts an init value, so _parse_integer_limit() becomes:
> > 
> > 	unsigned int _parse_integer_limit(const char *s, unsigned int base,
> > 					  unsigned long long *p, size_t max_chars)
> > 	{
> > 		return _parse_integer_limit_init(s, base, 0, p, max_chars);
> > 	}
> > 
> > with init = 0:
> > 
> > 	static unsigned int _parse_integer_limit_init(const char *s, unsigned int base,
> > 						      unsigned long long init,
> > 						      unsigned long long *p,
> > 						      size_t max_chars)
> > 	{
> > 		unsigned long long res;
> > 		unsigned int rv;
> > 
> > 		res = init;
> > 		/* ...
> > 		 * the rest is the same implementation as _parse_integer_limit()
> > 		 * ...
> > 		 */
> > 		return rv;
> > 	}
> > 
> > That allows to accumulate the final value into the same variable, which makes
> > things simpler and decreases the amount of overflow checks.
> > 
> > The scale can now be a bigger value, like 0.00000000000000000000000000000000423
> > can be parsed with scale = 35, resulting into 423.
> > 
> > The truncation loop is still there... I think this implementation is better,
> > and I am not sure what is the input limit that you would consider ok to allow
> > non-zero digits to be truncated once the scale can now be something bigger than 19.
> > As long as the output fits into a u64 variable, the parser still works.  
> 
> The truncation loop is at least stricting the input on digits!
> Any comments on that?
> 
> > 
> > I am also adding new test cases for that!  
> 
> I have a v13 ready with this. I'll give it a go soon...
>