Anthony Liguori wrote:
> Jamie Lokier wrote:
> >Anthony Liguori wrote:
> >
> >>After checking that we can demarshal, marshal again and compared to
> >>the expected decoded value. This doesn't work so well for floats
> >>because they cannot be accurately represented in decimal but we try
> >>our best.
> >>
> >
> >Good sprintf/scanf/strtod implementations do guarantee that what's
> >printed and then parsed gets back the same floating point value, as
> >long as you have printed sufficient decimal digits.
> >
> >I'm not sure if FLT_DIG/DLB_DIG are the right number of digits,
> >though. Glibc's documentation of those is confusing and they might
> >mean something a little different.
> >
>
> Eh, I played around quite a bit and the results were disappointing.
>
> $ printf "%f\n" 43.32
> 43.320000
> $ printf "%0.32f\n" 43.32
> 43.31999999999999999972244424384371
True enough. I'm not sure how to ask for the minimum number of digits
for unambiguous representation.
Point still stands though: Try *parsing* both those outputs, and the
double result is exactly the same in each case.
$ printf '%0.32f\n' $(printf '%0.32f\n' 43.32)
43.31999999999999999972244424384371
So you can do this:
unambiguous decimal text -> double -> same text
and this:
double -> unambiguous decimal text -> same double
As long as "unambiguous decimal" was printed with enough digits.
-- Jamie
