Re: [musl] Do you recommend using fmt_fp() and

[ardi <ardillasdelmonte@gmail.com>]

On Fri, Aug 19, 2022 at 5:19 AM Rich Felker <dalias@libc.org> wrote:
>
> On Thu, Aug 18, 2022 at 05:51:26PM +0200, ardi wrote:
> > Hi,
> >
> > I'm looking for a small and robust dtoa-like implementation for quad
> > floats (IEEE binary128)  [...]
> >
> I think the fmt_fp code is a very good choice for this. It's basically
> the minimal, dependency-free, most straightforward way of doing
> exact/correctly-rounded binary floating point to decimal conversion,
> and it doess not depend in any way on the format of the long double
> type, just knowing the parameters (MANT_DIG, MAX_EXP), and being able
> to do a very minimal amount of math on the floating point type to
> extract the mantissa and to determine rounding behavior to match the
> floating point type's.
>
> If you don't have the equivalent of frexpl you can even do that part
> with portable arithmetic on the floating point type. At one point long
> ago I think I even had a version of the code that did that, but it
> doesn't seem to have ever been in musl proper. It probably predated
> musl.
>
> The same should apply to the floatscan.c code if you need the other
> direction conversion too. It's just a direct dependency-free version
> of the minimal bignum logic needed to do it.

Thanks a lot. I'm working on adapting it at the moment!

One question, though, because I don't know the musl internals, and
I'm defining a custom FILE struct that has only the fields used by
the shgetc.c source file: Can the __uflow() invocation at line 23
of shgetc.c be called for string pseudo-FILEs, or is it guaranteed that
it will be called for real FILE objects only?

I ask the question because __uflow() drives into the stdio internals,
and I'd prefer to avoid that (I'm implementing the fp<>string code
for strings only).

Kind regards and thanks a lot!

César