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Message-ID: <20220823173020.GJ7074@brightrain.aerifal.cx> Date: Tue, 23 Aug 2022 13:30:20 -0400 From: Rich Felker <dalias@...c.org> To: ardi <ardillasdelmonte@...il.com> Cc: musl@...ts.openwall.com Subject: Re: Do you recommend using fmt_fp() and On Tue, Aug 23, 2022 at 07:00:41PM +0200, ardi wrote: > On Fri, Aug 19, 2022 at 5:19 AM Rich Felker <dalias@...c.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! See how strtod.c uses sh_fromstring and how sh_fromstring does not define any io callbacks (so that calling __uflow would necessarily result in a call to a null or ununitialized function pointer). From that, it's clear that it must not be reachable (or the existing code in musl would be broken). Rich
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