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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Floating point to string, and back via `%a'...
Date: Wed, 22 Jul 2015 17:08:20 -0700
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Keith Thompson <kst-u@mib.org> writes:
> "Chris M. Thomasson" <nospam@nospam.nospam> writes:
> [...]
>>> It looks like MSVC simply has a bug.  You could calculate the number of
>>> digits you need but that would be fiddly to get right in every single
>>> case.
>>
>> Right. FWIW, %a does not work on MSVC for sscanf always returns zero.
>
> Does MSVC just not implement %a?  Did you try changing to %la (you may
> have already answered that).
>
> Try this test program:
>
> #include <stdio.h>
> int main(void) {
>     char buf[256];
>     double in  = 1.0123456789;
>     double out = 1.0123456789;
>         int result;
>
>         sprintf(buf, "%a", in);
>         result = sscanf(buf, "%la", &out);
>
>         printf("in  = %.10f = %a\n", in, in);
>         printf("buf = \"%s\"\n", buf);
>         printf("sscanf returned %d\n", result);
>         printf("out = %.10f = %a\n", out, out);
>
>         if (in == out) {
>                 puts("in == out");
>         }
>         else {
>                 puts("in != out");
>         }
>         return 0;
> }

Here's a better version.  It doesn't examine the value of `out` unless
sscanf succeeds.

#include <stdio.h>
#include <float.h>
int main(void) {
    char buf[256];
    double in  = 1.0123456789;
    double out = FLT_MAX; /* arbitrary value */
    int result;

    sprintf(buf, "%a", in);
    result = sscanf(buf, "%la", &out);

    printf("in  = %.50f = %a\n", in, in);
    printf("buf = \"%s\"\n", buf);
    printf("sscanf returned %d\n", result);
    if (result == 1) {
        printf("out = %.50f = %a\n", out, out);
        if (in == out) {
            puts("in == out");
        }
        else {
            puts("in != out");
        }
    }
    return 0;
}

> On my system, the output is:
>
> in  = 1.0123456789 = 0x1.0329161f1f56cp+0
> buf = "0x1.0329161f1f56cp+0"
> sscanf returned 1
> out = 1.0123456789 = 0x1.0329161f1f56cp+0
> in == out
>
> What output do you get using MSVC?  If it's significantly different,
> there may well be a bug in MSVC (presumably in its runtime library).

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"