[Bug 28422] scanf family of functions provides only 7 digits of precision for converting doubles and long doubles

[wine-bugs at winehq.org]

http://bugs.winehq.org/show_bug.cgi?id=28422

--- Comment #6 from Alan W. Irwin <irwin at beluga.phys.uvic.ca> 2012-11-02 18:00:51 CDT ---
Piotr Caban said:
> This bug should be fixed, please retest.

This fix is not quite right since it still loses precision compared to the
equivalent Linux library.

The following simple test programme demonstrates the issue:

#include <stdlib.h>
#include <stdio.h>

int
main(void)
{
  long double x;
  char   buffer[1000];
  while(fscanf(stdin, "%s", buffer) >= 1)
  {
    printf("%s is input string\n", buffer);
    if (sscanf(buffer, "%Le", &x) != 1) exit(1);
    printf("%30.22Le is output string\n", x);
  }
  return 0;
}


For the git clone of the master Wine repo (which now has your fix) the result
is:

bash.exe-3.1$ echo 0.1 |./a.exe
0.1 is input string
 1.0000000000000000555112e-001 is output string

The corresponding Linux result is

 wine at raven> echo 0.1 |./a.out
0.1 is input string
  1.0000000000000000000136e-01 is output string

The git Wine result has a relative difference of 6.e-17 between input and
output and the Linux result has relative difference of 1.e-20 between input and
output which implies 3-4 decimal digits of long double precision are lost with
the git Wine version.

I also have a much more extensive (but less easy to interpret) case involving
doubles (as opposed to long doubles) that indicates the git-Wine scanf version
is not quite as numerically precise as the Linux version.

>From these results I wonder if your fix has some double (as opposed to long
double) arithmetic in it for both the double and long double conversions.  Over
the weekend I will look harder at that possibility, but I hope you do as well.

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