Greetings!

I have a need to control the rounding direction being used for floating point calculations. In C I would just call fegetround() and fesetround(). I have used Inline::C with some success, but I wonder if there isn't a better way to accomplish the same thing.

Is there some magic flag or module that will let me manipulate the rounding mode? I think my options are:

  1. Keep using Inline::C
  2. Write a new module in C to expose the functions defined in fenv.h
  3. ...

I've included a small program below that shows my current solution. This works with Strawberry Perl although I have yet to test it anywhere else so YMMV.

Sample Code

#!/usr/bin/perl
use warnings;
use strict;

use Inline C => <<'END_C';

#include <fenv.h>

int _getround()
{
    return fegetround();
}

int _setround(int mode)
{
    return fesetround(mode);
}

int _round_down()
{
    return fesetround(FE_DOWNWARD);
}

END_C

## simulate my object
my $one   = 1.0;
my $three = 3.0;

## save the rounding mode and set to round towards negative infinity
my $mode = _getround();
my $changed = _round_down();

## perform the calculations
my $lb = $one / $three;
my $ub = -( -$one / $three );

## restore the original rounding mode to be nice
_setround($mode);

## display the results
printf( "\$lb = %.23f\n", $lb );
printf( "\$ub = %.23f\n", $ub );
print "\$lb == \$ub ", $lb == $ub ? 'true' : 'false', "\n";
print "\$changed = $changed\n";

[download]

 

Sample Output

P:\>fround
$mode = 0
$lb = 0.33333333333333331000000
$ub = 0.33333333333333337000000
$lb == $ub false
$changed = 0

[download]

Owl looked at him, and wondered whether to push him off the tree; but, feeling that he could always do it afterwards, he tried once more to find out what they were talking about.

In reply to Is there a better way to call fe(get|set)round? by HollyKing

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