Hi,
The demo script (named 'double.pl'): 
use warnings;
use strict;

use Inline C => Config =>
    BUILD_NOISY => 1;

use Inline C => <<'EOC';

SV * foo(SV * nv) {
   double x = (double)SvNV(nv);
   return newSVnv(x);
}

EOC

print perl_foo(2.5), "\n";
print perl_foo(1.1), "\n"; # LINE 17

sub perl_foo {
    my $ret = foo($_[0]);
    if($_[0] - $ret) {warn "Loss of precision"} # LINE 21
    return $ret;
}

[download]
That script does pretty much what I want. On a perl built without long double support it simply prints out: 
2.5
1.1

[download]
On a (linux, mdk-9.1) perl built with long double support it prints out (transcribed, not copy'n'pasted): 
2.5
Loss of precision at double.pl line 21
1.10000000000000009

[download]
The key points to the exercise are:

1) That the warning is emitted for perl_foo(1.1) but not for perl_foo(2.5) - ie I want to see the warning on the -Duselongdouble build of perl only when there's an actual loss of precision;

2) The detection of the loss of precision is being handled by the perl code and not by the C code (over which, in the real life situation, I have no control);

3) The warning should never be seen on a perl that was not built with long double support.

The questions:

1) The warning is reported as coming from line 21. I would prefer to see line 17 reported as the line that generated the warning. How do I go about achieving that ? (I ask that question with a pained expression on my face, for I feel I should know the answer - or be able to find it for myself.)

2) Is simply subtracting the return value from the argument a reliable way of testing whether precision has been lost ? (It seems obvious to me that the answer to that question is "yes" ... but I'm always wary of the validity of answers that seem obvious :-)

3) Is there a better way that a -Duselongdouble build of perl could determine that foo(1.1) would lead to a loss of precision ? (It's only a minor issue, but I would prefer that a -Duselongdouble build of perl could tell in advance that foo(1.1) was going to lead to a loss of precision, rather than having to wait until foo(1.1) had executed.)

Cheers,
Rob

In reply to XS, C doubles and -Duselongdouble by syphilis

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