in reply to Re: pack() returns an unusable string
in thread pack() returns an unusable string

I would guess that your packed string '$p' has either a single quote "'" in it or an escape or $ or @ at the end that eats the closing single quote

I should add that there's nothing special about the given NV value of 2.4.
The behaviour I'm seeing is happening for all NV values that I've tested.

To get a dump of $s: 
# test.pl

use warnings;
use Devel::Peek;

$template = 'D<';
$nv = 2.4;
$p = pack $template, $nv;
$s = "'$p'";

Dump $s;

system $^X, '-wle', "print unpack('H*', $s);";

[download]
That outputs the following (which does, at least, contain an "@"): 
C:\_32\pscrpt>perl test.pl
SV = PV(0x4bcf98) at 0x571248
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x4b7e88 "'\232\231\231\231\231\231\231\231\0@\0\0\0\0\0\0'"\0
  CUR = 18
  LEN = 20
Can't find string terminator "'" anywhere before EOF at -e line 1.

[download]
But if I change the value of $nv to 2.4e100, I get: 
C:\_32\pscrpt>perl test.pl
SV = PV(0x3ecf98) at 0x383348
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x3e7e88 "'\vL\177v\257zb\206\264>\0\0\0\0\0\0'"\0
  CUR = 18
  LEN = 20
Can't find string terminator "'" anywhere before EOF at -e line 1.

[download]
Interestingly, changing the value to 3.1e-100 results in a different warning: 
C:\_32\pscrpt>perl test.pl
SV = PV(0x26cf98) at 0x5311f8
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x267e88 "'9\302\271\243\"\211\224\255\264>\0\0\0\0\0\0'"\0
  CUR = 18
  LEN = 20
Use of uninitialized value $_ in print at -e line 1.

[download]
Perhaps something to do with the internal double quote.

For a value of 13.16e-100, I get: 
C:\_32\pscrpt>perl test.pl
SV = PV(0x38cf98) at 0x4c51b8
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x387e88 "'\245^,\356\266\0208\270\266>\0\0\0\0\0\0'"\0
  CUR = 18
  LEN = 20
Can't find string terminator "'" anywhere before EOF at -e line 1.

[download]
Is there some way to pass the string returned by pack() to that system command ?
I assume that it ought to be possible, and that I'm just doing something dumb.

Cheers,
Rob

Replies are listed 'Best First'.
Re^3: pack() returns an unusable string
by tybalt89 (Monsignor) on May 27, 2021 at 01:34 UTC

    And here's a version with Data::Dumper and some config vars that also works. 

    #!/usr/bin/perl -l

use strict; # https://perlmonks.org/?node_id=11133064
use warnings;

my $template = 'd<';
my $nv = 2.4;
$nv = 4135;                     # NOTE errors for this number
my $p = pack $template, $nv;
use Data::Dumper;
$Data::Dumper::Terse = $Data::Dumper::Useqq = 1;
my $s = Dumper $p;
print ">$s<\n";

system $^X, '-wle', "use strict; print unpack('H*', $s);";

    [download]

    Outputs: 

    >"\0\0\0\0\0'\260\@"
<

000000000027b040

    [download]

    note that it adds a newline, but perl doesn't care.

[reply]
[d/l]
[select]
      And here's a version with Data::Dumper and some config vars that also works

      Neither this nor the rendition using Data::Dump works for me on Windows: 
      C:\_32\pscrpt>type test3.pl

use strict; # https://perlmonks.org/?node_id=11133064
use warnings;

my $template = 'd<';
my $nv = 2.4;
$nv = 4135;                     # NOTE errors for this number
my $p = pack $template, $nv;
use Data::Dumper;
$Data::Dumper::Terse = $Data::Dumper::Useqq = 1;
my $s = Dumper $p;
print ">$s<\n";

system $^X, '-wle', "use strict; print unpack('H*', $s);";

C:\_32\pscrpt>perl test3.pl
>"\0\0\0\0\0'\260\@"
<
syntax error at -e line 1, at EOF
Execution of -e aborted due to compilation errors.

C:\_32\pscrpt>

      [download]
      I guess it's something to do with Windows command line quoting rules.
      Mind you, having it work on Windows specifically is not important.
      Now that I have a working example, I can while away some time porting your fix to Windows when I have nothing better to do.

      Thanks tybalt89 !!

      Cheers,
      Rob

      Thanks
[reply]
[d/l]

        Just another guess (to tweak GrandFather) because I don't have a Windows system to test on. 

        #!/usr/bin/perl -l

use strict; # https://perlmonks.org/?node_id=11133064
use warnings;

my $template = 'd<';
my $nv = 2.4;
$nv = 4135;                     # NOTE errors for this number
my $p = pack $template, $nv;
use Data::Dump qw(pp);
my $s = (pp $p) =~ s/"(.*)"/qq($1)/sr; # tweak for different delimiter
print ">$s<\n";

system $^X, '-wle', "use strict; print unpack('H*', $s);";

        [download]
[reply]
[d/l]
Re^3: pack() returns an unusable string
by tybalt89 (Monsignor) on May 27, 2021 at 01:11 UTC

    This seems to work :) 

    #!/usr/bin/perl -l

use strict; # https://perlmonks.org/?node_id=11133064
use warnings;

my $template = 'd<';
my $nv = 2.4;
$nv = 4135;                     # NOTE errors for this number
my $p = pack $template, $nv;
use Data::Dump qw(pp);
my $s = pp $p;
print ">$s<\n";

system $^X, '-wle', "use strict; print unpack('H*', $s);";

    [download]

    Outputs: 

    >"\0\0\0\0\0'\xB0\@"<

000000000027b040

    [download]
[reply]
[d/l]
[select]

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