Yes, it should work; but, strangely, it doesn’t:

#! perl
use strict;
use warnings;
use Data::Dump;
use Win32API::File qw(getLogicalDrives GetVolumeInformation);

my @drives = getLogicalDrives();

for my $d (@drives)
{
#   my @x = (undef) x 7;
    my @x;
    $#x = 6;
#   dd \@x;
    GetVolumeInformation($d, @x);
    print "$d $x[0] ($x[5])\n";
}
[download]

Output:

 1:16 >perl 1442_SoPW.pl
Modification of a read-only value attempted at C:/Perl/Strawberry/stra
+wberry-perl-5.22.0.1-64bit-PDL/perl/lib/Win32API/File.pm line 269.

 1:16 >
[download]

But with the line dd \@x; uncommented, it does work:

 1:16 >perl 1442_SoPW.pl
[undef, undef, undef, undef, undef, undef, undef]
C:\  (NTFS)
[undef, undef, undef, undef, undef, undef, undef]
D:\  ()
[undef, undef, undef, undef, undef, undef, undef]
E:\  ()

 1:16 >
[download]

What’s going on?

What’s going on?

The explicit method populates the array with 7 physical pointers to undef:

[0] Perl> @x = (undef)x7;;
[0] Perl> use Devel::Peek;;
[No such file or directory] Perl> Dump \@x;;
SV = RV(0x3c735d0) at 0x3c735c0
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x3c6ac88
  SV = PVAV(0x3cfafd8) at 0x3c6ac88
    REFCNT = 2
    FLAGS = (RMG)
    MAGIC = 0x3cee198
      MG_VIRTUAL = &PL_vtbl_arylen_p
      MG_TYPE = PERL_MAGIC_arylen_p(@)
      MG_FLAGS = 0x02
        REFCOUNTED
      MG_OBJ = 0x3c6a410
      SV = PVMG(0x3b1a078) at 0x3c6a410
        REFCNT = 1
        FLAGS = (GMG,SMG,pIOK)
        IV = 6
        NV = 0
        PV = 0
        MAGIC = 0x3ceea08
          MG_VIRTUAL = &PL_vtbl_arylen
          MG_TYPE = PERL_MAGIC_arylen(#)
          MG_OBJ = 0x3c6ac88
    ARRAY = 0x3cf1398
    FILL = 6
    MAX = 7
    ARYLEN = 0x3c6a410
    FLAGS = (REAL)
    Elt No. 0
    SV = NULL(0x0) at 0x3cb4b98
      REFCNT = 1
      FLAGS = ()
    Elt No. 1
    SV = NULL(0x0) at 0x3c73578
      REFCNT = 1
      FLAGS = ()
    Elt No. 2
    SV = NULL(0x0) at 0x3cb49b8
      REFCNT = 1
      FLAGS = ()
    Elt No. 3
    SV = NULL(0x0) at 0x3cb4af0
      REFCNT = 1
      FLAGS = ()
[download]

Whereas, when you pre-extend the array, no physical pointer are populated; the undef's are implied by the absence of pointers:

[0] Perl> undef @x;;
[0] Perl> $#x = 6;;
[0] Perl> Dump \@x;;
SV = RV(0x3c735d0) at 0x3c735c0
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x3c6ac88
  SV = PVAV(0x3cfafd8) at 0x3c6ac88
    REFCNT = 2
    FLAGS = (RMG)
    MAGIC = 0x3cee198
      MG_VIRTUAL = &PL_vtbl_arylen_p
      MG_TYPE = PERL_MAGIC_arylen_p(@)
      MG_FLAGS = 0x02
        REFCOUNTED
      MG_OBJ = 0x3c6a410
      SV = PVMG(0x3b1a078) at 0x3c6a410
        REFCNT = 1
        FLAGS = (GMG,SMG,pIOK)
        IV = 6
        NV = 0
        PV = 0
        MAGIC = 0x3ceea08
          MG_VIRTUAL = &PL_vtbl_arylen
          MG_TYPE = PERL_MAGIC_arylen(#)
          MG_OBJ = 0x3c6ac88
    ARRAY = 0x3cf1398
    FILL = 6
    MAX = 6
    ARYLEN = 0x3c6a410
    FLAGS = (REAL)
    Elt No. 0
    Elt No. 1
    Elt No. 2
    Elt No. 3
[download]

Perl code would treat the two the as the same, but user-written XS code makes assumptions.

Thanks for checking it.

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)

In the absence of evidence, opinion is indistinguishable from prejudice.

Perl code would treat the two the as the same, but user-written XS code makes assumptions.

When I read this, I felt an urge to find Perl code that behaves differently for these arrays :)

Turns out that what this API function is doing would require explicit undefs even if the function was written in Perl. This is a subroutine that modifies its input parameters. In Perl you might write it like this:

use Data::Dumper;

my @a;
@a = (undef) x 5;
my @b;
$#b = 4;

sub modify {
    $_ = 5 for @_;
}

modify(@a);
print Dumper \@a;
modify(@b);
print Dumper \@b;
[download]

Here the subroutine "modify" behaves similar to GetVolumeInformation as it modifies the scalars in the array passed to it.

So the arrays @a and @b should be identical. However, modify() will replace @a with five times 5, but it cannot write to @b:

$VAR1 = [
          5,
          5,
          5,
          5,
          5
        ];
Modification of a read-only value attempted at script.pl line 12.
[download]

Update: Since this seems to depend on the version: I am using Perl 5.20.2 on Debian.