in reply to Re^2: Perl 6: Static/Dynamic Strong/Weak Type Systems
in thread Perl 6: Static/Dynamic Strong/Weak Type Systems

I've never seen p and P in pack before and I can't figure out what they actually do (as in I have read the docs, tried some code but still don't understand). I thought 
#! /usr/bin/perl -l

$a="fergal";
$p=\$a+0;
print $p;
$x=unpack("P9", pack 'j', $p);
print length($x);

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might read the first 9 bytes of the SV structure for $a but it just has length 0. Do you have a demo?

Anyway, this is not really what I'm getting at. You can't use this to try treat the memory for a perl array variable as a perl string variable, at most this allows you to treat the memory for a perl array variable as the C string inside a perl string variable.

Imagine you could twiddle the pointer inside a perl reference value, it still wouldn't be weakly typed. If you point the ref at a bit of memory that contains a perl string variable then it will become a string-ref if you point it at an array variable it will become an array-ref (I think this is the case although I'm not an XS whizz and I don't have time to test it as I'm going on holidays in an hour!). So for perl to be weakly typed I would have to wrong and the language would have to have this reference twiddling feature built in.

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Re^4: Perl 6: Static/Dynamic Strong/Weak Type Systems
by rcaputo (Chaplain) on Apr 16, 2006 at 18:26 UTC

    Demo? I've had this code lying around for ages:

    #!/usr/bin/perl -wl

use strict;

my $target = 'IHBT';

print "target contains       : $target";

my $target_address = \$target + 0; # nummify
my $native_version    = pack 'L', $target_address;
my $pointer = pack 'P', $native_version;

print "target address        : ", sprintf('%x', $target_address);

my $sv = unpack('P4', $native_version);
print "deref'd packed (sv)   : ", sprintf('%x', unpack('L', $sv));

my $pv = unpack('P4', $sv);
print "deref'd sv (pv)       : ", sprintf('%x', unpack('L', $pv));

my $value = unpack('P' . length($target), $pv);
print "deref'd pv (value)    : ``$value''\n";

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    I hope it helps. :)

    --
    Rocco Caputo - http://poe.perl.org/

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Re^4: Perl 6: Static/Dynamic Strong/Weak Type Systems
by diotalevi (Canon) on Apr 17, 2006 at 04:51 UTC

    You're just missing a few pointer dereferences. The pack("j",...) does the int->ptr conversion. The first unpack does SvANY( $p ) to find $a. The next one doesSvANY( $a ) to find its xpv struct. The next and final unpack is dereffing the xpv_pv pointer to read the C string.

    $x = unpack "P9", unpack "P4", unpack "P4", pack "j", $p;

    This is all *reading* addressable memory. Writing back to it is a completely different trick involving faking structs for B to convince perl that strings that you control are real perl structures. You can then fake up a perl string where the pointer goes to any arbitrary location.

    ⠤⠤ ⠙⠊⠕⠞⠁⠇⠑⠧⠊

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