Re: Code Loop?

[hippo]

The code can be shortened to simply:

use 5.010;
say $#@x[1];

Since this compiles even with strict you know that you've done something odd. Corion's explanation is spot on.

[LanX]

It would be nice to have a strict feature to catch such special variable "zombies", but I'm not sure how to implement it.

Point is that $@ is a special global package variable, with the glob *main::@ , hence this glob has slots for an array, hash, function,... without being declared.

like

D:\Users\lanx>perl -E "use strict; say @@"


D:\Users\lanx>perl -E "use strict; say *@"
*main::@

IMHO in at least 95% of the cases these slots are used accidentally.

Some use them intentionally, but could activate a pragma in these cases. °

°) dunno many obfuscators who use strict anyway ;-)

[Corion]

You could poison them with a tie to catch uses at runtime:

package Tie::Poison;
sub TIESCALAR {
...
}

sub TIEARRAY {
...
}

sub FETCH {
    my $var = tied $_[0];
    die "Prohibited variable '$$var' used";
}

package unusual_variable_names;
our @unusual = qw(
    @@
    @$
    ...    
);

sub unimport {
    for my $name (@unusual) {
        no strict 'refs';
        tie @{ $name }, 'Tie::Poison', $name;
    };
};

...

package main;
no unusual_variable_names;

[LanX]

yep but as you said it's only runtime.

But it should be feasible to write a an op-code walker catching them at compile time in the UNITCHECK phase.

D:\>perl -MO=Terse -E "use strict; say @@"
LISTOP (0x2820630) leave [1]
    OP (0x2820600) enter
    COP (0x2820670) nextstate
    LISTOP (0x2820700) say
        OP (0x28206d0) pushmark
        UNOP (0x2820740) rv2av [2]                      <-- array
            PADOP (0x2820778) gv  GV (0x1dede0) *@      <-- of glob *@
-e syntax OK

D:\>perl -MO=Concise -E "use strict; say @@"
7  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 48 -e:1) v:%,*,&,{,x*,x&,x$,$,469764096 ->3
6     <@> say vK ->7
3        <0> pushmark s ->4
5        <1> rv2av[t2] lK/3 ->6                         <-- array
4           <#> gv[*@] s ->5                            <-- of glob *@
-e syntax OK

[bitman]

Ahhh, so it's $#@ (as Corion says) times the (random) address of an anonymous array containing the 1?

[LanX]

> times the (random) address of an anonymous array containing the 1?

yes since x is imposing numeric context on it's RHS

 
D:\>perl -E "say [1]; say 0+[1]"
ARRAY(0x24e6d8)
2418704