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Postings from April 2018
On 10 April 2018 at 21:59, Dave Mitchell <davem@iabyn.com> wrote:
> On Tue, Apr 10, 2018 at 02:59:50PM +0200, Christian Walde wrote:
> > If you really look at all 3 points i made, in aggregate, and don't see
> > how this is a problem and a danger, then i don't think i can come up
> > with other word combinations to make you see it.
>
> Can you come up with a hypothetical scenario, e.g. a multi-threaded
> program that uses libraries to connect to a database and retrieve and
> parse some XML data. Then got through it step by step so that I can see
> why using perl and CPAN is dangerous, but using (e.g.) java and a DB and
> XML library is safe? What is the crucial difference between the two that
> flips it from being safe to unsafe?
>
Here's one trivial hacked-together example of code which I would argue
leads to "unexpected" results, at least from the perspective of a C or Java
programmer experienced in the ways of threads:
#!/usr/bin/perl
use strict;
use warnings;
use threads;
use threads::shared;
my %items : shared;
my $lockvar : shared;
my $t = threads->create(sub {
lock $lockvar;
cond_wait($lockvar);
my @data : shared = qw(example content here);
$items{0 + \@data} = \@data;
});
my $seen_item;
while(1) {
do {
lock $lockvar;
cond_broadcast($lockvar);
} until keys %items;
print "Item with address " . $_ . " actually refers to " . (0 +
$items{0 + $_}) . "\n" for keys %items;
last if defined $seen_item and $seen_item ne (values %items)[0];
($seen_item) = values %items;
}
$t->join;
=pod
Sample output (5.26.1):
Item with address 31700632 actually refers to 31360376
Item with address 31700632 actually refers to 30235168
=cut
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