I am interested in "focused crawling" (crawling web pages of some specific topic and ignoring all the others) and have written a "focused crawler" recently. Perl is a reasonable alternative to writing web crawler for its LWP module and CPAN. However, when I planed to implement multithread strategy in crawling, I was confused by perl 5.8.4's "threads" module, especially threads::shared. How is a object reference shared by multiple threads? I want to utilize "Cache::File::Heap" module to sort the urls in "crawling frontier" by heuristic prediction of its "harvest outcome". Below is the relevant code part: 
#!/usr/bin/perl -w
use strict;
use threads;
use threads::shared;
use Cache::File::Heap;

my $heap = Cache::File::Heap->new('frontier');
my $heap_lock : shared = 0; 

...

sub go {#crawling thread's control flow
       ....
       #xtracted best promising url
       {
           lock $heap_lock;
           my($value, $url) = $heap->extract_minimum;
       }

       ...
       #after downloading and extract hyperlinks
       {
           lock $heap_lock;
           $heap->add($value, $url) for 
        }
        ...
}

my @threads;
for(1..10) {
    push @threads, threads->new(\&go);
}
for(@threads) {
    $_->join;
}

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All is fine, just untill all the threads joined by main thread and main thread exists. Following error message appears: Scalar leaks : -1 Segmentation fault. My question is : How to share object reference (such as Cache::File::Heap) ?? Cache::File::Heap is the wrapper of BerkeleyDB's BTREE, is BerkeleyDB thread-safe?

In reply to Multithread Web Crawler by xuqy

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