How can I asynchronously monitor a file in Perl?

I am wondering if it is possible, and if so how, one could create a perl script that constantly monitors a file/d开发者_JAVA技巧b, and then call a subroutine to perform text processing if the file is changed. I'm pretty sure this would be possible using sockets, but this needs to be used for a webchat application on a site running on a shared host, and I'm not so sure sockets would be allowed on it.

The basic idea is:

• create a listener for a chat file/database
• when the file is updated with a new message, call a subroutine
• the called subroutine will send the new message back to the browser to be displayed

Thanks in advance.

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Many operating systems run a service that allows applications to register a request to be notified when a file or path has been updated. This is generally called a File Alteration Monitor. See the linked wikipedia page for some systems available. Recent linux systems use Inotify, previously Dnotify or gamin were used. OS X uses FSEvents. Windows has a similar system. I don't know of any module or mechanism that works cross platform for all these systems, but there are specific modules available on CPAN, such as SGI::FAM and File::Tail::FAM.

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I'd do this with a cron job and a Makefile that invoked a Perl script. The handy thing is that you get the Perl script's atime automatically as the timestamp to compare against, since the script's atime is updated when it is invoked.

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use POE qw(Wheel::FollowTail);
POE::Session->create(
    inline_states => {
      _start => sub {
        $_[HEAP]{tailor} = POE::Wheel::FollowTail->new(
          Filename => "/var/log/thttpd.log",
          InputEvent => "got_log_line",
          ResetEvent => "got_log_rollover",
        );
      },
      got_log_line => sub {
        #print "Log: $_[ARG0]\n";
        parseline($_[ARG0]);
      },
      got_log_rollover => sub {
        #print "Log rolled over.\n";
      },
    }
  );

POE::Kernel->run();
exit;

#parseline()...etc.