plazm has asked for the wisdom of the Perl Monks concerning the following question:

Hello!

Here comes the code: 

use vars qw($LF);

sub new
{
        my $proto = shift;
        my $class = ref($proto) || $proto;

        open(FH, ">".LOCK_FILE) or die "ERROR: cannot open lock file: 
+".LOCK_FILE.": $!\n";

        my $this = {
                LF => \*FH
        };

        bless($this, $class);

        return $this;
}
....

#
# code_lock()
# Lock a section of code.
#

sub code_lock
{
        my $this = attr shift;

        my $ret = flock $LF, LOCK_EX;
        return scalar $ret;
}

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This doesn't work because $FL appears to be undefined in code_lock(). If I change flock() call to the following: 

flock $this->{'FL'}, LOCK_EX;

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this works just fine! I am really wondering what is the problem here.

Thanks!

Replies are listed 'Best First'.
Re: a problem with a filehandle reference
by Rhandom (Curate) on Apr 19, 2001 at 23:39 UTC
    If you would like to make life more simple, you should use either IO::File or Filehandle or Symbol::gensym, as in the following. 
    use IO::File ();

# assuming LOCK_FILE is a constant with the filename

my $fh = IO::File->new( LOCK_FILE, 'w' );
# now you can pass $fh directly to your sub
# as in code_lock( $fh );
# sub code_lock { my $fh = shift(); }

    [download]
    You could also create the same sort of thing with Filehandle or with the following code. 
    use Symbol();
my $fh = Symbol::gensym();
open($fh,">".LOCK_FILE);
# pass it as in the above

    [download]
[reply]
[d/l]
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      Thanks! This should simplify the things. I am just really wondering why the approach doesn't work with filehandles. Is there something wrong with attr or am I doing stupid things? A.
[reply]
        Without experimenting, you may need to do things like 
        local *FH = \$this->{FL};
flock FH, LOCK_EX;

        [download]
        or 
        flock { $this->{FL} }, LOCK_EX;

        [download]
        You have the same types of problems when trying to print on a handle stored in a hash.
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[d/l]
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Re: a problem with a filehandle reference
by suaveant (Parson) on Apr 19, 2001 at 23:06 UTC
    You never assign anything to $LF (and in your example you keep switching LF and FL)
                    - Ant
[reply]
      Sorry, in the example it should be read as 'LF' and $LF. Actually, I believe I do assign. 
      my $this = attr shift;

      [download]
      should create localized $LF.
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[d/l]
        I am not familiar with attr... and I can't find it in perldoc -f or the camel book... what does it do?
                        - Ant
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