Problem with ForkManager

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

Hello,

I am using Parallel ForkManager and I would like to use a more complex callback to run on finish, so I would like to define it before hand to make my code more readable. When I do this however I get the following error:

Can't use string ("1") as a subroutine ref while "strict refs" in use 
+at $HOME/local/perl/lib/perl5/site_perl/5.12.3/Parallel/ForkManager.p
+m line 561 (#1)

(F) Only hard references are allowed by "strict refs".  Symbolic refer
+ences are disallowed.  See perlref.
[download]

I don't understand what's going on. Can somebody explain? Below is a minimal example to reproduce the error:

use strict;
use warnings;
use diagnostics;
use Parallel::ForkManager;

my $pm = new Parallel::ForkManager(2);

# This works
pm->run_on_finish( sub { print "Hello!\n"; } );

# This doesn't
#sub hello { print "Hello!\n"; }
#$pm->run_on_finish( &hello );

foreach my $child ( 0 .. 4 ) {
    my $pid = $pm->start and next;
    
    $pm->finish;
}

$pm->wait_all_children;
[download]

Comment on Problem with ForkManager Select or Download Code

Replies are listed 'Best First'.
Re: Problem with ForkManager by BrowserUk (Patriarch) on May 10, 2011 at 18:11 UTC
This: `&hello` calls The subroutine named `hello`. You need `\&hello` to obtain the address. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l] [select]
Re: Problem with ForkManager by chrestomanci (Priest) on May 10, 2011 at 18:38 UTC
You need to pass in a function reference to run_on_finish(). You could write: `my $hello_ref = sub { print "Hello!\n"; }; $pm->run_on_finish($hello_ref);` [download] or: `sub hello { print "Hello!\n"; } $pm->run_on_finish(\&hello);` [download]	[reply] [d/l] [select]
Re^2: Problem with ForkManager by Anonymous Monk on May 10, 2011 at 19:36 UTC
Wonderful, thanks. I would never have gotten it.	[reply]
Re: Problem with ForkManager by locked_user sundialsvc4 (Abbot) on May 11, 2011 at 01:55 UTC
Just as an aside ... you will from time to time encounter some code (e.g. in CPAN...) which is designed to allow you to designate the subroutine to be called using either a string or a code-reference. The code in question is designed so that, if given a string, it treats that string as a method name, and it tries to call that method against some object that has also been provided. As usual, “TMTOWTDI.™”