value uninitilized when using Parallel::ForkManager module

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

my $pwm_count_hash = undef;
PWM:
for my $pwm (@ciona_pwm_array) {
    $pm->start and next PWM;
    ($pwm_count_hash->{$pwm}) = count_pwm_num($pwm,$seq_1,$seq_2);
    #print $pwm,"\t",$pwm_count_hash->{$pwm},"\n";
    $pm->finish;
}

$pm->wait_all_children;

foreach my $id (keys %{$pwm_count_hash}) {
      print $id,"\t",$pwm_count_hash->{$id},"\n";
      # no printing here!
}
[download]

I declared a hash reference ($pwm_count_hash) outside of parallel processing fork.However, when terminating all child process, this value ($pwm_count_hash) was still undefined. The foreach loop after forking processing lead to no result. I expect that this value had been defined after each child process.

I think this may be related to child-parent talk. But I have a little knowledge of it. Please let me know how to initialize $pwm_count_hash in the child process.

Comment on value uninitilized when using Parallel::ForkManager module Download Code

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Re: value uninitilized when using Parallel::ForkManager module by Anonymous Monk on Apr 14, 2015 at 06:51 UTC
when you fork everybody gets their own independent/unrelated/unshared copy of $pwm_count_hash . As each process updates $pwm_count_hash, the others don't see these updates, variables aren't shared like that See Re: Please Explain the Parallel::ForkManager Idiom my $pid = $pm->start and next;, Mr. Peabody Explains fork()	[reply]
Re: value uninitilized when using Parallel::ForkManager module by Laurent_R (Canon) on Apr 14, 2015 at 06:37 UTC
It seems to me that this code line: `($pwm_count_hash->{$pwm}) = count_pwm_num($pwm,$seq_1,$seq_2);` [download] is never executed, because of the `next` instruction of the previous line (unless `$pm->start` fails). Je suis Charlie.	[reply] [d/l] [select]
Re^2: value uninitilized when using Parallel::ForkManager module by afoken (Chancellor) on Apr 14, 2015 at 16:07 UTC
It seems to me that this code line: `$pwm_count_hash->{$pwm}) = count_pwm_num($pwm,$seq_1,$seq_2);` is never executed, because of the next instruction of the previous line (unless $pm->start fails). This is how Parallel::ForkManager works: `$pm->start` hides a `fork()` call and returns true (the child's PID) in the parent process and false in the child process. `$pm->start and next` makes sure that all following code is executed only in the child process, `$pm->finish()` at the end of the loop hides `exit()` and makes sure that the loop is executed only once in the child process. (Actually, Parallel::ForkManager does a little bit more. See the documentation.) Alexander -- Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)	[reply] [d/l] [select]
Re^3: value uninitilized when using Parallel::ForkManager module by Laurent_R (Canon) on Apr 14, 2015 at 17:45 UTC
Thank you very much for the information, afoken, I did not know, because I never used Parallel::ForkManager, but I should probably have guessed something like that, since I wrote forking programs, albeit quite sometime ago and not in Perl. Je suis Charlie.	[reply]
Re: value uninitilized when using Parallel::ForkManager module by atcroft (Abbot) on Apr 14, 2015 at 18:49 UTC
Parallel::ForkManager has a mechanism for sending data back to the parent from the child processes as of version 0.7.6, which sounds like what you are really wanting to do. (There is a section of the documentation on it, as well as an example. Adapting the example was not, in my experience, a difficult exercise.) Hope that helps.	[reply]