Perl - multithreading / fork / synchronize problem: - multithreading

Question

I'm trying to work as an example for the following code:
my $milon;
my $pid = fork();
die if not defined $pid;
if (not $pid)
{
$milon->{$pid} = $pid;
exit;
}
$milon->{3} = 4;
my $finished = wait();
print( "debug10: TEST = ", Dumper($milon));
output:
debug10: TEST = $VAR1 = {
'3' => 4
};
How do I make the dictionary keep both 3 => 4 and also the $pid => $pid?
It doesn't have to be forking, it could be multithreading or NonBlocking IO, whichever is better according to what you think.
This is an example of course, I just want to conclude from this example to my real code.

Answer 1

You need some memory that is shared between your threads/processes. The easiest is probably to use interpreter-based threads and threads::shared. For instance:
use threads;
use threads::shared;
my %milon :shared;
for (1 .. 2) {
threads->create(sub {
my $tid = threads->tid();
$milon{$tid} = $tid;
});
}
$milon{3} = 4;
$_->join for threads->list; # Wait for all threads to be done
print Dumper \%milon;
This outputs:
$VAR1 = {
'1' => 1,
'2' => 2,
'3' => 4
};

Answer 2

Following sample code demonstrates usage of fork() to compute square of a number from an array #numbers (total 100) in parallel execution.
REAPER function assigned to $SIG{CHLD} signal cleans up completed child processes to avoid zombie processes hanging around in process table.
Investigate if fork() approach will fit your problem/task.
use strict;
use warnings;
use POSIX qw(strftime :sys_wait_h);
use Time::HiRes qw(usleep);
my $limit = 10;
my $threads = $limit;
my #numbers = map { int(rand(100)) } 1..100;
sub REAPER {
local $!;
while( (my $pid = waitpid(-1, WNOHANG) ) > 0 && WIFEXITED($?) ) {
$threads++;
}
$SIG{CHLD} = \&REAPER;
}
$SIG{CHLD} = \&REAPER;
for ( #numbers ) {
while( $threads == 0 or $threads > $limit ) { usleep(1) }
my $pid = fork();
die $! unless defined $pid;
if( $pid ) {
# parent
$threads--;
} else {
# child
my $n = compute_square($_);
printf "Process %6d: %3d => %d\n", $$, $_, $n;
exit 0;
}
}
sub compute_square {
my $num = shift;
return $num*$num;
}