Question

I would like to maintain a pidlist array of children I have forked and then remove them as they exit (to restrict how many forked processes i have at any given time). I thought I might be clever and do this by using @main::pid_list in either a delete or splice, but no joy. I can successfully pop an element, but obviously it is not going to remove the correct pid. Any ideas how to handle this or would I be better doing this some entirely different way?

#!/usr/bin/perl -w
use POSIX ":sys_wait_h";
use Data::Dumper;

# Only allow 5 processes running at a time

sub REAPER {
    my $child = shift;
    while (($child = waitpid(-1, WNOHANG)) > 0) {
        # Need to remove child from pidlist here
        #pop(@main::pid_list);                     #This works
        #delete($main::pid_list[$child]);          #This does not

    }
    $SIG{CHLD} = \&REAPER;
}

@pid_list = ();
@files = (1 .. 20);

foreach my $file (@files) {
    my $processed = 'false';
    while ($processed eq 'false') {

        print "Working on file: $file\n";
        $SIG{CHLD} = \&REAPER;
        if (scalar(@pid_list) < 5) {
            $pid = fork();
            if ( $pid == 0 ) {
                print "$$: Child Processing file #" . $file . "\n";
                sleep(10);
                print "$$: Child done processing file #" . $file . "\n";
                exit(0);
            }
            push(@pid_list, $pid);
            print Dumper(@pid_list);
            $processed = 'true';
        }
        sleep(1);
    }
}

# Since we are at the end we need to wait for the last process to end
print "PID: $$ End of parent program\n";

exit 0;

Answer 1

Use a hash table instead of an array.

sub REAPER {
    my $child = shift;
    while (($child = waitpid(-1, WNOHANG)) > 0) {
        # Need to remove child from pidlist here
        delete $main::pid_list{$child};
    }
    $SIG{CHLD} = \&REAPER;
}

...

if ((scalar keys %main::pid_list) < 5) {
    ...
    if ($pid != 0) {
        ...
       exit(0);
    }
    $main::pid_list{$pid}++;
}