Question

So I have a program where i spawn off some children who do some useful tasks. I then spawn off another child who needs to wait for the first children to stop before doing its work. The parent program then continues running and at the end waits for the last forked child to stop.

I’m getting an issue where the child who needs to wait on the others doesn’t.

use strict;
use warnings;
use diagnostics;

my $pid1;
my $child1 = fork();
if ($child1) {
    # parent
    #print "pid is $pid, parent $$\n";
    $pid1 = $child1;
} elsif ($child1 == 0) {
        # child1
        # do something
        sleep 20;
        print "Child1\n";

        exit 0;
} else {
        die "couldnt fork: $!\n";
}

my $pid2;
my $child2 = fork();
if ($child2) {
    # parent
    #print "pid is $pid, parent $$\n";
    $pid2 = $child2;
} elsif ($child2 == 0) {
        # child2
        # wait for child1 to finish
        my $tmp = waitpid($pid1, 0);

        # do something else
        print "Child2\n";

        exit 0;
} else {
        die "couldnt fork: $!\n";
}

# do more stuff

# wait for child2 to finish
my $tmp = waitpid($pid2, 0);

Is there an easy way to do this? Possibly without having to wrap the first child in the second?

Answer 1

The easy way to do this is with Forks::Super.

use Forks::Super;
my $child1 = fork();
if ($child1 != 0) {
    # ... parent code ...
} else {
    # ... child code ...
    exit;
}

my $child2 = fork { 
    depend_on => $child1, 
    on_busy => 'queue',
    sub => sub {
       # ... code to execute in 2nd child ...
    }
};
# ... more code to run in the parent ...
# ... and at the end of the program:
waitall;

In Forks::Super, waitpid is still called in the parent (behind the scenes). But when the first child is finished, Forks::Super will know it is time to start launch the second child process in the background.