how to return values from several forks

[Marc Marc]

I'm working on a program that simulates a certain model with several paramters and returns one result value. I want to check how changing each parameter affects the result. What I did before was running the simulation with the base set of paramters to get a baseline and then change one parameter and run it again. Now I found out about fork() and can just run fork() for each set of parameters. This runs a lot faster, so it's preferrable.
I want to return not the absolute result values of each parameter set but the difference to the base. I have learned how to let two processes communicate from http://docstore.mik.ua/orelly/perl/cookbook/ch16_11.htm, but it fails when I try to spawn several forks and get their result back.

in pseudocode:

for parameter = each parameter_set
  fork()
if parent
  base = sim(base_parameters)
  wait for return values
  print return1 - base, return2 - base etc
else
  result = sim(parameter)
  return result

[chorny]

Try using module like Child.

[Marc Marc]

Thanks for the answer chorny, but I need the script to be able to run on other machines without them installing extra modules, so that won't work.

I have however found a solution that works if I know the number of children I need to spawn before runtime. It's far from elegant, but it works ;)

#!/usr/bin/perl -w
# multifork test
 
use IO::Handle;
 
#prepare pipes, one per child
pipe(READER0, WRITER0);
WRITER0->autoflush(1);
pipe(READER1, WRITER1);
WRITER1->autoflush(1);
pipe(READER2, WRITER2);
WRITER2->autoflush(1);
pipe(READER3, WRITER3);
WRITER3->autoflush(1);
 
if ($pid[0] = fork) { # i'm the parent
    close WRITER0;
    if ($pid[1] = fork) { # i'm still the parent
        close WRITER1;
        if($pid[2] = fork) { # i'm still the parent
            close WRITER2;
            if($pid[3] = fork) { # i'm still the parent
                close WRITER3;
            } else { # i'm child 3
                die "cannot fork: $!" unless defined $pid[3];
                close READER3;
                # do childish stuff here
                print WRITER3 "Child Pid $$ is sending this\n";
                close WRITER3;
                exit;
            }
        } else { # i'm child 2
            die "cannot fork: $!" unless defined $pid[2];
            close READER2;
            # do childish stuff here
            print WRITER2 "Child Pid $$ is sending this\n";
            close WRITER2;
            exit;
        }
    } else { # i'm child 1
        die "cannot fork: $!" unless defined $pid[1];
        close READER1;
        # do childish stuff here
        print WRITER1 "Child Pid $$ is sending this\n";
        close WRITER1;
        exit;
    }
} else { # i'm child 0
    die "cannot fork: $!" unless defined $pid[0];
    close READER0;
    # do childish stuff here
    print WRITER0 "Child Pid $$ is sending this\n";
    close WRITER0;
    exit;
}
 
# listen to your children:
chomp($line[0] = <READER0>);
chomp($line[1] = <READER1>);
chomp($line[2] = <READER2>);
chomp($line[3] = <READER3>);
# print out what the children said
print "Parent Pid $$ just read this: $line[0]\n";
print "Parent Pid $$ just read this: $line[1]\n";
print "Parent Pid $$ just read this: $line[2]\n";
print "Parent Pid $$ just read this: $line[3]\n";
#wait for children to finish
for ($n=0;$n<=3;$n++) {
    waitpid($pid[$n], 0);
}