Re: controlling child processes

[johngg]

I have a script that does tape backups. It forks a child process for each tape device on the host so that multiple tape devices are written concurrently rather than consecutively. This is the skeleton of the process, which uses pipes so that the children can log messages that the parent can read later.

use IO::Pipe;
...
foreach $tapeDevice (@tapeDevices)
{
    $pipeFH = IO::Pipe->new();
    ...
    if($pid = fork())
    {
        # This is the parent.
        $pipeFH->reader();
        push @pipes, $pipeFH;
        ...
    }
    elsif(defined $pid)
    {
        # This is the child.
        $pipeFH->writer();
        # Do lots of stuff writing to tape
        ...
        $pipeFH->print("some message or other");
        ...
    }
    else
    {
        # Fork failed for some reason.
        die "fork: $!\n";
    }
}
# We have now forked a child for each tape device so
# wait for our children to finish. Print exit status.
while((my $returnPID = wait()) != -1)
{
    print "Child $returnPID returned status ",
       $? >> 8, "\n";
}
# All children have returned, get messages.
foreach $pipe (@pipes)
{
    while(defined($_ = $pipe->getline()))
    {
        # Do something
        ...
    }
}

That is the bare bones of the mechanism. I have not attempted to reconcile use strict; and my or our here as it is just an illustration.

I hope this is useful.

Cheers,

JohnGG