Notice I never install a SIGCHLD handler. That is because the parent will process them itself. Also, the waitpids are done non-blocking. The sleep makes sure we do not loop too tightly.

As splinky said, if you call a subroutine, make sure it exits or you call exit after it returns. The original code I took this from actually exec'd and so I didn't worry about it.

$num_children is a command line arg, not shown here so you don't have to see my lame mp3 ripping code. It, obviously, controls how many child procs will be spawned.

# Interesting stuff has happened to load the job queue and par
# the command line, and whatever else you want to happen.  
sub SPAWN {
    $counter++;
    if ( $cpid = fork ) {
        $tracker{$cpid} = $ref->[1];
    }
    else {
    # This can be a call like this, or
    my_sub( @_ );
    exit;
    # You can exec some external proc like
    # exec "/bin/ls -FC /var";
    }
}

my $dpid = 0;

while( @job_q ) {
    my $ref = shift @job_q;
    #--
    # Initial loop to spawn children
    #--
    if ( $counter < $num_children ) {
        &SPAWN( $ref );
    }
    else {
        do {
            sleep 5;
            $dpid = waitpid( -1, 1 );
        } until $dpid;
        last if ( $dpid == -1 );  # No children left
        printf "%s has finished, spawning next child (%d left)\n", $tr
+acker{$dpi
d}, scalar @job_q;
        &SPAWN( $ref );
    }
}

do {    
    sleep 5;
    $dpid = waitpid( -1, 1 );
    printf("%s has finished\n", $tracker{$dpid}) if(defined( $tracker{
+$dpid} )
    );
} until $dpid == -1;
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I will warn you this isn't the most robust code. I haven't had problems with it, but it is something only I use. It is a slightly different solution to the problem though.

mikfire

use POSIX ":sys_wait_h";
sub REAPER {
  my $child;
  while ($child = waitpid(-1,WNOHANG)) {
    $Kid_Status{$child} = $?;
  }
  $SIG{CHLD} = \&REAPER;  # still loathe sysV
}
$SIG{CHLD} = \&REAPER;

..............

for (1..6) {
  do {
    $pid = fork();
    unless (defined $pid) {
      warn "cannot fork: $!";
      sleep 10;
    }
  } until defined $pid;

  if ($pid) {  # parent
    print "forked process $pid\n";
  } 
  else {     # child
    my_sub(@parms);
    exit;  # don't forget this
  }
}
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Untested. Have fun.

*Woof*

#!/opt/perl/bin/perl


# This is the simple child-process subroutine.
sub ABC
{
        sleep(2);
        print "abc with counter = $counter.\n";
}


# This section spawns the initial 6 children.

for ($counter = 1; $counter <= 6; $counter++)
{
        $pid = fork();

        if ($pid) # parent
        {
        $child{$pid} = $counter;
        $procnum[$counter] = $pid;
        print "forked process $counter.\n";
        }

        else # child
        {
        ABC();
        exit $counter;
        }
}


# This section spawns additional children 7-20.

$diecount = 0;

while ($counter <= 20)
{
        $doneproc = wait();
        $doneval = $? >> 8;
        $pid = fork();

        if ($pid) # parent
        {
        $child{$pid} = $counter;
        print "child $doneval ($doneproc) exited, forking process $cou
+nter.\n";
        $counter++;
        $diecount++;
        }

        else # child
        {
        ABC();
        exit $counter;
        }
}


# This section waits for all children to die.

while ($diecount <= 20)
{
        wait();
        $diecount++;
}

print "exiting.\n";
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BazB added code tags.