use strict;
use warnings;
use Carp;
use File::Copy;
use Fcntl qw/LOCK_SH LOCK_EX LOCK_NB LOCK_UN/;

sub x_copy # NOT USED
{
  my ($src, $dst) = @_;
  open (my $f, "<", $src) or confess;
  binmode $f;
  read $f, my $buf, -s $f or confess ;
  close $f;
  open (my $o, ">", $dst) or confess;
  binmode $o;
  print $o $buf;
  close $o;
}


sub getlock
{
  open my $f, ">", "lock.tmp" or confess;
  flock $f, LOCK_EX or confess;
  shift->();
  flock $f, LOCK_UN or confess;
  close $f;
}


for (1..5) {
 unless (fork()) {
 for (1..5) {
  my $filename = "somefile$_.tmp";
  my $f=undef;
  getlock sub {
   unless (-e $filename) {
    open ($f, ">", $filename) or confess;
    binmode $f;
   }
  };
  if ($f) {
  print ($f "x") for (1..40_000_000);
  close $f;
  }
  my $newfilename = "c_${$}_$filename";
  unless (-e $newfilename) {
    copy($filename, $newfilename) or confess;
    die if -s $filename != -s $newfilename;
  }
 }
 exit;
 }
}

while (wait != -1) { };

##</code><code>##

die if -s $filename != -s $newfilename;

##</code><code>##

read $f, my $buf, -s $f or confess ;