The problem is that select() isn't re-entrant: it gets interrupted by the signal handler and doesn't resume, so you end up with an empty @ready array. That means you fall out of the while (@ready) {} loop; the sub returns, createProcess() is called again, the file handles are re-opened: the previous command is terminated.

To work around that, add a flag variable which gets reset in the handler: 

my $redo; # flag

sub status () {
  $SIG{USR1} = \&status;
  print STDERR "Currently running:  $StartedProcess\n";
  $redo = 1; # re-set flag
}

# Opens external program, and as long as it exists, checks
# if there is data in the pipe (STDOUT/STDERR)
sub createProcess () {
  my($line, $selector, @ready, $fh);

  $StartedProcess = open3(*CMD_IN, *CMD_OUT, *CMD_ERR, "./external.pp"
+);
  close(CMD_IN);

  $selector = IO::Select->new();
  $selector->add(*CMD_ERR, *CMD_OUT);
  $redo = 1; # set flag
  while($redo) {
    $redo = 0; # clear flag
    while(@ready = $selector->can_read) {
      # do some operations on STDOUT/STDERR here
    }
    warn "after inner loop\n";
  }
  warn "closing CMD_OUT and CMD_ERR\n";
  close(CMD_OUT);
  close(CMD_ERR);
}

[download]

Also, you should set up a SIGCHLD handler which clears the $selector after the child is gone.

In reply to Re^8: Signal to parent-process. Does it affect it's children? by shmem
in thread Signal to parent-process. Does it affect it's children? by rapide

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