Ah, you're correct. Until something waits on the process it'll still have an entry in the process table and hence be "alive" as far as sending signal 0 goes. Sending signal 0 is probably of more use in checking if an unrelated process (or at least non-descendent process) is alive.

Update: And you can see this behavior with the code below.

#!/usr/bin/perl

my $pid = fork;
die "fork: $!\n" unless defined $pid;

if( $pid ) {
  ## Parent
  print "Parent $$, child is $pid\n";

  for( 1..5 ) {
    check_alive( $pid );
    sleep 2;
  }
} else {
  ## Child
  print "Child here, pid $$; sleeping 7 secs\n";
  sleep 5;
  print "Child exiting\n";
  exit 0;
}

print STDERR "Parent, waiting on child\n";

waitpid( $pid, 0 )
  or warn "waitpid: $!\n";

check_alive( $pid );

exit 0;

sub check_alive {
  my $pid = shift;
  my $kill = kill 0 => $pid or warn "kill: $!\n";

  $kill = $kill == 1 ? "alive" : "$!";

  print STDERR scalar localtime, ": kill sez $kill\n";
}

__END__

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In reply to Re^3: Finding if a child process has terminated by Fletch
in thread Finding if a child process has terminated by cog

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