Handling the SIGHUP Signal in Daemons

A process that usually starts when the system boots and shuts down when the system is shut down is called a daemon (Disk And Execution MONitor). If a daemon process has a configuration file which is modified after the process has been started, there should be a way to tell that process to re-read its configuration file, without stopping the process. Many daemons provide this mechanism using the SIGHUP signal handler. When you want to tell the daemon to re-read the file you simply send it the SIGHUP signal.

Not all platforms automatically reinstall their (native) signal handlers after a signal delivery. This means that the handler works only the first time the signal is sent. The solution to this problem is to use POSIX signal handlers if available, their behaviour is well-defined.

The following example implements a simple daemon, which restarts itself every time the SIGHUP signal is received. The actual code is located in the subroutine code(), which simply prints some debug info to show that it works and should be replaced with the real code.

  #!/usr/bin/perl -w

  use POSIX ();
  use FindBin ();
  use File::Basename ();
  use File::Spec::Functions;

  $|=1;

  # make the daemon cross-platform, so exec always calls the script
  # itself with the right path, no matter how the script was invoked.
  my $script = File::Basename::basename($0);
  my $SELF = catfile $FindBin::Bin, $script;

  # POSIX unmasks the sigprocmask properly
  my $sigset = POSIX::SigSet->new();
  my $action = POSIX::SigAction->new('sigHUP_handler',
                                     $sigset,
                                     &POSIX::SA_NODEFER);
  POSIX::sigaction(&POSIX::SIGHUP, $action);

  sub sigHUP_handler {
      print "got SIGHUP\n";
      exec($SELF, @ARGV) or die "Couldn't restart: $!\n";
  }

  code();

  sub code {
      print "PID: $$\n";
      print "ARGV: @ARGV\n";
      my $c = 0;
      while (++$c) {
          sleep 2;
          print "$c\n";
      }
  }
  __END__

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