## 7.1.1 Example

Here is a minimal example program. It uses the `alarm()` function to limit the time spent waiting to open a file; this is useful if the file is for a serial device that may not be turned on, which would normally cause the `os.open()` to hang indefinitely. The solution is to set a 5-second alarm before opening the file; if the operation takes too long, the alarm signal will be sent, and the handler raises an exception.

      import signal, os, FCNTL

      def handler(signum, frame):
          print 'Signal handler called with signal', signum
          raise IOError, "Couldn't open device!"

      # Set the signal handler and a 5-second alarm
      signal.signal(signal.SIGALRM, handler)
      signal.alarm(5)

      # This open() may hang indefinitely
      fd = os.open('/dev/ttyS0', FCNTL.O_RDWR)

      signal.alarm(0)          # Disable the alarm