"""
 signal.py

 An example of a python program that stops after a given time.
 See https://docs.python.org/2/library/signal.html

 Tested with python 2.

 Jim Mahoney | Apr 18 2018 | MIT License | cs.marlboro.college
"""
import signal, time

class TimeHasRunOut(Exception):
    """ The exception raised when the timer expires """
    # This is a user defined exception.
    # Python exceptions are used in try/except clauses
    # to handle errors or other events that change the
    # regular flow of program execution.
    pass

def raise_TimeHasRunOut(signal_number, frame):
    """ signal handler - when the alarm goes out, raise an exception """
    raise TimeHasRunOut()

def start_alarm(time_in_sec):
    """ Start the alarm timer """
    # The approach here is to use the operating system's built-in signaling
    # mechanism. Any process can be sent a signal, and can have a function
    # defined to run when it gets a signal. Signals are things like "quit",
    # which is what happens when you type control-C at a prompt.

    # When this process gets an ALARM signal, run raise_TimeHasRunOut(s,f)
    signal.signal(signal.SIGALRM, raise_TimeHasRunOut)
    
    # Tell the system to send this process an ALARM in this many seconds.
    signal.setitimer(signal.ITIMER_REAL, time_in_sec)

def main():
    print "signal.py - an example of a signal interrupting a process."
    start_alarm(5.0)  # Set an alarm to go off in 5 sec.
    try:                                # Do this until an exception is raised.
        for i in range(100):
            print i
            time.sleep(0.5)
    except TimeHasRunOut:               # Handle this sort of exception here.
        print "The alarm has gone off."

main()