Mehul Kar

Feb 08, 2017

Neat Recursion Trick with UNIX Signals

programming

I learned a neat little trick to do recursion with UNIX signals that I probably wouldn’t have thought of myself.

Let’s look at a script.py that looks like this:

import signal
import time

class MyClass:
  def main(signum=None, frame=None):
    # bind this function as a callback to SIGALARM
    signal.signal(signal.SIGALRM, self.main)

    # send a SIGALARM in 10 seconds
    signal.alarm(10)


if __name__ == "__main__":
  # kick it off
  MyClass().main()

  # keep alive
  while True:
    time.sleep(10)

Let’s break it down:

if __name__ == '__main__':
  # kick it off
  MyClass().main()

  # keep alive
  while True:
    time.sleep(10)

Here we call the MyClass().main() function and then start an infinite loop with a while True loop. This is just a dumb way to keep the python script.py process alive forever.

class MyClass():
  def main(signum=None, frame=None):
    # bind this function as a callback to SIGALARM
    signal.signal(signal.SIGALRM, self.main)

    # send a SIGALARM in 10 seconds
    signal.alarm(10)

The main function here does 2 things:

  1. Binds self.main as a callback to SIGALARM. This means that if a SIGALARM is received by the current process, it should call self.main().
  2. Sends SIGALARM to the current process in 10 seconds.

The combination of these two lines mean that every 10 seconds the main() function of the same MyClass() instance will be called.

You could probably ask why a UNIX signal is being used to trigger the call to main() again, and why we couldn’t just call the function again. One interesting application of this technique is that by using UNIX signals, we can trigger this recursion from other processes as well. Specifically, a use case would be to trigger the recursion from a child or forked process that knows about its parent.

For example, in MyClass().main(), if we were to create a subprocess that runs python child.py like this:

import time
import signal
import subprocess
def main():
  signal.signal(signal.SIGALRM, self.main)

  ## create subprocess
  subprocess.call(['python', 'child.py'])

  signal.alarm(10)

if __name__ == '__main__':
  MyClass().main()
  while True:
    time.sleep(10)

and if this is what child.py looks like:

import os
import signal

def child():
  print("I'm the child!")

  """
    Do some work here
  """

  # Send SIGALARM to the parent PID
  os.kill(os.getppid(), signal.SIGALRM)

if __name__ == '__main__':
  child()

When child() runs, it does some work and then sends SIGALARM to its parent PID. This signal is sent to the python script.py process and is handled by the callback we attached in MyClass().main(), calling MyClass().main() again.

So this trick opens up some interesting possibilities in asynchronous programming with multiple processes.

I’m not sure how contrived this example is–at least a little bit– but I thought it was a clever implementation.

Enjoy!

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