Python signal Module

The signal module in Python provides mechanisms to use and handle signals. Signals are software interrupts delivered to a process, typically to notify it of events like a user pressing Ctrl+C (which sends the SIGINT signal), or a timer expiration (which sends the SIGALRM signal).

Table of Contents

1. Introduction
2. Key Functions and Constants
   • signal.signal
   • signal.getsignal
   • signal.pause
   • signal.setitimer
   • signal.getitimer
   • signal.SIG_IGN
   • signal.SIG_DFL
   • signal.SIGINT
   • signal.SIGTERM
3. Examples
   • Handling SIGINT (Ctrl+C)
   • Using Timers with SIGALRM
   • Ignoring Signals
   • Handling Custom Signals
4. Real-World Use Case
5. Conclusion
6. References

Introduction

Signals are a way for the operating system to communicate with a process. The signal module in Python provides an interface to set up handlers for different signals. This is useful for tasks like cleaning up resources on exit, handling timeout events, or dealing with interruptions.

Key Functions and Constants

signal.signal

Sets up a signal handler for a given signal.

import signal

def handler(signum, frame):
    print(f'Signal handler called with signal {signum}')

signal.signal(signal.SIGINT, handler)

signal.getsignal

Returns the current signal handler for a given signal.

current_handler = signal.getsignal(signal.SIGINT)

signal.pause

Pauses the program until a signal is received.

signal.pause()

signal.setitimer

Sets an interval timer.

signal.setitimer(signal.ITIMER_REAL, 5)  # 5 seconds

signal.getitimer

Gets the current value of an interval timer.

remaining_time = signal.getitimer(signal.ITIMER_REAL)

signal.SIG_IGN

A constant to ignore a signal.

signal.signal(signal.SIGINT, signal.SIG_IGN)

signal.SIG_DFL

A constant to reset the signal to its default action.

signal.signal(signal.SIGINT, signal.SIG_DFL)

signal.SIGINT

A constant for the interrupt signal (Ctrl+C).

signal.SIGTERM

A constant for the termination signal.

Examples

Handling SIGINT (Ctrl+C)

import signal
import time

def handler(signum, frame):
    print('Signal handler called with signal', signum)

signal.signal(signal.SIGINT, handler)

print('Press Ctrl+C')
while True:
    time.sleep(1)

Using Timers with SIGALRM

import signal
import time

def handler(signum, frame):
    print('Alarm signal received')

signal.signal(signal.SIGALRM, handler)
signal.setitimer(signal.ITIMER_REAL, 5)  # 5 seconds

print('Waiting for alarm...')
signal.pause()

Ignoring Signals

import signal
import time

signal.signal(signal.SIGINT, signal.SIG_IGN)

print('Press Ctrl+C (it will be ignored)')
while True:
    time.sleep(1)

Handling Custom Signals

import signal
import os
import time

def handler(signum, frame):
    print(f'Received signal {signum}')

signal.signal(signal.SIGUSR1, handler)

print(f'Sending SIGUSR1 to process {os.getpid()}')
os.kill(os.getpid(), signal.SIGUSR1)

time.sleep(1)

Real-World Use Case

Graceful Shutdown of a Server

import signal
import time
import threading

shutdown_event = threading.Event()

def handler(signum, frame):
    print('Signal received, shutting down...')
    shutdown_event.set()

signal.signal(signal.SIGINT, handler)
signal.signal(signal.SIGTERM, handler)

print('Server is running. Press Ctrl+C to stop.')
while not shutdown_event.is_set():
    # Simulate server work
    print('Working...')
    time.sleep(2)

print('Server has shut down gracefully.')

Conclusion

The signal module in Python is used for handling asynchronous events such as interrupts and timers. By setting up custom signal handlers, you can manage how your program responds to various signals, making it more robust and responsive to external events.

References

• Python signal module documentation