Signal Handling

Introduction

The signal module provides mechanisms to handle asynchronous events like keyboard interrupts and system notifications. Signal handlers define custom responses to signals.

Basic Signal Handling

import signal
import sys

def signal_handler(signum, frame):
    print(f"Received signal {signum}")
    sys.exit(0)

# Handle SIGINT (Ctrl+C)
signal.signal(signal.SIGINT, signal_handler)

# Handle SIGTERM
signal.signal(signal.SIGTERM, signal_handler)

print("Running... Press Ctrl+C")
signal.pause()

Signal Types

import signal

# Available signals
print(signal.SIGINT)   # 2 - Interrupt (Ctrl+C)
print(signal.SIGTERM)  # 15 - Termination request
print(signal.SIGKILL)  # 9 - Kill (cannot be caught)
print(signal.SIGHUP)   # 1 - Hangup
print(signal.SIGALRM)  # 14 - Alarm clock

Alarm Signals

import signal
import time

def alarm_handler(signum, frame):
    print("Wake up!")

signal.signal(signal.SIGALRM, alarm_handler)

signal.alarm(3)  # Set alarm for 3 seconds
print("Waiting...")
signal.pause()

Custom Signal Handlers

import signal

class SignalHandler:
    def __init__(self):
        self.triggered = False
    
    def __call__(self, signum, frame):
        self.triggered = True
        print(f"Signal {signum} handled")

handler = SignalHandler()
signal.signal(signal.SIGTERM, handler)

# Raise SIGTERM to test
signal.raise_signal(signal.SIGTERM)
print(handler.triggered)

Ignoring Signals

import signal

# Ignore SIGINT (Ctrl+C won't stop program)
signal.signal(signal.SIGINT, signal.SIG_IGN)

print("Running... Ctrl+C won't stop this")
import time
time.sleep(10)
print("Done")

Process Signals from OS

import signal
import os
import sys

def handle_usr1(signum, frame):
    print("USR1 received - performing checkpoint")
    # Save state

signal.signal(signal.SIGUSR1, handle_usr1)

# From another terminal: kill -USR1 <pid>
print(f"My PID: {os.getpid()}")

while True:
    signal.pause()

Practice Problems

1. Handle SIGINT gracefully
2. Set alarm with signal.alarm
3. Create custom signal handler class
4. Ignore specific signals
5. Handle SIGUSR1 for user-defined signals