Signals

Overview

This week introduces signals as a fundamental mechanism for asynchronous inter-process communication in Unix/Linux systems.
Students will learn how signals are generated, how processes can handle them, and how to send signals between processes. We will explore the standard signal API and signal handling mechanisms.

By the end of this week, students will understand how processes communicate through signals, how to handle exceptional events, and how to register custom signal handlers.

Key Concepts

What are Signals?

• Signals as software interrupts for asynchronous event notification
• Uses of signals: termination, suspension, error notification, timer events, user-defined IPC
• Signal lifecycle: generation → delivery → handling
• Synchronous vs. asynchronous signals
• Signals as a form of lightweight IPC between processes

Standard Signals

• Process control signals:
  • SIGTERM — polite termination request
  • SIGKILL — forced termination (uncatchable)
  • SIGSTOP / SIGCONT — suspend and resume execution
• Error and fault signals:
  • SIGSEGV — segmentation fault
  • SIGFPE — floating-point exception
  • SIGILL — illegal instruction
  • SIGBUS — bus error
• User and timer signals:
  • SIGUSR1 / SIGUSR2 — user-defined signals
  • SIGALRM — timer expiration (from alarm())
• Job control signals:
  • SIGINT — interrupt from keyboard (Ctrl+C)
  • SIGQUIT — quit from keyboard (Ctrl+\)
  • SIGTSTP — terminal stop (Ctrl+Z)
• I/O and pipe signals:
  • SIGPIPE — write to a pipe with no reader
  • SIGIO — asynchronous I/O event
• Child process signals:
  • SIGCHLD — child process terminated or stopped

Signal Handling

• Default actions: terminate, ignore, stop, continue, core dump
• Custom signal handlers:
  • sigaction() — modern, reliable signal handling API
  • signal() — traditional (unreliable) signal handler registration [do not use]
• Handler function signature: void handler(int signum)
• Persistent vs. one-shot handlers
• Signal disposition: default, ignore (SIG_IGN), custom handler (SIG_DFL)

Sending and Raising Signals

• kill() — send a signal to a process or process group
• raise() — send a signal to the calling process
• killpg() — send a signal to a process group
• pthread_kill() — send a signal to a specific thread
• Command-line tools: kill, killall, pkill

Practice / Lab

Signal Handling

• Write a program that handles SIGINT (Ctrl+C) and prints a message instead of terminating.
• Register a handler using sigaction() and observe the behavior.
• Use the kill command from the shell to send signals to your running program.

Inter-Process Signaling

• Write a parent process that forks a child and sends SIGUSR1 using kill().
• Have the child process handle SIGUSR1 and respond with SIGUSR2 to the parent.

Timers

• Use alarm() to schedule SIGALRM and implement a timeout mechanism.

Homework

• IPC-1: Signal Echo

References & Resources

Required

• Signals in C programming
• Signal Handling in Linux
• Signals in C Language
• Signal handling — sigaction
• Kerrisk, The Linux Programming Interface
  • Chapter 20: Signals — Fundamental Concepts
  • Chapter 21: Signals — Signal Handlers
• Advanced Programming in the UNIX Environment — W. Richard Stevens, Chapter 10 “Signals”

Recommended

• Linux manual page - signal(7)
• Linux manual page - sigaction(2)
• Linux manual page - kill(2)
• Linux manual page - raise(3)
• Linux manual page - alarm(2)
• POSIX Signals (Wikipedia)
• Understanding sigaction

Quiz (Self-check)

1. What is the difference between SIGTERM and SIGKILL?
2. Why is sigaction() preferred over signal() for signal handling?
3. What are the default actions a process can take when receiving a signal?
4. How do you send a signal from one process to another?
5. What is the purpose of SIGUSR1 and SIGUSR2?
6. What happens when you press Ctrl+C in a terminal running a program?
7. How does the alarm() function use signals?

Suggested Tools

• kill — send signals to processes by PID
• killall — send signals to processes by name
• pkill — send signals based on pattern matching
• trap — shell built-in for handling signals in scripts
• strace — trace signal delivery and system calls
• gdb — debug signal handling behavior