system-programming/10_Signals

Chapter 10 – Signals (Summary)

Based on Linux System Programming, 2nd Edition by Robert Love
This chapter dives into the mechanics, pitfalls, and POSIX-compliant interfaces of signals in Linux.

---

What Are Signals?

• Signals are asynchronous software interrupts used for interprocess communication (IPC). They notify processes of events such as:
  • User-generated events (e.g., pressing Ctrl‑C sends SIGINT)
  • Internal faults (e.g., divide by zero triggers SIGFPE)
  • Kernel actions or process-related events (e.g., child termination sends SIGCHLD)
• Signals require special handling because they can occur at any moment, disrupting normal flow.

---

Signal Handling Models

• Early Unix systems struggled with unreliable signals—deliveries could be missed or lost.
• POSIX standardized a reliable signal model, ensuring signals are not lost and behavior is predictable.

---

Setting Signal Handlers

• Use the sigaction() interface (preferred) to set up handlers in a robust and portable way.
• signal() is outdated and has inconsistent behavior across platforms; use only for trivial cases like ignoring or using the default handler.
• SIGKILL and SIGSTOP cannot be caught, ignored, or blocked.

---

Signal Masks and Blocking

• Each process has a signal mask (via sigset_t) that defines which signals are currently blocked.
• Signal blocking is essential when running critical code to avoid race conditions.
• Use sigprocmask() to modify the mask safely.

---

Common Signal APIs

int kill(pid_t pid, int signo);   // Send a signal to another process
int killpg(int pgrp, int signo);  // Send a signal to all processes in a given process group. Equivalent to kill(-pgrp, signo).
int raise(int signo);             // Send a signal to self
unsigned int alarm(unsigned int seconds); // Schedule a SIGALRM
int pause(void);                  // Wait indefinitely until a signal arrives
void abort(void);                 // Raise SIGABRT and terminate

These calls allow fine-grained control over signal generation and handling.

---

Reentrancy and Safe Signal Handling

• Signal handlers can interrupt your program at any point, even in the middle of non-reentrant functions, causing undefined behavior.
• Only call async-signal-safe functions within handlers (e.g., write, _exit).
• Avoid unsafe operations like malloc(), printf(), or strtok() inside signal handlers.

---

Process Behavior with Signals

• Signal dispositions are inherited across fork() (child inherits parent’s handlers) but reset to default on exec().

---

Summary of Key Concepts

Concept	Description
Signals	Asynchronous notifications from kernel or processes
POSIX Signal Model	Reliable, standardized approach to signal delivery
sigaction()	Robust interface to set signal handlers
Signal Masks	Block/unblock signals to prevent interruptions
Async-signal-safe calls	Safe functions to call inside signal handlers
Core Functions	kill, raise, alarm, pause, abort

---

Why This Matters

Signals are indispensable for handling asynchronous events, timed operations, and interprocess interactions. Proper use of reliable signal handling, blocking, and safe coding practices ensures your applications remain responsive and stable under interruptions.

---

References

• Linux System Programming by Robert Love (2nd Ed.) — Chapter 10
• POSIX and Linux signal handling docs: signal(), sigaction(), sigprocmask()