Signals

A signal is an asynchronous notification delivered to a process — a lightweight "software interrupt". The kernel, another process, or the terminal can send one at any time; the process can catch it, ignore it, or let the default action happen. Signals are the first and simplest form of inter-process communication we meet.

What is a signal?

Signals come from many sources — the kernel (on a fault), the terminal driver (Ctrl+C), another process (kill), a timer (alarm) — and follow a simple lifecycle: generation → delivery → handling. When one is delivered, the process's normal flow is interrupted to run the signal's disposition, then resumes.

Common signals

Sending signals

• kill(pid, sig) — send a signal to a process (or process group)
• raise(sig) — send one to yourself
• pthread_kill(thread, sig) — target a specific thread
• From the shell: kill, killall, pkill

Handling signals

A process chooses a disposition per signal: the default action, ignore it (SIG_IGN), or run a custom handler. Use sigaction() — the modern, reliable API (the old signal() has portability pitfalls, so avoid it):

#include <signal.h>
#include <stdio.h>

void on_sigint(int sig) {
    // (only async-signal-safe calls belong here)
    write(1, "caught SIGINT\n", 14);
}

int main(void) {
    struct sigaction sa = {0};
    sa.sa_handler = on_sigint;
    sigemptyset(&sa.sa_mask);   // signals blocked during the handler
    sa.sa_flags = SA_RESTART;   // auto-restart interrupted syscalls
    sigaction(SIGINT, &sa, NULL);

    for (;;) pause();           // wait for signals
}

The struct sigaction also offers sa_sigaction (an extended handler with siginfo_t when you set SA_SIGINFO), sa_mask (extra signals to block while the handler runs), and flags like SA_RESTART and SA_NODEFER.

Timers

alarm(seconds) schedules a one-shot SIGALRM; setitimer() provides repeating, finer-grained timers. Combined with a handler, they implement timeouts.

Practice / lab

• Catch SIGINT with sigaction and print a message instead of terminating; send it with Ctrl+C and with kill -INT <pid>.
• Fork a child, have the parent send SIGUSR1 with kill(), and let the child respond with SIGUSR2.
• Use alarm() to implement a timeout around a blocking read.

Homework

• IPC-1: Signal Echo — print details of the sending process from inside a SIGUSR1 handler.

References & resources

Required

• Signals in C (GeeksforGeeks)
• Signal handling with sigaction (video)
• Understanding sigaction (Baeldung)
• The Linux Programming Interface (Kerrisk) — Ch. 20–21: signals

Optional / enrichment

• signal(7) · sigaction(2) · kill(2) · alarm(2)
• Signal (IPC) (Wikipedia)

Self-check

1. What is the difference between SIGTERM and SIGKILL?
2. Why is sigaction() preferred over signal()?
3. What are the possible dispositions for a signal?
4. Why must you only call async-signal-safe functions inside a handler?
5. How does alarm() use signals to implement a timeout?