sigaction

Learn advanced signal handling with sigaction.

The sigaction structure

sigaction is the modern, portable alternative to signal. With the struct sigaction you specify handler, mask and flags separately.

First declare struct sigaction sa;. Assign the handler with sa.sa_handler = handler;. Empty the mask with sigemptyset(&sa.sa_mask). Reset flags with sa.sa_flags = 0;. Then register with sigaction(SIGUSR1, &sa, NULL). This is more controlled and safer than signal.

sa_flags and mask

sa_flags changes the handler behavior. SA_RESTART automatically restarts interrupted system calls. SA_SIGINFO passes extra info to the handler.

sa_mask is the set of signals blocked while the handler runs. With sigemptyset no other signal is blocked during the handler. You can add specific signals with sigaddset. This control is not available in signal.

Exercise Catch with sigaction

Define int caught = 0; and void handler(int sig) with caught = 1;. Declare struct sigaction sa;; set sa.sa_handler = handler;, sigemptyset(&sa.sa_mask);, sa.sa_flags = 0;. Register with sigaction(SIGUSR1, &sa, NULL);. Call raise(SIGUSR1); and printf("caught=%d\n", caught);.

Show example solution

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

int caught = 0;

void handler(int sig) {
    caught = 1;
}

int main(void) {
    struct sigaction sa;
    sa.sa_handler = handler;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = 0;
    sigaction(SIGUSR1, &sa, NULL);
    raise(SIGUSR1);
    printf("caught=%d\n", caught);
    return 0;
}

Test yourself

sa_handler assignment

sa.sa_handler = ____;

Show answer

sa.sa_handler = handler;

Match sigaction fields

sigaction

Modern handler
sa_handler

Handler field
sa_flags

Flags field
sigemptyset

Empty mask
What advantage does sigaction have over signal?
- It is faster
- Requires less code
- More controlled and portable
- Changes the signal number
Show answer

More controlled and portable — sigaction is more controlled and consistent across systems.

Next lesson: kill

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