sigaction
- C course
- Advanced
- Lesson 15/83
- 5 min
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.
Practice 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);.
Starter code
#include <stdio.h>
#include <signal.h>
int caught = 0;
// Write handler function
// Declare sigaction struct
// Register and raise
int main(void) {
return 0;
}Sample 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;
}Expected output
caught=1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 sa_handler assignment
sa.sa_handler = ____;sa.sa_handler = handler;Explanation: sa_handler is assigned the handler function.
2 Match sigaction fields
| Term | Meaning |
|---|---|
sigaction |
Modern handler |
sa_handler |
Handler field |
sa_flags |
Flags field |
sigemptyset |
Empty mask |
3 What advantage does sigaction have over signal?
- It is faster
- Requires less code
- More controlled and portable Correct answer
- Changes the signal number
Explanation: sigaction is more controlled and consistent across systems.