Signals in C

A handler runs on interrupt, and it may only touch async-signal-safe things.

#define _POSIX_C_SOURCE 200809L

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

static volatile sig_atomic_t stop_requested = 0;

static void on_signal(int signum) {
    (void) signum;
    stop_requested = 1;
}

int main(void) {
    struct sigaction action;
    sigemptyset(&action.sa_mask);
    action.sa_flags = 0;
    action.sa_handler = on_signal;

    if (sigaction(SIGINT, &action, NULL) != 0) {
        perror("sigaction");
        return 1;
    }

    printf("working, press ctrl-c\n");
    for (int i = 0; i < 5 && !stop_requested; i++) {
        sleep(1);
    }
    printf("stopped %s\n", stop_requested ? "early" : "normally");
    return 0;
}

How it works

1. sigaction is POSIX, so the feature macro comes first.
2. The flag it sets must be volatile sig_atomic_t.
3. Do the real work back in the main loop, not in the handler.

Keywords and builtins used here

• NULL
• for
• if
• int
• main
• on_signal
• return
• sig_atomic_t
• sigaction
• static
• struct
• void
• volatile

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Step 4 of 4 in Processes & signals, step 10 of 14 in Systems programming.

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