Unix signal handling example in C, SIGINT, SIGALRM, SIGHUP...

signal.c

/**
 * More info?
 * a.dotreppe@aspyct.org
 * http://aspyct.org
 *
 * Hope it helps :)
 */

#include <stdio.h>
#include <stdlib.h> 
#include <signal.h> // sigaction(), sigsuspend(), sig*()
#include <unistd.h> // alarm()

void handle_signal(int signal);
void handle_sigalrm(int signal);
void do_sleep(int seconds);

/* Usage example
 * 
 * First, compile and run this program:
 *     $ gcc signal.c
 *     $ ./a.out
 * 
 * It will print out its pid. Use it from another terminal to send signals
 *     $ kill -HUP <pid>
 *     $ kill -USR1 <pid>
 *     $ kill -ALRM <pid>
 *
 * Exit the process with ^C ( = SIGINT) or SIGKILL, SIGTERM
 */
int main() {
    struct sigaction sa;

    // Print pid, so that we can send signals from other shells
    printf("My pid is: %d\n", getpid());

    // Setup the sighub handler
    sa.sa_handler = &handle_signal;

    // Restart the system call, if at all possible
    sa.sa_flags = SA_RESTART;

    // Block every signal during the handler
    sigfillset(&sa.sa_mask);

    // Intercept SIGHUP and SIGINT
    if (sigaction(SIGHUP, &sa, NULL) == -1) {
        perror("Error: cannot handle SIGHUP"); // Should not happen
    }

    if (sigaction(SIGUSR1, &sa, NULL) == -1) {
        perror("Error: cannot handle SIGUSR1"); // Should not happen
    }

    // Will always fail, SIGKILL is intended to force kill your process
    if (sigaction(SIGKILL, &sa, NULL) == -1) {
        perror("Cannot handle SIGKILL"); // Will always happen
        printf("You can never handle SIGKILL anyway...\n");
    }

    if (sigaction(SIGINT, &sa, NULL) == -1) {
        perror("Error: cannot handle SIGINT"); // Should not happen
    }

    for (;;) {
        printf("\nSleeping for ~3 seconds\n");
        do_sleep(3); // Later to be replaced with a SIGALRM
    }
}

void handle_signal(int signal) {
    const char *signal_name;
    sigset_t pending;

    // Find out which signal we're handling
    switch (signal) {
        case SIGHUP:
            signal_name = "SIGHUP";
            break;
        case SIGUSR1:
            signal_name = "SIGUSR1";
            break;
        case SIGINT:
            printf("Caught SIGINT, exiting now\n");
            exit(0);
        default:
            fprintf(stderr, "Caught wrong signal: %d\n", signal);
            return;
    }

    /*
     * Please note that printf et al. are NOT safe to use in signal handlers.
     * Look for async safe functions.
     * http://man7.org/linux/man-pages/man7/signal-safety.7.html
     */
    printf("Caught %s, sleeping for ~3 seconds\n"
           "Try sending another SIGHUP / SIGINT / SIGALRM "
           "(or more) meanwhile\n", signal_name);
    
    /*
     * All signals are blocked during this handler
     * (because of the `sigfillset(&sa.sa_mask);` in `main()`).
     * But, at least on OSX, if you send 2 other SIGHUP,
     * only one will be delivered: signals are not queued
     * However, if you send HUP, INT, HUP,
     * you'll see that both INT and HUP are queued
     * Even more, on my system, HUP has priority over INT
     *
     * There are two types of signals: standard signals,
     * and realtime signals. Realtime signals are SIGRTMIN (34)
     * and above.
     * While there can be only one traditional signal waiting at
     * a time for each type (i.e. one HUP, one INT etc), there
     * However, there can be multiple realtime signals queued
     * for each type (e.g. 3 SIGRTMIN, 4 SIGRTMAX...)
     *
     * See the other file for a list of all the signals available.
     */
    do_sleep(3);
    printf("Done sleeping for %s\n", signal_name);

    // So what did you send me while I was asleep?
    sigpending(&pending);
    if (sigismember(&pending, SIGHUP)) {
        printf("A SIGHUP is waiting\n");
    }
    if (sigismember(&pending, SIGUSR1)) {
        printf("A SIGUSR1 is waiting\n");
    }

    printf("Done handling %s\n\n", signal_name);
}

void handle_sigalrm(int signal) {
    if (signal != SIGALRM) {
        fprintf(stderr, "Caught wrong signal: %d\n", signal);
    }

    printf("Got sigalrm, do_sleep() will end\n");
}

void do_sleep(int seconds) {
    struct sigaction sa;
    sigset_t mask;
    
    sa.sa_handler = &handle_sigalrm; // Intercept and ignore SIGALRM
    sa.sa_flags = SA_RESETHAND; // Remove the handler after first signal
    sigfillset(&sa.sa_mask);
    sigaction(SIGALRM, &sa, NULL);
    
    // Get the current signal mask
    sigprocmask(0, NULL, &mask);

    // Unblock SIGALRM
    sigdelset(&mask, SIGALRM);

    // Wait with this mask
    alarm(seconds);
    sigsuspend(&mask);

    printf("sigsuspend() returned\n");
}

signals.txt

For reference, here is a list of all the signals on Linux.
The signals from SIGRTMIN and above are real time signals.

$ uname -a
Linux localhost 4.9.0-6-amd64 #1 SMP Debian 4.9.82-1+deb9u3 (2018-03-02) x86_64 GNU/Linux

$ kill -l
 1) SIGHUP       2) SIGINT       3) SIGQUIT      4) SIGILL       5) SIGTRAP
 6) SIGABRT      7) SIGBUS       8) SIGFPE       9) SIGKILL     10) SIGUSR1
11) SIGSEGV     12) SIGUSR2     13) SIGPIPE     14) SIGALRM     15) SIGTERM
16) SIGSTKFLT   17) SIGCHLD     18) SIGCONT     19) SIGSTOP     20) SIGTSTP
21) SIGTTIN     22) SIGTTOU     23) SIGURG      24) SIGXCPU     25) SIGXFSZ
26) SIGVTALRM   27) SIGPROF     28) SIGWINCH    29) SIGIO       30) SIGPWR
31) SIGSYS      34) SIGRTMIN    35) SIGRTMIN+1  36) SIGRTMIN+2  37) SIGRTMIN+3
38) SIGRTMIN+4  39) SIGRTMIN+5  40) SIGRTMIN+6  41) SIGRTMIN+7  42) SIGRTMIN+8
43) SIGRTMIN+9  44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9  56) SIGRTMAX-8  57) SIGRTMAX-7
58) SIGRTMAX-6  59) SIGRTMAX-5  60) SIGRTMAX-4  61) SIGRTMAX-3  62) SIGRTMAX-2
63) SIGRTMAX-1  64) SIGRTMAX

Thanks a lot.

Instead of calling printf() from within your signal handler (bad idea) you could implement a FIFO of messages to be written and check that (calling printf() on non-empty) from your main event loop. (Basically any case where you're going to handle signals in a non-terminating fashion entails implementing one or more event loops to handle the interruption by signals and resume whatever was being done by main()).

Thanks. 👍

Thanks, man, this is useful for me

BH1SCW

Thanks 👍

I must be missing an important concept here. After reading these comments and the signal man pages it seems multiple people say that using printf in a signal handler is a bad idea, but not why. Could someone elaborate?

@kohbo the man page you want is: http://man7.org/linux/man-pages/man7/signal-safety.7.html.

Thanks a lot it really helped to understand the signal handling by reading your code.I now understand much better how it is done.

It is very good example! Thanks!
Please, add before codes line (for correct execute all functions):
#define _GNU_SOURCE

Hi @AlexGin. Care elaborating why we need this _GNU_SOURCE?