#signal #cpp #linux

signal_handlers.cpp

//
// Example of linux signal handlers
//
// Authored by Daniel Shih <[email protected]>
//

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

static bool caught = false; // check if the signal was caught and stop looping
static struct sigaction old_signal_handlers[32];

// handler for SIGSEGV and SIGABRT
static void
death_handler (int signum, siginfo_t* sigdata, void* sigctx)
{
    printf ("Signal %d received\n", signum);
    signal (signum, SIG_DFL); // restore default signal handler of signum
    kill (getpid(), signum); // kill itself using signum
}

// handler of SIGTSTP and SIGINT
static void
stop_handler (int signum, siginfo_t* sigdata, void* unused)
{
    caught = true;
    printf ("Signal %d received\n", signum);

    // return to exit gracefully
    return;
}

// handler for SIGALRM
static void
alarm_handler (int signum)
{
    caught = true;
    printf ("Signal %d received\n", signum);

    // return to exit gracefully
    return;
}

// install signal handlers
static void
InstallSignalHandlers()
{
    // define and initialize signal set
    sigset_t signalset;
    sigfillset (&signalset);

    // define signal handler "action"
    struct sigaction action;
    action.sa_sigaction = stop_handler;
    // make sa_sigaction (not sa_handler) to handle signals
    action.sa_flags = SA_SIGINFO;
    // block signals in signalset
    action.sa_mask = signalset;

    // install SIGTSTP and SIGINIT handlers to "action" using sigaction()
    sigaction (SIGTSTP, &action, &old_signal_handlers[SIGTSTP]); // ctrl-z
    sigaction (SIGINT, &action, &old_signal_handlers[SIGINT]); // ctrl-c

    // define another signal handler "deathaction"
    struct sigaction deathaction;
    deathaction.sa_sigaction = death_handler;
    deathaction.sa_flags = SA_SIGINFO;
    deathaction.sa_mask = signalset;

    // install SIGSEGV nad SIGABRT handlers to "deathaction" using sigaction()
    // segmentation fault, or kill -11
    sigaction (SIGSEGV, &deathaction, &old_signal_handlers[SIGSEGV]);
    // raise(SIGABRT), abort(), or kill -6
    sigaction (SIGABRT, &deathaction, &old_signal_handlers[SIGABRT]);

    // register SIGALRM handler using signal() ***not recommended***
    signal (SIGALRM, alarm_handler); // alarm(secs)

    return;
}

// main process
int
main(int argc, char** argv)
{
    if (argc != 2)
    {
        printf ("usage: %s [seconds to alarm]\n", argv[0]);
        return -1;
    }

    // install signal handlers
    InstallSignalHandlers();

    // set alarm
    alarm(atoi(argv[1]));

    // get pid
    int pid = getpid();

    // loop to wait signal
    for (int second = 1; !caught; second++)
    {
        sleep(1);
        printf ("process %d started %d seconds\n", pid, second);
    };

    // going to exit
    printf ("exiting...\n");

    return 0;
}