/**
 * More info?
 * a.dotreppe@aspyct.org
 * http://aspyct.org
 * @aspyct (twitter)
 *
 * 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 wait_for_signal(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>
 *     $ ^C  # SIGINT
 *
 * Exit the process with SIGKILL, SIGTERM
 */
int main() {
    struct sigaction sa;

    // Print pid, so that we can send signals from other shells
    pid_t pid = 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, SIGINT, and SIGUSR1
    if (sigaction(SIGHUP, &sa, NULL) == -1) {
        perror("Error: cannot handle SIGHUP"); // Should not happen
    }

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

    for (;;) {
        printf("\nMy pid is: %d\n", pid);
        printf("Suspending for ~15 seconds\n");
        wait_for_signal(15); // Later to be replaced with a SIGALRM
    }
}

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

    printf("handle_signal\n");

    // Find out which signal we're handling
    switch (signal) {
        case SIGHUP:
            signal_name = "SIGHUP";
            break;
        case SIGUSR1:
            signal_name = "SIGUSR1";
            break;
        case SIGINT:
  	    signal_name = "SIGINT (but I don't feel like exiting now)";
            break;
        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.
     */
    printf("Caught %s\n", signal_name);
}

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

    printf("Got SIGALRM\n");
}

void wait_for_signal(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");
}