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https://gist.github.com/aspyct/3462238

 

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

/**
  * 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 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.
  */
  printf("Caught %s, sleeping for ~3 seconds\n"
  "Try sending another SIGHUP / SIGINT / SIGALRM "
  "(or more) meanwhile\n", signal_name);
  /*
  * Indeed, all signals are blocked during this handler
  * 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
  */
  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");
  }

 

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