#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <signal.h>
#include <spawn.h>
#include <unistd.h>
#include <sys/wait.h>
#include <errno.h>
#include <stdatomic.h>

static volatile sig_atomic_t children_left = 0;

static void
int_to_string(int number, char *buf)
{
  int divisor = 1;
  int digit_count = 1;
  int i = 0;

  while (number / divisor >= 10)
  {
      divisor *= 10;
      digit_count++;
  }

  while (digit_count--)
  {
      buf[i++] = '0' + (number / divisor);
      number %= divisor;
      divisor /= 10;
  }
}

static void
child_died()
{
        char buf[] = "4294967295";
        int total;

        __sync_fetch_and_sub(&children_left, 1);

        total = children_left;
        int_to_string(total, buf);
        access(buf, R_OK);
}

int
main(int argc __attribute__((__unused__)), char *argv[])
{
        struct sigaction sa = { 0 };
        int wait_time;

        sigemptyset(&sa.sa_mask);
        sa.sa_sigaction = child_died;
        sa.sa_flags = SA_RESTART | SA_SIGINFO | SA_NODEFER | SA_NOCLDSTOP;

        sigaction(SIGCHLD, &sa, NULL);

        pid_t pid;
        char * const args[] = { "/bin/sleep", "1", NULL };
        int status = -1;
        int i;
        for (i = 0 ; i < 256 ; i++) {
                status = posix_spawn(&pid, args[0], NULL, NULL, args, NULL);
                if (status != 0) {
                        fprintf(stderr, "%s: posix_spawn() FAILED, "
                                "status=%d\n", argv[0], status);
                } else {
                        children_left++;
                }
        }

        wait_time = 3;
        while (wait_time > 0) {
            wait_time = sleep(wait_time);
        }

        if (children_left != 0) {
            fprintf(stderr, "%d children didn't notify!\n", children_left);
            return EXIT_FAILURE;
        }

        return EXIT_SUCCESS;
}