Short SIGCHLD sample

// Short SIGCHLD sample. For details, visit:
// <https://www.ipa.go.jp/security/awareness/vendor/programmingv1/b07_04.html>

#include <sys/types.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void setup_SIGCHLD(void);
void catch_SIGCHLD(int signo);

int main(void)
{
  int i;

  puts("main start.");

  setup_SIGCHLD();

  for (i = 0; i < 3; i++)
  {
    pid_t child_pid = fork();

    if(child_pid == -1) {
      puts("fork error.");
    }
    else if(child_pid == 0) {
      sleep(1);
      exit(0);
    }

    printf("child %d spawned.\n", child_pid);
    sleep(1);
  }

  puts("Press Ctrl+C to exit.");
  while(1) {
    sleep(1);
  }
}

void setup_SIGCHLD(void)
{
  struct sigaction act;
  memset(&act, 0, sizeof(act));
  act.sa_handler = catch_SIGCHLD;
  sigemptyset(&act.sa_mask);
  act.sa_flags = SA_NOCLDSTOP | SA_RESTART;
  sigaction(SIGCHLD, &act, NULL);
}

void catch_SIGCHLD(int signo)
{
  pid_t child_pid = 0;
  puts("SIGCHLD trapped.");

#if 1
  do {
    int child_ret;
    child_pid = waitpid(-1, &child_ret, WNOHANG);
    printf("waitpid => %d, %d\n", child_pid, child_ret);
  } while(child_pid > 0);
#endif
}