How can I prevent zombie child processes?

Question

I am writing a server that uses fork() to spawn handlers for client connections. The server does not need to know about what happens to the forked processes – they work on their own, and when they're done, they should just die instead of becoming zombies. What is an easy way to accomplish this?

Answer 1

There are several ways, but using sigaction with SA_NOCLDWAIT in the parent process is probably the easiest one:

struct sigaction sigchld_action = {
  .sa_handler = SIG_DFL,
  .sa_flags = SA_NOCLDWAIT
};
sigaction(SIGCHLD, &sigchld_action, NULL);

Answer 2

Use double forks. Have your children immediately fork another copy and have the original child process exit.

http://thinkiii.blogspot.com/2009/12/double-fork-to-avoid-zombie-process.html

This is simpler than using signals, in my opinion, and more understandable.

void safe_fork()
{
  pid_t pid;
  if (!pid=fork()) {
    if (!fork()) {
      /* this is the child that keeps going */
      do_something(); /* or exec */
    } else {
      /* the first child process exits */
      exit(0);
    }
  } else {
    /* this is the original process */  
    /* wait for the first child to exit which it will immediately */
    waitpid(pid);
  }
}