Fork process: Resume parent process while children terminated (Linux)

I’ve written a C++ application that waits until an event occurs (e.g. wait for incoming connection). After that event occured, it will continue by forking a child process that handles that event.

So, my code looks essentially like this:

int main(void) {
  while(1) {
    Event e = waitUntilSomethingHappens(); // blocks execution until event occurs
    pid_t pid = fork(); 
    if (pid == 0) {
      doChildStuff(e); 
      exit(0); 
      return 0; 
    }
  }
  return 0;
}

My expectation now was that the child process will terminate (because of exit(0) and/or return). Indeed, it leaves the while loop, but it seems not to be terminated. When I hit ps -e, both processes are displayed, whereas the child process is marked as <defunct>.

Why doesn’t it disappear? What can I do to let it disappear?

Answer

You have to use one of the wait calls to get a notification for the state change of the child.

From the manual:

A child that terminates, but has not been waited for becomes a “zombie”. The kernel maintains a minimal set of information about the zombie process (PID, termination status, resource usage information) in order to allow the parent to later perform a wait to obtain information about the child. As long as a zombie is not removed from the system via a wait, it will consume a slot in the kernel process table, and if this table fills, it will not be possible to create further processes. If a parent process terminates, then its “zombie” children (if any) are adopted by init(8), which automatically performs a wait to remove the zombies.