Closing a socket

[Swiftle]

Hi, I'm doing a client/server with threads instead of forks. The goal is to have multiple clients connect to the server and for each client the server makes a new thread. Although it's working atm, there are things that bother me, especially the sockets. The first thing I want to know is (when using the standard fork model) why the listening socket is closed immediately after the fork?

while(1){
    
    new_fd = accept(sockfd, 
		    (struct sockaddr *)&their_addr,
		    &sin_size);

    if (new_fd == -1) {
      perror("Serveur: accept");
    }
    
    printf("Serveur:  connection recue du client %s\n",
	   inet_ntoa(their_addr.sin_addr));
    
    if (fork()==0) {  
      /* this is the child process */
      close(sockfd); 

      if (send(new_fd, "Hi client!\n",11,0)==-1) {
	perror("Serveur: send");
	return EXIT_FAILURE;
      }

      return EXIT_SUCCESS;
    }
    
    close(new_fd);  
  }

The second thing I want to know is since I'm using threads I don't copy anything, so if I close the listening socket it's closed for the entire program. My question: is letting the socket open OK or not/ do I need multiple sockets?