Q:

Converting to Multi-Threaded Socket Application

As I am currently doing this project in only C, I've up untill this point only used my webserver as a single threaded application. However, I dont want that anymore! So I have the following code that handles my Work.
void BeginListen()
{
        CreateSocket();

        BindSocket();

        ListenOnSocket();

        while ( 1 )
        {
            ProcessConnections();
        }
}

Now I've added fork(); before the start of ProcessConnection(); which helpes me allowing multiple connections! However, when I add code for daemoning the application found in this answer. I've encounted a little problem, using fork() will create a copy of my whole  running app, which is the purpose of fork(). So, I'd like to solve this problem.
My ProcessConnection() looks like this 
void ProcessConnections()
{
        fork();

        addr_size = sizeof(connector);

        connecting_socket = accept(current_socket, (struct sockaddr *)&connector, &addr_size);

        if ( connecting_socket < 0 )
        {
                perror("Accepting sockets");
                exit(-1);
        }

        HandleCurrentConnection(connecting_socket);

        DisposeCurrentConnection();
}

How would I do to simply just add a couple of lines above or after connecting=socket = accept... in order to make it accept more than one connection at the time? Can i use fork(); but when it comes down to DisposeCurrentConnection(); I want to kill that process and just have the parent-thread running.

A:

I'm not a 100% sure what it is that you're trying to do, buy off the top of my head, I'd prefer to do the fork after the accept, and simply exit() when you're done. Keep in mind though, that you need to react to the SIGCHLD signal when the child process exits, otherwise you'll have a ton of zombie-processes hanging around, waiting to deliver their exit-status to the parent process. C-pseudo-code:
for (;;) {
  connecting_socket = accept(server_socket);
  if (connecting_socket < 0)
    {
      if (errno == EINTR)
        continue;
      else
        {
          // handle error
          break;
        }
    }

  if (! (child_pid = fork ()))
    {
       // child process, do work with connecting socket
       exit (0);
    }
  else if (child_pid > 0)
    {
      // parent process, keep track of child_pid if necessary.
    }
  else
    {
      // fork failed, unable to service request, send 503 or equivalent.
    }
}

The child_pid is needed to (as already mentioned) to kill the child-process, but also if you wish to use waitpid to collect the exit status.
Concerning the zombie-processes, if you're not interested in what happened to the process, you could install a signal hander for SIGCHLD and just loop on waitpid with -1 until it there are no more child-processes, like this
while (-1 != waitpid (-1, NULL, WNOHANG))
  /* no loop body */ ;

The waitpid function will return the pid of the child that exited, so if you wish you can correlate this to some other information about the connection (if you did keep track of the pid). Keep in mind that accept will probably exit with errno set to EINTR, without a valid connection if a SIGCHLD is caught, so remember to check for this on accepts return.
EDIT:
Don't forget to check for error conditions, i.e. fork returns -1.

