[Freeciv-Dev] Re: (PR#3391) SDL client polls the network

[Rafał Bursig <bursig@xxxxxxxxx>]

> > Currently the SDL client monitors the network connection by polling
> > every 100 milliseconds for data.  This leads to a lot of lag in
> updates.
> >
> > The correct thing to do is to get an interrupt or callback that is
> > activated when network data is available.  This is what the other
> > clients do (although it is possible that the underlying libraries
> just
> > do polling, we would hope not).  However, in my brief research I
> don't
> > see how to do this with SDL.  Is it possible that this functionality
> is
> > not portable?  How could anyone ever program on a system that didn't
> > provide this?
>
> Rafal asked me yesterday the question. I searched a bit and found
> nothing. The canonical solution looks like this:
>  Thread 1:
>     1) select on network input
>     2) push an event in the SDL event queue
>     3) goto 1)
>  Thread 2:
>     1) wait for SDL event queue
>     2) act according to the event (mouse, network)
>     3) goto 1)
>
> The other is polling (see our current code).
>
> I completely agree that this is a big design error in SDL.
>
> Lets take a look 
> (http://www.libsdl.org/cgi/cvsweb.cgi/SDL12/src/events/SDL_events.c?rev=1.9&content-type=text/x-cvsweb-markup):
> /* Run the system dependent event loops */
> void SDL_PumpEvents(void)
> {
>   if (!SDL_EventThread) {
>     SDL_VideoDevice *video = current_video;
>     SDL_VideoDevice *this  = current_video;
>
>     /* Get events from the video subsystem */
>     if (video) {
>       video->PumpEvents(this);
>     }
>
>     /* Queue pending key-repeat events */
>     SDL_CheckKeyRepeat();
>
> #ifndef DISABLE_JOYSTICK
>     /* Check for joystick state change */
>     if (SDL_numjoysticks && (SDL_eventstate & SDL_JOYEVENTMASK)) {
>       SDL_JoystickUpdate();
>     }
> #endif
>   }
> }
>
> int SDL_WaitEvent(SDL_Event * event)
> {
>    while (1) {
>       SDL_PumpEvents();
>       switch (SDL_PeepEvents(event, 1, SDL_GETEVENT, SDL_ALLEVENTS)) {
>         case -1: return 0;
>         case 1:  return 1;
>         case 0:  SDL_Delay(10);
>       }
>    }
> }
>
> So this is even worse. It has a built in polling every 10ms.
Hi

As I say before SDL is Simple DirectMedia Layer that was design to work 
on many enviroment.
Event code was create also for work on no-threaded system ( Windows 
???? ) and with single thread program. In this role work great but it 
doesn't mean its good solution :(
Some time before Bob Pendelton fight with this problem and he creatae 
thread based event code
(see: http://gameprogrammer.com/game.html )

we can use his code ( is small see attach ) and write somthing like :

/* Secoudary thread */
static int socket_thread(void *socket)
{
  struct timeval tv;
  fd_set civfdset;
  SDL_Event ev;
    ev.type = SDL_USEREVENT;
  ev.user.code = 0;
  ev.user.data1 = 0;
  ev.user.data2 = 0;
    while (net_socket >= 0) {
    FD_ZERO(&civfdset);
    FD_SET(net_socket, &civfdset);
    tv.tv_sec = 0;
    tv.tv_usec = 0;

    if (select(FD_SETSIZE, &civfdset, NULL, NULL, &tv)) {
      if (FD_ISSET(net_socket, &civfdset)) {
        FE_PushEvent(&ev);
      }
    }       } /* while */

  return 0;
}

/* Primary Thread */

void add_net_input(int sock)
{
  freelog(LOG_DEBUG, "Connection UP (%d)", sock);
    net_socket = sock;
    pThread = SDL_CreateThread(socket_thread, NULL);

}

void remove_net_input(void)
{
  net_socket = (-1);
  freelog(LOG_DEBUG, "Connection DOWN... ");
    SDL_WaitThread(pThread, NULL);

}  
and in main event loop :
    FE_WaitEvent(&Main.event);
    switch (Main.event.type) {
    case SDL_QUIT:
      return;
    case SDL_USEREVENT:
      input_from_server(net_socket);
    break;
...

But when I use it I can't make debug with gdb :(

Rafal


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