[SDL] Re: [SDLnet] server shutdown produces strange client behavior (Windows XP)

[Benjamin Deutsch]

Hello,

>> ---
>> while (true) {
>>     int nr = SDLNet_CheckSockets(set, 0);
>>     int rdy = SDLNet_SocketReady(server);
>>
>>     if (( nr > 0) && (rdy != 0)) {
>>         cout << "nr: " << nr << "rdy: " << rdy << endl;
>>         // receive and print data
>>     }
>> }
>> ---
>>
>> Whenever the server suddenly closes, the client gets stuck in a loop 
>> telling me both nr and rdy are 1. So there is activity on the server 
>> socket, according to my client. But there is no data to receive. Is 
>> there a way for my client to handle this problem? I would like my 
>> client to say something like 'server offline' and quit. But I haven't 
>> found any function that shows me whether a connection has gone 
>> offline.
> 
> I did some additional testing, and it seems that when the client 
> suddenly closes the server gets into this loop as well. At the client,
> SDLNet_CheckSockets(set, 0) continuously  returns 1 and 
> SDLNet_SocketReady(server) too. But the received message is always 0 
> bytes long. Is this normal behavior? When a program is stuck in a loop 
> continuously receiving 0 byte messages, does this mean the other side 
> has incorrectly shut down the connection?

Incorrectly or correctly. On Win32 and Unix-like platforms, SDL_net uses 
recv() for reading from the socket (check the CVS code with the web 
interface: http://www.libsdl.org/cgi/cvsweb.cgi/SDL_net/ -> lates 
version of SDL_TCP). Try looking at the man page for recv, also 
available through Google: http://www.google.com/search?q=man+recv

Basically, recv has three possible return values r:
  r > 0 : Exactly r bytes (possibly less than you asked for) were read
  r = 0 : "End of file", the connection has been closed
  r < 0 : An error has occurred.

So a socket which has been closed will immediately return 0, and 
therefor is considered "ready" by select(). I think it's platform 
dependent how often 0 is returned before it becomes an error to read 
from the socket. Generally, after the first 0, you should assume the 
connection has been closed, and close the socket on the client side.

By the way, an excellent guide to networking is Beej's guide, if you 
don't know it yet: http://www.ecst.csuchico.edu/~beej/guide/net/ . It's 
Unix-centric, but the same principles apply to Win32 and many other 
operating systems, so it's a worthwhile read, in my opinion.

Good luck,
   Benjamin Deutsch