problem with select() and sockets...

[Adam Balgach]

I am having a bit of a problem, and it might have to do with my
understanding of how select works...

basically what i am trying to simulate is a system:
client->gateway->server model. The client breaks a file up into
sections and sends them to the server, through teh gateway which just
forwards info. Now upon getting the info, the server sends back an
ACK through the gateway, when the client sees the ack it sends the
next part (simeple stop_and_wait protocol) now my server is set to
randomly drop packets. and here is the problem i am having.

when no packets get dropped, everything works fine. if the first ACK
gets dropped from teh server then the client goes into a weird loop,
even if the server resends that ACK, the client never sees info coming
to it.

here is the code:

int sock;
fd_set fds;
struct time_val tv;

tv.tv_sec=3; //set timeout to 3sec
FD_ZERO(&fds);
if ((sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) //open new
socket
WriteLogDie("so cket() failed to open a new socket",client_ log);

FD_SET(sock, &fds);

if (bind(sock, (struct sockaddr *) &ClientAddr,
sizeof(ClientAd dr)) < 0) /*bind to local addr*/

if (packet==0)
SendFileName();
if (packet>0)
SendData();

select (sock+1, &fds,NULL,NULL, &tv);

if(select(sock+ 1,&fds,NULL, NULL, &tv)
RecievePacket() ;
else {
//TIMEOUT
cout << "got a timeout\n";
do {
ResendPacket();
cout << "Resending\ n";
}while (select(sock+1, &fds,NULL,NULL, &tv));
}

This code is nested in a hwile loop that just checks to see when were
done sending. This works perfectly if no packets are dropped. but if
the first ACK from the server is dropped, it goes into a loop, and
even if the gateway forwards information with the ACK later to the
client, it never shows that select() chooses to get info from the
socket. i dont know why. does anyone?

Thanks,
Adam

[Artie Gold]

Adam Balgach wrote:

I am having a bit of a problem, and it might have to do with my
understanding of how select works...

Whoops. It looks like you misspelled
news:comp.unix. programmer

(right down the hall).

[IOW, C++ knows of neither sockets nor select -- they are POSIXisms.]

HTH,
--ag

--
Artie Gold -- Austin, Texas

"If you don't think it matters, you're not paying attention."