Programming timeout with sockets

simasimon · 05-01-2008, 11:14 AM

hi
i´m experiencing some client/server application programming with C. my idea is to be able to share files between client/server, but there is a problem i couldn´t solve for quiet some time.

i am trying to implement a connection timeout while receiving files.. the idea is that if i am receiving a file from the server suppose, and the server goes down, after X amount of seconds the client (which was receiving the file) should timeout.

here is what i tried

Code:

 nfds = sock + 1;
 errno = 0;
 FD_ZERO(&rfds);
        
 while(j < file_size){
       int st;
        
       FD_SET(sock, &rfds);

       printf("llamando a select \n");
       if((st = select(nfds, &rfds, (fd_set *)0, (fd_set *)0, &tv)) <= 0){
                if (errno == EINTR)
                          continue;
                else if (st == 0)
                          error ("Connection timeout\n");
                else
                          printf("Select Error: %d\n", errno);
        }
        
        if(FD_ISSET(sock, &rfds)){
        cc = read (sock, buf, buf_size);
        if (cc < 0)
                error ("Error reading socket\n");
        j += cc;
        if (fwrite (buf, sizeof (char), cc, file) < cc)
                error ("Error writing file\n");
        }
}

what happends is the following. i initiate the transfer of a big file. while transfer is going on, y temrinate the server... but the select function instead of making timeout as i expected, it just kept "rceiving" data, and it never timed out... any ideas?

ta0kira · 05-01-2008, 01:47 PM

First of all, you need to change this:

Code:

       if((st = select(nfds, &rfds, (fd_set *)0, (fd_set *)0, &tv)) <= 0){
                if (errno == EINTR)
                          continue;
                else if (st == 0)
                          error ("Connection timeout\n");
                else
                          printf("Select Error: %d\n", errno);
}

because continue will keep adding the descriptor to rfds:

Code:

while ((st = select(nfds, &rfds, (fd_set *)0, (fd_set *)0, &tv)) < 0 && errno == EINTR);
if (st <= 0)
 { 
if (st <  0) printf("Select Error: %d\n", errno);
else         error ("Connection timeout\n");
break;
 }
}

You should also FD_ZERO after every cycle. Lastly, you need to shutdown sockets when one end exits. Merely closing the descriptor or letting the program exit leaves the socket extant, so the other end doesn't see that the other end is permanently closed until shutdown occurs or the kernel realizes that nothing else can write to the socket.
ta0kira

PS You can replace the while (... && errno == EINTR) with TEMP_FAILURE_RETRY(...).

PPS You also need to re-init the timeout value at the beginning of each cycle since it might be modified as a result of select returning early.

simasimon · 05-02-2008, 09:26 AM

select continues returning 1 after i kill the client

ta0kira · 05-02-2008, 01:31 PM

Did you try it with the changes I suggested?
ta0kira

Wim Sturkenboom · 05-02-2008, 02:22 PM

Long ago that I programmed something like that, but if I remember correctly:

When the other side hangs up, select will indicate that the socket is readable and you have to read it. When you read it, you will get zero bytes, which indicates that the other side closed the connection and you can close the socket.
You do read, but you don't check for zero bytes (only less than zero).

simasimon · 05-02-2008, 02:36 PM

Yes i tried thank you taokira but wim saved the day
it works know, thanks a lot!!!

ta0kira · 05-03-2008, 12:16 AM

Quote:

Originally Posted by Wim Sturkenboom

When the other side hangs up, select will indicate that the socket is readable and you have to read it. When you read it, you will get zero bytes, which indicates that the other side closed the connection and you can close the socket.
You do read, but you don't check for zero bytes (only less than zero).

Yes, I did forget about that.
ta0kira