Socket Blocking and Timeout on Select

Question

I am currently creating an echo server that disconnects clients after a maxWaitTime of being in idle.

I was hoping the program would block the socket until the client sent data but when I run the program in gdb it goes through the select and blocks on Readline.

I know retval = 0 whenever it goes through the select and that the fd_set sock goes to [256, (31 zeroes)] and after the select, sock goes to [32 zeroes].

The accepting of the connection happens in another function and the connection descriptor is passed to the echo function.

If you are able to help point me in the right direction or let me know how I can disconnect a client after a certain amount of time please let me know.

If you require any further information please let me know.

Thanks in advance!

    FD_ZERO(&sock);
    FD_SET(sockfd,&sock);

    int opt = 3;

    setsockopt(sockfd, SOL_SOCKET, SO_RCVLOWAT,&opt,sizeof(opt));

    timeout.tv_sec = maxWaitTime;
    timeout.tv_usec = 0;

    for ( ; ; ) {
            FD_SET(sockfd,&sock);

            printf("Set is %d\n",FD_ISSET(sockfd,&sock));

            int retval;
            retval = select(1, &sock, NULL, NULL, &timeout);

            if(retval)
            {
                    quitProgram(number);
            }
            else
            {
            printf("n is %d\n",retval);

            if ( (n = Readline(sockfd, line, MAXLINE)) == 0)
            {
                    return;         /* connection closed by other end */
            }

            Writen(sockfd, line, n);

    }

`

If this is on a POSIX machine (e.g. OSX or Linux) then the first argument to `select`should the the highest numbered socket plus one. In your case `sockfd + 1`. Also, I guess your timeout `maxWaitTime` is non-zero? — Some programmer dude, Sep 26 '12 at 12:55
read the man page of select() very carefully. 3 important things are: 1. The first argument to select must be the largest socket descriptor plus one 2. select returns 0 when the timeout occurs. 3. On Linux, the timeout argument is decremented by select() - thus your current code will eventually run with a timeout of 0 if you're on Linux — nos, Sep 26 '12 at 12:56
Another point, `select` returns `-1` on error, zero on timeout only, and a positive number if sockets are ready (actually the number of sockets ready in the three sets combined). So you can't just check if the return is non-zero for error. — Some programmer dude, Sep 26 '12 at 12:58

score 7 · Accepted Answer · answered Sep 26 '12 at 22:03

As others have commented, you have some logic holes in your code. By your own admission:

I know retval = 0 whenever it goes through the select and that the fd_set sock goes to [256, (31 zeroes)] and after the select, sock goes to [32 zeroes].

That should have been an indication to you that something was going wrong. The socket was not in the fd_set after select() exited, which meant the socket was not readible yet. retval=0 means select() timed out.

You have to reset not only the fd_set every time select() is called, but also the timeval as well. Try this instead:

int opt = 3; 
setsockopt(sockfd, SOL_SOCKET, SO_RCVLOWAT,&opt,sizeof(opt)); 

for ( ; ; )
{ 
    timeout.tv_sec = maxWaitTime; 
    timeout.tv_usec = 0; 

    FD_ZERO(&sock); 
    FD_SET(sockfd,&sock); 

    int retval = select(sockfd+1, &sock, NULL, NULL, &timeout); 
    if (retval <= 0) 
    { 
        quitProgram(number); /* error or connection timed out */
    } 
    else 
    { 
        if ( (n = Readline(sockfd, line, MAXLINE)) <= 0) 
        { 
            return; /* error or connection closed by other end */ 
        } 

        Writen(sockfd, line, n); 
    } 
}

Socket Blocking and Timeout on Select

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