[Live-devel] Correct use of 'select' to avoid packet loss in Linux+BSD; correct use of WSAGetLastError and codes

[Aurélien Nephtali]

On Thu, Apr 9, 2009 at 9:52 AM, Ross Finlayson <finlayson at live555.com> wrote:
> Thanks for the suggestion; I like this.
>
> Unless anyone sees a problem with this, I will include it in the next
> release of the software.

Hello,

On Linux, select() is implemented above poll(). In net/core/datagram.c
(linux 2.6.26 sources), in datagram_poll() you can see the lines that
check for readable event on the socket :

       /* readable? */
        if (!skb_queue_empty(&sk->sk_receive_queue) ||
            (sk->sk_shutdown & RCV_SHUTDOWN))
                mask |= POLLIN | POLLRDNORM;

The event is set if the socket input buffer is not empty. On FreeBSD
it is the same thing :

sys/kern/uipc_socket.c in sopoll_generic() :

        if (events & (POLLIN | POLLRDNORM))
                if (soreadable(so))
                        revents |= events & (POLLIN | POLLRDNORM);

sys/sys/socketvar.h :

#define soreadable(so) \
    ((so)->so_rcv.sb_cc >= (so)->so_rcv.sb_lowat || \
        ((so)->so_rcv.sb_state & SBS_CANTRCVMORE) || \
        !TAILQ_EMPTY(&(so)->so_comp) || (so)->so_error)

-- 
Aurélien Nephtali