Date: Fri, 23 Jan 2004 16:50:33 +0100 From: Andre Oppermann <andre@freebsd.org> To: Stuart Pook <Stuart.Pook@infres.enst.fr> Cc: freebsd-hackers@freebsd.org Subject: Re: send(2) does not block, send(2) man page wrong? Message-ID: <401142C9.70502@freebsd.org> In-Reply-To: <E1Ak3BD-0006pn-00@roo> References: <E1Ak3BD-0006pn-00@roo>
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Stuart Pook wrote:
> The documentation for send(2) says
>
> If no messages space is available at the socket to hold the message to be
> transmitted, then send() normally blocks, unless the socket has been
> placed in non-blocking I/O mode. The select(2) call may be used to
> determine when it is possible to send more data.
>
> I cannot get send (or sendto which is what really interests me)
> to block on FreeBSD 4.9. When I send as fast as I can to a socket,
> send rapidly fails with ENOBUFS. I am not surprised that the kernel is
> running out of mbufs but I am surprised that send does not block until
> more become available.
>
> Select does not block either. It always says that I can write to the
> socket and then send fails with ENOBUFS.
send() for UDP should block if the socket is filled and the interface
can't drain the data fast enough.
> The udp_output function in /sys/netinet/udp_usrreq.c, seems clear:
>
> /*
> * Calculate data length and get a mbuf
> * for UDP and IP headers.
> */
> M_PREPEND(m, sizeof(struct udpiphdr), M_DONTWAIT);
> if (m == 0) {
> error = ENOBUFS;
> if (addr)
> splx(s);
> goto release;
> }
>
> There is no sign of send blocking waiting for a mbuf or of it returning
> EAGAIN if the socket is non-blocking.
>
> Is the documentation for send(2) wrong or is there some way to make
> send and sendto block?
Good question. There is not feedback loop like in tcp, so handling this
blocking and releasing would be a little bit harder to do for UDP.
> I have used setsockopt(s, SOL_SOCKET, SO_SNDBUF) to reduce the size
> of the output queue for the socket but send still returns ENOBUFS and
> never blocks or returns EAGAIN.
>
> I note that send on Linux and Solaris blocks and that on these systems
> select can be used to wait until the send will not block.
>
> I have written a test program,
> http://www.infres.enst.fr/~pook/send/server.c, that shows that send does
> not block on FreeBSD. It does with Linux and Solaris.
Do you know what the behaviour of Net- and/or OpenBSD is?
--
Andre
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