Date: Mon, 19 Jan 2004 17:53:05 +0100 From: Stuart Pook <Stuart.Pook@infres.enst.fr> To: freebsd-hackers@freebsd.org Subject: send(2) does not block, send(2) man page wrong? Message-ID: <E1Aice9-0002by-00@roo>
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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 is 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.
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?
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.
thanks for your help
Stuart
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