send(2) does not block, send(2) man page wrong?

[Ruslan Ermilov]

On Fri, Jan 23, 2004 at 04:25:07PM +0100, 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.
> 
> 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.
> 
UDP sockets don't use send buffer (like TCP does) for storing data,
only receive buffer, so SO_SNDBUF only affects the maximum size of
one write operation.


Cheers,
-- 
Ruslan Ermilov
FreeBSD committer
ru at FreeBSD.org
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