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

[Andre Oppermann]

Stuart Pook wrote:
>>send() for UDP should block if the socket is filled and the interface
>>can't drain the data fast enough.
> 
> It doesn't (at least I cannot make it block)

This stuff is rather complex.  A send() on a UDP socket processes right
down to the if_output.  If that fails because the ifqueue is full, the
packet will be free()d right away.  No luck with blocking and retrying.

>>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.
> 
> Send(2) indicates that it should do so.

True.  The only thing I can offer is that when one packet produces an
ENOBUFS, the socket will block on the next one for a couple of milliseconds.
Doing the full program requires significant changes to the current structure
of the BSD network code.

You could do the same in userland with a call to nanosleep(2) when you get
a ENOBUFS.

>>>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?
> 
> NetBSD is the same as FreeBSD.  I have not tested OpenBSD.
> MacOS X is similiar to FreeBSD in that send doesn't block, howver
> the send does not give an error: the packet is just thrown away.

Browsing through the code I see that none of the BSDs are able to block
on an UDP send.

-- 
Andre