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

[Julian Elischer]

On Fri, 23 Jan 2004, Andre Oppermann wrote:

> 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.

from memory that is what ping -f does..


> 
> >>>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
> 
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