On Tue, 7 Oct 2008, John Baldwin wrote:
On Tuesday 07 October 2008 04:57:55 pm Robert Watson wrote:
rwatson 2008-10-07 20:57:55 UTC
FreeBSD src repository
Modified files:
sys/kern uipc_socket.c
Log:
SVN rev 183675 on 2008-10-07 20:57:55Z by rwatson
In soreceive_dgram, when a 0-length buffer is passed into recv(2) and
no data is ready, return 0 rather than blocking or returning EAGAIN.
This is consistent with the behavior of soreceive_generic (soreceive)
in earlier versions of FreeBSD, and restores this behavior for UDP.
Discussed with: jhb, sam
MFC after: 3 days
I do find this behavior odd though. I would expect
recv(fd, NULL, 0)
to discard the next packet from the socket if one is available rather than
returning success and not doing anything (and it seems that this is what it
does both before and now). Similarly, I would expect recv(fd, NULL, 0) to
block on a blocking socket if there isn't a packet available. It would be
orthogonal then to return EAGAIN in this case (no packet available,
zero-length user buffer) on a non-blocking socket.
It seems that Solaris dropped this behavior (return 0) from their recv()
system call sometime after SunOS 4.0 from comments in the OpenSolaris
source. From reading __skb_recv_datagram(), it seems that Linux 2.6 returns
EAGAIN. NetBSD and OS X both have the odd behavior. OpenBSD has the odd
behavior, but with a caveat of sorts having to do with control messages.
OpenBSD cvsweb annotate is down though, so I haven't found the reason for
their change.
Yes, I agree it's odd, and I'm not sure I like it. I discovered the problem
while writing edge-case regression tests for socket receive to better exercise
soreceive_dgram, at first concluding it was a bug in soreceive_generic! My
feeling, though, is that I should leave behavior "compatible" for 7.1, and
perhaps we should change it for 7.2.
Robert N M Watson
Computer Laboratory
University of Cambridge
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