On 4/08/2016 1:18 AM, Lundberg, Johannes wrote:
​Hi Alan

Thanks for the reply.

Can I still use the same receiving function for sendmsg/send and tell what
kind of message is coming?
How would I tell if there is an fd attached or not?

Even if I set cmsg_level and cmsg_type it won't let me send it. The problem
is having a zero length attachment on freebsd....
I can't send -1 as fd because that errors to invalid file descriptor.

On Wed, Aug 3, 2016 at 10:12 AM, Alan Somers <asom...@freebsd.org> wrote:

On Wed, Aug 3, 2016 at 10:54 AM, Lundberg, Johannes
<johan...@brilliantservice.co.jp> wrote:

I'm porting a project to fbsd and I have problem with this part that
in linux but not fbsd when fd = -1.


I get "invalid argument" from sendmsg() when setting CMSG_LEN(0).

Anyone have a clue how to correctly do this on fbsd?



It sounds like you're trying to send an empty cmsg.  The error may
happen because your msg_controllen field is inconsistent with your
cmsg_len field.  You're setting msg_controllen as if there were a full
cmsg, but  then cmsg_len says that there is no cmsg.  Or maybe the
error is because (just guessing) FreeBSD doesn't allow sending empty
or undefined cmsgs.  Notice that cmsg_level and cmsg_type are
undefined in the case where fd == -1.  POSIX doesn't say whether
sendmsg supports empty cmsgs, but why bother?  You could just use send
instead of sendmsg if you're not sending a file descriptor.


I think it's a standards interpretation thing.

what data do you send WITH the message? I assume you have some in-band data as well.

if you have no FD, just use "sendto()."

the other end will still be able to do a recvmesg() but will discover no added info.

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