> then, how the kernel know the alignment constraint userland have
> used?

By looking at the cmsg_len value.

> there's no way to tell, as cmsg_len() does not include padding

That's exactly what I was asking for the rationale for, and implicitly
suggesting changing.

Did you actually read what I wrote?

> (see RFC2292 page 19 for diagram)

Thank you.  Finally a spec.

Yes, looking at 2292, I see.  My question now becomes "why was it done
this way, rather than including the padding in cmsg_len".  But as you
didn't write 2292, I can't expect you to answer that.

Now, would it really have been so difficult to answer "because RFC2292
says so" to my first message?

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