Re: sendmmsg() on NetBSD vs Linux

[Robert Elz <kre%munnari.OZ.AU@localhost>]

    Date:        Sun, 1 Nov 2015 15:41:04 +0000 (UTC)
    From:        mlelstv%serpens.de@localhost (Michael van Elst)
    Message-ID:  <n15bqg$3og$1%serpens.de@localhost>

  | That is a bug in the NetBSD sendmmsg() implementation.

I'm not sure that's necessarily true.   On neither Linux nor NetBSD
is the msg_hdr field of the struct mmsghdr declared const (the
mmsghdr itself cannot be, as the msg_len field is intended to be modified).

To me this suggests that user code should not rely upon the sendmmsg()
call leaving the msg_hdr field untouched (on either linux or NetBSD).

Certainly nothing in the man pages (on either system) suggests that
doing so is safe.

Relying on undocumented behaviour of the actual linux implementation
is not wise.

Whether or not NetBSD should be copying back the modified msg_hdr is
a whole other question - it seems to me, that the sys call would run
faster if only the msg_len fields were copied back, the msghdr struct
is 3 pointers, 2 socklen_t's and 2 int's (plus likely alignment padding).
Maybe doing one big copyout() is better than a bunch of much smaller
ones, and maybe it depends upon the actual way that the kernel/user
mem mapping is achieved - it might be better one way on some ports and
the other on others.

kre