> 1) It hasn't mattered until this point in the discussion, but I
> actually use writev(2) for the write. Thus we would need a sendv(2)
> system call.

No - sendmsg() with the various flag and control fields set
appropriately amounts to sendv():

ssize_t sendv(int s, struct iovec *msg, int msglen, int flags)
{
 struct msghdr mh;

 mh.msg_name = 0;
 mh.msg_namelen = 0;
 mh.msg_iov = msg;
 mh.msg_iovlen = msglen;
 mh.msg_control = 0;
 mh.msg_controllen = 0;
 mh.msg_flags = 0;
 return(sendmsg(s,&mh,flags));
}

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