Retrieving IPv6 hop limit / flow header

[rhendriks]

set_recv_hoplimit_v6() can be used to

Set the value of the IPV6_RECVHOPLIMIT option for this socket.
The received hop limit is returned as ancillary data by recvmsg() only if the application has enabled the IPV6_RECVHOPLIMIT socket option

However, it is unclear how to retrieve the hop limit data when calling recvmsg()

It is also unclear how to set the flow flow label field for outgoing packets.
And how to retrieve the flow label value when calling recvmsg()