socket option IP_MSFILTER questions and observations

Hi,

I'm trying to use source specific multicast by setting the IP_MSFILTER 
socket option. I noticed i need to join the group with IP_ADD_MEMBERSHIP 
before setting the source filter. This is not the behavior i expected 
after reading RFC3678.
A potential issue i'm thinking of with this approach: an IGMP report 
could be sent after joining a group but before the filter is set.
Is there a way to prevent this?

Thanks,
Cedric

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