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1. 1. 1. Pass the interface number to use for transmission,
         Pass NULL otherwise.
      2. Return the interface number associated with the selected source address.

p_addr_src

Pointer to a variable to : 

Pass a suggested IPv6 source address,
Pass NULL otherwise.

p_addr_dest

Pointer to the IPv6 destination address.

p_addr_nexthop

Pointer to Next Hop IPv6 address found in the process of founding the best source address.

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