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to atomically return either an existing set of IP multicast options for the PCB, or a newlly allocated set with default values. The inpcb is returned locked. This function may sleep. Call ip_moptions() to acquire a reference to a PCB's socket options, and perform the update of the options while holding the PCB lock. Release the lock before returning. Remove garbage collection of multicast options when values return to the default, as this complicates locking substantially. Most applications allocate a socket either to be multicast, or not, and don't tend to keep around sockets that have previously been used for multicast, then used for unicast. This closes a number of race conditions involving multiple threads or processes modifying the IP multicast state of a socket simultaenously. MFC after: 7 days
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