New SHM layout from gpsd

[Hal Murray]

The 31 bit idea seems strange/ugly to me.  How did you decide to do it that 
way?

Why is it better than 32 unsigned bits?  Is there some case that works with 31 
bits that breaks with 32?


I think there is a case that works for 32 unsigned that doesn't work for 31.  
Consider code that gets updated to use 64 bit time_t but they forget to update 
the SHM interface.  That will pick up the 32nd bit and do the right think for 
another 68 years.

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An alternative would be to make the new high-bit slots into 64 bits and make 
the rule use-them, ignore the old slot.  That would eat 2 more dummy words.


-- 
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