Double FFFFFFFFFFFFFFFF is *a* IEEE 754 NaN. Anything of the form [7F]FFFxxxxxxxxxxxx where "x" is nonzero is a NaN.

There are at least two kinds of NaN: signaled and quiet. IEEE 754 doesn't define the how to encode them, but recommends using the most significant bit of "x" (0 for signaled).

In reply to Re^2: How to create nan/inf by ikegami
in thread How to create nan/inf by syphilis

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