These functions extract the exponent from the
internal floating-point representation of
x and return it as a floating-point value.
The integer constant
FLT_RADIX, defined in
<float.h>, indicates the radix used for the system’s floating-point representation.
If
FLT_RADIX is 2,
logb(x) is equal to
floor(log2(x)), except that it is probably faster.

If
x is subnormal,
logb() returns the exponent
x would have if it were normalized.

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