These functions extract the exponent of x and return it as a
floating-point value. If FLT_RADIX is two, logb(x)
is equal to floor(log2(x)), except it's probably faster.
If x is de-normalized, logb() returns the exponent x would
have if it were normalized.
If x is zero, -HUGE_VAL (resp. -HUGE_VALF, -HUGE_VALL) is returned, and a
pole error occurs. If x is infinite, plus infinity is returned. If
x is NaN, NaN is returned.
In order to check for errors, set errno to zero and call
feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return,
if errno is non-zero or fetestexcept(FE_INVALID | FE_DIVBYZERO |
FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.
If an error occurs and (math_errhandling & MATH_ERRNO) is non-zero,
then errno is set toERANGE. If an error occurs and
(math_errhandling & MATH_ERREXCEPT) is non-zero, then the
divide-by-zero floating-point exception is raised.
A pole error occurs when x is zero.