These functions return the exponent part of their argument as a signed integer.
When no error occurs, these functions are equivalent to the corresponding
logb() functions, cast to (int). An error will occur for zero and
infinity and NaN, and possibly for overflow.
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 to EDOM. If an error occurs and
(math_errhandling & MATH_ERREXCEPT) is non-zero, then the invalid
floating-point exception is raised.
A domain error occurs when x is zero or infinite (or too large, or too
small) or NaN. If x is zero, the constant FP_ILOGB0 is returned.
If x is NaN, the constant FP_ILOGBNAN is returned. If x
is infinite (or too large), INT_MAX is returned. If x is too small,
INT_MIN is returned.