The logb() function computes the exponent of x, which is the integral part of log_r |x|, as a signed floating point value, for non-zero x, where r is the radix of the machine's floating-point arithmetic.

Upon successful completion, logb() returns the exponent of x.

If x is 0.0, logb() returns -HUGE_VAL and sets errno to EDOM.

If x is +-Inf, logb() returns +Inf.

If x is NaN, logb() returns NaN.

For exceptional cases, matherr(3M) tabulates the values to be returned as dictated by various Standards.

The logb() function will fail if:

EDOM

The x argument is 0.0.

See attributes(5) for descriptions of the following attributes:

ilogb(3M), matherr(3M), attributes(5)