#include <math.h>

/*
special cases:
        logb(+-0) = -inf, and raise divbyzero
        logb(+-inf) = +inf
        logb(nan) = nan
*/

double logb(double x)
{
        if (!isfinite(x))
                return x * x;
        if (x == 0)
                return -1/(x*x);
        return ilogb(x);
}