#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);
}