log2

static VALUE
math_log2(VALUE obj, VALUE x)
{
    double d0, d;

    Need_Float(x);
    d0 = RFLOAT_VALUE(x);
    /* check for domain error */
    if (d0 < 0.0) domain_error("log2");
    /* check for pole error */
    if (d0 == 0.0) return DBL2NUM(-INFINITY);
    d = log2(d0);
    return DBL2NUM(d);
}