lgamma

static VALUE
math_lgamma(VALUE unused_obj, VALUE x)
{
    double d;
    int sign=1;
    VALUE v;
    d = Get_Double(x);
    /* check for domain error */
    if (isinf(d)) {
        if (signbit(d)) domain_error("lgamma");
        return rb_assoc_new(DBL2NUM(INFINITY), INT2FIX(1));
    }
    v = DBL2NUM(lgamma_r(d, &sign));
    return rb_assoc_new(v, INT2FIX(sign));
}