to_der

static VALUE
ossl_x509attr_to_der(VALUE self)
{
    X509_ATTRIBUTE *attr;
    VALUE str;
    int len;
    unsigned char *p;

    GetX509Attr(self, attr);
    if((len = i2d_X509_ATTRIBUTE(attr, NULL)) <= 0)
        ossl_raise(eX509AttrError, NULL);
    str = rb_str_new(0, len);
    p = (unsigned char *)RSTRING_PTR(str);
    if(i2d_X509_ATTRIBUTE(attr, &p) <= 0)
        ossl_raise(eX509AttrError, NULL);
    rb_str_set_len(str, p - (unsigned char*)RSTRING_PTR(str));

    return str;
}