to_der

static VALUE
ossl_x509ext_to_der(VALUE obj)
{
    X509_EXTENSION *ext;
    unsigned char *p;
    long len;
    VALUE str;

    GetX509Ext(obj, ext);
    if((len = i2d_X509_EXTENSION(ext, NULL)) <= 0)
        ossl_raise(eX509ExtError, NULL);
    str = rb_str_new(0, len);
    p = (unsigned char *)RSTRING_PTR(str);
    if(i2d_X509_EXTENSION(ext, &p) < 0)
        ossl_raise(eX509ExtError, NULL);
    ossl_str_adjust(str, p);

    return str;
}