OpenSSL::SSL::SSLSocket#alpn_protocol

static VALUE
ossl_ssl_alpn_protocol(VALUE self)
{
    SSL *ssl;
    const unsigned char *out;
    unsigned int outlen;

    GetSSL(self, ssl);

    SSL_get0_alpn_selected(ssl, &out, &outlen);
    if (!outlen)
        return Qnil;
    else
        return rb_str_new((const char *) out, outlen);
}