int tls_construct_client_hello(SSL *s)
{
    unsigned char *buf;
    unsigned char *p, *d;
    int i;
    int protverr;
    unsigned long l;
    int al = 0;
#ifndef OPENSSL_NO_COMP
    int j;
    SSL_COMP *comp;
#endif
    SSL_SESSION *sess = s->session;

    buf = (unsigned char *)s->init_buf->data;

    /* Work out what SSL/TLS/DTLS version to use */
    protverr = ssl_set_client_hello_version(s);
    if (protverr != 0) {
        SSLerr(SSL_F_TLS_CONSTRUCT_CLIENT_HELLO, protverr);
        goto err;
    }

    if ((sess == NULL) || !ssl_version_supported(s, sess->ssl_version) ||
        /*
         * In the case of EAP-FAST, we can have a pre-shared
         * "ticket" without a session ID.
         */
        (!sess->session_id_length && !sess->tlsext_tick) ||
        (sess->not_resumable)) {
        if (!ssl_get_new_session(s, 0))
            goto err;
    }
    /* else use the pre-loaded session */

    p = s->s3->client_random;

    /*
     * for DTLS if client_random is initialized, reuse it, we are
     * required to use same upon reply to HelloVerify
     */
    if (SSL_IS_DTLS(s)) {
        size_t idx;
        i = 1;
        for (idx = 0; idx < sizeof(s->s3->client_random); idx++) {
            if (p[idx]) {
                i = 0;
                break;
            }
        }
    } else
        i = 1;

    if (i && ssl_fill_hello_random(s, 0, p, sizeof(s->s3->client_random)) <= 0)
        goto err;

    /* Do the message type and length last */
    d = p = ssl_handshake_start(s);

    /*-
     * version indicates the negotiated version: for example from
     * an SSLv2/v3 compatible client hello). The client_version
     * field is the maximum version we permit and it is also
     * used in RSA encrypted premaster secrets. Some servers can
     * choke if we initially report a higher version then
     * renegotiate to a lower one in the premaster secret. This
     * didn't happen with TLS 1.0 as most servers supported it
     * but it can with TLS 1.1 or later if the server only supports
     * 1.0.
     *
     * Possible scenario with previous logic:
     *      1. Client hello indicates TLS 1.2
     *      2. Server hello says TLS 1.0
     *      3. RSA encrypted premaster secret uses 1.2.
     *      4. Handshake proceeds using TLS 1.0.
     *      5. Server sends hello request to renegotiate.
     *      6. Client hello indicates TLS v1.0 as we now
     *         know that is maximum server supports.
     *      7. Server chokes on RSA encrypted premaster secret
     *         containing version 1.0.
     *
     * For interoperability it should be OK to always use the
     * maximum version we support in client hello and then rely
     * on the checking of version to ensure the servers isn't
     * being inconsistent: for example initially negotiating with
     * TLS 1.0 and renegotiating with TLS 1.2. We do this by using
     * client_version in client hello and not resetting it to
     * the negotiated version.
     */
    *(p++) = s->client_version >> 8;
    *(p++) = s->client_version & 0xff;

    /* Random stuff */
    memcpy(p, s->s3->client_random, SSL3_RANDOM_SIZE);
    p += SSL3_RANDOM_SIZE;

    /* Session ID */
    if (s->new_session)
        i = 0;
    else
        i = s->session->session_id_length;
    *(p++) = i;
    if (i != 0) {
        if (i > (int)sizeof(s->session->session_id)) {
            SSLerr(SSL_F_TLS_CONSTRUCT_CLIENT_HELLO, ERR_R_INTERNAL_ERROR);
            goto err;
        }
        memcpy(p, s->session->session_id, i);
        p += i;
    }

    /* cookie stuff for DTLS */
    if (SSL_IS_DTLS(s)) {
        if (s->d1->cookie_len > sizeof(s->d1->cookie)) {
            SSLerr(SSL_F_TLS_CONSTRUCT_CLIENT_HELLO, ERR_R_INTERNAL_ERROR);
            goto err;
        }
        *(p++) = s->d1->cookie_len;
        memcpy(p, s->d1->cookie, s->d1->cookie_len);
        p += s->d1->cookie_len;
    }

    /* Ciphers supported */
    i = ssl_cipher_list_to_bytes(s, SSL_get_ciphers(s), &(p[2]));
    if (i == 0) {
        SSLerr(SSL_F_TLS_CONSTRUCT_CLIENT_HELLO, SSL_R_NO_CIPHERS_AVAILABLE);
        goto err;
    }
#ifdef OPENSSL_MAX_TLS1_2_CIPHER_LENGTH
    /*
     * Some servers hang if client hello > 256 bytes as hack workaround
     * chop number of supported ciphers to keep it well below this if we
     * use TLS v1.2
     */
    if (TLS1_get_version(s) >= TLS1_2_VERSION
        && i > OPENSSL_MAX_TLS1_2_CIPHER_LENGTH)
        i = OPENSSL_MAX_TLS1_2_CIPHER_LENGTH & ~1;
#endif
    s2n(i, p);
    p += i;

    /* COMPRESSION */
#ifdef OPENSSL_NO_COMP
    *(p++) = 1;
#else

    if (!ssl_allow_compression(s) || !s->ctx->comp_methods)
        j = 0;
    else
        j = sk_SSL_COMP_num(s->ctx->comp_methods);
    *(p++) = 1 + j;
    for (i = 0; i < j; i++) {
        comp = sk_SSL_COMP_value(s->ctx->comp_methods, i);
        *(p++) = comp->id;
    }
#endif
    *(p++) = 0;                 /* Add the NULL method */

    /* TLS extensions */
    if (ssl_prepare_clienthello_tlsext(s) <= 0) {
        SSLerr(SSL_F_TLS_CONSTRUCT_CLIENT_HELLO, SSL_R_CLIENTHELLO_TLSEXT);
        goto err;
    }
    if ((p =
         ssl_add_clienthello_tlsext(s, p, buf + SSL3_RT_MAX_PLAIN_LENGTH,
                                    &al)) == NULL) {
        ssl3_send_alert(s, SSL3_AL_FATAL, al);
        SSLerr(SSL_F_TLS_CONSTRUCT_CLIENT_HELLO, ERR_R_INTERNAL_ERROR);
        goto err;
    }

    l = p - d;
    if (!ssl_set_handshake_header(s, SSL3_MT_CLIENT_HELLO, l)) {
        ssl3_send_alert(s, SSL3_AL_FATAL, SSL_AD_HANDSHAKE_FAILURE);
        SSLerr(SSL_F_TLS_CONSTRUCT_CLIENT_HELLO, ERR_R_INTERNAL_ERROR);
        goto err;
    }

    return 1;
 err:
    ossl_statem_set_error(s);
    return 0;
}