save_client_hello.c

static int save_client_hello(ptls_on_client_hello_t *self, ptls_t *tls, ptls_iovec_t server_name, const ptls_iovec_t *protocols,
                             size_t num_protocols, const uint16_t *signature_algorithms, size_t num_signature_algorithms)
{
    ptls_set_server_name(tls, (const char *)server_name.base, server_name.len);
#if 0 // you might want to save ALPN as well
    ptls_set_negotiated_protocol(tls, (const char *)protocols[0].base, protocols[0].len);
#endif
    return 0;
}

static ptls_context_t ctx = ...;
static ptls_on_client_hello_t on_client_hello = {save_client_hello};

ctx.on_client_hello = &on_client_hello;