e TLS or DTLS protocols.

6.5.1.1 TLS protocol
....................

There are no special requirements for the TLS protocol operation in
non-blocking mode if a non-blocking socket is used.

It is recommended, however, for future compatibility, when in
non-blocking mode, to call the *note gnutls_init:: function with the
‘GNUTLS_NONBLOCK’ flag set (see *note Session initialization::).

6.5.1.2 Datagram TLS protocol
.............................

When in non-blocking mode the function, the *note gnutls_init:: function
must be called with the ‘GNUTLS_NONBLOCK’ flag set (see *note Session
initialization::).

In contrast with the TLS protocol, the pull timeout function is
required, but will only be called with a timeout of zero.  In that case
it should indicate whether there are data to be received or not.  When
not using the default pull function, then *note
gnutls_transport_set_pull_timeout_function:: should be called.

Although in the TLS protocol implementation each call to receive or send
function implies to restoring the same function that was interrupted, in
the DTLS protocol this requirement isn't true.  There are cases where a
retransmission is required, which are indicated by a received message
and thus *note gnutls_record_get_direction:: must be called to decide
which direction to check prior to restoring a function call.

 -- Function: int gnutls_record_get_direction (gnutls_session_t SESSION)
     SESSION: is a ‘gnutls_session_t’ type.

     This function is useful to determine whether a GnuTLS function was
     interrupted while sending or receiving, so that ‘select()’ or
     ‘poll()’ may be called appropriately.

     It provides information about the internals of the record protocol
     and is only useful if a prior gnutls function call, e.g.
     ‘gnutls_handshake()’ , was interrupted and returned
     ‘GNUTLS_E_INTERRUPTED’ or ‘GNUTLS_E_AGAIN’ .  After such an
     interrupt applications may call ‘select()’ or ‘poll()’ before
     restoring the interrupted GnuTLS function.

     This function's output is unreliable if you are using the same
     ‘session’ in different threads for sending and receiving.

     *Returns:* 0 if interrupted while trying to read data, or 1 while
     trying to write data.

When calling *note gnutls_handshake:: through a multi-plexer, to be able
to handle properly the DTLS handshake retransmission timers, the
function *note gnutls_dtls_get_timeout:: should be used to estimate when
to call *note gnutls_handshake:: if no data have been received.