+SSL_shutdown() tries to send the "close notify" shutdown alert to the peer.
+Whether the operation succeeds or not, the SSL_SENT_SHUTDOWN flag is set and
+a currently open session is considered closed and good and will be kept in the
+session cache for further reuse.
+
+The shutdown procedure consists of 2 steps: the sending of the "close notify"
+shutdown alert and the reception of the peer's "close notify" shutdown
+alert. According to the TLS standard, it is acceptable for an application
+to only send its shutdown alert and then close the underlying connection
+without waiting for the peer's response (this way resources can be saved,
+as the process can already terminate or serve another connection).
+When the underlying connection shall be used for more communications, the
+complete shutdown procedure (bidirectional "close notify" alerts) must be
+performed, so that the peers stay synchronized.
+
+SSL_shutdown() supports both uni- and bidirectional shutdown by its 2 step
+behaviour.
+
+=over 4
+
+=item When the application is the first party to send the "close notify"
+alert, SSL_shutdown() will only send the alert and then set the
+SSL_SENT_SHUTDOWN flag (so that the session is considered good and will
+be kept in cache). SSL_shutdown() will then return with 0. If a unidirectional
+shutdown is enough (the underlying connection shall be closed anyway), this
+first call to SSL_shutdown() is sufficient. In order to complete the
+bidirectional shutdown handshake, SSL_shutdown() must be called again.
+The second call will make SSL_shutdown() wait for the peer's "close notify"
+shutdown alert. On success, the second call to SSL_shutdown() will return
+with 1.
+
+=item If the peer already sent the "close notify" alert B<and> it was
+already processed implicitly inside another function
+(L<SSL_read(3)|SSL_read(3)>), the SSL_RECEIVED_SHUTDOWN flag is set.
+SSL_shutdown() will send the "close notify" alert, set the SSL_SENT_SHUTDOWN
+flag and will immediately return with 1.
+Whether SSL_RECEIVED_SHUTDOWN is already set can be checked using the
+SSL_get_shutdown() (see also L<SSL_set_shutdown(3)|SSL_set_shutdown(3)> call.
+
+=back
+
+It is therefore recommended, to check the return value of SSL_shutdown()
+and call SSL_shutdown() again, if the bidirectional shutdown is not yet
+complete (return value of the first call is 0). As the shutdown is not
+specially handled in the SSLv2 protocol, SSL_shutdown() will succeed on
+the first call.
+
+The behaviour of SSL_shutdown() additionally depends on the underlying BIO.
+
+If the underlying BIO is B<blocking>, SSL_shutdown() will only return once the
+handshake step has been finished or an error occurred.
+
+If the underlying BIO is B<non-blocking>, SSL_shutdown() will also return