Unidirectional shutdown is allowed according to the RFC.

session cache for further reuse.

The shutdown procedure consists of 2 steps: the sending of the "close notify"

shutdown alert and the receipt ion of the peer's "close notify" shutdown

alert:

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 the 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. In order to

complete the shutdown handshake, SSL_shutdown() must be called again.

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 call of e.g.

B<SSL_read(3)|SSL_read(3)>, SSL_shutdown() will send the "close notify"

alert and will immediately return with 1.

alert, set the SSL_SENT_SHUTDOWN flag and will immediately return with 1.

=back

=item 0

The shutdown is not yet finished. Call SSL_shutdown() for a second time.

The shutdown is not yet finished. Call SSL_shutdown() for a second time,

if a bidirectional shutdown shall be performed.

The output of L<SSL_get_error(3)|SSL_get_error(3)> may be misleading, as an

erroneous SSL_ERROR_SYSCALL may be flagged even though no error occurred.