doc/ssl/SSL_shutdown.pod - third_party/openssl - Git at Google

 =pod

 =head1 NAME

 SSL_shutdown - shut down a TLS/SSL connection

 =head1 SYNOPSIS

  #include <openssl/ssl.h>

  int SSL_shutdown(SSL *ssl);

 =head1 DESCRIPTION

 SSL_shutdown() shuts down an active TLS/SSL connection. It sends the
 "close notify" shutdown alert to the peer.

 =head1 NOTES

 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).

 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
 when the underlying BIO could not satisfy the needs of SSL_shutdown()
 to continue the handshake. In this case a call to SSL_get_error() with the
 return value of SSL_shutdown() will yield B<SSL_ERROR_WANT_READ> or
 B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after
 taking appropriate action to satisfy the needs of SSL_shutdown().
 The action depends on the underlying BIO. When using a non-blocking socket,
 nothing is to be done, but select() can be used to check for the required
 condition. When using a buffering BIO, like a BIO pair, data must be written
 into or retrieved out of the BIO before being able to continue.

 SSL_shutdown() can be modified to only set the connection to "shutdown"
 state but not actually send the "close notify" alert messages,
 see L<SSL_CTX_set_quiet_shutdown(3)|SSL_CTX_set_quiet_shutdown(3)>.
 When "quiet shutdown" is enabled, SSL_shutdown() will always succeed
 and return 1.

 =head1 RETURN VALUES

 The following return values can occur:

 =over 4

 =item Z<>0

 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.

 =item Z<>1

 The shutdown was successfully completed. The "close notify" alert was sent
 and the peer's "close notify" alert was received.

 =item E<lt>0

 The shutdown was not successful because a fatal error occurred either
 at the protocol level or a connection failure occurred. It can also occur if
 action is need to continue the operation for non-blocking BIOs.
 Call L<SSL_get_error(3)|SSL_get_error(3)> with the return value B<ret>
 to find out the reason.

 =back

 =head1 SEE ALSO

 L<SSL_get_error(3)|SSL_get_error(3)>, L<SSL_connect(3)|SSL_connect(3)>,
 L<SSL_accept(3)|SSL_accept(3)>, L<SSL_set_shutdown(3)|SSL_set_shutdown(3)>,
 L<SSL_CTX_set_quiet_shutdown(3)|SSL_CTX_set_quiet_shutdown(3)>,
 L<SSL_clear(3)|SSL_clear(3)>, L<SSL_free(3)|SSL_free(3)>,
 L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>

 =cut
	=pod

	=head1 NAME

	SSL_shutdown - shut down a TLS/SSL connection

	=head1 SYNOPSIS

	#include <openssl/ssl.h>

	int SSL_shutdown(SSL *ssl);

	=head1 DESCRIPTION

	SSL_shutdown() shuts down an active TLS/SSL connection. It sends the
	"close notify" shutdown alert to the peer.

	=head1 NOTES

	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).

	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
	when the underlying BIO could not satisfy the needs of SSL_shutdown()
	to continue the handshake. In this case a call to SSL_get_error() with the
	return value of SSL_shutdown() will yield B<SSL_ERROR_WANT_READ> or
	B<SSL_ERROR_WANT_WRITE>. The calling process then must repeat the call after
	taking appropriate action to satisfy the needs of SSL_shutdown().
	The action depends on the underlying BIO. When using a non-blocking socket,
	nothing is to be done, but select() can be used to check for the required
	condition. When using a buffering BIO, like a BIO pair, data must be written
	into or retrieved out of the BIO before being able to continue.

	SSL_shutdown() can be modified to only set the connection to "shutdown"
	state but not actually send the "close notify" alert messages,
	see L<SSL_CTX_set_quiet_shutdown(3)\|SSL_CTX_set_quiet_shutdown(3)>.
	When "quiet shutdown" is enabled, SSL_shutdown() will always succeed
	and return 1.

	=head1 RETURN VALUES

	The following return values can occur:

	=over 4

	=item Z<>0

	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.

	=item Z<>1

	The shutdown was successfully completed. The "close notify" alert was sent
	and the peer's "close notify" alert was received.

	=item E<lt>0

	The shutdown was not successful because a fatal error occurred either
	at the protocol level or a connection failure occurred. It can also occur if
	action is need to continue the operation for non-blocking BIOs.
	Call L<SSL_get_error(3)\|SSL_get_error(3)> with the return value B<ret>
	to find out the reason.

	=back

	=head1 SEE ALSO

	L<SSL_get_error(3)\|SSL_get_error(3)>, L<SSL_connect(3)\|SSL_connect(3)>,
	L<SSL_accept(3)\|SSL_accept(3)>, L<SSL_set_shutdown(3)\|SSL_set_shutdown(3)>,
	L<SSL_CTX_set_quiet_shutdown(3)\|SSL_CTX_set_quiet_shutdown(3)>,
	L<SSL_clear(3)\|SSL_clear(3)>, L<SSL_free(3)\|SSL_free(3)>,
	L<ssl(3)\|ssl(3)>, L<bio(3)\|bio(3)>

	=cut