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=pod

=head1 NAME

SSL_do_handshake - perform a TLS/SSL handshake

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_do_handshake(SSL *ssl);

=head1 DESCRIPTION

SSL_do_handshake() will wait for a SSL/TLS handshake to take place. If the
connection is in client mode, the handshake will be started. The handshake
routines may have to be explicitly set in advance using either
L<SSL_set_connect_state(3)|SSL_set_connect_state(3)> or
L<SSL_set_accept_state(3)|SSL_set_accept_state(3)>.

=head1 NOTES

The behaviour of SSL_do_handshake() depends on the underlying BIO.

If the underlying BIO is B<blocking>, SSL_do_handshake() will only return
once the handshake has been finished or an error occurred, except for SGC
(Server Gated Cryptography). For SGC, SSL_do_handshake() may return with -1,
but SSL_get_error() will yield B<SSL_ERROR_WANT_READ/WRITE> and
SSL_do_handshake() should be called again.

If the underlying BIO is B<non-blocking>, SSL_do_handshake() will also return
when the underlying BIO could not satisfy the needs of SSL_do_handshake()
to continue the handshake. In this case a call to SSL_get_error() with the
return value of SSL_do_handshake() 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_do_handshake().
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.

=head1 RETURN VALUES

The following return values can occur:

=over 4

=item 1

The TLS/SSL handshake was successfully completed, a TLS/SSL connection has been
established.

=item 0

The TLS/SSL handshake was not successful but was shut down controlled and
by the specifications of the TLS/SSL protocol. Call SSL_get_error() with the
return value B<ret> to find out the reason.

=item E<lt>0

The TLS/SSL handshake was not successful because a fatal error occurred either
at the protocol level or a connection failure occurred. The shutdown was
not clean. It can also occur of action is need to continue the operation
for non-blocking BIOs. Call SSL_get_error() 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(3)|ssl(3)>, L<bio(3)|bio(3)>,
L<SSL_set_connect_state(3)|SSL_set_connect_state(3)>

=cut