=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 an 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)> or
L<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.

If the underlying BIO is B<nonblocking>, 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 nonblocking 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 Z<>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 Z<>1

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

=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 if action is needed to continue the operation
for nonblocking 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)>, L<SSL_connect(3)>,
L<SSL_accept(3)>, L<ssl(7)>, L<bio(7)>,
L<SSL_set_connect_state(3)>

=head1 COPYRIGHT

Copyright 2002-2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut
