=pod

=head1 NAME

SSL_connect - initiate the TLS/SSL handshake with an TLS/SSL server

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_connect(SSL *ssl);

=head1 DESCRIPTION

SSL_connect() initiates the TLS/SSL handshake with a server. The communication
channel must already have been set and assigned to the B<ssl> by setting an
underlying B<BIO>.

=head1 NOTES

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

If the underlying BIO is B<blocking>, SSL_connect() will only return once the
handshake has been finished or an error occurred.

If the underlying BIO is B<non-blocking>, SSL_connect() will also return
when the underlying BIO could not satisfy the needs of SSL_connect()
to continue the handshake, indicating the problem by the return value -1.
In this case a call to SSL_get_error() with the
return value of SSL_connect() 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_connect().
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.

Many systems implement Nagle's algorithm by default which means that it will
buffer outgoing TCP data if a TCP packet has already been sent for which no
corresponding ACK has been received yet from the peer. This can have performance
impacts after a successful TLSv1.3 handshake or a successful TLSv1.2 (or below)
resumption handshake, because the last peer to communicate in the handshake is
the client. If the client is also the first to send application data (as is
typical for many protocols) then this data could be buffered until an ACK has
been received for the final handshake message.

The B<TCP_NODELAY> socket option is often available to disable Nagle's
algorithm. If an application opts to disable Nagle's algorithm consideration
should be given to turning it back on again later if appropriate. The helper
function BIO_set_tcp_ndelay() can be used to turn on or off the B<TCP_NODELAY>
option.

=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 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)>, L<SSL_accept(3)>,
L<SSL_shutdown(3)>, L<ssl(7)>, L<bio(7)>,
L<SSL_set_connect_state(3)>,
L<SSL_do_handshake(3)>,
L<SSL_CTX_new(3)>

=head1 COPYRIGHT

Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (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
