.\"	$OpenBSD: SSL_do_handshake.3,v 1.4 2016/12/16 15:39:08 jmc Exp $
.\"	OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
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.\" This file was written by Martin Sjoegren <martin@strakt.com>.
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.Dd $Mdocdate: December 16 2016 $
.Dt SSL_DO_HANDSHAKE 3
.Os
.Sh NAME
.Nm SSL_do_handshake
.Nd perform a TLS/SSL handshake
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft int
.Fn SSL_do_handshake "SSL *ssl"
.Sh DESCRIPTION
.Fn 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
.Xr SSL_set_connect_state 3
or
.Xr SSL_set_accept_state 3 .
.Pp
The behaviour of
.Fn SSL_do_handshake
depends on the underlying
.Vt BIO .
.Pp
If the underlying
.Vt BIO
is
.Em blocking ,
.Fn SSL_do_handshake
will only return once the handshake has been finished or an error occurred.
.Pp
If the underlying
.Vt BIO
is
.Em non-blocking ,
.Fn SSL_do_handshake
will also return when the underlying
.Vt BIO
could not satisfy the needs of
.Fn SSL_do_handshake
to continue the handshake.
In this case a call to
.Xr SSL_get_error 3
with the return value of
.Fn SSL_do_handshake
will yield
.Dv SSL_ERROR_WANT_READ
or
.Dv SSL_ERROR_WANT_WRITE .
The calling process then must repeat the call after taking appropriate action
to satisfy the needs of
.Fn SSL_do_handshake .
The action depends on the underlying
.Vt BIO .
When using a non-blocking socket, nothing is to be done, but
.Xr select 2
can be used to check for the required condition.
When using a buffering
.Vt BIO ,
like a
.Vt BIO
pair, data must be written into or retrieved out of the
.Vt BIO
before being able to continue.
.Sh RETURN VALUES
The following return values can occur:
.Bl -tag -width Ds
.It 0
The TLS/SSL handshake was not successful but was shut down controlled and
by the specifications of the TLS/SSL protocol.
Call
.Xr SSL_get_error 3
with the return value
.Fa ret
to find out the reason.
.It 1
The TLS/SSL handshake was successfully completed,
and a TLS/SSL connection has been established.
.It <0
The TLS/SSL handshake was not successful because either a fatal error occurred
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
.Vt BIO Ns s .
Call
.Xr SSL_get_error 3
with the return value
.Fa ret
to find out the reason.
.El
.Sh SEE ALSO
.Xr BIO_new 3 ,
.Xr ssl 3 ,
.Xr SSL_accept 3 ,
.Xr SSL_connect 3 ,
.Xr SSL_get_error 3 ,
.Xr SSL_set_connect_state 3