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.\" $OpenBSD: SSL_write.3,v 1.3 2016/12/06 12:24:33 schwarze Exp $
.\" OpenSSL 99d63d46 Oct 26 13:56:48 2016 -0400
.\"
.\" This file was written by Lutz Jaenicke <jaenicke@openssl.org>.
.\" Copyright (c) 2000, 2001, 2002 The OpenSSL Project. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\"
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\"
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in
.\" the documentation and/or other materials provided with the
.\" distribution.
.\"
.\" 3. All advertising materials mentioning features or use of this
.\" software must display the following acknowledgment:
.\" "This product includes software developed by the OpenSSL Project
.\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
.\"
.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
.\" endorse or promote products derived from this software without
.\" prior written permission. For written permission, please contact
.\" openssl-core@openssl.org.
.\"
.\" 5. Products derived from this software may not be called "OpenSSL"
.\" nor may "OpenSSL" appear in their names without prior written
.\" permission of the OpenSSL Project.
.\"
.\" 6. Redistributions of any form whatsoever must retain the following
.\" acknowledgment:
.\" "This product includes software developed by the OpenSSL Project
.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)"
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR
.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
.\" OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: December 6 2016 $
.Dt SSL_WRITE 3
.Os
.Sh NAME
.Nm SSL_write
.Nd write bytes to a TLS/SSL connection
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft int
.Fn SSL_write "SSL *ssl" "const void *buf" "int num"
.Sh DESCRIPTION
.Fn SSL_write
writes
.Fa num
bytes from the buffer
.Fa buf
into the specified
.Fa ssl
connection.
.Pp
If necessary,
.Fn SSL_write
will negotiate a TLS/SSL session, if not already explicitly performed by
.Xr SSL_connect 3
or
.Xr SSL_accept 3 .
If the peer requests a re-negotiation,
it will be performed transparently during the
.Fn SSL_write
operation.
The behaviour of
.Fn SSL_write
depends on the underlying
.Vt BIO .
.Pp
For the transparent negotiation to succeed, the
.Fa ssl
must have been initialized to client or server mode.
This is being done by calling
.Xr SSL_set_connect_state 3
or
.Xr SSL_set_accept_state 3
before the first call to an
.Xr SSL_read 3
or
.Fn SSL_write
function.
.Pp
If the underlying
.Vt BIO
is
.Em blocking ,
.Fn SSL_write
will only return once the write operation has been finished or an error
occurred, except when a renegotiation take place, in which case a
.Dv SSL_ERROR_WANT_READ
may occur.
This behaviour can be controlled with the
.Dv SSL_MODE_AUTO_RETRY
flag of the
.Xr SSL_CTX_set_mode 3
call.
.Pp
If the underlying
.Vt BIO
is
.Em non-blocking ,
.Fn SSL_write
will also return when the underlying
.Vt BIO
could not satisfy the needs of
.Fn SSL_write
to continue the operation.
In this case a call to
.Xr SSL_get_error 3
with the return value of
.Fn SSL_write
will yield
.Dv SSL_ERROR_WANT_READ
or
.Dv SSL_ERROR_WANT_WRITE .
As at any time a re-negotiation is possible, a call to
.Fn SSL_write
can also cause read operations!
The calling process then must repeat the call after taking appropriate action
to satisfy the needs of
.Fn SSL_write .
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 BIO before being able
to continue.
.Pp
.Fn SSL_write
will only return with success when the complete contents of
.Fa buf
of length
.Fa num
have been written.
This default behaviour can be changed with the
.Dv SSL_MODE_ENABLE_PARTIAL_WRITE
option of
.Xr SSL_CTX_set_mode 3 .
When this flag is set,
.Fn SSL_write
will also return with success when a partial write has been successfully
completed.
In this case the
.Fn SSL_write
operation is considered completed.
The bytes are sent and a new
.Fn SSL_write
operation with a new buffer (with the already sent bytes removed) must be
started.
A partial write is performed with the size of a message block,
which is 16kB.
.Pp
When an
.Fn SSL_write
operation has to be repeated because
.Xr SSL_get_error 3
returned
.Dv SSL_ERROR_WANT_READ
or
.Dv SSL_ERROR_WANT_WRITE ,
it must be repeated with the same arguments.
.Pp
When calling
.Fn SSL_write
with
.Fa num Ns =0
bytes to be sent, the behaviour is undefined.
.Sh RETURN VALUES
The following return values can occur:
.Bl -tag -width Ds
.It >0
The write operation was successful.
The return value is the number of bytes actually written to the TLS/SSL
connection.
.It 0
The write operation was not successful.
Probably the underlying connection was closed.
Call
.Xr SSL_get_error 3
with the return value to find out whether an error occurred or the connection
was shut down cleanly
.Pq Dv SSL_ERROR_ZERO_RETURN .
.It <0
The write operation was not successful, because either an error occurred or
action must be taken by the calling process.
Call
.Xr SSL_get_error 3
with the return value 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_CTX_new 3 ,
.Xr SSL_CTX_set_mode 3 ,
.Xr SSL_get_error 3 ,
.Xr SSL_read 3 ,
.Xr SSL_set_connect_state 3
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