1.\" $OpenBSD: SSL_write.3,v 1.5 2018/03/27 17:35:50 schwarze Exp $ 2.\" OpenSSL 99d63d46 Oct 26 13:56:48 2016 -0400 3.\" 4.\" This file was written by Lutz Jaenicke <jaenicke@openssl.org>. 5.\" Copyright (c) 2000, 2001, 2002 The OpenSSL Project. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in 16.\" the documentation and/or other materials provided with the 17.\" distribution. 18.\" 19.\" 3. All advertising materials mentioning features or use of this 20.\" software must display the following acknowledgment: 21.\" "This product includes software developed by the OpenSSL Project 22.\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)" 23.\" 24.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to 25.\" endorse or promote products derived from this software without 26.\" prior written permission. For written permission, please contact 27.\" openssl-core@openssl.org. 28.\" 29.\" 5. Products derived from this software may not be called "OpenSSL" 30.\" nor may "OpenSSL" appear in their names without prior written 31.\" permission of the OpenSSL Project. 32.\" 33.\" 6. Redistributions of any form whatsoever must retain the following 34.\" acknowledgment: 35.\" "This product includes software developed by the OpenSSL Project 36.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)" 37.\" 38.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY 39.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 40.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 41.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR 42.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 43.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 44.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 45.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 46.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 47.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 48.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED 49.\" OF THE POSSIBILITY OF SUCH DAMAGE. 50.\" 51.Dd $Mdocdate: March 27 2018 $ 52.Dt SSL_WRITE 3 53.Os 54.Sh NAME 55.Nm SSL_write 56.Nd write bytes to a TLS/SSL connection 57.Sh SYNOPSIS 58.In openssl/ssl.h 59.Ft int 60.Fn SSL_write "SSL *ssl" "const void *buf" "int num" 61.Sh DESCRIPTION 62.Fn SSL_write 63writes 64.Fa num 65bytes from the buffer 66.Fa buf 67into the specified 68.Fa ssl 69connection. 70.Pp 71If necessary, 72.Fn SSL_write 73will negotiate a TLS/SSL session, if not already explicitly performed by 74.Xr SSL_connect 3 75or 76.Xr SSL_accept 3 . 77If the peer requests a re-negotiation, 78it will be performed transparently during the 79.Fn SSL_write 80operation. 81The behaviour of 82.Fn SSL_write 83depends on the underlying 84.Vt BIO . 85.Pp 86For the transparent negotiation to succeed, the 87.Fa ssl 88must have been initialized to client or server mode. 89This is being done by calling 90.Xr SSL_set_connect_state 3 91or 92.Xr SSL_set_accept_state 3 93before the first call to an 94.Xr SSL_read 3 95or 96.Fn SSL_write 97function. 98.Pp 99If the underlying 100.Vt BIO 101is 102.Em blocking , 103.Fn SSL_write 104will only return once the write operation has been finished or an error 105occurred, except when a renegotiation take place, in which case a 106.Dv SSL_ERROR_WANT_READ 107may occur. 108This behaviour can be controlled with the 109.Dv SSL_MODE_AUTO_RETRY 110flag of the 111.Xr SSL_CTX_set_mode 3 112call. 113.Pp 114If the underlying 115.Vt BIO 116is 117.Em non-blocking , 118.Fn SSL_write 119will also return when the underlying 120.Vt BIO 121could not satisfy the needs of 122.Fn SSL_write 123to continue the operation. 124In this case a call to 125.Xr SSL_get_error 3 126with the return value of 127.Fn SSL_write 128will yield 129.Dv SSL_ERROR_WANT_READ 130or 131.Dv SSL_ERROR_WANT_WRITE . 132As at any time a re-negotiation is possible, a call to 133.Fn SSL_write 134can also cause read operations! 135The calling process then must repeat the call after taking appropriate action 136to satisfy the needs of 137.Fn SSL_write . 138The action depends on the underlying 139.Vt BIO . 140When using a non-blocking socket, nothing is to be done, but 141.Xr select 2 142can be used to check for the required condition. 143When using a buffering 144.Vt BIO , 145like a 146.Vt BIO 147pair, data must be written into or retrieved out of the BIO before being able 148to continue. 149.Pp 150.Fn SSL_write 151will only return with success when the complete contents of 152.Fa buf 153of length 154.Fa num 155have been written. 156This default behaviour can be changed with the 157.Dv SSL_MODE_ENABLE_PARTIAL_WRITE 158option of 159.Xr SSL_CTX_set_mode 3 . 160When this flag is set, 161.Fn SSL_write 162will also return with success when a partial write has been successfully 163completed. 164In this case the 165.Fn SSL_write 166operation is considered completed. 167The bytes are sent and a new 168.Fn SSL_write 169operation with a new buffer (with the already sent bytes removed) must be 170started. 171A partial write is performed with the size of a message block, 172which is 16kB. 173.Pp 174When an 175.Fn SSL_write 176operation has to be repeated because 177.Xr SSL_get_error 3 178returned 179.Dv SSL_ERROR_WANT_READ 180or 181.Dv SSL_ERROR_WANT_WRITE , 182it must be repeated with the same arguments. 183.Pp 184When calling 185.Fn SSL_write 186with 187.Fa num Ns =0 188bytes to be sent, the behaviour is undefined. 189.Sh RETURN VALUES 190The following return values can occur: 191.Bl -tag -width Ds 192.It >0 193The write operation was successful. 194The return value is the number of bytes actually written to the TLS/SSL 195connection. 196.It 0 197The write operation was not successful. 198Probably the underlying connection was closed. 199Call 200.Xr SSL_get_error 3 201with the return value to find out whether an error occurred or the connection 202was shut down cleanly 203.Pq Dv SSL_ERROR_ZERO_RETURN . 204.It <0 205The write operation was not successful, because either an error occurred or 206action must be taken by the calling process. 207Call 208.Xr SSL_get_error 3 209with the return value to find out the reason. 210.El 211.Sh SEE ALSO 212.Xr BIO_new 3 , 213.Xr ssl 3 , 214.Xr SSL_accept 3 , 215.Xr SSL_connect 3 , 216.Xr SSL_CTX_new 3 , 217.Xr SSL_CTX_set_mode 3 , 218.Xr SSL_get_error 3 , 219.Xr SSL_read 3 , 220.Xr SSL_set_connect_state 3 221.Sh HISTORY 222.Fn SSL_write 223appeared in SSLeay 0.4 or earlier and has been available since 224.Ox 2.4 . 225