.\" $OpenBSD: SSL_read.3,v 1.7 2020/05/26 19:45:58 schwarze Exp $ .\" full merge up to: OpenSSL 99d63d46 Oct 26 13:56:48 2016 -0400 .\" partial merge up to: OpenSSL 18bad535 Apr 9 15:13:55 2019 +0100 .\" .\" This file was written by Lutz Jaenicke and .\" Matt Caswell . .\" Copyright (c) 2000, 2001, 2008, 2016 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, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" 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: May 26 2020 $ .Dt SSL_READ 3 .Os .Sh NAME .Nm SSL_read , .Nm SSL_peek .Nd read bytes from a TLS connection .Sh SYNOPSIS .In openssl/ssl.h .Ft int .Fn SSL_read "SSL *ssl" "void *buf" "int num" .Ft int .Fn SSL_peek "SSL *ssl" "void *buf" "int num" .Sh DESCRIPTION .Fn SSL_read tries to read .Fa num bytes from the specified .Fa ssl into the buffer .Fa buf . .Pp .Fn SSL_peek is identical to .Fn SSL_read except that no bytes are removed from the underlying BIO during the read, such that a subsequent call to .Fn SSL_read will yield at least the same bytes once again. .Pp In the following, .Fn SSL_read and .Fn SSL_peek are called .Dq read functions . .Pp If necessary, a read function will negotiate a TLS 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 read function operation. The behaviour of the read functions 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 done by calling .Xr SSL_set_connect_state 3 or .Xr SSL_set_accept_state 3 before the first call to a read function. .Pp The read functions works based on the TLS records. The data are received in records (with a maximum record size of 16kB). Only when a record has been completely received, it can be processed (decrypted and checked for integrity). Therefore data that was not retrieved at the last read call can still be buffered inside the TLS layer and will be retrieved on the next read call. If .Fa num is higher than the number of bytes buffered, the read functions will return with the bytes buffered. If no more bytes are in the buffer, the read functions will trigger the processing of the next record. Only when the record has been received and processed completely will the read functions return reporting success. At most the contents of the record will be returned. As the size of a TLS record may exceed the maximum packet size of the underlying transport (e.g., TCP), it may be necessary to read several packets from the transport layer before the record is complete and the read call can succeed. .Pp If the underlying .Vt BIO is blocking, a read function will only return once the read operation has been finished or an error occurred, except when a renegotiation takes place, in which case an .Dv SSL_ERROR_WANT_READ may occur. This behavior 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 non-blocking, a read function will also return when the underlying .Vt BIO could not satisfy the needs of the function to continue the operation. In this case a call to .Xr SSL_get_error 3 with the return value of the read function will yield .Dv SSL_ERROR_WANT_READ or .Dv SSL_ERROR_WANT_WRITE . As at any time a re-negotiation is possible, a read function may also cause write operations. The calling process must then repeat the call after taking appropriate action to satisfy the needs of the read function. 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. .Pp .Xr SSL_pending 3 can be used to find out whether there are buffered bytes available for immediate retrieval. In this case a read function can be called without blocking or actually receiving new data from the underlying socket. .Pp When a read function operation has to be repeated because of .Dv SSL_ERROR_WANT_READ or .Dv SSL_ERROR_WANT_WRITE , it must be repeated with the same arguments. .Sh RETURN VALUES The following return values can occur: .Bl -tag -width Ds .It >0 The read operation was successful. The return value is the number of bytes actually read from the TLS connection. .It 0 The read operation was not successful. The reason may either be a clean shutdown due to a .Dq close notify alert sent by the peer (in which case the .Dv SSL_RECEIVED_SHUTDOWN flag in the ssl shutdown state is set (see .Xr SSL_shutdown 3 and .Xr SSL_set_shutdown 3 ) . It is also possible that the peer simply shut down the underlying transport and the shutdown is incomplete. 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 read 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_pending 3 , .Xr SSL_set_connect_state 3 , .Xr SSL_set_shutdown 3 , .Xr SSL_shutdown 3 , .Xr SSL_write 3 .Sh HISTORY .Fn SSL_read appeared in SSLeay 0.4 or earlier. .Fn SSL_peek first appeared in SSLeay 0.6.6. Both functions have been available since .Ox 2.4 .