.\" $OpenBSD: SSL_read.3,v 1.8 2021/10/24 15:10:13 schwarze Exp $ .\" full merge up to: OpenSSL 5a2443ae Nov 14 11:37:36 2016 +0000 .\" partial merge up to: OpenSSL 24a535ea Sep 22 13:14:20 2020 +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. 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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: October 24 2021 $ .Dt SSL_READ 3 .Os .Sh NAME .Nm SSL_read_ex , .Nm SSL_read , .Nm SSL_peek_ex , .Nm SSL_peek .Nd read bytes from a TLS connection .Sh SYNOPSIS .In openssl/ssl.h .Ft int .Fn SSL_read_ex "SSL *ssl" "void *buf" "size_t num" "size_t *readbytes" .Ft int .Fn SSL_read "SSL *ssl" "void *buf" "int num" .Ft int .Fn SSL_peek_ex "SSL *ssl" "void *buf" "size_t num" "size_t *readbytes" .Ft int .Fn SSL_peek "SSL *ssl" "void *buf" "int num" .Sh DESCRIPTION .Fn SSL_read_ex and .Fn SSL_read try to read .Fa num bytes from the specified .Fa ssl into the buffer .Fa buf . On success .Fn SSL_read_ex stores the number of bytes actually read in .Pf * Fa readbytes . .Pp .Fn SSL_peek_ex and .Fn SSL_peek are identical to .Fn SSL_read_ex and .Fn SSL_read , respectively, except that no bytes are removed from the underlying BIO during the read, such that a subsequent call to .Fn SSL_read_ex or .Fn SSL_read will yield at least the same bytes once again. .Pp In the following, .Fn SSL_read_ex , .Fn SSL_read , .Fn SSL_peek_ex , 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 work 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 .Fn SSL_read_ex and .Fn SSL_peek_ex return 1 for success or 0 for failure. Success means that one or more application data bytes have been read from the SSL connection. Failure means that no bytes could be read from the SSL connection. Failures can be retryable (e.g. we are waiting for more bytes to be delivered by the network) or non-retryable (e.g. a fatal network error). In the event of a failure, call .Xr SSL_get_error 3 to find out the reason which indicates whether the call is retryable or not. .Pp For .Fn SSL_read and .Fn SSL_peek , 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 . .Pp .Fn SSL_read_ex and .Fn SSL_peek_ex first appeared in OpenSSL 1.1.1 and have been available since .Ox 7.1 .