| Current File : //usr/local/share/man/man3/SSL_read.3 |
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.\" ========================================================================
.\"
.IX Title "SSL_READ 3"
.TH SSL_READ 3 "2017-05-25" "1.1.0f" "OpenSSL"
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
SSL_read \- read bytes from a TLS/SSL connection
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/ssl.h>
\&
\& int SSL_read(SSL *ssl, void *buf, int num);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fISSL_read()\fR tries to read \fBnum\fR bytes from the specified \fBssl\fR into the
buffer \fBbuf\fR.
.SH "NOTES"
.IX Header "NOTES"
If necessary, \fISSL_read()\fR will negotiate a \s-1TLS/SSL\s0 session, if
not already explicitly performed by \fISSL_connect\fR\|(3) or
\&\fISSL_accept\fR\|(3). If the
peer requests a re-negotiation, it will be performed transparently during
the \fISSL_read()\fR operation. The behaviour of \fISSL_read()\fR depends on the
underlying \s-1BIO.\s0
.PP
For the transparent negotiation to succeed, the \fBssl\fR must have been
initialized to client or server mode. This is being done by calling
\&\fISSL_set_connect_state\fR\|(3) or \fISSL_set_accept_state()\fR
before the first call to an \fISSL_read()\fR or \fISSL_write\fR\|(3)
function.
.PP
\&\fISSL_read()\fR works based on the \s-1SSL/TLS\s0 records. The data are received in
records (with a maximum record size of 16kB for SSLv3/TLSv1). Only when a
record has been completely received, it can be processed (decryption and
check of integrity). Therefore data that was not retrieved at the last
call of \fISSL_read()\fR can still be buffered inside the \s-1SSL\s0 layer and will be
retrieved on the next call to \fISSL_read()\fR. If \fBnum\fR is higher than the
number of bytes buffered, \fISSL_read()\fR will return with the bytes buffered.
If no more bytes are in the buffer, \fISSL_read()\fR will trigger the processing
of the next record. Only when the record has been received and processed
completely, \fISSL_read()\fR will return reporting success. At most the contents
of the record will be returned. As the size of an \s-1SSL/TLS\s0 record may exceed
the maximum packet size of the underlying transport (e.g. \s-1TCP\s0), it may
be necessary to read several packets from the transport layer before the
record is complete and \fISSL_read()\fR can succeed.
.PP
If the underlying \s-1BIO\s0 is \fBblocking\fR, \fISSL_read()\fR will only return, once the
read operation has been finished or an error occurred, except when a
renegotiation take place, in which case a \s-1SSL_ERROR_WANT_READ\s0 may occur.
This behaviour can be controlled with the \s-1SSL_MODE_AUTO_RETRY\s0 flag of the
\&\fISSL_CTX_set_mode\fR\|(3) call.
.PP
If the underlying \s-1BIO\s0 is \fBnon-blocking\fR, \fISSL_read()\fR will also return
when the underlying \s-1BIO\s0 could not satisfy the needs of \fISSL_read()\fR
to continue the operation. In this case a call to
\&\fISSL_get_error\fR\|(3) with the
return value of \fISSL_read()\fR will yield \fB\s-1SSL_ERROR_WANT_READ\s0\fR or
\&\fB\s-1SSL_ERROR_WANT_WRITE\s0\fR. As at any time a re-negotiation is possible, a
call to \fISSL_read()\fR can also cause write operations! The calling process
then must repeat the call after taking appropriate action to satisfy the
needs of \fISSL_read()\fR. The action depends on the underlying \s-1BIO.\s0 When using a
non-blocking socket, nothing is to be done, but \fIselect()\fR can be used to check
for the required condition. When using a buffering \s-1BIO,\s0 like a \s-1BIO\s0 pair, data
must be written into or retrieved out of the \s-1BIO\s0 before being able to continue.
.PP
\&\fISSL_pending\fR\|(3) can be used to find out whether there
are buffered bytes available for immediate retrieval. In this case
\&\fISSL_read()\fR can be called without blocking or actually receiving new
data from the underlying socket.
.SH "WARNING"
.IX Header "WARNING"
When an \fISSL_read()\fR operation has to be repeated because of
\&\fB\s-1SSL_ERROR_WANT_READ\s0\fR or \fB\s-1SSL_ERROR_WANT_WRITE\s0\fR, it must be repeated
with the same arguments.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
The following return values can occur:
.IP "> 0" 4
.IX Item "> 0"
The read operation was successful.
The return value is the number of bytes actually read from the \s-1TLS/SSL\s0
connection.
.IP "<= 0" 4
.IX Item "<= 0"
The read operation was not successful, because either the connection was closed,
an error occurred or action must be taken by the calling process.
Call \fISSL_get_error\fR\|(3) with the return value \fBret\fR to find out the reason.
.Sp
Old documentation indicated a difference between 0 and \-1, and that \-1 was
retryable.
You should instead call \fISSL_get_error()\fR to find out if it's retryable.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fISSL_get_error\fR\|(3), \fISSL_write\fR\|(3),
\&\fISSL_CTX_set_mode\fR\|(3), \fISSL_CTX_new\fR\|(3),
\&\fISSL_connect\fR\|(3), \fISSL_accept\fR\|(3)
\&\fISSL_set_connect_state\fR\|(3),
\&\fISSL_pending\fR\|(3),
\&\fISSL_shutdown\fR\|(3), \fISSL_set_shutdown\fR\|(3),
\&\fIssl\fR\|(3), \fIbio\fR\|(3)
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2000\-2016 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.