.\"	$NetBSD: SSL_read.3,v 1.16 2017/01/27 23:00:49 spz Exp $
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.\" ========================================================================
.\"
.IX Title "SSL_read 3"
.TH SSL_read 3 "2017-01-27" "1.0.2k" "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 "LIBRARY"
libcrypto, -lcrypto
.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"
.PD 0
.IP "<0" 4
.IX Item "<0"
.PD
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
SSLv2 (deprecated) does not support a shutdown alert protocol, so it can
only be detected, whether the underlying connection was closed. It cannot
be checked, whether the closure was initiated by the peer or by something
else.
.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), \fIopenssl_bio\fR\|(3)