SSL_read - read bytes from a TLS/SSL connection.
#include <openssl/ssl.h> int SSL_read(SSL *ssl, void *buf, int num);
SSL_read(3openssl) OpenSSL SSL_read(3openssl)
NAME
SSL_read - read bytes from a TLS/SSL connection.
SYNOPSIS
#include <openssl/ssl.h>
int SSL_read(SSL *ssl, void *buf, int num);
DESCRIPTION
SSL_read() tries to read num bytes from the specified ssl into the
buffer buf.
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
+---------------+--------------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+--------------------------+
|Availability | library/security/openssl |
+---------------+--------------------------+
|Stability | Pass-through uncommitted |
+---------------+--------------------------+
NOTES
If necessary, SSL_read() will negotiate a TLS/SSL session, if not
already explicitly performed by SSL_connect(3) or SSL_accept(3). If the
peer requests a re-negotiation, it will be performed transparently
during the SSL_read() operation. The behaviour of SSL_read() depends on
the underlying BIO.
For the transparent negotiation to succeed, the ssl must have been
initialized to client or server mode. This is being done by calling
SSL_set_connect_state(3) or SSL_set_accept_state() before the first
call to an SSL_read() or SSL_write(3) function.
SSL_read() works based on the SSL/TLS 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 SSL_read() can still be buffered inside the SSL layer and
will be retrieved on the next call to SSL_read(). If num is higher than
the number of bytes buffered, SSL_read() will return with the bytes
buffered. If no more bytes are in the buffer, SSL_read() will trigger
the processing of the next record. Only when the record has been
received and processed completely, SSL_read() will return reporting
success. At most the contents of the record will be returned. As the
size of an SSL/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
SSL_read() can succeed.
If the underlying BIO is blocking, SSL_read() will only return, once
the read operation has been finished or an error occurred, except when
a renegotiation take place, in which case a SSL_ERROR_WANT_READ may
occur. This behaviour can be controlled with the SSL_MODE_AUTO_RETRY
flag of the SSL_CTX_set_mode(3) call.
If the underlying BIO is non-blocking, SSL_read() will also return when
the underlying BIO could not satisfy the needs of SSL_read() to
continue the operation. In this case a call to SSL_get_error(3) with
the return value of SSL_read() will yield SSL_ERROR_WANT_READ or
SSL_ERROR_WANT_WRITE. As at any time a re-negotiation is possible, a
call to SSL_read() can also cause write operations! The calling process
then must repeat the call after taking appropriate action to satisfy
the needs of SSL_read(). The action depends on the underlying BIO. When
using a non-blocking socket, nothing is to be done, but select() can be
used to check for the required condition. When using a buffering BIO,
like a BIO pair, data must be written into or retrieved out of the BIO
before being able to continue.
SSL_pending(3) can be used to find out whether there are buffered bytes
available for immediate retrieval. In this case SSL_read() can be
called without blocking or actually receiving new data from the
underlying socket.
Source code for open source software components in Oracle Solaris can
be found at https://www.oracle.com/downloads/opensource/solaris-source-
code-downloads.html.
This software was built from source available at
https://github.com/oracle/solaris-userland. The original community
source was downloaded from
https://www.openssl.org/source/openssl-1.0.2ze.tar.gz.
Further information about this software can be found on the open source
community website at https://www.openssl.org/.
WARNING
When an SSL_read() operation has to be repeated because of
SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE, it must be repeated with
the same arguments.
RETURN VALUES
The following return values can occur:
> 0 The read operation was successful. The return value is the number
of bytes actually read from the TLS/SSL connection.
<= 0
<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 SSL_get_error(3) with the return value
ret to find out the reason.
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.
Old documentation indicated a difference between 0 and -1, and that
-1 was retryable. You should instead call SSL_get_error() to find
out if it's retryable.
SEE ALSO
SSL_get_error(3), SSL_write(3), SSL_CTX_set_mode(3), SSL_CTX_new(3),
SSL_connect(3), SSL_accept(3) SSL_set_connect_state(3), SSL_pending(3),
SSL_shutdown(3), SSL_set_shutdown(3), ssl(3), bio(3)
1.0.2ze 2022-05-03 SSL_read(3openssl)