SSL_read(3) OpenSSL SSL_read(3)NAMESSL_read - read bytes from a TLS/SSL connection.
SYNOPSIS
#include <openssl/ssl.h>
int SSL_read(SSL *ssl, void *buf, int num);
DESCRIPTIONSSL_read() tries to read num bytes from the specified ssl
into the buffer buf.
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.
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 The read operation was not successful. The reason may
either be a clean shutdown due to a "close notify"
alert sent by the peer (in which case the
SSL_RECEIVED_SHUTDOWN flag in the ssl shutdown state
is set (see SSL_shutdown(3), SSL_set_shutdown(3)). It
is also possible, that the peer simply shut down the
underlying transport and the shutdown is incomplete.
Call SSL_get_error() with the return value ret to find
out, whether an error occurred or the connection was
shut down cleanly (SSL_ERROR_ZERO_RETURN).
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.
<0 The read operation was not successful, because either
an error occurred or action must be taken by the
calling process. Call SSL_get_error() with the return
value ret to find out the reason.
SEE ALSOSSL_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_shutdown(3),
SSL_set_shutdown(3), ssl(3), bio(3)13/Sep/2001 0.9.6j SSL_read(3)
