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SSL_read(3) OpenSSL SSL_read(3)
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SSL_read - read bytes from a TLS/SSL connection.
int SSL_read(SSL *ssl, void *buf, int num);
SSL_read() tries to read num bytes from the specified ssl into the
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 dur-
ing 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 ini-
tialized 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 con-
tinue 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.
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.
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 pos-
sible, 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.
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_shut-
down(3), SSL_set_shutdown(3), ssl(3), openssl_bio(3)
0.9.9-dev 2002-06-09 SSL_read(3)