SSL_read
SSL_read(3) OpenSSL SSL_read(3)
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.
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 neces-
sary 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 pro-
cess then must repeat the call after taking appropriate action to sat-
isfy 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 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_shut-
down(3), SSL_set_shutdown(3), ssl(3), bio(3)
0.9.7a 2001-09-13 SSL_read(3)
