SSL_READ(3) OpenSSL SSL_READ(3)
SSL_read - read bytes from a TLS/SSL connection.
#include <openssl/ssl.h>
int SSL_read(SSL *ssl, void *buf, int num);
SSL_read() tries to read num bytes from the specified ssl
into the buffer buf.
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() opera-
tion. 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 pro-
cessing 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 tran-
sport 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.
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SSL_READ(3) OpenSSL SSL_READ(3)
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 underly-
ing 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 con-
nection.
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 under-
lying 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 under-
lying 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)
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SSL_READ(3) OpenSSL SSL_READ(3)
SSL_set_connect_state(3), SSL_shutdown(3),
SSL_set_shutdown(3), ssl(3), bio(3)
MirOS BSD #10-current 2005-02-05 3
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