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 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 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_shutdown(3), SSL_set_shutdown(3), ssl(3), bio(3)
