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 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)
0.9.7l 2001-09-13 SSL_read(3)
|