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.
SSL_pending(3) can be used to find out whether there are buffered bytes
available for immediate retrieval. In this case
SSL_read() can be
called without blocking or actually receiving new data from the underlying
socket.
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
- <0
- The read operation was not successful, because either the connection was
closed, an error occurred or action must be taken by the calling process.
Call SSL_get_error(3) with the return value ret to find out
the reason.
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.
Old documentation indicated a difference between 0 and -1, and that -1 was
retryable. You should instead call SSL_get_error() to find out if
it's retryable.
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_pending(3),
SSL_shutdown(3),
SSL_set_shutdown(3),
ssl(3),
bio(3)