SSL_read
Section: OpenSSL (3)
Updated: 2017-05-25
Index
Return to Main Contents
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)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- NOTES
-
- WARNING
-
- RETURN VALUES
-
- SEE ALSO
-
|
|
- Running on 
-
:
: 