SSL_READ(3) BSD Library Functions Manual SSL_READ(3)NAMESSL_read — read bytes from a TLS/SSL connection
SYNOPSIS
#include <openssl/ssl.h>
int
SSL_read(SSL *ssl, void *buf, int num);
DESCRIPTIONSSL_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 under‐
lying BIO.
For the transparent negotiation to succeed, the ssl must have been ini‐
tialized to client or server mode. This is being done by calling
SSL_set_connect_state(3) or SSL_set_accept_state(3) before the first call
to SSL_read() or SSL_write(3).
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 after
a record has been completely received can it be processed (decrypted and
checked for integrity). Therefore data 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 num‐
ber of bytes buffered, SSL_read() will return with the bytes buffered.
If no more bytes are in the buffer, SSL_read() will trigger the process‐
ing of the next record. Only when the record has been received and pro‐
cessed completely will SSL_read() 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 trans‐
port 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 has occurred, except when a
renegotiation take place, in which case a SSL_ERROR_WANT_READ may occur.
This behavior 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(2) 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 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) and 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 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 to find out the reason.
SEE ALSObio(3), ssl(3), SSL_accept(3), SSL_connect(3), SSL_CTX_new(3),
SSL_CTX_set_mode(3), SSL_get_error(3), SSL_pending(3),
SSL_set_connect_state(3), SSL_set_shutdown(3), SSL_shutdown(3),
SSL_write(3)BSD April 29, 2024 BSD
