SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek - read bytes from a TLS/SSL
connection
#include <openssl/ssl.h>
int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
int SSL_read(SSL *ssl, void *buf, int num);
int SSL_peek_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
int SSL_peek(SSL *ssl, void *buf, int num);
SSL_read_ex() and
SSL_read() try to read
num bytes from the
specified
ssl into the buffer
buf. On success
SSL_read_ex() will store the number of bytes actually read in
*readbytes.
SSL_peek_ex() and
SSL_peek() are identical to
SSL_read_ex()
and
SSL_read() respectively except no bytes are actually removed from
the underlying BIO during the read, so that a subsequent call to
SSL_read_ex() or
SSL_read() will yield at least the same bytes.
In the paragraphs below a "read function" is defined as one of
SSL_read_ex(),
SSL_read(),
SSL_peek_ex() or
SSL_peek().
If necessary, a read function 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
read function operation. The behaviour of the read functions 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 invocation of a read function.
The read functions work based on the SSL/TLS records. The data are received in
records (with a maximum record size of 16kB). Only when a record has been
completely received, can it be processed (decryption and check of integrity).
Therefore, data that was not retrieved at the last read call can still be
buffered inside the SSL layer and will be retrieved on the next read call. If
num is higher than the number of bytes buffered then the read functions
will return with the bytes buffered. If no more bytes are in the buffer, the
read functions will trigger the processing of the next record. Only when the
record has been received and processed completely will the read functions
return reporting success. At most the contents of one 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 the read call can
succeed.
If
SSL_MODE_AUTO_RETRY has been switched off and a non-application data
record has been processed, the read function can return and set the error to
SSL_ERROR_WANT_READ. In this case there might still be unprocessed data
available in the
BIO. If read ahead was set using
SSL_CTX_set_read_ahead(3), there might also still be unprocessed data
available in the
SSL. This behaviour can be controlled using the
SSL_CTX_set_mode(3) call.
If the underlying BIO is
blocking, a read function will only return once
the read operation has been finished or an error occurred, except when a
non-application data record has been processed and
SSL_MODE_AUTO_RETRY
is not set. Note that if
SSL_MODE_AUTO_RETRY is set and only
non-application data is available the call will hang.
If the underlying BIO is
nonblocking, a read function will also return
when the underlying BIO could not satisfy the needs of the function to
continue the operation. In this case a call to
SSL_get_error(3) with
the return value of the read function will yield
SSL_ERROR_WANT_READ or
SSL_ERROR_WANT_WRITE. As at any time it's possible that non-application
data needs to be sent, a read function can also cause write operations. The
calling process then must repeat the call after taking appropriate action to
satisfy the needs of the read function. The action depends on the underlying
BIO. When using a nonblocking 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 the read function can be
called without blocking or actually receiving new data from the underlying
socket.
SSL_read_ex() and
SSL_peek_ex() will return 1 for success or 0 for
failure. Success means that 1 or more application data bytes have been read
from the SSL connection. Failure means that no bytes could be read from the
SSL connection. Failures can be retryable (e.g. we are waiting for more bytes
to be delivered by the network) or non-retryable (e.g. a fatal network error).
In the event of a failure call
SSL_get_error(3) to find out the reason
which indicates whether the call is retryable or not.
For
SSL_read() and
SSL_peek() 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, 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.
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.
SSL_get_error(3),
SSL_write_ex(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(7),
bio(7)
The
SSL_read_ex() and
SSL_peek_ex() functions were added in
OpenSSL 1.1.1.
Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy in the
file LICENSE in the source distribution or at
<
https://www.openssl.org/source/license.html>.