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SSL_read(3) OpenSSL SSL_read(3)
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SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek - read bytes from a
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
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
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
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.
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
Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (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
1.1.1i 2020-12-10 SSL_read(3)