Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
SSL_READ(3)			    OpenSSL			   SSL_READ(3)

       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

       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

       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

1.1.1o				  2022-05-03			   SSL_READ(3)


Want to link to this manual page? Use this URL:

home | help