5 SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek
6 - read bytes from a TLS/SSL connection
10 #include <openssl/ssl.h>
12 int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
13 int SSL_read(SSL *ssl, void *buf, int num);
15 int SSL_peek_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
16 int SSL_peek(SSL *ssl, void *buf, int num);
20 SSL_read_ex() and SSL_read() try to read B<num> bytes from the specified B<ssl>
21 into the buffer B<buf>. On success SSL_read_ex() will store the number of bytes
22 actually read in B<*readbytes>.
24 SSL_peek_ex() and SSL_peek() are identical to SSL_read_ex() and SSL_read()
25 respectively except no bytes are actually removed from the underlying BIO during
26 the read, so that a subsequent call to SSL_read_ex() or SSL_read() will yield
27 at least the same bytes.
31 In the paragraphs below a "read function" is defined as one of SSL_read_ex(),
32 SSL_read(), SSL_peek_ex() or SSL_peek().
34 If necessary, a read function will negotiate a TLS/SSL session, if not already
35 explicitly performed by L<SSL_connect(3)> or L<SSL_accept(3)>. If the
36 peer requests a re-negotiation, it will be performed transparently during
37 the read function operation. The behaviour of the read functions depends on the
40 For the transparent negotiation to succeed, the B<ssl> must have been
41 initialized to client or server mode. This is being done by calling
42 L<SSL_set_connect_state(3)> or SSL_set_accept_state() before the first
43 invocation of a read function.
45 The read functions work based on the SSL/TLS records. The data are received in
46 records (with a maximum record size of 16kB). Only when a record has been
47 completely received, can it be processed (decryption and check of integrity).
48 Therefore data that was not retrieved at the last read call can still be
49 buffered inside the SSL layer and will be retrieved on the next read
50 call. If B<num> is higher than the number of bytes buffered then the read
51 functions will return with the bytes buffered. If no more bytes are in the
52 buffer, the read functions will trigger the processing of the next record.
53 Only when the record has been received and processed completely will the read
54 functions return reporting success. At most the contents of the record will
55 be returned. As the size of an SSL/TLS record may exceed the maximum packet size
56 of the underlying transport (e.g. TCP), it may be necessary to read several
57 packets from the transport layer before the record is complete and the read call
60 If the underlying BIO is B<blocking>, a read function will only return once the
61 read operation has been finished or an error occurred, except when a
62 renegotiation takes place, in which case a SSL_ERROR_WANT_READ may occur. This
63 behaviour can be controlled with the SSL_MODE_AUTO_RETRY flag of the
64 L<SSL_CTX_set_mode(3)> call.
66 If the underlying BIO is B<non-blocking>, a read function will also return when
67 the underlying BIO could not satisfy the needs of the function to continue the
68 operation. In this case a call to L<SSL_get_error(3)> with the
69 return value of the read function will yield B<SSL_ERROR_WANT_READ> or
70 B<SSL_ERROR_WANT_WRITE>. As at any time a re-negotiation is possible, a
71 a read function can also cause write operations! The calling process then must
72 repeat the call after taking appropriate action to satisfy the needs of the read
73 function. The action depends on the underlying BIO. When using a non-blocking
74 socket, nothing is to be done, but select() can be used to check for the
75 required condition. When using a buffering BIO, like a BIO pair, data must be
76 written into or retrieved out of the BIO before being able to continue.
78 L<SSL_pending(3)> can be used to find out whether there
79 are buffered bytes available for immediate retrieval. In this case
80 the read function can be called without blocking or actually receiving
81 new data from the underlying socket.
85 When a read function operation has to be repeated because L<SSL_get_error(3)>
86 returned B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE>, it must be repeated
87 with the same arguments.
91 SSL_read_ex() and SSL_peek_ex() will return 1 for success or 0 for failure.
92 Success means that 1 or more application data bytes have been read from the SSL
94 Failure means that no bytes could be read from the SSL connection.
95 Failures can be retryable (e.g. we are waiting for more bytes to
96 be delivered by the network) or non-retryable (e.g. a fatal network error).
97 In the event of a failure call L<SSL_get_error(3)> to find out the reason which
98 indicates whether the call is retryable or not.
100 For SSL_read() and SSL_peek() the following return values can occur:
106 The read operation was successful.
107 The return value is the number of bytes actually read from the TLS/SSL
112 The read operation was not successful, because either the connection was closed,
113 an error occurred or action must be taken by the calling process.
114 Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason.
116 Old documentation indicated a difference between 0 and -1, and that -1 was
118 You should instead call SSL_get_error() to find out if it's retryable.
124 L<SSL_get_error(3)>, L<SSL_write_ex(3)>,
125 L<SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)>,
126 L<SSL_connect(3)>, L<SSL_accept(3)>
127 L<SSL_set_connect_state(3)>,
129 L<SSL_shutdown(3)>, L<SSL_set_shutdown(3)>,
134 Copyright 2000-2016 The OpenSSL Project Authors. All Rights Reserved.
136 Licensed under the OpenSSL license (the "License"). You may not use
137 this file except in compliance with the License. You can obtain a copy
138 in the file LICENSE in the source distribution or at
139 L<https://www.openssl.org/source/license.html>.