SSL_peek_ex(3ssl)

1SSL_READ(3)                         OpenSSL                        SSL_READ(3)
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NAME

6       SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek - read bytes from a
7       TLS/SSL connection
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SYNOPSIS

10        #include <openssl/ssl.h>
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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);
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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);
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DESCRIPTION

19       SSL_read_ex() and SSL_read() try to read num bytes from the specified
20       ssl into the buffer buf. On success SSL_read_ex() will store the number
21       of bytes actually read in *readbytes.
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23       SSL_peek_ex() and SSL_peek() are identical to SSL_read_ex() and
24       SSL_read() respectively except no bytes are actually removed from the
25       underlying BIO during the read, so that a subsequent call to
26       SSL_read_ex() or SSL_read() will yield at least the same bytes.
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NOTES

29       In the paragraphs below a "read function" is defined as one of
30       SSL_read_ex(), SSL_read(), SSL_peek_ex() or SSL_peek().
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32       If necessary, a read function will negotiate a TLS/SSL session, if not
33       already explicitly performed by SSL_connect(3) or SSL_accept(3). If the
34       peer requests a re-negotiation, it will be performed transparently
35       during the read function operation. The behaviour of the read functions
36       depends on the underlying BIO.
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38       For the transparent negotiation to succeed, the ssl must have been
39       initialized to client or server mode. This is being done by calling
40       SSL_set_connect_state(3) or SSL_set_accept_state() before the first
41       invocation of a read function.
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43       The read functions work based on the SSL/TLS records. The data are
44       received in records (with a maximum record size of 16kB). Only when a
45       record has been completely received, can it be processed (decryption
46       and check of integrity).  Therefore data that was not retrieved at the
47       last read call can still be buffered inside the SSL layer and will be
48       retrieved on the next read call. If num is higher than the number of
49       bytes buffered then the read functions will return with the bytes
50       buffered. If no more bytes are in the buffer, the read functions will
51       trigger the processing of the next record.  Only when the record has
52       been received and processed completely will the read functions return
53       reporting success. At most the contents of one record will be returned.
54       As the size of an SSL/TLS record may exceed the maximum packet size of
55       the underlying transport (e.g. TCP), it may be necessary to read
56       several packets from the transport layer before the record is complete
57       and the read call can succeed.
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59       If SSL_MODE_AUTO_RETRY has been switched off and a non-application data
60       record has been processed, the read function can return and set the
61       error to SSL_ERROR_WANT_READ.  In this case there might still be
62       unprocessed data available in the BIO.  If read ahead was set using
63       SSL_CTX_set_read_ahead(3), there might also still be unprocessed data
64       available in the SSL.  This behaviour can be controlled using the
65       SSL_CTX_set_mode(3) call.
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67       If the underlying BIO is blocking, a read function will only return
68       once the read operation has been finished or an error occurred, except
69       when a non-application data record has been processed and
70       SSL_MODE_AUTO_RETRY is not set.  Note that if SSL_MODE_AUTO_RETRY is
71       set and only non-application data is available the call will hang.
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73       If the underlying BIO is non-blocking, a read function will also return
74       when the underlying BIO could not satisfy the needs of the function to
75       continue the operation.  In this case a call to SSL_get_error(3) with
76       the return value of the read function will yield SSL_ERROR_WANT_READ or
77       SSL_ERROR_WANT_WRITE.  As at any time it's possible that non-
78       application data needs to be sent, a read function can also cause write
79       operations.  The calling process then must repeat the call after taking
80       appropriate action to satisfy the needs of the read function.  The
81       action depends on the underlying BIO.  When using a non-blocking
82       socket, nothing is to be done, but select() can be used to check for
83       the required condition.  When using a buffering BIO, like a BIO pair,
84       data must be written into or retrieved out of the BIO before being able
85       to continue.
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87       SSL_pending(3) can be used to find out whether there are buffered bytes
88       available for immediate retrieval.  In this case the read function can
89       be called without blocking or actually receiving new data from the
90       underlying socket.
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RETURN VALUES

93       SSL_read_ex() and SSL_peek_ex() will return 1 for success or 0 for
94       failure.  Success means that 1 or more application data bytes have been
95       read from the SSL connection.  Failure means that no bytes could be
96       read from the SSL connection.  Failures can be retryable (e.g. we are
97       waiting for more bytes to be delivered by the network) or non-retryable
98       (e.g. a fatal network error).  In the event of a failure call
99       SSL_get_error(3) to find out the reason which indicates whether the
100       call is retryable or not.
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102       For SSL_read() and SSL_peek() the following return values can occur:
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104       > 0 The read operation was successful.  The return value is the number
105           of bytes actually read from the TLS/SSL connection.
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107       <= 0
108           The read operation was not successful, because either the
109           connection was closed, an error occurred or action must be taken by
110           the calling process.  Call SSL_get_error(3) with the return value
111           ret to find out the reason.
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113           Old documentation indicated a difference between 0 and -1, and that
114           -1 was retryable.  You should instead call SSL_get_error() to find
115           out if it's retryable.
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HISTORY

118       SSL_read_ex() and SSL_peek_ex() were added in OpenSSL 1.1.1.
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COPYRIGHT

126       Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.
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128       Licensed under the OpenSSL license (the "License").  You may not use
129       this file except in compliance with the License.  You can obtain a copy
130       in the file LICENSE in the source distribution or at
131       <https://www.openssl.org/source/license.html>.
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1351.1.1                             2018-09-11                       SSL_READ(3)