1SSL_read(3)                         OpenSSL                        SSL_read(3)
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NAME

6       SSL_read - read bytes from a TLS/SSL connection.
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SYNOPSIS

9        #include <openssl/ssl.h>
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11        int SSL_read(SSL *ssl, void *buf, int num);
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DESCRIPTION

14       SSL_read() tries to read num bytes from the specified ssl into the
15       buffer buf.
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NOTES

18       If necessary, SSL_read() will negotiate a TLS/SSL session, if not
19       already explicitly performed by SSL_connect(3) or SSL_accept(3). If the
20       peer requests a re-negotiation, it will be performed transparently
21       during the SSL_read() operation. The behaviour of SSL_read() depends on
22       the underlying BIO.
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24       For the transparent negotiation to succeed, the ssl must have been
25       initialized to client or server mode. This is being done by calling
26       SSL_set_connect_state(3) or SSL_set_accept_state() before the first
27       call to an SSL_read() or SSL_write(3) function.
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29       SSL_read() works based on the SSL/TLS records. The data are received in
30       records (with a maximum record size of 16kB for SSLv3/TLSv1). Only when
31       a record has been completely received, it can be processed (decryption
32       and check of integrity). Therefore data that was not retrieved at the
33       last call of SSL_read() can still be buffered inside the SSL layer and
34       will be retrieved on the next call to SSL_read(). If num is higher than
35       the number of bytes buffered, SSL_read() will return with the bytes
36       buffered.  If no more bytes are in the buffer, SSL_read() will trigger
37       the processing of the next record. Only when the record has been
38       received and processed completely, SSL_read() will return reporting
39       success. At most the contents of the record will be returned. As the
40       size of an SSL/TLS record may exceed the maximum packet size of the
41       underlying transport (e.g. TCP), it may be necessary to read several
42       packets from the transport layer before the record is complete and
43       SSL_read() can succeed.
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45       If the underlying BIO is blocking, SSL_read() will only return, once
46       the read operation has been finished or an error occurred, except when
47       a renegotiation take place, in which case a SSL_ERROR_WANT_READ may
48       occur.  This behaviour can be controlled with the SSL_MODE_AUTO_RETRY
49       flag of the SSL_CTX_set_mode(3) call.
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51       If the underlying BIO is non-blocking, SSL_read() will also return when
52       the underlying BIO could not satisfy the needs of SSL_read() to
53       continue the operation. In this case a call to SSL_get_error(3) with
54       the return value of SSL_read() will yield SSL_ERROR_WANT_READ or
55       SSL_ERROR_WANT_WRITE. As at any time a re-negotiation is possible, a
56       call to SSL_read() can also cause write operations! The calling process
57       then must repeat the call after taking appropriate action to satisfy
58       the needs of SSL_read(). The action depends on the underlying BIO. When
59       using a non-blocking socket, nothing is to be done, but select() can be
60       used to check for the required condition. When using a buffering BIO,
61       like a BIO pair, data must be written into or retrieved out of the BIO
62       before being able to continue.
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64       SSL_pending(3) can be used to find out whether there are buffered bytes
65       available for immediate retrieval. In this case SSL_read() can be
66       called without blocking or actually receiving new data from the
67       underlying socket.
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WARNING

70       When an SSL_read() operation has to be repeated because of
71       SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE, it must be repeated with
72       the same arguments.
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RETURN VALUES

75       The following return values can occur:
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77       > 0 The read operation was successful.  The return value is the number
78           of bytes actually read from the TLS/SSL connection.
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80       <= 0
81       <0  The read operation was not successful, because either the
82           connection was closed, an error occurred or action must be taken by
83           the calling process.  Call SSL_get_error(3) with the return value
84           ret to find out the reason.
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86           SSLv2 (deprecated) does not support a shutdown alert protocol, so
87           it can only be detected, whether the underlying connection was
88           closed. It cannot be checked, whether the closure was initiated by
89           the peer or by something else.
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91           Old documentation indicated a difference between 0 and -1, and that
92           -1 was retryable.  You should instead call SSL_get_error() to find
93           out if it's retryable.
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SEE ALSO

96       SSL_get_error(3), SSL_write(3), SSL_CTX_set_mode(3), SSL_CTX_new(3),
97       SSL_connect(3), SSL_accept(3) SSL_set_connect_state(3), SSL_pending(3),
98       SSL_shutdown(3), SSL_set_shutdown(3), ssl(3), bio(3)
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1021.0.2o                            2020-08-01                       SSL_read(3)
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