1SSL_write(3)                        OpenSSL                       SSL_write(3)
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NAME

6       SSL_write - write bytes to a TLS/SSL connection.
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SYNOPSIS

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

14       SSL_write() writes num bytes from the buffer buf into the specified ssl
15       connection.
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NOTES

18       If necessary, SSL_write() 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_write() operation. The behaviour of SSL_write() depends
22       on 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(3) or SSL_write() function.
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29       If the underlying BIO is blocking, SSL_write() will only return, once
30       the write operation has been finished or an error occurred, except when
31       a renegotiation take place, in which case a SSL_ERROR_WANT_READ may
32       occur.  This behaviour can be controlled with the SSL_MODE_AUTO_RETRY
33       flag of the SSL_CTX_set_mode(3) call.
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35       If the underlying BIO is non-blocking, SSL_write() will also return,
36       when the underlying BIO could not satisfy the needs of SSL_write() to
37       continue the operation. In this case a call to SSL_get_error(3) with
38       the return value of SSL_write() will yield SSL_ERROR_WANT_READ or
39       SSL_ERROR_WANT_WRITE. As at any time a re-negotiation is possible, a
40       call to SSL_write() can also cause read operations! The calling process
41       then must repeat the call after taking appropriate action to satisfy
42       the needs of SSL_write(). The action depends on the underlying BIO.
43       When using a non-blocking socket, nothing is to be done, but select()
44       can be used to check for the required condition. When using a buffering
45       BIO, like a BIO pair, data must be written into or retrieved out of the
46       BIO before being able to continue.
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48       SSL_write() will only return with success, when the complete contents
49       of buf of length num has been written. This default behaviour can be
50       changed with the SSL_MODE_ENABLE_PARTIAL_WRITE option of
51       SSL_CTX_set_mode(3). When this flag is set, SSL_write() will also
52       return with success, when a partial write has been successfully
53       completed. In this case the SSL_write() operation is considered
54       completed. The bytes are sent and a new SSL_write() operation with a
55       new buffer (with the already sent bytes removed) must be started.  A
56       partial write is performed with the size of a message block, which is
57       16kB for SSLv3/TLSv1.
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WARNING

60       When an SSL_write() operation has to be repeated because of
61       SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE, it must be repeated with
62       the same arguments.
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64       When calling SSL_write() with num=0 bytes to be sent the behaviour is
65       undefined.
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RETURN VALUES

68       The following return values can occur:
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70       > 0 The write operation was successful, the return value is the number
71           of bytes actually written to the TLS/SSL connection.
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73       <= 0
74           The write operation was not successful, because either the
75           connection was closed, an error occurred or action must be taken by
76           the calling process.  Call SSL_get_error() with the return value
77           ret to find out the reason.
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79           SSLv2 (deprecated) does not support a shutdown alert protocol, so
80           it can only be detected, whether the underlying connection was
81           closed. It cannot be checked, why the closure happened.
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83           Old documentation indicated a difference between 0 and -1, and that
84           -1 was retryable.  You should instead call SSL_get_error() to find
85           out if it's retryable.
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SEE ALSO

88       SSL_get_error(3), SSL_read(3), SSL_CTX_set_mode(3), SSL_CTX_new(3),
89       SSL_connect(3), SSL_accept(3) SSL_set_connect_state(3), ssl(3), bio(3)
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931.0.2o                            2019-09-10                      SSL_write(3)
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