1SSL_write(3) OpenSSL SSL_write(3)
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6 SSL_write - write bytes to a TLS/SSL connection.
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9 #include <openssl/ssl.h>
10 int SSL_write(SSL *ssl, const void *buf, int num);
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14 SSL_write() writes num bytes from the buffer buf into the specified ssl
15 connection.
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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.
23 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.
28 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.
34 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.
47 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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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.
63 When calling SSL_write() with num=0 bytes to be sent the behaviour is
65 undefined.
66
68 The following return values can occur:
69 > 0 The write operation was successful, the return value is the number
71 of bytes actually written to the TLS/SSL connection.
72 <= 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.
78 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.
82 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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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)
901.0.2o 2020-08-01 SSL_write(3)