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SSL_write(3) OpenSSL SSL_write(3)
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SSL_write_ex, SSL_write - write bytes to a TLS/SSL connection
int SSL_write_ex(SSL *s, const void *buf, size_t num, size_t *written);
int SSL_write(SSL *ssl, const void *buf, int num);
SSL_write_ex() and SSL_write() write num bytes from the buffer buf into
the specified ssl connection. On success SSL_write_ex() will store the
number of bytes written in *written.
In the paragraphs below a "write function" is defined as one of either
SSL_write_ex(), or SSL_write().
If necessary, a write function will negotiate a TLS/SSL session, if not
already explicitly performed by SSL_connect(3) or SSL_accept(3). If the
peer requests a re-negotiation, it will be performed transparently
during the write function operation. The behaviour of the write
functions depends on the underlying BIO.
For the transparent negotiation to succeed, the ssl must have been
initialized to client or server mode. This is being done by calling
SSL_set_connect_state(3) or SSL_set_accept_state() before the first
call to a write function.
If the underlying BIO is blocking, the write functions will only
return, once the write operation has been finished or an error
If the underlying BIO is nonblocking the write functions will also
return when the underlying BIO could not satisfy the needs of the
function to continue the operation. In this case a call to
SSL_get_error(3) with the return value of the write function will yield
SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE. As at any time a re-
negotiation is possible, a call to a write function can also cause read
operations! The calling process then must repeat the call after taking
appropriate action to satisfy the needs of the write function. The
action depends on the underlying BIO. When using a nonblocking socket,
nothing is to be done, but select() can be used to check for the
required condition. When using a buffering BIO, like a BIO pair, data
must be written into or retrieved out of the BIO before being able to
The write functions will only return with success when the complete
contents of buf of length num has been written. This default behaviour
can be changed with the SSL_MODE_ENABLE_PARTIAL_WRITE option of
SSL_CTX_set_mode(3). When this flag is set the write functions will
also return with success when a partial write has been successfully
completed. In this case the write function operation is considered
completed. The bytes are sent and a new write call with a new buffer
(with the already sent bytes removed) must be started. A partial write
is performed with the size of a message block, which is 16kB.
When a write function call has to be repeated because SSL_get_error(3)
returned SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE, it must be
repeated with the same arguments. The data that was passed might have
been partially processed. When SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER was
set using SSL_CTX_set_mode(3) the pointer can be different, but the
data and length should still be the same.
You should not call SSL_write() with num=0, it will return an error.
SSL_write_ex() can be called with num=0, but will not send application
data to the peer.
SSL_write_ex() will return 1 for success or 0 for failure. Success
means that all requested application data bytes have been written to
the SSL connection or, if SSL_MODE_ENABLE_PARTIAL_WRITE is in use, at
least 1 application data byte has been written to the SSL connection.
Failure means that not all the requested bytes have been written yet
(if SSL_MODE_ENABLE_PARTIAL_WRITE is not in use) or no bytes could be
written to the SSL connection (if SSL_MODE_ENABLE_PARTIAL_WRITE is in
use). Failures can be retryable (e.g. the network write buffer has
temporarily filled up) or non-retryable (e.g. a fatal network error).
In the event of a failure call SSL_get_error(3) to find out the reason
which indicates whether the call is retryable or not.
For SSL_write() the following return values can occur:
> 0 The write operation was successful, the return value is the number
of bytes actually written to the TLS/SSL connection.
The write operation was not successful, because either the
connection was closed, an error occurred or action must be taken by
the calling process. Call SSL_get_error() with the return value
ret to find out the reason.
Old documentation indicated a difference between 0 and -1, and that
-1 was retryable. You should instead call SSL_get_error() to find
out if it's retryable.
SSL_get_error(3), SSL_read_ex(3), SSL_read(3) SSL_CTX_set_mode(3),
SSL_CTX_new(3), SSL_connect(3), SSL_accept(3) SSL_set_connect_state(3),
The SSL_write_ex() function was added in OpenSSL 1.1.1.
Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file LICENSE in the source distribution or at
1.1.1i 2020-12-10 SSL_write(3)