SSL_write - write bytes to a TLS/SSL connection
#include <openssl/ssl.h>
int SSL_write(SSL *ssl, const void *buf, int num);
SSL_write()
writes num bytes from the buffer buf into the specified
ssl connection.
If necessary, SSL_write()
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 SSL_write()
operation. The behaviour of SSL_write()
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 an SSL_read(3) or SSL_write()
function.
If the underlying BIO is blocking, SSL_write()
will only return, once the
write operation has been finished or an error occurred, except when a
renegotiation take place, in which case a SSL_ERROR_WANT_READ may occur.
This behaviour can be controlled with the SSL_MODE_AUTO_RETRY flag of the
SSL_CTX_set_mode(3) call.
If the underlying BIO is non-blocking, SSL_write()
will also return,
when the underlying BIO could not satisfy the needs of SSL_write()
to continue the operation. In this case a call to
SSL_get_error(3) with the
return value of SSL_write()
will yield SSL_ERROR_WANT_READ or
SSL_ERROR_WANT_WRITE. As at any time a re-negotiation is possible, a
call to SSL_write()
can also cause read operations! The calling process
then must repeat the call after taking appropriate action to satisfy the
needs of SSL_write()
. The action depends on the underlying BIO. When using a
non-blocking 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 continue.
SSL_write()
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,
SSL_write()
will also return with success, when a partial write has been
successfully completed. In this case the SSL_write()
operation is considered
completed. The bytes are sent and a new SSL_write()
operation 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 for SSLv3/TLSv1.
When an SSL_write()
operation has to be repeated because of
SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE, it must be repeated
with the same arguments.
When calling SSL_write()
with num=0 bytes to be sent the behaviour is
undefined.
The following return values can occur:
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(3), SSL_CTX_set_mode(3), SSL_CTX_new(3), SSL_connect(3), SSL_accept(3) SSL_set_connect_state(3), ssl(3), bio(3)
Copyright 2000-2016 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 https://www.openssl.org/source/license.html.