SSL_write - write bytes to a TLS/SSL connection.
int SSL_write(SSL *ssl, const void *buf, int num);
SSL_write() writes num bytes from the buffer buf into the specified
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
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
SSL_set_accept_state() before the first call to an SSL_read(3) or
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
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,
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
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.
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.
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. Probably the underlying connection
was closed. Call
SSL_get_error() with the return value ret to find out, whether an error occurred or the connection was shut down
SSLv2 (deprecated) does not support a shutdown alert protocol, so it can
only be detected, whether the underlying connection was closed. It cannot
be checked, why the closure happened.
The write operation was not successful, because either 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.
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