SSL_write - write bytes to a TLS/SSL connection.
int SSL_write(SSL *ssl, const void *buf, int num);
bytes from the buffer buf
If necessary, SSL_write()
will negotiate a TLS/SSL session, if not
already explicitly performed by SSL_connect(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
first call to an SSL_read(3)
If the underlying BIO is blocking
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
return, when the underlying BIO could not satisfy the needs of
to continue the operation. In this case a call to
with the return value of SSL_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.
will only return with success, when the complete contents of
of length num
has been written. This default behaviour can
be changed with the SSL_MODE_ENABLE_PARTIAL_WRITE option of
. When this flag is set, SSL_write()
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
When an SSL_write()
operation has to be repeated because of
, it must be repeated
with the same arguments.
When calling SSL_write()
with num=0 bytes to be sent the behaviour is
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 cleanly (SSL_ERROR_ZERO_RETURN).
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