SSL_write(3) OpenSSL SSL_write(3)
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 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
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:
>0 The write operation was successful, the return value is the number of bytes actually
written to the TLS/SSL connection.
0 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
<0 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.
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), openssl_bio(3)
1.0.1i 2014-06-05 SSL_write(3)