Cryptography and SSL/TLS Toolkit



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 or SSL_accept. 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 or SSL_set_accept_state() before the first call to an SSL_read 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 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 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. 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. 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).


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, SSL_read, SSL_CTX_set_mode, SSL_CTX_new, SSL_connect, SSL_accept SSL_set_connect_state, ssl, bio