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).

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.


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