#include <openssl/ssl.h> int SSL_write_ex(SSL *s, const void *buf, size_t num, size_t *written); int SSL_write(SSL *ssl, const void *buf, int num);
If necessary, a write function 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 write function operation. The behaviour of the write functions 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 a write function.
If the underlying BIO is blocking, the write functions will only return, once the write operation has been finished or an error occurred.
If the underlying BIO is nonblocking the write functions will also return when the underlying BIO could not satisfy the needs of the function to continue the operation. In this case a call to SSL_get_error(3) with the return value of the write function will yield SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE. As at any time a re-negotiation is possible, a call to a write function can also cause read operations! The calling process then must repeat the call after taking appropriate action to satisfy the needs of the write function. The action depends on the underlying BIO. When using a nonblocking 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.
The write functions 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 the write functions will also return with success when a partial write has been successfully completed. In this case the write function operation is considered completed. The bytes are sent and a new write call 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.
You should not call SSL_write() with num=0, it will return an error. SSL_write_ex() can be called with num=0, but will not send application data to the peer.
For SSL_write() the following return values can occur:
Old documentation indicated a difference between 0 and -1, and that -1 was retryable. You should instead call SSL_get_error() to find out if it's retryable.
Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.