#include <openssl/ssl.h>
int SSL_write(SSL *ssl, const void *buf, int num);
DESCRIPTION
SSL_write() writes num bytes from the buffer buf into the specified ssl connection.
NOTES
If necessary, SSL_write() will negotiate a TLS/SSL session, if not already explicitly performed by 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_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_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 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 WARNING