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:
- > 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, because either the connection was closed, 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.
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.
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.