SSL_pending — obtain number of readable bytes buffered in an SSL object
SSL_pending() returns the number of bytes which are available
inside ssl for immediate read.
Data are received in blocks from the peer. Therefore data
can be buffered inside ssl and are ready for immediate retrieval
The number of bytes pending is returned.
SSL_pending() takes into account only bytes from the TLS/SSL
record that is currently being processed (if any). If the SSL object's read_ahead flag is set, additional protocol bytes may have been
read containing more TLS/SSL records; these are ignored by SSL_pending().
Up to OpenSSL 0.9.6, SSL_pending() does not check if the record
type of pending data is application data.