+Data is received in whole blocks known as records from the peer. A whole record
+is processed (e.g. decrypted) in one go and is buffered by OpenSSL until it is
+read by the application via a call to L<SSL_read(3)>.
+
+SSL_pending() returns the number of bytes which have been processed, buffered
+and are available inside B<ssl> for immediate read.
+
+If the B<SSL> object's I<read_ahead> flag is set (see
+L<SSL_CTX_set_read_ahead(3)>), additional protocol bytes (beyond the current
+record) may have been read containing more TLS/SSL records. This also applies to
+DTLS and pipelining (see L<SSL_CTX_set_split_send_fragment(3)>). These
+additional bytes will be buffered by OpenSSL but will remain unprocessed until
+they are needed. As these bytes are still in an unprocessed state SSL_pending()
+will ignore them. Therefore it is possible for no more bytes to be readable from
+the underlying BIO (because OpenSSL has already read them) and for SSL_pending()
+to return 0, even though readable application data bytes are available (because
+the data is in unprocessed buffered records).
+
+SSL_has_pending() returns 1 if B<s> has buffered data (whether processed or
+unprocessed) and 0 otherwise. Note that it is possible for SSL_has_pending() to
+return 1, and then a subsequent call to SSL_read() to return no data because the
+unprocessed buffered data when processed yielded no application data (for
+example this can happen during renegotiation). It is also possible in this
+scenario for SSL_has_pending() to continue to return 1 even after an SSL_read()
+call because the buffered and unprocessed data is not yet processable (e.g.
+because OpenSSL has only received a partial record so far).