Clarify behaviour with respect to SSL/TLS records.

must be used before the first call to an SSL_read() or

L<SSL_write(3)|SSL_write(3)> function.

SSL_read() works based on the SSL/TLS records. The data are received in

records (with a maximum record size of 16kB for SSLv3/TLSv1). Only when a

record has been completely received, it can be processed (decryption and

check of integrity). Therefore data that was not retrieved at the last

call of SSL_read() can still be buffered inside the SSL layer and will be

retrieved on the next call to SSL_read(). If B<num> is higher than the

number of bytes buffered, SSL_read() will return with the bytes buffered.

If no more bytes are in the buffer, SSL_read() will trigger the processing

of the next record. Only when the record has been received and processed

completely, SSL_read() will return reporting success. At most the contents

of the record will be returned.

If the underlying BIO is B<blocking>, SSL_read() will only return, once the

read operation has been finished or an error occurred, except when a

renegotiation take place, in which case a SSL_ERROR_WANT_READ may occur.