In the paragraphs below a "read function" is defined as one of SSL_read_ex(), SSL_read(), SSL_peek_ex()
or SSL_peek().
If necessary, a read function will negotiate a TLS/SSL session, if not already explicitly performed by
SSL_connect(3) or SSL_accept(3). If the peer requests a re-negotiation, it will be performed
transparently during the read function operation. The behaviour of the read functions 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(3) or SSL_set_accept_state() before the first
invocation of a read function.
The read functions work based on the SSL/TLS records. The data are received in records (with a maximum
record size of 16kB). Only when a record has been completely received, can it be processed (decryption
and check of integrity). Therefore, data that was not retrieved at the last read call can still be
buffered inside the SSL layer and will be retrieved on the next read call. If num is higher than the
number of bytes buffered then the read functions will return with the bytes buffered. If no more bytes
are in the buffer, the read functions will trigger the processing of the next record. Only when the
record has been received and processed completely will the read functions return reporting success. At
most the contents of one record will be returned. As the size of an SSL/TLS record may exceed the maximum
packet size of the underlying transport (e.g. TCP), it may be necessary to read several packets from the
transport layer before the record is complete and the read call can succeed.
If SSL_MODE_AUTO_RETRY has been switched off and a non-application data record has been processed, the
read function can return and set the error to SSL_ERROR_WANT_READ. In this case there might still be
unprocessed data available in the BIO. If read ahead was set using SSL_CTX_set_read_ahead(3), there
might also still be unprocessed data available in the SSL. This behaviour can be controlled using the
SSL_CTX_set_mode(3) call.
If the underlying BIO is blocking, a read function will only return once the read operation has been
finished or an error occurred, except when a non-application data record has been processed and
SSL_MODE_AUTO_RETRY is not set. Note that if SSL_MODE_AUTO_RETRY is set and only non-application data is
available the call will hang.
If the underlying BIO is nonblocking, a read function will also return when the underlying BIO could not
satisfy the needs of the function to continue the operation. In this case a call to SSL_get_error(3)
with the return value of the read function will yield SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE. As at
any time it's possible that non-application data needs to be sent, a read function can also cause write
operations. The calling process then must repeat the call after taking appropriate action to satisfy the
needs of the read function. The action depends on the underlying BIO. When using a nonblocking 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_pending(3) can be used to find out whether there are buffered bytes available for immediate
retrieval. In this case the read function can be called without blocking or actually receiving new data
from the underlying socket.
When used with a QUIC SSL object, calling an I/O function such as SSL_read() allows internal network
event processing to be performed. It is important that this processing is performed regularly. If an
application is not using thread assisted mode, an application should ensure that an I/O function such as
SSL_read() is called regularly, or alternatively ensure that SSL_handle_events() is called regularly. See
openssl-quic(7) and SSL_handle_events(3) for more information.
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