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            #include <openssl/ssl.h>
            int SSL_read(SSL *ssl, void *buf, int num);


           SSL_read() tries to read num bytes from the specified ssl into the
           buffer buf.


           If necessary, SSL_read() 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 SSL_read() operation. The behaviour of SSL_read() 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
           call to an SSL_read() or 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 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. 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
           SSL_read() can succeed.
           If the underlying BIO is 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.  This behaviour can be controlled with the SSL_MODE_AUTO_RETRY
           flag of the SSL_CTX_set_mode(3) call.
           If the underlying BIO is non-blocking, SSL_read() will also return when
           the underlying BIO could not satisfy the needs of SSL_read() to
           continue the operation. In this case a call to SSL_get_error(3) with
           the return value of SSL_read() will yield SSL_ERROR_WANT_READ or
           SSL_ERROR_WANT_WRITE. As at any time a re-negotiation is possible, a
           call to SSL_read() can also cause write operations! The calling process
           then must repeat the call after taking appropriate action to satisfy
           the needs of SSL_read(). The action depends on the underlying BIO. When
           using a non-blocking socket, nothing is to be done, but select() can be


           The following return values can occur:
           >0  The read operation was successful; the return value is the number
               of bytes actually read from the TLS/SSL connection.
           0   The read operation was not successful. The reason may either be a
               clean shutdown due to a "close notify" alert sent by the peer (in
               which case the SSL_RECEIVED_SHUTDOWN flag in the ssl shutdown state
               is set (see SSL_shutdown(3), SSL_set_shutdown(3)). It is also
               possible, that the peer simply shut down the underlying transport
               and the shutdown is incomplete. Call SSL_get_error() with the
               return value ret to find out, whether an error occurred or the
               connection was shut down cleanly (SSL_ERROR_ZERO_RETURN).
               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, whether the closure was initiated by
               the peer or by something else.
           <0  The read operation was not successful, because either 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.


           SSL_get_error(3), SSL_write(3), SSL_CTX_set_mode(3), SSL_CTX_new(3),
           SSL_connect(3), SSL_accept(3) SSL_set_connect_state(3), SSL_pending(3),
           SSL_shutdown(3), SSL_set_shutdown(3), ssl(3), bio(3)

    1.0.1e 2017-03-22 SSL_read(3)


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