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SSL_read(3)                         OpenSSL                        SSL_read(3)

       SSL_read - read bytes from a TLS/SSL connection.

       libcrypto, -lcrypto

        #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
       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 SSL_read() can be
       called without blocking or actually receiving new data from the
       underlying socket.

       When an SSL_read() operation has to be repeated because of
       SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE, it must be repeated with
       the same arguments.

       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
       <0  The read operation was not successful, because either the
           connection was closed, an error occurred or action must be taken by
           the calling process.  Call SSL_get_error(3) with the return value
           ret to find out the reason.

           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.

           Old documentation indicated a difference between 0 and -1, and that
           -1 was retryable.  You should instead call SSL_get_error() to find
           out if it's retryable.

       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), openssl_bio(3)

1.0.2k                            2017-01-27                       SSL_read(3)