BIO_s_bio(3)                 OpenSSL                 BIO_s_bio(3)


     BIO_s_bio, BIO_make_bio_pair, BIO_destroy_bio_pair,
     BIO_shutdown_wr, BIO_set_write_buf_size,
     BIO_get_write_buf_size, BIO_new_bio_pair,
     BIO_get_write_guarantee, BIO_ctrl_get_write_guarantee,
     BIO_get_read_request, BIO_ctrl_get_read_request,
     BIO_ctrl_reset_read_request - BIO pair BIO


      #include <openssl/bio.h>

      BIO_METHOD *BIO_s_bio(void);

      #define BIO_make_bio_pair(b1,b2)   (int)BIO_ctrl(b1,BIO_C_MAKE_BIO_PAIR,0,b2)
      #define BIO_destroy_bio_pair(b)    (int)BIO_ctrl(b,BIO_C_DESTROY_BIO_PAIR,0,NULL)

      #define BIO_shutdown_wr(b) (int)BIO_ctrl(b, BIO_C_SHUTDOWN_WR, 0, NULL)

      #define BIO_set_write_buf_size(b,size) (int)BIO_ctrl(b,BIO_C_SET_WRITE_BUF_SIZE,size,NULL)
      #define BIO_get_write_buf_size(b,size) (size_t)BIO_ctrl(b,BIO_C_GET_WRITE_BUF_SIZE,size,NULL)

      int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2);

      #define BIO_get_write_guarantee(b) (int)BIO_ctrl(b,BIO_C_GET_WRITE_GUARANTEE,0,NULL)
      size_t BIO_ctrl_get_write_guarantee(BIO *b);

      #define BIO_get_read_request(b)    (int)BIO_ctrl(b,BIO_C_GET_READ_REQUEST,0,NULL)
      size_t BIO_ctrl_get_read_request(BIO *b);

      int BIO_ctrl_reset_read_request(BIO *b);


     BIO_s_bio() returns the method for a BIO pair. A BIO pair is
     a pair of source/sink BIOs where data written to either half
     of the pair is buffered and can be read from the other half.
     Both halves must usually by handled by the same application
     thread since no locking is done on the internal data

     Since BIO chains typically end in a source/sink BIO it is
     possible to make this one half of a BIO pair and have all
     the data processed by the chain under application control.

     One typical use of BIO pairs is to place TLS/SSL I/O under
     application control, this can be used when the application
     wishes to use a non standard transport for TLS/SSL or the
     normal socket routines are inappropriate.

     Calls to BIO_read() will read data from the buffer or
     request a retry if no data is available.

1.0.2t               Last change: 2019-09-10                    1

BIO_s_bio(3)                 OpenSSL                 BIO_s_bio(3)

     Calls to BIO_write() will place data in the buffer or
     request a retry if the buffer is full.

     The standard calls BIO_ctrl_pending() and
     BIO_ctrl_wpending() can be used to determine the amount of
     pending data in the read or write buffer.

     BIO_reset() clears any data in the write buffer.

     BIO_make_bio_pair() joins two separate BIOs into a connected

     BIO_destroy_pair() destroys the association between two
     connected BIOs. Freeing up any half of the pair will
     automatically destroy the association.

     BIO_shutdown_wr() is used to close down a BIO b. After this
     call no further writes on BIO b are allowed (they will
     return an error). Reads on the other half of the pair will
     return any pending data or EOF when all pending data has
     been read.

     BIO_set_write_buf_size() sets the write buffer size of BIO b
     to size.  If the size is not initialized a default value is
     used. This is currently 17K, sufficient for a maximum size
     TLS record.

     BIO_get_write_buf_size() returns the size of the write

     BIO_new_bio_pair() combines the calls to BIO_new(),
     BIO_make_bio_pair() and BIO_set_write_buf_size() to create a
     connected pair of BIOs bio1, bio2 with write buffer sizes
     writebuf1 and writebuf2. If either size is zero then the
     default size is used.  BIO_new_bio_pair() does not check
     whether bio1 or bio2 do point to some other BIO, the values
     are overwritten, BIO_free() is not called.

     BIO_get_write_guarantee() and BIO_ctrl_get_write_guarantee()
     return the maximum length of data that can be currently
     written to the BIO. Writes larger than this value will
     return a value from BIO_write() less than the amount
     requested or if the buffer is full request a retry.
     BIO_ctrl_get_write_guarantee() is a function whereas
     BIO_get_write_guarantee() is a macro.

     BIO_get_read_request() and BIO_ctrl_get_read_request()
     return the amount of data requested, or the buffer size if
     it is less, if the last read attempt at the other half of
     the BIO pair failed due to an empty buffer.  This can be
     used to determine how much data should be written to the BIO
     so the next read will succeed: this is most useful in

1.0.2t               Last change: 2019-09-10                    2

BIO_s_bio(3)                 OpenSSL                 BIO_s_bio(3)

     TLS/SSL applications where the amount of data read is
     usually meaningful rather than just a buffer size. After a
     successful read this call will return zero.  It also will
     return zero once new data has been written satisfying the
     read request or part of it.  Note that
     BIO_get_read_request() never returns an amount larger than
     that returned by BIO_get_write_guarantee().

     BIO_ctrl_reset_read_request() can also be used to reset the
     value returned by BIO_get_read_request() to zero.


     Both halves of a BIO pair should be freed. That is even if
     one half is implicit freed due to a BIO_free_all() or
     SSL_free() call the other half needs to be freed.

     When used in bidirectional applications (such as TLS/SSL)
     care should be taken to flush any data in the write buffer.
     This can be done by calling BIO_pending() on the other half
     of the pair and, if any data is pending, reading it and
     sending it to the underlying transport. This must be done
     before any normal processing (such as calling select() ) due
     to a request and BIO_should_read() being true.

     To see why this is important consider a case where a request
     is sent using BIO_write() and a response read with
     BIO_read(), this can occur during an TLS/SSL handshake for
     example. BIO_write() will succeed and place data in the
     write buffer. BIO_read() will initially fail and
     BIO_should_read() will be true. If the application then
     waits for data to be available on the underlying transport
     before flushing the write buffer it will never succeed
     because the request was never sent!

     BIO_eof() is true if no data is in the peer BIO and the peer
     BIO has been shutdown.


     BIO_new_bio_pair() returns 1 on success, with the new BIOs
     available in bio1 and bio2, or 0 on failure, with NULL
     pointers stored into the locations for bio1 and bio2. Check
     the error stack for more information.

     [XXXXX: More return values need to be added here]


     The BIO pair can be used to have full control over the
     network access of an application. The application can call
     select() on the socket as required without having to go
     through the SSL-interface.

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BIO_s_bio(3)                 OpenSSL                 BIO_s_bio(3)

      BIO *internal_bio, *network_bio;
      BIO_new_bio_pair(internal_bio, 0, network_bio, 0);
      SSL_set_bio(ssl, internal_bio, internal_bio);

      application |   TLS-engine
         |        |
         +----------> SSL_operations()
                  |     /\    ||
                  |     ||    \/
                  |   BIO-pair (internal_bio)
         +----------< BIO-pair (network_bio)
         |        |
       socket     |

       SSL_free(ssl);                /* implicitly frees internal_bio */

     As the BIO pair will only buffer the data and never directly
     access the connection, it behaves non-blocking and will
     return as soon as the write buffer is full or the read
     buffer is drained. Then the application has to flush the
     write buffer and/or fill the read buffer.

     Use the BIO_ctrl_pending(), to find out whether data is
     buffered in the BIO and must be transfered to the network.
     Use BIO_ctrl_get_read_request() to find out, how many bytes
     must be written into the buffer before the SSL_operation()
     can successfully be continued.


     As the data is buffered, SSL_operation() may return with a
     ERROR_SSL_WANT_READ condition, but there is still data in
     the write buffer. An application must not rely on the error
     value of SSL_operation() but must assure that the write
     buffer is always flushed first. Otherwise a deadlock may
     occur as the peer might be waiting for the data before being
     able to continue.


     SSL_set_bio(3), ssl(3), bio(3), BIO_should_retry(3),

1.0.2t               Last change: 2019-09-10                    4

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