BIO_s_bio(3)

     NAME
          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

     SYNOPSIS
           #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);


     DESCRIPTION
          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
          structures.

          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.

          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
          pair.

          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
          buffer.

          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
          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.

     NOTES
          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!

     RETURN VALUES
          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]

     EXAMPLE
          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.

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

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

            ...
            SSL_free(ssl);                /* implicitly frees internal_bio */
            BIO_free(network_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.

     WARNING
          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.

     SEE ALSO
          SSL_set_bio(3), ssl(3), bio(3), BIO_should_retry(3),
          BIO_read(3)