BIO_new_bio_pair(3)

     BIO_new_bio_pair(3)    0.9.6h (2000-09-16)    BIO_new_bio_pair(3)

     NAME
          BIO_new_bio_pair - create a new BIO pair

     SYNOPSIS
           #include <openssl/bio.h>

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

     DESCRIPTION
          BIO_new_bio_pair() creates a buffering BIO pair. It has two
          endpoints between data can be buffered. Its typical use is
          to connect one endpoint as underlying input/output BIO to an
          SSL and access the other one controlled by the program
          instead of accessing the network connection directly.

          The two new BIOs bio1 and bio2 are symmetric with respect to
          their functionality. The size of their buffers is determined
          by writebuf1 and writebuf2. If the size give is 0, 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.

          The two BIOs, even though forming a BIO pair and must be
          BIO_free()'ed separately. This can be of importance, as some
          SSL-functions like SSL_set_bio() or SSL_free() call
          BIO_free() implicitly, so that the peer-BIO is left
          untouched and must also be BIO_free()'ed.

     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);
           SSL_operations();
           ...

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

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     BIO_new_bio_pair(3)    0.9.6h (2000-09-16)    BIO_new_bio_pair(3)

            ...
            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.

     IMPORTANT
          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.

     RETURN VALUES
          The following return values can occur:

          1   The BIO pair was created successfully. The new BIOs are
              available in bio1 and bio2.

          o   The operation failed. The NULL pointer is stored into
              the locations for bio1 and bio2. Check the error stack
              for more information.

     SEE ALSO
          SSL_set_bio(3), ssl(3), bio(3), BIO_ctrl_pending(3),
          BIO_ctrl_get_read_request(3)

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