=pod

=head1 NAME

BIO_new_bio_pair - create a new BIO pair

=head1 SYNOPSIS

 #include <openssl/bio.h>

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

=head1 DESCRIPTION

BIO_new_bio_pair() creates a buffering BIO pair based on the
L<SSL_set_bio(3)|SSL_set_bio(3)> method. The BIO pair has two endpoints between which
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 B<bio1> and B<bio2> are symmetric with respect to their
functionality. The size of their buffers is determined by B<writebuf1> and
B<writebuf2>. If the size give is 0, the default size is used.

BIO_new_bio_pair() does not check whether B<bio1> or B<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.

=head1 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     |

  ...
  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.

=head1 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.

=head1 RETURN VALUES

The following return values can occur:

=over 4

=item 1

The BIO pair was created successfully. The new BIOs are available in
B<bio1> and B<bio2>.

=item 0

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

=back

=head1 SEE ALSO

L<SSL_set_bio(3)|SSL_set_bio(3)>, L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>,
L<BIO_ctrl_pending(3)|BIO_ctrl_pending(3)>,
L<BIO_ctrl_get_read_request(3)|BIO_ctrl_get_read_request(3)>

=cut
