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+=pod
+
+=head1 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
+
+=head1 SYNOPSIS
+
+ #include <openssl/bio.h>
+
+ const BIO_METHOD *BIO_s_bio(void);
+
+ int BIO_make_bio_pair(BIO *b1, BIO *b2);
+ int BIO_destroy_bio_pair(BIO *b);
+ int BIO_shutdown_wr(BIO *b);
+
+ int BIO_set_write_buf_size(BIO *b, long size);
+ size_t BIO_get_write_buf_size(BIO *b, long size);
+
+ int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2);
+
+ int BIO_get_write_guarantee(BIO *b);
+ size_t BIO_ctrl_get_write_guarantee(BIO *b);
+ int BIO_get_read_request(BIO *b);
+ size_t BIO_ctrl_get_read_request(BIO *b);
+ int BIO_ctrl_reset_read_request(BIO *b);
+
+=head1 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_ex() will read data from the buffer or request a retry if no
+data is available.
+
+Calls to BIO_write_ex() 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<b>. After this call no further
+writes on BIO B<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<b> to B<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 B<bio1>, B<bio2>
+with write buffer sizes B<writebuf1> and B<writebuf2>. If either size is
+zero then 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.
+
+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_ex() 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.
+
+=head1 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_ex() and a response read with BIO_read_ex(), this can occur during an
+TLS/SSL handshake for example. BIO_write_ex() will succeed and place data in the
+write buffer. BIO_read_ex() 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_make_bio_pair(), BIO_destroy_bio_pair(), BIO_shutdown_wr(),
+BIO_set_write_buf_size(), BIO_get_write_buf_size(),
+BIO_get_write_guarantee(), and BIO_get_read_request() are implemented
+as macros.
+
+=head1 RETURN VALUES
+
+BIO_new_bio_pair() returns 1 on success, with the new BIOs available in
+B<bio1> and B<bio2>, or 0 on failure, with NULL pointers stored into the
+locations for B<bio1> and B<bio2>. Check the error stack for more information.
+
+[XXXXX: More return values need to be added here]
+
+=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, internal_bio);
+ SSL_operations(); /* e.g SSL_read and SSL_write */
+ ...
+
+ application | TLS-engine
+ | |
+ +----------> SSL_operations()
+ | /\ ||
+ | || \/
+ | BIO-pair (internal_bio)
+ | BIO-pair (network_bio)
+ | || /\
+ | \/ ||
+ +-----------< BIO_operations()
+ | |
+ | |
+ 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 transferred 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 WARNING
+
+As the data is buffered, SSL_operation() may return with an 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 SEE ALSO
+
+L<SSL_set_bio(3)>, L<ssl(7)>, L<bio(7)>,
+L<BIO_should_retry(3)>, L<BIO_read_ex(3)>
+
+=head1 COPYRIGHT
+
+Copyright 2000-2016 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (the "License"). You may not use
+this file except in compliance with the License. You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut