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>

        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);

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. 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_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.

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.

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]

EXAMPLES

       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 nonblocking 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.

WARNINGS

       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.

SEE ALSO

       SSL_set_bio(3), ssl(7), bio(7), BIO_should_retry(3), BIO_read_ex(3)

COPYRIGHT

       Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (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 <https://www.openssl.org/source/license.html>.