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BIO_s_bio(3)                        OpenSSL                       BIO_s_bio(3)

       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

       libcrypto, -lcrypto

        #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);

       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

       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

       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

       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.

       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.

       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

       [XXXXX: More return values need to be added here]

       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()
           |        |
           |        |

         SSL_free(ssl);                /* implicitly frees internal_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

       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.

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

       Copyright 2000-2020 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

1.1.1i                            2020-12-10                      BIO_s_bio(3)