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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>
- BIO_METHOD *BIO_s_bio(void);
- #define BIO_make_bio_pair(b1,b2) (int)BIO_ctrl(b1,BIO_C_MAKE_BIO_PAIR,0,b2)
- #define BIO_destroy_bio_pair(b) (int)BIO_ctrl(b,BIO_C_DESTROY_BIO_PAIR,0,NULL)
- #define BIO_shutdown_wr(b) (int)BIO_ctrl(b, BIO_C_SHUTDOWN_WR, 0, NULL)
- #define BIO_set_write_buf_size(b,size) (int)BIO_ctrl(b,BIO_C_SET_WRITE_BUF_SIZE,size,NULL)
- #define BIO_get_write_buf_size(b,size) (size_t)BIO_ctrl(b,BIO_C_GET_WRITE_BUF_SIZE,size,NULL)
- int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2);
- #define BIO_get_write_guarantee(b) (int)BIO_ctrl(b,BIO_C_GET_WRITE_GUARANTEE,0,NULL)
- size_t BIO_ctrl_get_write_guarantee(BIO *b);
- #define BIO_get_read_request(b) (int)BIO_ctrl(b,BIO_C_GET_READ_REQUEST,0,NULL)
- 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() will read data from the buffer or request a retry if no
- data is available.
- Calls to BIO_write() 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() 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() and a response read with BIO_read(), this can occur during an
- TLS/SSL handshake for example. BIO_write() will succeed and place data in the write
- buffer. BIO_read() 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!
- =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();
- ...
- 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 WARNING
- 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 SEE ALSO
- L<SSL_set_bio(3)|SSL_set_bio(3)>, L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>,
- L<BIO_should_retry(3)|BIO_should_retry(3)>, L<BIO_read(3)|BIO_read(3)>
- =cut
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