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
BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t
size_t BIO_ctrl_get_write_guarantee(BIO *b);
size_t BIO_ctrl_get_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 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
BIO_destroy_pair() destroys the association between two connected
BIOs. Freeing up any half of the pair will automatically destroy
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() 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
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!
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.
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);
application | TLS-engine
| /\ ||
| || \/
| BIO-pair (internal_bio)
+----------< BIO-pair (network_bio)
SSL_free(ssl); /* implicitly frees internal_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
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.
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.
SSL_set_bio(3), ssl(3), bio(3), BIO_should_retry(3), BIO_read(3)