BIO_s_bio, BIO_make_bio_pair, BIO_destroy_bio_pair,
BIO_ctrl_reset_read_request - BIO pair BIO
void ); #define BIO_make_bio_pair(b1,b2)
#define BIO_shutdown_wr(b) (int)BIO_ctrl(b, BIO_C_SHUTDOWN_WR,
size_t writebuf2 ); #define BIO_get_write_guarantee(b)
BIO *b ); #define BIO_get_read_request(b)
BIO *b ); int BIO_ctrl_reset_read_request(
BIO *b );
The BIO_s_bio() function 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.
The BIO_reset() function clears any data in the write
The BIO_make_bio_pair() function joins two separate BIOs
into a connected pair.
The BIO_destroy_pair() function destroys the association
between two connected BIOs. Freeing up any half of the
pair will automatically destroy the association.
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.
The BIO_set_write_buf_size() function 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.
The BIO_get_write_buf_size() function returns the size of
the write buffer.
The BIO_new_bio_pair() function combines the calls to
BIO_new(), BIO_make_bio_pair(), and
BIO_set_write_buf_size() to create a connected pair of
BIOs, bio1 and bio2, with write buffer sizes writebuf1 and
writebuf2. If either size is zero then the default size is
BIO_get_write_guarantee() and BIO_ctrl_get_write_guarantee()
return the maximum length of data that can be 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. BIO_get_read_request() never
returns an amount larger than that returned by
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. 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.
Functions: SSL_set_bio(3), ssl(3), bio(3),
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