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_get_write_guaran‐
tee, BIO_ctrl_get_write_guarantee, BIO_get_read_request,
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, 0, NULL)
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 inap‐
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 buf‐
The BIO_reset() function clears any data in the write buffer.
The BIO_make_bio_pair() function joins two separate BIOs into a con‐
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
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 used.
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 meaning‐
ful 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_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. 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.
Functions: SSL_set_bio(3), ssl(3), bio(3),
BIO_ctrl_get_read_request(3), BIO_new_bio_pair(3), BIO_should_retry(3),