BIO_s_bio(3) OpenSSL BIO_s_bio(3)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_ctrl_reset_read_request - BIO pair BIO
#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);
DESCRIPTIONBIO_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 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‐
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 associa‐
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 cur‐
rently 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 over‐
written, 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 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. 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() 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!
RETURN VALUESBIO_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 informa‐
[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);
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 applica‐
tion 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 contin‐
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_opera‐
tion() 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.
SEE ALSOSSL_set_bio(3), ssl(3), bio(3), BIO_should_retry(3), BIO_read(3)1.0.0h 2003-05-23 BIO_s_bio(3)