#use wml::openssl-macros area=docs page=BIO_s_bio
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
#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);
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.
BIO_read() will read data from the buffer or request
a retry if no data is available.
BIO_write() will place data in the buffer or request
a retry if the buffer is full.
The standard calls
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
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_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_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_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_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
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
To see why this is important consider a case where a request is sent using
BIO_write() and a response read with
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
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.
[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); 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.
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
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):}