|BIO_S_BIO(3)||Library Functions Manual||BIO_S_BIO(3)|
const BIO_METHOD *
size_t writebuf1, BIO **bio2,
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 be 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(3) will read data from the buffer or request a retry if no data is available.
Calls to BIO_write(3) will place data in the buffer or request a retry if the buffer is full.
BIO_reset(3) 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 association.
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_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_new_bio_pair() does not check
whether bio1 or bio2 point to
some other BIO; the values are overwritten and
BIO_free(3) 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(3) 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.
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.
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(3) 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(2)) due to a request and BIO_should_read(3) being true.
To see why this is important, consider a case where a request is sent using BIO_write(3) and a response read with BIO_read(3), this can occur during a TLS/SSL handshake for example. BIO_write(3) will succeed and place data in the write buffer. BIO_read(3) will initially fail and BIO_should_read(3) will be true. If the application then waits for data to become available on the underlying transport before flushing the write buffer, it will never succeed because the request was never sent.
BIO_eof(3) is true if no data is in the peer BIO and the peer BIO has been shutdown.
BIO_get_read_request() are implemented as
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.
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(); /* e.g. SSL_read() and SSL_write() */ ... application | TLS-engine | | +----------> SSL_operations() | /\ || | || \/ | BIO-pair (internal_bio) | BIO-pair (network_bio) | || /\ | \/ || +-----------< BIO_operations() | | 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(3) to find out
whether data is buffered in the BIO and must be transferred to the network.
BIO_ctrl_get_read_request() to find out how many
bytes must be written into the buffer before the SSL operations can
successfully be continued.
BIO_ctrl_reset_read_request() first appeared in OpenSSL 0.9.4 and have been available since OpenBSD 2.6.
appeared in OpenSSL 0.9.5 and has been available since
BIO_shutdown_wr() first appeared in
OpenSSL 0.9.6 and has been available since OpenBSD
ERROR_SSL_WANT_READcondition, but there is still data in the write buffer. An application must not rely on the error value of the 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.
|May 1, 2018||OpenBSD-current|