BIO_S_BIO(3) BSD Library Functions Manual 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)
BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2,
#define BIO_get_write_guarantee(b) \
(int)BIO_ctrl(b, BIO_C_GET_WRITE_GUARANTEE, 0, NULL)
#define BIO_get_read_request(b) \
(int)BIO_ctrl(b, BIO_C_GET_READ_REQUEST, 0, NULL)
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
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 stan‐
dard transport for TLS/SSL or the normal socket routines are inappropri‐
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.
The standard calls BIO_ctrl_pending(3) and BIO_ctrl_wpending(3) can be
used to determine the amount of pending data in the read or write buffer.
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 associa‐
BIO_shutdown_wr() is used to close down a BIO b. After this call no fur‐
ther 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_new(3), 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_new_bio_pair() does not check
whether bio1 or bio2 do point to some other BIO, the values are overwrit‐
ten, BIO_free(3) 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(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_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 applica‐
tions 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_get_write_guarantee().
BIO_ctrl_reset_read_request() can also be used to reset the value
returned by BIO_get_read_request() to zero.
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 loca‐
tions for bio1 and bio2. Check the error stack for more information.
Both halves of a BIO pair should be freed. Even if one half is implic‐
itly freed due to a BIO_free_all(3) or SSL_free(3) call, the other half
still 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(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 an 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.
The BIO pair can be used to have full control over the network access of
an application. The application can call select(2) 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 application has
to flush the write buffer and/or fill the read buffer.
Use BIO_ctrl_pending(3) 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(3) can successfully be continued.
SEE ALSObio(3), BIO_read(3), BIO_should_retry(3), ssl(3), SSL_set_bio(3)CAVEATS
As the data is buffered, SSL_operation(3) may return with an
ERROR_SSL_WANT_READ condition, but there is still data in the write buf‐
fer. An application must not rely on the error value of SSL_operation(3)
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.
BSD April 21, 2019 BSD