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
const BIO_METHOD *BIO_s_bio(void);
int BIO_make_bio_pair(BIO *b1, BIO *b2);
int BIO_destroy_bio_pair(BIO *b);
int BIO_shutdown_wr(BIO *b);
int BIO_set_write_buf_size(BIO *b, long size);
size_t BIO_get_write_buf_size(BIO *b, long size);
int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2);
int BIO_get_write_guarantee(BIO *b);
size_t BIO_ctrl_get_write_guarantee(BIO *b);
int BIO_get_read_request(BIO *b);
size_t BIO_ctrl_get_read_request(BIO *b);
int BIO_ctrl_reset_read_request(BIO *b);
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
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
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()
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()
be used to determine the amount of pending data in the read or write buffer.
clears any data in the write buffer.
joins two separate BIOs into a connected pair.
destroys the association between two connected BIOs.
Freeing up any half of the pair will automatically destroy the association.
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.
sets the write buffer size of BIO b
. If the size is not initialized a default value is used. This is
currently 17K, sufficient for a maximum size TLS record.
returns the size of the write buffer.
combines the calls to BIO_new()
to create a
connected pair of BIOs bio1
with write buffer sizes
. If either size is zero then the default
size is used. BIO_new_bio_pair()
does not check whether bio1
do point to some other BIO, the values are overwritten,
is not called.
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()
than the amount requested or if the buffer is full request a retry.
is a function whereas
is a macro.
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
never returns an amount larger than that
returned by BIO_get_write_guarantee()
can also be used to reset the value
returned by BIO_get_read_request()
Both halves of a BIO pair should be freed. That is even if one half is implicit
freed due to a BIO_free_all()
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
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()
To see why this is important consider a case where a request is sent using
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
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!
is true if no data is in the peer BIO and the peer BIO has been
are implemented as macros.
returns 1 on success, with the new BIOs available in
, or 0 on failure, with NULL pointers stored into
the locations for bio1
. Check the error stack for more
[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(); //e.g SSL_read and SSL_write
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 the BIO_ctrl_pending()
, to find out whether data is buffered in the
BIO and must be transferred to the network. Use
to find out, how many bytes must be written
into the buffer before the SSL_operation()
can successfully be
As the data is buffered, SSL_operation()
may return with an
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()
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
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Licensed under the OpenSSL license (the "License"). You may not use
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file LICENSE in the source distribution or at