BIO_new_bio_pair - create a new BIO pair
int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2);
BIO_new_bio_pair() creates a buffering BIO pair based on the
SSL_set_bio(3) method. The BIO pair has two endpoints between which data can be buffered.
Its typical use is to connect one endpoint as underlying input/output BIO
to an SSL and access the other one controlled by the program instead of
accessing the network connection directly.
The two new BIOs bio1 and bio2 are symmetric with respect to their functionality. The size of their
buffers is determined by writebuf1 and
writebuf2. If the size give is 0, 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.
The two BIOs, even though forming a BIO pair and must be
BIO_free()'ed separately. This can be of importance, as some
BIO_free() implicitly, so that the peer-BIO is left
untouched and must also be
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 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.
The following return values can occur:
The BIO pair was created successfully. The new BIOs are available in
bio1 and bio2.
The operation failed. The NULL pointer is stored into the locations for
bio1 and bio2. Check the error stack for more information.
SSL_set_bio(3), ssl(3), bio(3),