BIO_new_bio_pair(3)BIO_new_bio_pair(3)NAMEBIO_new_bio_pair - Create a new BIO pair
size_t writebuf2 );
The BIO_new_bio_pair() function creates a buffering BIO pair. It has
two endpoints between data can be buffered. Its typical use is to con‐
nect one endpoint as underlying input/output BIO to an SSL and access
the other one controlled by the program instead of accessing the net‐
work 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 given is 0, the default size is used.
The BIO_new_bio_pair() function does not check whether bio1 or bio2
point to another BIO. The values are overwritten, and the BIO_free()
function is not called.
The two BIOs, even though forming a BIO pair, must be freed separately,
using the BIO_free() function. This is important because some SSL func‐
tions, such as SSL_set_bio() and SSL_free(), call BIO_free() implic‐
itly, so that the peer-BIO is left untouched and also must be freed
As the data is buffered, the SSL_operation() function might return 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 the
SSL_operation() function, 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: 1
The BIO pair was created successfully. The new BIOs are avail‐
able in bio1 and bio2. 0
The operation failed. The NULL pointer is stored into the loca‐
tions for bio1 and bio2. Check the error stack for more informa‐
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);
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 applica‐
tion has to flush the write buffer and/or fill the read buffer.
Use the BIO_ctrl_pending() function to find out whether data is
buffered in the BIO and must be transfered to the network. Use the
BIO_ctrl_get_read_request() function to find out how many bytes must be
written into the buffer before the SSL_operation() can continue.
Functions: SSL_set_bio(3), ssl(3), bio(3), BIO_ctrl_pending(3),