BIO_new_bio_pair(3)

NAME
BIO_new_bio_pair - create a new BIO pair

SYNOPSIS

 #include <openssl/bio.h>
 int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2);

DESCRIPTION
BIO_new_bio_pair() creates a buffering BIO pair. It has two endpoints between 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 SSL-functions like SSL_set_bio() or SSL_free() call BIO_free() implicitly, so that the peer-BIO is left untouched and must also be BIO_free()'ed.

EXAMPLE
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);
 SSL_operations();
 ...
 application |   TLS-engine
    |        |
    +----------> SSL_operations()
             |     /\    ||
             |     ||    \/
             |   BIO-pair (internal_bio)
    +----------< BIO-pair (network_bio)
    |        |
  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.

Use the 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 SSL_operation() can successfully be continued.

IMPORTANT
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.

RETURN VALUES
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.

SEE ALSO
SSL_set_bio(3), ssl(3), bio(3), BIO_ctrl_pending(3), BIO_ctrl_get_read_request(3)