BIO_s_bio() manual page detailing BIO pair.

=pod

=head1 NAME

BIO_s_bio - BIO pair

=head1 SYNOPSIS

 #include <openssl/bio.h>

 BIO_METHOD *BIO_s_bio(void);

 #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_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)

 int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2);

 #define BIO_get_write_guarantee(b) (int)BIO_ctrl(b,BIO_C_GET_WRITE_GUARANTEE,0,NULL)

 size_t BIO_ctrl_get_write_guarantee(BIO *b);

 #define BIO_get_read_request(b)    (int)BIO_ctrl(b,BIO_C_GET_READ_REQUEST,0,NULL)

 size_t BIO_ctrl_get_read_request(BIO *b);

 int BIO_ctrl_reset_read_request(BIO *b);

=head1 DESCRIPTION

BIO_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.

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 SSL I/O under application control, this

can be used when the application wishes to use a non standard trasport for

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() and BIO_ctrl_wpending() can be used to

determine the amount of pending data in the read or write buffer.

BIO_reset() 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 both halves of the pair will automatically destroy the association.

BIO_set_write_buf_size() sets the write buffer size of BIO B<b> to B<size>.

If the size is not initialised 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(), BIO_make_bio_pair() and

BIO_set_write_buf_size() to create a connected pair of BIOs B<bio1>, B<bio2>

with write buffer sizes B<writebuf1> and B<writebuf2>. If either size is

zero then the default size is used.

BIO_get_write_guarantee() and BIO_ctrl_get_write_guarentee() 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() 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 failed due to an

empty buffer. This can be used to determine how much data should be written to the

other half of the pair so the next read will succeed: this is most useful in 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.

BIO_ctrl_reset_read_request() can also be used to reset the value returned by

BIO_get_read_request() to zero.

=head1 NOTES

Both halves of a BIO pair should be freed. That is even if one half is implicity

freed due to a BIO_free_all() or SSL_free() call the other half needs to be freed.

When used in bidirectional applications (such as SSL) care should be taken to

flush any data in the write buffer. This can be done by calling BIO_pending()

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() being true.

To see why this is important consider a case where a request is sent using

BIO_write() and a response read with BIO_read(), this can occur during an

SSL handshake for example. BIO_write() will succeed and place data in the write

buffer. BIO_read() will initially fail and BIO_should_read() will be true. If

the application then waits for data to be available on the underlying transport

before flusing the write buffer it will never succeed because the request was

never sent!

=head1 EXAMPLE

TBA

=head1 SEE ALSO

L<SSL_set_bio(3)|SSL_set_bio(3)>, L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>,

L<BIO_should_retry(3)|BIO_should_retry(3)>, L<BIO_read(3)|BIO_read(3)>

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