BIO_s_bio() manual page detailing BIO pair.

This combines several manual pages provided by
Lutz Jaenicke <Lutz.Jaenicke@aet.TU-Cottbus.DE>,
various comments by Bodo to the lists and a bit
of source examination by me.

doc/crypto/BIO_s_bio.pod

+=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)>
+
+=cut