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BIO_s_bio(3)


NAME

  BIO_s_bio, BIO_make_bio_pair, BIO_destroy_bio_pair, BIO_shutdown_wr,
  BIO_set_write_buf_size, BIO_get_write_buf_size, BIO_get_write_guarantee,
  BIO_ctrl_get_write_guarantee, BIO_get_read_request,
  BIO_ctrl_reset_read_request - BIO pair BIO


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_shutdown_wr(b) (int)BIO_ctrl(b, BIO_C_SHUTDOWN_WR, 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 );


DESCRIPTION

  The BIO_s_bio() function 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. Both halves must usually by
  handled by the same application thread since no locking is done on the
  internal data structures.

  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 TLS/SSL I/O under application
  control, this can be used when the application wishes to use a non standard
  transport for TLS/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.

  The BIO_reset() function clears any data in the write buffer.

  The BIO_make_bio_pair() function joins two separate BIOs into a connected
  pair.

  The BIO_destroy_pair() function destroys the association between two
  connected BIOs. Freeing up any half of the pair will automatically destroy
  the association.

  The BIO_shutdown_wr() is used to close down a BIO b. After this call no
  further writes on BIO b are allowed. They will return an error. Reads on
  the other half of the pair will return any pending data or EOF when all
  pending data has been read.

  The BIO_set_write_buf_size() function sets the write buffer size of BIO b
  to size. If the size is not initialized a default value is used. This is
  currently 17K, sufficient for a maximum size TLS record.

  The BIO_get_write_buf_size() function returns the size of the write buffer.

  The BIO_new_bio_pair() function combines the calls to BIO_new(),
  BIO_make_bio_pair(), and BIO_set_write_buf_size() to create a connected
  pair of BIOs, bio1 and bio2, with write buffer sizes writebuf1 and
  writebuf2. If either size is zero then the default size is used.

  BIO_get_write_guarantee() and BIO_ctrl_get_write_guarantee() return the
  maximum length of data that can be 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 attempt
  at the other half of the BIO pair failed due to an empty buffer.  This can
  be used to determine how much data should be written to the BIO so the next
  read will succeed. This is most useful in TLS/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.  It also will return
  zero once new data has been written satisfying the read request or part of
  it.  BIO_get_read_request() never returns an amount larger than that
  returned by BIO_get_write_guarantee().

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


NOTES

  Both halves of a BIO pair should be freed. Even if one half is implicit
  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 TLS/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
  TLS/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 flushing the write buffer it will never succeed
  because the request was never sent.


SEE ALSO

  Functions: SSL_set_bio(3), ssl(3), bio(3), BIO_ctrl_get_read_request(3),
  BIO_new_bio_pair(3), BIO_should_retry(3), BIO_read(3)

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