.Dd $Mdocdate: August 2 2015 $
.Dt BIO_S_BIO 3
.Os
.Sh NAME
.Nm BIO_s_bio ,
.Nm BIO_make_bio_pair ,
.Nm BIO_destroy_bio_pair ,
.Nm BIO_shutdown_wr ,
.Nm BIO_set_write_buf_size ,
.Nm BIO_get_write_buf_size ,
.Nm BIO_new_bio_pair ,
.Nm BIO_get_write_guarantee ,
.Nm BIO_ctrl_get_write_guarantee ,
.Nm BIO_get_read_request ,
.Nm BIO_ctrl_get_read_request ,
.Nm BIO_ctrl_reset_read_request
.Nd BIO pair BIO
.Sh SYNOPSIS
.In openssl/bio.h
.Ft BIO_METHOD *
.Fo BIO_s_bio
.Fa void
.Fc
.Bd -unfilled
#define	BIO_make_bio_pair(b1, b2) \e
	(int)BIO_ctrl(b1, BIO_C_MAKE_BIO_PAIR, 0, b2)
#define	BIO_destroy_bio_pair(b) \e
	(int)BIO_ctrl(b, BIO_C_DESTROY_BIO_PAIR, 0, NULL)
#define	BIO_shutdown_wr(b) \e
	(int)BIO_ctrl(b, BIO_C_SHUTDOWN_WR, 0, NULL)
#define	BIO_set_write_buf_size(b, size) \e
	(int)BIO_ctrl(b, BIO_C_SET_WRITE_BUF_SIZE, size, NULL)
#define	BIO_get_write_buf_size(b, size) \e
	(size_t)BIO_ctrl(b, BIO_C_GET_WRITE_BUF_SIZE, size, NULL)
.Ed
.Pp
.Ft int
.Fo BIO_new_bio_pair
.Fa "BIO **bio1"
.Fa "size_t writebuf1"
.Fa "BIO **bio2"
.Fa "size_t writebuf2"
.Fc
.Bd -unfilled
#define	BIO_get_write_guarantee(b) \e
	(int)BIO_ctrl(b, BIO_C_GET_WRITE_GUARANTEE, 0, NULL)
.Ed
.Pp
.Ft size_t
.Fo BIO_ctrl_get_write_guarantee
.Fa "BIO *b"
.Fc
.Bd -unfilled
#define	BIO_get_read_request(b) \e
	(int)BIO_ctrl(b, BIO_C_GET_READ_REQUEST, 0, NULL)
.Ed
.Pp
.Ft size_t
.Fo BIO_ctrl_get_read_request
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_ctrl_reset_read_request
.Fa "BIO *b"
.Fc
.Sh DESCRIPTION
.Fn 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.
Both halves must usually be handled by the same application thread
since no locking is done on the internal data structures.
.Pp
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.
.Pp
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.
.Pp
Calls to
.Xr BIO_read 3
will read data from the buffer or request a retry if no data is available.
.Pp
Calls to
.Xr BIO_write 3
will place data in the buffer or request a retry if the buffer is full.
.Pp
The standard calls
.Xr BIO_ctrl_pending 3
and
.Xr BIO_ctrl_wpending 3
can be used to determine the amount of pending data
in the read or write buffer.
.Pp
.Xr BIO_reset 3
clears any data in the write buffer.
.Pp
.Fn BIO_make_bio_pair
joins two separate BIOs into a connected pair.
.Pp
.Fn BIO_destroy_pair
destroys the association between two connected BIOs.
Freeing up any half of the pair will automatically destroy the association.
.Pp
.Fn BIO_shutdown_wr
is used to close down a BIO
.Fa b .
After this call no further writes on BIO
.Fa 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.
.Pp
.Fn BIO_set_write_buf_size
sets the write buffer size of BIO
.Fa b
to
.Fa size .
If the size is not initialized a default value is used.
This is currently 17K, sufficient for a maximum size TLS record.
.Pp
.Fn BIO_get_write_buf_size
returns the size of the write buffer.
.Pp
.Fn BIO_new_bio_pair
combines the calls to
.Xr BIO_new 3 ,
.Fn BIO_make_bio_pair
and
.Fn BIO_set_write_buf_size
to create a connected pair of BIOs
.Fa bio1
and
.Fa bio2
with write buffer sizes
.Fa writebuf1
and
.Fa writebuf2 .
If either size is zero, then the default size is used.
.Fn BIO_new_bio_pair
does not check whether
.Fa bio1
or
.Fa bio2
do point to some other BIO, the values are overwritten,
.Xr BIO_free 3
is not called.
.Pp
.Fn BIO_get_write_guarantee
and
.Fn BIO_ctrl_get_write_guarantee
return the maximum length of data
that can be currently written to the BIO.
Writes larger than this value will return a value from
.Xr BIO_write 3
less than the amount requested or if the buffer is full request a retry.
.Fn BIO_ctrl_get_write_guarantee
is a function whereas
.Fn BIO_get_write_guarantee
is a macro.
.Pp
.Fn BIO_get_read_request
and
.Fn 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.
Note that
.Fn BIO_get_read_request
never returns an amount larger than that returned by
.Fn BIO_get_write_guarantee .
.Pp
.Fn BIO_ctrl_reset_read_request
can also be used to reset the value returned by
.Fn BIO_get_read_request
to zero.
.Sh RETURN VALUES
.Fn BIO_new_bio_pair
returns 1 on success, with the new BIOs available in
.Fa bio1
and
.Fa bio2 ,
or 0 on failure, with NULL pointers stored into the locations for
.Fa bio1
and
.Fa bio2 .
Check the error stack for more information.
.\" XXX More return values need to be added here.
.Sh NOTES
Both halves of a BIO pair should be freed.
Even if one half is implicitly freed due to a
.Xr BIO_free_all 3
or
.Xr SSL_free 3
call, the other half still needs to be freed.
.Pp
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
.Xr BIO_pending 3
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
.Xr select 2 )
due to a request and
.Xr BIO_should_read 3
being true.
.Pp
To see why this is important,
consider a case where a request is sent using
.Xr BIO_write 3
and a response read with
.Xr BIO_read 3 ,
this can occur during a TLS/SSL handshake for example.
.Xr BIO_write 3
will succeed and place data in the write buffer.
.Xr BIO_read 3
will initially fail and
.Xr BIO_should_read 3
will be true.
If the application then waits for data to become available
on the underlying transport before flushing the write buffer,
it will never succeed because the request was never sent.
.Sh EXAMPLE
The BIO pair can be used to have full control
over the network access of an application.
The application can call
.Xr select 2
on the socket as required without having to go through the SSL-interface.
.Bd -literal -offset 2n
BIO *internal_bio, *network_bio;
\&...
BIO_new_bio_pair(internal_bio, 0, network_bio, 0);
SSL_set_bio(ssl, internal_bio, internal_bio);
SSL_operations();
\&...

application |   TLS-engine
   |        |
   +----------> SSL_operations()
            |     /\e    ||
            |     ||    \e/
            |   BIO-pair (internal_bio)
   +----------< BIO-pair (network_bio)
   |        |
 socket     |

\&...
SSL_free(ssl);		/* implicitly frees internal_bio */
BIO_free(network_bio);
\&...
.Ed
.Pp
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.
.Pp
Use
.Xr BIO_ctrl_pending 3
to find out whether data is buffered in the BIO
and must be transferred to the network.
Use
.Fn BIO_ctrl_get_read_request
to find out how many bytes must be written into the buffer before the
.Xr SSL_operation 3
can successfully be continued.
.Sh SEE ALSO
.Xr bio 3 ,
.Xr BIO_read 3 ,
.Xr BIO_should_retry 3 ,
.Xr ssl 3 ,
.Xr SSL_set_bio 3
.Sh CAVEATS
As the data is buffered,
.Xr SSL_operation 3
may return with an
.Dv ERROR_SSL_WANT_READ
condition, but there is still data in the write buffer.
An application must not rely on the error value of
.Xr SSL_operation 3
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.