BIO_destroy_bio_pair (3)
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(The comments found at the beginning of the groff file "man3/BIO_destroy_bio_pair.3ssl".)
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_new_bio_pair,
BIO_get_write_guarantee, BIO_ctrl_get_write_guarantee, BIO_get_read_request,
BIO_ctrl_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
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 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.
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 any half of the pair will automatically destroy the association.
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.
BIO_set_write_buf_size() 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.
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 bio1, bio2
with write buffer sizes writebuf1 and writebuf2. If either size is
zero then 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.
BIO_get_write_guarantee() and 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
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.
Note that
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. That is 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!
RETURN VALUES
BIO_new_bio_pair() returns 1 on success, with the new BIOs available in
bio1 and
bio2, or 0 on failure, with
NULL
pointers stored into the
locations for
bio1 and
bio2. Check the error stack for more information.
[
XXXXX:
More return values need to be added here]
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, 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.
WARNING
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.
SEE ALSO
SSL_set_bio(3),
ssl(3),
bio(3),
BIO_should_retry(3),
BIO_read(3)