=pod

=head1 NAME

BIO_s_dgram_pair, BIO_new_bio_dgram_pair, BIO_dgram_set_no_trunc,
BIO_dgram_get_no_trunc, BIO_dgram_get_effective_caps, BIO_dgram_get_caps,
BIO_dgram_set_caps, BIO_dgram_set_mtu, BIO_dgram_get_mtu - datagram pair BIO

=head1 SYNOPSIS

 #include <openssl/bio.h>

 const BIO_METHOD *BIO_s_dgram_pair(void);

 int BIO_new_bio_dgram_pair(BIO **bio1, size_t writebuf1,
                            BIO **bio2, size_t writebuf2);
 int BIO_dgram_set_no_trunc(BIO *bio, int enable);
 int BIO_dgram_get_no_trunc(BIO *bio);
 uint32_t BIO_dgram_get_effective_caps(BIO *bio);
 uint32_t BIO_dgram_get_caps(BIO *bio);
 int BIO_dgram_set_caps(BIO *bio, uint32_t caps);
 int BIO_dgram_set_mtu(BIO *bio, unsigned int mtu);
 unsigned int BIO_dgram_get_mtu(BIO *bio);

=head1 DESCRIPTION

BIO_s_dgram_pair() returns the method for a BIO datagram pair. A BIO datagram
pair is similar to a BIO pair (see L<BIO_s_bio(3)>) but has datagram semantics.
Broadly, this means that the length of the buffer passed to a write call will
match that retrieved by a read call. If the buffer passed to a read call is too
short, the datagram is truncated or the read fails, depending on how the BIO is
configured.

The BIO datagram pair attaches certain metadata to each write, such as source
and destination addresses. This information may be retrieved on read.

A typical application of a BIO datagram pair is to allow an application to keep
all datagram network I/O requested by libssl under application control.

The BIO datagram pair is designed to support multithreaded use where certain
restrictions are observed; see THREADING.

The BIO datagram pair allows each half of a pair to signal to the other half
whether they support certain capabilities; see CAPABILITY INDICATION.

BIO_new_bio_dgram_pair() combines the calls to L<BIO_new(3)>,
L<BIO_make_bio_pair(3)> and L<BIO_set_write_buf_size(3)> 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.

L<BIO_make_bio_pair(3)> may be used to join two datagram pair BIOs into a pair.
The two BIOs must both use the method returned by BIO_s_dgram_pair() and neither
of the BIOs may currently be associated in a pair.

L<BIO_destroy_bio_pair(3)> destroys the association between two connected BIOs.
Freeing either half of the pair will automatically destroy the association.

L<BIO_reset(3)> clears any data in the write buffer of the given BIO. This means
that the opposite BIO in the pair will no longer have any data waiting to be
read.

The BIO maintains a fixed size internal write buffer. When the buffer is full,
further writes will fail until the buffer is drained via calls to
L<BIO_read(3)>. The size of the buffer can be changed using
L<BIO_set_write_buf_size(3)> and queried using L<BIO_get_write_buf_size(3)>.

Note that the write buffer is partially consumed by metadata stored internally
which is attached to each datagram, such as source and destination addresses.
The size of this overhead is undefined and may change between releases.

The standard L<BIO_ctrl_pending(3)> call has modified behaviour and returns the
size of the next datagram waiting to be read in bytes. An application can use
this function to ensure it provides an adequate buffer to a subsequent read
call. If no datagram is waiting to be read, zero is returned.

This BIO does not support sending or receiving zero-length datagrams. Passing a
zero-length buffer to BIO_write is treated as a no-op.

L<BIO_eof(3)> returns 1 only if the given BIO datagram pair BIO is not currently
connected to a peer BIO.

L<BIO_get_write_guarantee(3)> and L<BIO_ctrl_get_write_guarantee(3)> return how
large a datagram the next call to L<BIO_write(3)> can accept. If there is not
enough space in the write buffer to accept another datagram equal in size to the
configured MTU, zero is returned (see below). This is intended to avoid a
situation where an application attempts to read a datagram from a network
intending to write it to a BIO datagram pair, but where the received datagram
ends up being too large to write to the BIO datagram pair.

BIO_dgram_set_no_trunc() and BIO_ctrl_get_no_trunc() set and retrieve the
truncation mode for the given half of a BIO datagram pair. When no-truncate mode
is enabled, BIO_read() will fail if the buffer provided is inadequate to hold
the next datagram to be read. If no-truncate mode is disabled (the default), the
datagram will be silently truncated. This default behaviour maintains
compatibility with the semantics of the Berkeley sockets API.

BIO_dgram_set_mtu() and BIO_dgram_get_mtu() may be used to set an informational
MTU value on the BIO datagram pair. If BIO_dgram_set_mtu() is used on a BIO
which is currently part of a BIO datagram pair, the MTU value is set on both
halves of the pair. The value does not affect the operation of the BIO datagram
pair (except for BIO_get_write_guarantee(); see above) but may be used by other
code to determine a requested MTU. When a BIO datagram pair BIO is created, the
MTU is set to an unspecified but valid value.

L<BIO_flush(3)> is a no-op.

=head1 NOTES

The halves of a BIO datagram pair have independent lifetimes and must be
separately freed.

=head1 THREADING

L<BIO_recvmmsg(3)>, L<BIO_sendmmsg(3)>, L<BIO_read(3)>, L<BIO_write(3)>,
L<BIO_pending(3)>, L<BIO_get_write_guarantee(3)> and L<BIO_flush(3)> may be used
by multiple threads simultaneously on the same BIO datagram pair. Specific
L<BIO_ctrl(3)> operations (namely BIO_CTRL_PENDING, BIO_CTRL_FLUSH and
BIO_C_GET_WRITE_GUARANTEE) may also be used. Invoking any other BIO call, or any
other L<BIO_ctrl(3)> operation, on either half of a BIO datagram pair while any
other BIO call is also in progress to either half of the same BIO datagram pair
results in undefined behaviour.

=head1 CAPABILITY INDICATION

The BIO datagram pair can be used to enqueue datagrams which have source and
destination addresses attached. It is important that the component consuming one
side of a BIO datagram pair understand whether the other side of the pair will
honour any source and destination addresses it attaches to each datagram. For
example, if datagrams are queued with destination addresses set but simply read
by simple calls to L<BIO_read(3)>, the destination addresses will be discarded.

Each half of a BIO datagram pair can have capability flags set on it which
indicate whether source and destination addresses will be honoured by the reader
and whether they will be provided by the writer. These capability flags should
be set via a call to BIO_dgram_set_caps(), and these capabilities will be
reflected in the value returned by BIO_dgram_get_effective_caps() on the
opposite BIO. If necessary, the capability value previously set can be retrieved
using BIO_dgram_get_caps(). Note that BIO_dgram_set_caps() on a given BIO
controls the capabilities advertised to the peer, and
BIO_dgram_get_effective_caps() on a given BIO determines the capabilities
advertised by the peer of that BIO.

The following capabilities are available:

=over 4

=item B<BIO_DGRAM_CAP_HANDLES_SRC_ADDR>

The user of the datagram pair BIO promises to honour source addresses provided
with datagrams written to the BIO pair.

=item B<BIO_DGRAM_CAP_HANDLES_DST_ADDR>

The user of the datagram pair BIO promises to honour destination addresses provided
with datagrams written to the BIO pair.

=item B<BIO_DGRAM_CAP_PROVIDES_SRC_ADDR>

The user of the datagram pair BIO advertises the fact that it will provide source
addressing information with future writes to the BIO pair, where available.

=item B<BIO_DGRAM_CAP_PROVIDES_DST_ADDR>

The user of the datagram pair BIO advertises the fact that it will provide
destination addressing information with future writes to the BIO pair, where
available.

=back

If a caller attempts to specify a destination address (for example, using
L<BIO_sendmmsg(3)>) and the peer has not advertised the
B<BIO_DGRAM_CAP_HANDLES_DST_ADDR> capability, the operation fails. Thus,
capability negotiation is mandatory.

If a caller attempts to specify a source address when writing, or requests a
destination address when receiving, and local address support has not been
enabled, the operation fails; see L<BIO_dgram_set_local_addr_enable(3)>.

If a caller attempts to enable local address support using
L<BIO_dgram_set_local_addr_enable(3)> and L<BIO_dgram_get_local_addr_cap(3)>
does not return 1 (meaning that the peer has not advertised both the
B<BIO_DGRAM_CAP_HANDLES_SRC_ADDR> and the B<BIO_DGRAM_CAP_PROVIDES_DST_ADDR>
capability), the operation fails.

B<BIO_DGRAM_CAP_PROVIDES_SRC_ADDR> and B<BIO_DGRAM_CAP_PROVIDES_DST_ADDR>
indicate that the application using that half of a BIO datagram pair promises to
provide source and destination addresses respectively when writing datagrams to
that half of the BIO datagram pair. However, these capability flags do not
affect the behaviour of the BIO datagram pair.

=head1 RETURN VALUES

BIO_new_bio_dgram_pair() returns 1 on success, with the new BIOs available in
B<bio1> and B<bio2>, or 0 on failure, with NULL pointers stored into the
locations for B<bio1> and B<bio2>. Check the error stack for more information.

BIO_dgram_set_no_trunc(), BIO_dgram_set_caps() and BIO_dgram_set_mtu() return 1
on success and 0 on failure.

BIO_dgram_get_no_trunc() returns 1 if no-truncate mode is enabled on a BIO, or 0
if no-truncate mode is not enabled or not supported on a given BIO.

BIO_dgram_get_effective_caps() and BIO_dgram_get_caps() return zero if no
capabilities are supported.

BIO_dgram_get_mtu() returns the MTU value configured on the BIO, or zero if the
operation is not supported.

=head1 SEE ALSO

L<BIO_s_bio(3)>, L<bio(7)>

=head1 COPYRIGHT

Copyright 2022 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut
