Ubuntu Manpage: BIO_s_dgram_pair, BIO_new_bio_dgram_pair, BIO_dgram_set_no_trunc, BIO_dgram_get_no

Provided by: libssl-doc_3.4.1-1ubuntu3_all

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

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);

DESCRIPTION

BIO_s_dgram_pair() returns the method for a BIO datagram pair. A BIO datagram pair is similar to a BIO
pair (see 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 BIO_new(3), BIO_make_bio_pair(3) and
BIO_set_write_buf_size(3) 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_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.

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

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 BIO_read(3). The size of the buffer can be changed using
BIO_set_write_buf_size(3) and queried using 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 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.

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

BIO_get_write_guarantee(3) and BIO_ctrl_get_write_guarantee(3) return how large a datagram the next call
to 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.

BIO_flush(3) is a no-op.

NOTES

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

THREADING

       BIO_recvmmsg(3), BIO_sendmmsg(3), BIO_read(3), BIO_write(3), BIO_pending(3), BIO_get_write_guarantee(3)
       and BIO_flush(3) may be used by multiple threads simultaneously on the same BIO datagram pair. Specific
       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 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.

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
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:

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.

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.

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.

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.

If a caller attempts to specify a destination address (for example, using BIO_sendmmsg(3)) and the peer
has not advertised the 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
BIO_dgram_set_local_addr_enable(3).

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

BIO_DGRAM_CAP_PROVIDES_SRC_ADDR and 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.

RETURN VALUES

       BIO_new_bio_dgram_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.

       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.

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
       <https://www.openssl.org/source/license.html>.

3.4.1                                              2025-04-03                             BIO_S_DGRAM_PAIR(3SSL)