.\" This man page is Copyright (C) 1998 Heiner Eisen.
.\"
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.\" $Id: x25.7,v 1.4 1999/05/18 10:35:12 freitag Exp $
.\"
.TH X25 7 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
x25 \- ITU-T X.25 / ISO-8208 protocol interface.
.SH SYNOPSIS
.B #include <sys/socket.h>
.br
.B #include <linux/x25.h>
.PP
.B x25_socket = socket(AF_X25, SOCK_SEQPACKET, 0);
.SH DESCRIPTION
X25 sockets provide an interface to the X.25 packet layer protocol.
This allows applications to
communicate over a public X.25 data network as standardized by
International Telecommunication Union's recommendation X.25
(X.25 DTE-DCE mode).
X25 sockets can also be used for communication
without an intermediate X.25 network (X.25 DTE-DTE mode) as described
in ISO-8208.
.PP
Message boundaries are preserved \(em a
.BR read (2)
from a socket will
retrieve the same chunk of data as output with the corresponding
.BR write (2)
to the peer socket.
When necessary, the kernel takes care
of segmenting and reassembling long messages by means of
the X.25 M-bit.
There is no hard-coded upper limit for the
message size.
However, reassembling of a long message might fail if
there is a temporary lack of system resources or when other constraints
(such as socket memory or buffer size limits) become effective.
If that
occurs, the X.25 connection will be reset.
.SS Socket addresses
The
.B AF_X25
socket address family uses the
.I struct sockaddr_x25
for representing network addresses as defined in ITU-T
recommendation X.121.
.PP
.in +4n
.EX
struct sockaddr_x25 {
    sa_family_t sx25_family;    /* must be AF_X25 */
    x25_address sx25_addr;      /* X.121 Address */
};
.EE
.in
.PP
.I sx25_addr
contains a char array
.I x25_addr[]
to be interpreted as a null-terminated string.
.I sx25_addr.x25_addr[]
consists of up to 15 (not counting the terminating null byte) ASCII
characters forming the X.121 address.
Only the decimal digit characters from \(aq0\(aq to \(aq9\(aq are allowed.
.SS Socket options
The following X.25-specific socket options can be set by using
.BR setsockopt (2)
and read with
.BR getsockopt (2)
with the
.I level
argument set to
.BR SOL_X25 .
.TP
.B X25_QBITINCL
Controls whether the X.25 Q-bit (Qualified Data Bit) is accessible by the
user.
It expects an integer argument.
If set to 0 (default),
the Q-bit is never set for outgoing packets and the Q-bit of incoming
packets is ignored.
If set to 1, an additional first byte is prepended
to each message read from or written to the socket.
For data read from
the socket, a 0 first byte indicates that the Q-bits of the corresponding
incoming data packets were not set.
A first byte with value 1 indicates
that the Q-bit of the corresponding incoming data packets was set.
If the first byte of the data written to the socket is 1, the Q-bit of the
corresponding outgoing data packets will be set.
If the first byte is 0,
the Q-bit will not be set.
.SH VERSIONS
The AF_X25 protocol family is a new feature of Linux 2.2.
.SH BUGS
Plenty, as the X.25 PLP implementation is
.BR CONFIG_EXPERIMENTAL .
.PP
This man page is incomplete.
.PP
There is no dedicated application programmer's header file yet;
you need to include the kernel header file
.IR <linux/x25.h> .
.B CONFIG_EXPERIMENTAL
might also imply that future versions of the
interface are not binary compatible.
.PP
X.25 N-Reset events are not propagated to the user process yet.
Thus,
if a reset occurred, data might be lost without notice.
.SH SEE ALSO
.BR socket (2),
.BR socket (7)
.PP
Jonathan Simon Naylor:
\(lqThe Re-Analysis and Re-Implementation of X.25.\(rq
The URL is
.UR ftp://ftp.pspt.fi\:/pub\:/ham\:/linux\:/ax25\:/x25doc.tgz
.UE .
.SH COLOPHON
This page is part of release 4.16 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.