284 lines
7.2 KiB
Groff
284 lines
7.2 KiB
Groff
.\" $NetBSD: unix.4,v 1.20 2009/08/23 19:32:16 joerg Exp $
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.\"
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.\" Copyright (c) 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)unix.4 8.1 (Berkeley) 6/9/93
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.\"
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.Dd October 30, 2006
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.Dt UNIX 4
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.Os
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.Sh NAME
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.Nm unix
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.Nd UNIX-domain protocol family
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/un.h
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.Sh DESCRIPTION
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The
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.Tn UNIX Ns -domain
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protocol family is a collection of protocols
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that provides local (on-machine) interprocess
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communication through the normal
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.Xr socket 2
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mechanisms.
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The
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.Tn UNIX Ns -domain
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family supports the
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.Dv SOCK_STREAM
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and
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.Dv SOCK_DGRAM
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socket types and uses
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filesystem pathnames for addressing.
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.Sh ADDRESSING
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.Tn UNIX Ns -domain
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addresses are variable-length filesystem pathnames of
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at most 104 characters.
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The include file
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.Aq Pa sys/un.h
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defines this address:
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.Bd -literal -offset indent
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struct sockaddr_un {
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u_char sun_len;
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u_char sun_family;
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char sun_path[104];
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};
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.Ed
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.Pp
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Binding a name to a
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.Tn UNIX Ns -domain
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socket with
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.Xr bind 2
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causes a socket file to be created in the filesystem.
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This file is
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.Em not
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removed when the socket is closed\(em\c
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.Xr unlink 2
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must be used to remove the file.
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.Pp
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The length of
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.Tn UNIX Ns -domain
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address, required by
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.Xr bind 2
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and
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.Xr connect 2 ,
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can be calculated by the macro
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.Fn SUN_LEN
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defined in
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.Aq Pa sys/un.h .
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The
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.Ar sun_path
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field must be terminated by a NUL character to be used with
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.Fn SUN_LEN ,
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but the terminating NUL is
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.Em not
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part of the address.
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The
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.Nx
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kernel ignores any user-set value in the
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.Va sun_len
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member of the structure.
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.Pp
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The
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.Tn UNIX Ns -domain
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protocol family does not support broadcast addressing or any form
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of
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.Dq wildcard
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matching on incoming messages.
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All addresses are absolute- or relative-pathnames
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of other
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.Tn UNIX Ns -domain
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sockets.
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Normal filesystem access-control mechanisms are also
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applied when referencing pathnames; e.g., the destination
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of a
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.Xr connect 2
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or
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.Xr sendto 2
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must be writable.
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.Sh PROTOCOLS
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The
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.Tn UNIX Ns -domain
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protocol family comprises simple
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transport protocols that support the
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.Dv SOCK_STREAM
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and
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.Dv SOCK_DGRAM
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abstractions.
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.Dv SOCK_STREAM
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sockets also support the communication of
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.Ux
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file descriptors through the use of the
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.Ar msg_control
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field in the
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.Ar msg
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argument to
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.Xr sendmsg 2
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and
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.Xr recvmsg 2 .
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.Pp
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Any valid descriptor may be sent in a message.
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The file descriptor(s) to be passed are described using a
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.Ar struct cmsghdr
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that is defined in the include file
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.Aq Pa sys/socket.h .
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The type of the message is
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.Dv SCM_RIGHTS ,
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and the data portion of the messages is an array of integers
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representing the file descriptors to be passed.
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The number of descriptors being passed is defined
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by the length field of the message;
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the length field is the sum of the size of the header
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plus the size of the array of file descriptors.
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.Pp
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The received descriptor is a
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.Em duplicate
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of the sender's descriptor, as if it were created with a call to
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.Xr dup 2 .
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Per-process descriptor flags, set with
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.Xr fcntl 2 ,
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are
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.Em not
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passed to a receiver.
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Descriptors that are awaiting delivery, or that are
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purposely not received, are automatically closed by the system
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when the destination socket is closed.
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.Pp
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A UNIX-domain socket supports two
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.Tn socket-level
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options for use with
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.Xr setsockopt 2
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and
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.Xr getsockopt 2 :
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.Pp
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The
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.Dv LOCAL_CREDS
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option may be enabled on a
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.Dv SOCK_DGRAM
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or a
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.Dv SOCK_STREAM
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socket.
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This option provides a mechanism for the receiver to
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receive the credentials of the process as a
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.Xr recvmsg 2
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control message.
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The msg_control field in the msghdr structure points
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to a buffer that contains a cmsghdr structure followed by a variable
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length sockcred structure, defined in
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.Aq Pa sys/socket.h
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as follows:
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.Bd -literal
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struct sockcred {
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uid_t sc_uid; /* real user id */
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uid_t sc_euid; /* effective user id */
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gid_t sc_gid; /* real group id */
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gid_t sc_egid; /* effective group id */
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int sc_ngroups; /* number of supplemental groups */
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gid_t sc_groups[1]; /* variable length */
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};
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.Ed
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.Pp
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The
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.Dv LOCAL_PEEREID
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option may be used with
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.Xr getsockopt 2
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to get the PID and effective user and group IDs of a
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.Dv SOCK_STREAM
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peer when it did
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.Xr connect 2
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or
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.Xr bind 2 .
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The returned structure is
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.Bd -literal
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struct unpcbid {
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pid_t unp_pid; /* process id */
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uid_t unp_euid; /* effective user id */
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gid_t unp_egid; /* effective group id */
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};
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.Ed
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as defined in
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.Aq Pa sys/un.h .
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.Pp
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The
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.Fn SOCKCREDSIZE
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macro computes the size of the sockcred structure for a specified number
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of groups.
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The cmsghdr fields have the following values:
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.Bd -literal
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cmsg_len = sizeof(struct cmsghdr) + SOCKCREDSIZE(ngroups)
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cmsg_level = SOL_SOCKET
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cmsg_type = SCM_CREDS
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.Ed
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.Sh EXAMPLES
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The following code fragment shows how to bind a socket to pathname:
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.Bd -literal -offset indent
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const char *pathname = "/path/to/socket";
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struct sockaddr_un addr;
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int ret;
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memset(\*[Am]addr, 0, sizeof(addr));
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addr.sun_family = AF_LOCAL;
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if (strlen(pathname) \*[Ge] sizeof(addr.sun_path))
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goto too_long;
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strncpy(addr.sun_path, pathname, sizeof(addr.sun_path));
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ret = bind(s, (const struct sockaddr *)\*[Am]addr, SUN_LEN(\*[Am]addr));
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if (ret != 0)
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goto bind_failed;
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\&...
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.Ed
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.Sh COMPATIBILITY
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The
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.Ar sun_len
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field exists only in system derived from 4.4BSD.
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On systems which don't have the
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.Fn SUN_LEN
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macro, the following definition is recommended:
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.Bd -literal -offset indent
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#ifndef SUN_LEN
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#define SUN_LEN(su) sizeof(struct(sockaddr_un))
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#endif
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.Ed
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.Sh SEE ALSO
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.Xr socket 2 ,
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.Xr intro 4
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.Rs
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.%T "An Introductory 4.4BSD Interprocess Communication Tutorial"
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.%A Stuart Sechrest
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.Re
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.Pq see Pa /usr/share/doc/psd/20.ipctut
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.Rs
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.%T "Advanced 4.4BSD IPC Tutorial"
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.%A Samuel J. Leffler
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.%A Robert S. Fabry
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.%A William N. Joy
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.%A Phil Lapsley
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.%A Steve Miller
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.%A Chris Torek
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.Re
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.Pq see Pa /usr/share/doc/psd/21.ipc
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