1.\" Copyright (c) 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)unix.4 8.1 (Berkeley) 6/9/93 33.\" $FreeBSD$ 34.\" 35.Dd October 5, 2009 36.Dt UNIX 4 37.Os 38.Sh NAME 39.Nm unix 40.Nd UNIX-domain protocol family 41.Sh SYNOPSIS 42.In sys/types.h 43.In sys/un.h 44.Sh DESCRIPTION 45The 46.Ux Ns -domain 47protocol family is a collection of protocols 48that provides local (on-machine) interprocess 49communication through the normal 50.Xr socket 2 51mechanisms. 52The 53.Ux Ns -domain 54family supports the 55.Dv SOCK_STREAM , 56.Dv SOCK_SEQPACKET , 57and 58.Dv SOCK_DGRAM 59socket types and uses 60file system pathnames for addressing. 61.Sh ADDRESSING 62.Ux Ns -domain 63addresses are variable-length file system pathnames of 64at most 104 characters. 65The include file 66.In sys/un.h 67defines this address: 68.Bd -literal -offset indent 69struct sockaddr_un { 70 u_char sun_len; 71 u_char sun_family; 72 char sun_path[104]; 73}; 74.Ed 75.Pp 76Binding a name to a 77.Ux Ns -domain 78socket with 79.Xr bind 2 80causes a socket file to be created in the file system. 81This file is 82.Em not 83removed when the socket is closed \(em 84.Xr unlink 2 85must be used to remove the file. 86.Pp 87The length of 88.Ux Ns -domain 89address, required by 90.Xr bind 2 91and 92.Xr connect 2 , 93can be calculated by the macro 94.Fn SUN_LEN 95defined in 96.In sys/un.h . 97The 98.Va sun_path 99field must be terminated by a 100.Dv NUL 101character to be used with 102.Fn SUN_LEN , 103but the terminating 104.Dv NUL 105is 106.Em not 107part of the address. 108.Pp 109The 110.Ux Ns -domain 111protocol family does not support broadcast addressing or any form 112of 113.Dq wildcard 114matching on incoming messages. 115All addresses are absolute- or relative-pathnames 116of other 117.Ux Ns -domain 118sockets. 119Normal file system access-control mechanisms are also 120applied when referencing pathnames; e.g., the destination 121of a 122.Xr connect 2 123or 124.Xr sendto 2 125must be writable. 126.Sh PROTOCOLS 127The 128.Ux Ns -domain 129protocol family is comprised of simple 130transport protocols that support the 131.Dv SOCK_STREAM , 132.Dv SOCK_SEQPACKET , 133and 134.Dv SOCK_DGRAM 135abstractions. 136.Dv SOCK_STREAM 137and 138.Dv SOCK_SEQPACKET 139sockets also support the communication of 140.Ux 141file descriptors through the use of the 142.Va msg_control 143field in the 144.Fa msg 145argument to 146.Xr sendmsg 2 147and 148.Xr recvmsg 2 . 149.Pp 150Any valid descriptor may be sent in a message. 151The file descriptor(s) to be passed are described using a 152.Vt "struct cmsghdr" 153that is defined in the include file 154.In sys/socket.h . 155The type of the message is 156.Dv SCM_RIGHTS , 157and the data portion of the messages is an array of integers 158representing the file descriptors to be passed. 159The number of descriptors being passed is defined 160by the length field of the message; 161the length field is the sum of the size of the header 162plus the size of the array of file descriptors. 163.Pp 164The received descriptor is a 165.Em duplicate 166of the sender's descriptor, as if it were created with a call to 167.Xr dup 2 . 168Per-process descriptor flags, set with 169.Xr fcntl 2 , 170are 171.Em not 172passed to a receiver. 173Descriptors that are awaiting delivery, or that are 174purposely not received, are automatically closed by the system 175when the destination socket is closed. 176.Pp 177The effective credentials (i.e., the user ID and group list) of a 178peer on a 179.Dv SOCK_STREAM 180socket may be obtained using the 181.Dv LOCAL_PEERCRED 182socket option. 183This may be used by a server to obtain and verify the credentials of 184its client, and vice versa by the client to verify the credentials 185of the server. 186These will arrive in the form of a filled in 187.Vt "struct xucred" 188(defined in 189.In sys/ucred.h ) . 190The credentials presented to the server (the 191.Xr listen 2 192caller) are those of the client when it called 193.Xr connect 2 ; 194the credentials presented to the client (the 195.Xr connect 2 196caller) are those of the server when it called 197.Xr listen 2 . 198This mechanism is reliable; there is no way for either party to influence 199the credentials presented to its peer except by calling the appropriate 200system call (e.g., 201.Xr connect 2 202or 203.Xr listen 2 ) 204under different effective credentials. 205.Pp 206.Tn UNIX 207domain sockets support a number of socket options which can be set with 208.Xr setsockopt 2 209and tested with 210.Xr getsockopt 2 : 211.Bl -tag -width ".Dv LOCAL_CONNWAIT" 212.It Dv LOCAL_CREDS 213This option may be enabled on 214.Dv SOCK_DGRAM , 215.Dv SOCK_SEQPACKET , 216or a 217.Dv SOCK_STREAM 218socket. 219This option provides a mechanism for the receiver to 220receive the credentials of the process as a 221.Xr recvmsg 2 222control message. 223The 224.Va msg_control 225field in the 226.Vt msghdr 227structure points to a buffer that contains a 228.Vt cmsghdr 229structure followed by a variable length 230.Vt sockcred 231structure, defined in 232.In sys/socket.h 233as follows: 234.Bd -literal 235struct sockcred { 236 uid_t sc_uid; /* real user id */ 237 uid_t sc_euid; /* effective user id */ 238 gid_t sc_gid; /* real group id */ 239 gid_t sc_egid; /* effective group id */ 240 int sc_ngroups; /* number of supplemental groups */ 241 gid_t sc_groups[1]; /* variable length */ 242}; 243.Ed 244.Pp 245The 246.Fn SOCKCREDSIZE 247macro computes the size of the 248.Vt sockcred 249structure for a specified number 250of groups. 251The 252.Vt cmsghdr 253fields have the following values: 254.Bd -literal 255cmsg_len = CMSG_LEN(SOCKCREDSIZE(ngroups)) 256cmsg_level = SOL_SOCKET 257cmsg_type = SCM_CREDS 258.Ed 259.It Dv LOCAL_CONNWAIT 260Used with 261.Dv SOCK_STREAM 262sockets, this option causes the 263.Xr connect 2 264function to block until 265.Xr accept 2 266has been called on the listening socket. 267.El 268.Sh SEE ALSO 269.Xr socket 2 , 270.Xr intro 4 271.Rs 272.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial" 273.%B PS1 274.%N 7 275.Re 276.Rs 277.%T "An Advanced 4.3 BSD Interprocess Communication Tutorial" 278.%B PS1 279.%N 8 280.Re 281