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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)unix.4 8.1 (Berkeley) 6/9/93 29.\" $FreeBSD: src/share/man/man4/unix.4,v 1.3.2.4 2002/04/22 08:59:53 dd Exp $ 30.\" 31.Dd November 1, 2015 32.Dt UNIX 4 33.Os 34.Sh NAME 35.Nm unix 36.Nd UNIX-domain protocol family 37.Sh SYNOPSIS 38.In sys/types.h 39.In sys/un.h 40.Sh DESCRIPTION 41The 42.Ux Ns -domain 43protocol family is a collection of protocols 44that provides local (on-machine) interprocess 45communication through the normal 46.Xr socket 2 47mechanisms. 48The 49.Ux Ns -domain 50family supports the 51.Dv SOCK_STREAM , 52.Dv SOCK_DGRAM 53and 54.Dv SOCK_SEQPACKET 55socket types and uses 56filesystem pathnames for addressing. 57.Sh ADDRESSING 58.Ux Ns -domain 59addresses are variable-length filesystem pathnames of 60at most 104 characters. 61The include file 62.In sys/un.h 63defines this address: 64.Bd -literal -offset indent 65struct sockaddr_un { 66u_char sun_len; 67sa_family_t sun_family; 68char sun_path[104]; 69}; 70.Ed 71.Pp 72Binding a name to a 73.Ux Ns -domain 74socket with 75.Xr bind 2 76causes a socket file to be created in the filesystem. 77This file is 78.Em not 79removed when the socket is closed \(em 80.Xr unlink 2 81must be used to remove the file. 82.Pp 83The 84.Ux Ns -domain 85protocol family does not support broadcast addressing or any form 86of 87.Dq wildcard 88matching on incoming messages. 89All addresses are absolute- or relative-pathnames 90of other 91.Ux Ns -domain 92sockets. 93Normal filesystem access-control mechanisms are also 94applied when referencing pathnames; e.g., the destination 95of a 96.Xr connect 2 97or 98.Xr sendto 2 99must be writable. 100.Sh PROTOCOLS 101The 102.Ux Ns -domain 103protocol family is comprised of simple 104transport protocols that support the 105.Dv SOCK_STREAM , 106.Dv SOCK_DGRAM 107and 108.Dv SOCK_SEQPACKET 109abstractions. 110.Dv SOCK_STREAM , 111.Dv SOCK_DGRAM 112and 113.Dv SOCK_SEQPACKET 114sockets also support the communication of 115.Ux 116file descriptors through the use of the 117.Va msg_control 118field in the 119.Fa msg 120argument to 121.Xr sendmsg 2 122and 123.Xr recvmsg 2 . 124.Pp 125Any valid descriptor may be sent in a message. 126The file descriptor(s) to be passed are described using a 127.Vt "struct cmsghdr" 128that is defined in the include file 129.In sys/socket.h . 130The type of the message is 131.Dv SCM_RIGHTS , 132and the data portion of the messages is an array of integers 133representing the file descriptors to be passed. 134The number of descriptors being passed is defined 135by the length field of the message; 136the length field is the sum of the size of the header 137plus the size of the array of file descriptors. 138.Pp 139The received descriptor is a 140.Em duplicate 141of the sender's descriptor, as if it were created via 142.Li dup(fd) 143or 144.Li fcntl(fd, F_DUPFD_CLOEXEC, 0) 145depending on whether 146.Dv MSG_CMSG_CLOEXEC 147is passed in the 148.Xr recvmsg 2 149call. 150Descriptors that are awaiting delivery, or that are 151purposely not received, are automatically closed by the system 152when the destination socket is closed. 153.Pp 154The effective credentials (i.e., the user ID and group list) of a 155peer on a 156.Dv SOCK_STREAM 157socket may be obtained using the 158.Dv LOCAL_PEERCRED 159socket option. 160This may be used by a server to obtain and verify the credentials of 161its client, and vice versa by the client to verify the credentials 162of the server. 163These will arrive in the form of a filled in 164.Vt "struct xucred" 165(defined in 166.In sys/ucred.h ) . 167The credentials presented to the server (the 168.Xr listen 2 169caller) are those of the client when it called 170.Xr connect 2 ; 171the credentials presented to the client (the 172.Xr connect 2 173caller) are those of the server when it called 174.Xr listen 2 . 175This mechanism is reliable; there is no way for either party to influence 176the credentials presented to its peer except by calling the appropriate 177system call (e.g., 178.Xr connect 2 179or 180.Xr listen 2 ) 181under different effective credentials. 182.Sh SEE ALSO 183.Xr socket 2 , 184.Xr intro 4 185.Rs 186.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial" 187.%B PS1 188.%N 7 189.Re 190.Rs 191.%T "An Advanced 4.3 BSD Interprocess Communication Tutorial" 192.%B PS1 193.%N 8 194.Re 195